WO1997042073A1

WO1997042073A1 - Hydrofoil craft

Info

Publication number: WO1997042073A1
Application number: PCT/US1997/007660
Authority: WO
Inventors: Peter R. Payne
Original assignee: Dynafoils, Inc.; CULLMAN, Marguerite, P.
Priority date: 1996-05-06
Filing date: 1997-05-06
Publication date: 1997-11-13
Also published as: AU3203997A

Abstract

A hydrofoil craft has a hull (110) and one or more foils (120, 130) for generating lift to support the hull. The drag on the foils can be greatly reduced compared to conventional foils to increase the efficiency of the hydrofoil craft.

Description

HYDROFOIL CRAFT

Background of the Invention

1. Field of the Invention

This invention relates to maritime vessels, and particularly to hydrofoil craft capable of operating with lower drag than conventional hydrofoil craft.

2. Description of the Related Art

Hydrofoil craft are theoretically capable of operating at speeds of several hundred knots, which is faster than conventional land vehicles, including high speed trains. However, as the speed of a hydrofoil craft increases, the drag incurred by the hydrofoils of the craft rapidly increases to high levels which make high-speed operation of conventional hydrofoil craft impractical.

Summary of the Invention

The present invention provides a hydrofoil craft capable of operating at high speeds with a lower drag than conventional hydrofoil craft. The present invention further provides a hydrofoil craft capable of operating in waves while experiencing reduced rolling motion.

The present invention also provides a hydrofoil craft capable of operating in a planing mode even in rough water without discomfort to passengers aboard the craft.

The present invention additionally provides a hydrofoil craft having an improved hull structure capable of supporting large pay loads.

The present invention yet further provides a hydrofoil craft having an improved support structure for a propeller shaft.

The present invention also provides a maritime vessel capable of being easily unbeached from a shore. The drag on a hydrofoil craft according to the present invention can be reduced to increase the efficiency of the hydrofoil craft in a variety of ways, which may be employed separately or in combination with one another. These include but are not limited to performing distributed suction on a submerged surface of a foil of the hydrofoil craft to stabilize a boundary layer on the surface, introducing microbubbles into a turbulent boundary layer on a submerged surface of the foil to reduce the skin friction, heating a submerged surface of the foil to above the ambient water temperature to stabilize the boundary layer on the surface, ventilating an upper surface of the foil so that only the lower surface of the foil is exposed to water, shaping the foil so that the upper surface of the foil does not produce cavitation while the lower surface has zero pressure drag, varying the span of the foil or the total span of a plurality of foils in accordance with the speed of the hydrofoil craft to maintain a favorable lift coefficient, and increasing the lift of the foil by disposing it in contact with water it in a region where the wake from another foil located in front of it has an upwards velocity component.

According to one form of the present invention, a hydrofoil craft includes a hull and a foil disposed beneath the hull and generating lift to support the hull and having an upper and lower surface and passages for fluid communicating between the lower surface of the foil and a region of the hydrofoil craft at a lower pressure than the lower surface during foil-borne operation of the hydrofoil. During operation of the hydrofoil craft, water is sucked through the lower surface of the foil due to the difference in pressure between the lower surface and the region at a lower pressure. The suction of the water stabilizes the boundary layer on the lower surface, enabling a laminar boundary layer to be maintained at higher Reynold's numbers.

According to another form of the present invention, a hydrofoil craft includes a hull, a foil disposed below the hull for generating lift to support the hull and having a plurality of holes formed in a surface thereof, and a conduit connected to the holes for supplying air to the holes such that microbubbles are discharged from the holes into a boundary layer on the surface of the foil. When injected into a turbulent boundary layer, the microbubbles decrease the density of the fluid in the boundary layer, thereby reducing the skin friction of the foil.

According to still another form of the present invention, a hydrofoil craft includes a hull, an engine disposed in the hull and having a cooling system, a foil disposed beneath the hull for generating lift to support the hull, and a conduit connected between the cooling system of the engine and the foil for circulating engine coolant through the foil to heat a surface of the foil to above the ambient water temperature to stabilize a boundary layer on the surface. Due to the stabilization of the boundary layer by the heating, a laminar boundary layer can be maintained at higher Reynold's numbers, resulting in reduced drag.

According to an additional form of the present invention, a method of operating a hydrofoil craft includes supporting a hull of a hydrofoil craft above a water surface with a foil planing on the water surface and connected to the hull by a support arm pivotably connected to the hull for pivoting about a transverse axis with respect to the hull as the hull is moving. Planing operation of the foil gives the hydrofoil craft excellent stability, while the pivotable connection of the support arm to the hull reduces accelerations of the hull caused by surface waves and increases the ride comfort.

According to another form of the present invention, a foil for use with a hydrofoil craft includes an upper surface having an inflection between its leading and trailing edges. Preferably, tangents to the leading and trailing edges are substantially parallel to each other. When the foil is at its design angle of attack at cruising speed, the upper surface produces substantially no lift so will not cavitate up to very high speeds. The lower surface of the foil is preferably shaped to produce substantially zero pressure drag when the foil is at the design angle of attack. The foil preferably has a ventilated base separating the trailing edge of the upper surface from the trailing edge of the lower surface. Due to the lack of cavitation on the upper surface and the zero pressure drag on the lower surface, the foil is highly efficient for high speed operation. According to yet another form of the present invention, a hydrofoil craft includes a hull, a foil for generating lift to support the hull, first and second support arms connected to the foil and pivotably connected to the hull for pivoting about a transverse axis extending in a widthwise direction of the hull, and first and second biasing mechanisms associated with the first and second support arms, respectively, for independently biasing the first and second support arms away from the hull. Because the support arms are independently biased, the hydrofoil craft can undergo less rolling motion in waves, increasing the ride comfort of the hydrofoil craft. The foil may also be divided into two or more foils disposed side by side, each connected to a corresponding support arm, and capable of moving independently of the adjoining foil(s).

In another form of the present invention, a hydrofoil craft includes a hull, a foil disposed beneath the hull for generating lift to support the hull, a first suspension movably mounted on the hull and having a support point biased away from the hull, and a second suspension including a support arm connected to the foil and pivotably connected to the first suspension at the support point for pivoting about a transverse axis extending in a widthwise direction of the hull and biased away from the first suspension. The provision of two suspensions can enable the hydrofoil craft to operate more effectively and increase the ride comfort.

In a further form of the present invention, a hydrofoil craft includes a hull, a first foil set having an adjustable total span for generating lift to support the hull, and at least one support arm connecting the foil set to the hull. The foil set may comprise a single foil with an adjustable span, or it may comprise a plurality of adjoining foils, the total span of which can be varied in accordance with the speed of the hydrofoil craft by lifting one or more of the foils above the water surface. The ability to vary the span of the foil set increases the efficiency of the hydrofoil craft by enabling the span to be set to the optimal length for a particular speed.

According to still another form of the present invention, a foil arrangement for a hydrofoil craft includes a foil having first and second spanwise ends, and a pair of support arms each having a first end secured to one of the ends of the spanwise ends of the foil and a second sloping towards the other support arm. Such a foil arrangement has high stiffness against lateral forces so it enables adjoining foils to be arranged end to end in close proximity for efficient operation.

In a further form of the present invention, a hydrofoil craft includes a hull, a first foil disposed beneath the hull in contact with water to generate lift, and a second foil disposed beneath the hull behind the first foil to generate lift in a region where a wake formed by the first foil has an upwards velocity component. The wake can significantly decrease the drag on the second foil, thereby increasing the efficiency of the hydrofoil craft. The second foil can be maintained in the region where the wake has an upwards velocity as the speed of the hydrofoil craft changes by varying the span of the first foil or by varying the total span of a plurality of foils disposed in front of the second foil. In another form of the present invention, a maritime vessel has a shaft support arrangement for a propeller shaft which prevents the shaft from whirling without the need for completely immobilized bearings to support the shaft. Not only can the shaft support arrangement reduce weight of structure for supporting the shaft, but it enables the shaft to be easily raised and lowered with respect to the hull of the vessel.

According to another form of the present invention, a maritime vessel includes a hull having a chamber in a lower portion thereof communicating with a bottom of the hull, and a pump for discharging water from the chamber to outside the hull with sufficient force to support the weight of the hull above a ground surface, such as a beach. When the hull is supported by the water discharged from the chamber, it can be easily moved along the ground surface to enable the vessel to be returned to a body of water. According to a still further form of the present invention, a maritime vessel includes a hull, an upright extending upwards from the hull, and a plurality of connecting members connected between the upright and the hull for transmitting a portion of the weight of the vessel to the upright. Such a structure is considerably lighter than a conventional hull structure. These and other features of the present invention will be described in detail below with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a side elevation of a first embodiment of a hydrofoil craft according to the present invention. Figure 2 is a side view of the main foil of the embodiment of Figure 1.

Figure 3 is an enlarged cross-sectional view of region A of Figure 2.

Figure 4 is a view of the bottom surface of the foil of Figure 2.

Figures 5 - 8 show various modifications of the region illustrated in Figure 3.

Figure 9 is a cross-sectional elevation of an embodiment in which the foil is equipped with a plenum which can be backflushed.

Figure 10 is a cross-sectional elevation of an embodiment in which distributed suction is applied to both surfaces of a foil.

Figure 11 is a cross-sectional elevation of an embodiment in which distributed suction is applied to one surface of a foil and microbubbles are released from a second surface of the foil.

Figure 12 is a cross-sectional elevation of an embodiment in which a foil is heated by the engine cooling system.

Figure 13 is a cross-sectional elevation of an embodiment in which a foil is heated electrically.

Figure 14 is a cross-sectional elevation of an embodiment in which both distributed suction and heating are applied to a foil.

Figure 15 is a side elevation of an embodiment of the present invention operating in a planing mode.

Figure 16 is a side elevation of the foil of the embodiment of Figure 15 as it passes through a wave. Figures 17 - 19 are respectively a plan view, a front elevation, and a side elevation of a foil which can be used in the present invention.

Figure 20 is a front elevation of a surface-piercing foil which can be used in the present invention.

Figure 21 is a side elevation of a foil which can be operated at high speeds without cavitation.

Figure 22 is a perspective view of an example of a foil arrangement for a hydrofoil craft which can alleviate rolling moments.

Figures 23 and 24 are schematic rear views of a hydrofoil craft equipped with the foil arrangement of Figure 22 in calm water and in a wave, respectively. Figure 25 schematically illustrates an embodiment having a pair of foils located side by side which can move independently of one another.

Figures 26 and 27 are schematic rear elevations of a hydrofoil craft equipped with the foils of Figure 25 operating in calm water and in a wave, respectively. Figure 28 is a schematic side view of a hydrofoil craft illustrating how the support arm is biased away from the hull in the embodiment of Figure 1.

Figures 29 and 30 are schematic side view of a hydrofoil craft having a double suspension.

Figure 31 is a graph showing the drag of a fixed-span foil as a function of speed. Figure 32 is a graph showing the drag as a function of speed for a foil having a span which is varied in accordance with speed so as to maintain an approximately constant lift coefficient above the speed of minimum drag.

Figure 33 is a graph showing the drag as a function of speed for a foil having a span which is varied in accordance with speed so as to attain a minimum total drag at any speed.

Figure 34 is a graph showing the variation of foil span with speed for Figures 32 and 33. Figure 35 is a schematic plan view of a foil having an adjustable span.

Figures 36 - 38 are an aft elevation, a side elevation, and a plan view of a set of foils which can be raised and lowered with respect to a hull to vary the total span of the foils in accordance with speed.

Figures 39A and 40 are respectively a front elevation and a side elevation of a foil equipped with support arms of high lateral stiffness, while Figures 39B - 39D are transverse cross sections of various possible shapes of the support arms.

Figures 41 and 42 are respectively an aft elevation and a side elevation of a set of foils equipped with wings on their tips.

Figure 43 is a front elevation of a set of foils arranged in the shape of a V. Figure 44 is a plan view of a set of foils arranged to create a swept shape.

Figure 45 is a plan view of a set of foils having a chord which decreases from the outer ends towards the center of the set.

Figure 46 is a plan view of a set of foils which are staggered in the fore and aft direction and overlap in the spanwise direction. Figures 47 - 49 are schematic front elevations of a hydrofoil craft according to the present invention during take-off, medium speed operation, and high speed cruising operation.

Figure 50 is a schematic side view of a front foil and an aft forward operating in a region where there is favorable interference. Figure 51 is a schematic side elevation of a hydrofoil craft having three sets of foils spaced in the lengthwise direction of its hull.

Figure 52 is a graph of the wake height contours behind a foil. Figure 53 is a side elevation of a hydrofoil craft equipped with a shaft support arrangement according to the present invention. Figure 54 is a transverse cross-sectional view taken along line II-II of Figure 53.

Figure 55 is a longitudinal cross-sectional view of the support arrangement of Figure 53. Figure 56 is a cut-away view of the forward end of the support arrangement.

Figure 57 is a cut-away view of the rear end of the support arrangement.

Figure 58 illustrates the shape of the outer periphery of the fairing at locations A and B of Figure 57. Figure 59 is a plan view of an embodiment of a shaft support arrangement for use with twin propeller shafts.

Figure 60 is a side elevation of the arrangement of Figure 59.

Figure 61 is an enlarged perspective view of one of the bearing assemblies of the embodiment of Figure 59. Figures 62 - 69 are partially cross-sectional perspective view of other embodiments of shaft support arrangements according to the present invention.

Figure 70 is a schematic perspective view of the hull of a maritime vessel according to the present invention.

Figures 71 and 72 are schematic transverse cross sections of possible shapes of a hull of a vessel according to the present invention.

Figure 73 is a schematic side view of the hull of Figure 70, showing members for closing the lengthwise ends of the chamber during a beaching operation.

Figure 74 is an enlarged schematic transverse cross section of a portion of the hull of Figure 70 during a beaching operation. Figure 75 is a view similar to Figure 74 showing the hull equipped with a skirt.

Figure 76 is a schematic side elevation of the hull of Figure 70 in a beached state.

Figure 77 is a schematic side elevation of the hull of Figure 70 equipped with a conduit for water which can be trailed behind the hull. Figure 78 is a schematic side elevation of an embodiment of a hydrofoil craft according to the present invention.

Figure 79 is a schematic perspective view of an embodiment in which an upright has a plurality of legs mounted on opposite beamwise sides of the hull.

Figure 80 is a schematic side elevation of an embodiment of the present invention in which an upright has a plurality of legs spaced in the lengthwise direction of the hull.

Figure 81 is a schematic side elevation of an embodiment equipped with a plurality of uprights spaced in the lengthwise direction of a hull. Figure 82 is a schematic perspective view of an embodiment in which connecting members extend to the centerline of a hull.

Figure 83 is a schematic perspective view of an embodiment in which connecting members extend to the beamwise sides of a hull.

Description of Preferred Embodiments

Figure 1 schematically illustrates an embodiment of a hydrofoil craft according to the present invention. The hydrofoil craft will typically include a hull 110, a plurality of sets of foils 120, 130 for generating lift to support the hull 110 above the water surface, support arms 121, 131 for connecting the foils 120, 130 to the hull 110, and a propulsion system which generates thrust to propel the hydrofoil craft.

The hydrofoil craft may have any type of hull 110 suitable to the conditions in which the craft is expected to operate and the load which the hull is expected to carry . It may have a single hull, or it may have a multi-hull structure, such as a catamaran or a trimaran structure. The hydrofoil craft will typically include at least two sets of foils which are spaced in the fore and aft direction of the hydrofoil craft, with the weight of the craft being supported by both sets of foils when the craft is foil-borne. For example, the hydrofoil craft of Figure 1 has a forward set of foils 120 comprising a single foil located forward of the center of gravity of the hull 110 and an aft set of foils 130 comprising two foils located aft of the center of gravity. However, the hydrofoil craft is not limited to having only two sets of foils. Each set of foils may include one or more foils. The weight of the hull 110 can be divided among the sets of foils in any desired manner. For example, the weight may be divided fairly evenly among the sets of foils, or one set may support a majority of the weight of the hull while another foil set serves primarily to adjust the pitch of the hydrofoil craft.

The foils are not restricted to any specific type and can be submerged foils, surface piercing foils, or planing foils, for example. The shapes of the foils can be selected according to the expected operating conditions. For example, they may be straight, swept, of constant or varying cord, planar or nonplanar (such as dihedral), and cambered or non-cambered. Furthermore, they may be designed for any desired mode of operation, such as subcavitating operation, supercavitating operation, ventilated operation (in which air is introduced to the top surface of the foils), or planing operation.

Each foil is connected to the hull 110 by one or more support arms 121 , 131 which transmit a portion of the weight of the hull to the foil. The support arms may be structured so as to maintain the position of the foils constant with respect to the hull during foil-borne operation, e.g. , the support arms may be rigidly connected to the hull 110. However, for foils supporting any significant portion of the weight of the hull 110, the support arms preferably have a structure which enables the foils to move with respect to the hull 110 in response to upgusts and downgusts of water velocity acting on the foil. This ability of the foils to move with respect to the hull 110 during foil-borne operation greatly decreases the vertical accelerations of the hull in rough water and increases the comfort of passengers on the hydrofoil craft. Various structures of support arms for enabling foils to move in such a manner are described in U.S. Patent No. 5,469,801 and U.S. Patent Application Serial Number 08/481,628, which are incoφorated by reference. The foils may be either rigidly or movably (e.g. , pivotably) connected to the support arms. In Figure 1 , the forward foil 120 is connected to the hull 110 by one or more sloping support arms 121 extending aft from the hull 110 and pivotably connected to the hull 110 for pivoting about a transverse axis 122 and is biased away from the hull 110 by a suitable biasing member, such as an adjustable shock strut 123. Each of the aft foils 130 is connected to the hull 110 by a corresponding support arm 131 which may be tilted and rotated by suitable actuators to enable the angle of attack of the aft foils 130 to be adjusted and to enable the support arms 131 to steer the hydrofoil craft, as described in U.S. Patent No. 5,469,801.

The hydrofoil craft may be propelled by any known mechanism, such as by propellers mounted in any convenient location (such as suspended beneath the hull or mounted on the foils), water jets, jet engines, or air fans. In Figure 1, the hydrofoil craft is propelled by a propeller 140 connected to an engine aboard the hull 110 by a sloping propeller shaft 141.

As described in U.S. Patent No. 5,469,801 and U.S. Patent Application No. 08/481,628, it is possible to operate a foil of a hydrofoil craft in a ventilated state in which atmospheric air is introduced to the upper surface of a submerged foil along one or more support arms for the foil. If sufficient air is provided to the upper surface of the foil, the operation of the foil will resemble that of a planing foil, with substantially the entire upper surface of the foil in contact with air and only the lower surface of the foil in contact with water. With water in contact substantially only with the lower surface of the foil, the pressure distribution under the foil favors the maintenance of a laminar boundary layer between the main body of the water and the lower surface of the foil provided that the lower surface is "mirror smooth" and the Reynold's number is not too large. The Reynold's number Rn is defined as [speed x chord] Iv, wherein v is the water's kinematic viscosity. Above a certain critical Reynold's number, a laminar boundary layer develops small flow disturbances which increase in amplitude until the entire boundary layer is filled with turbulence. The skin friction coefficient of a turbulent boundary layer on a smooth surface is much higher than for a laminar boundary layer. For example, at a Reynold's number of IO⁸ the skin friction coefficient in a laminar boundary layer is only approximately 6% of that in a turbulent boundary layer, and the percentage becomes even lower as the Reynold's number further increases. Therefore, in order to reduce the drag on a foil, it is desirable to maintain a laminar boundary layer at as high a Reynold's number as possible.

In the present invention, "distributed suction" may be employed in order to maintain a laminar boundary layer. In distributed suction, the layer of water actually in contact with the foil surface is drawn into the foil through small holes in the foil surface. One conceivable method of obtaining distributed suction is to connect a pump to the surface of the foil on which distributed suction is to be produced. A much simpler arrangement is to provide fluid communication between the surface of the foil on which distributed suction is to be produced (such as the lower surface of the foil) and a region of the hydrofoil craft which is at a lower pressure than this surface and to allow the natural difference in pressure to suck water from this surface. When distributed suction is to be produced on the lower surface of a foil, a convenient location which is at a lower pressure than the lower surface is the upper surface of the foil. Figure 2 illustrates a foil 200 of an embodiment of the present invention having a lower surface to which distributed suction is applied. The foil 200 is connected to an unillustrated hull 110 of the hydrofoil craft by one or more support arms 210, only one of which is shown for simplicity. The remainder of the hydrofoil craft may be the same as shown in Figure 1 and so has been omitted from the drawing. The foil 200, which may correspond to either of the foils 120, 130 of Figure 1, may be in a submerged state, as shown in the figure, in a partially submerged state, or in a planing state.

As shown in Figure 3, which is an enlarged cross-sectional view of region A of Figure 2, distributed suction can be produced along the lower surface of the foil 200 by forming passages in the form of a plurality of bleed holes 205 in the foil 200 between its upper and lower surfaces. The lower ends of the bleed holes 205 communicate with a laminar boundary layer 204 on the lower surface, while the upper ends communicate with the region above the upper surface of the foil 200. Because the foil 200 is generating lift, the pressure on the lower surface of the foil 200 is much higher than the pressure on the upper surface, so water automatically flows from the lower surface to the upper surface without the need for any powered suction device. If the foil 200 is submerged, the upper surface of the foil 200 may be in contact with water, or it may be covered by an air-filled cavity 202, as described in U.S. Patent No. 5,469,801 and U.S. Patent Application No. 08/481,628, in which case the upper surface of the foil 200 will be at substantially atmospheric pressure. An air-filled cavity 202 can be produced in any desired manner, such as in the ways described in U.S. Patent No. 5,469,801 and U.S. Patent Application No. 08/481,628, such as by introducing atmospheric air along an air pocket 201 formed on a blunt trailing edge of the illustrated support arm 210. The illustrated foil 200 is equipped with a step 203 near its leading edge for inducing flow separation from its upper surface, as described in U.S. Patent Application No.

08/481,628, but the foil 200 is not restricted to any particular shape. If the foil 200 is operating in a semisubmerged or a planing state, all or a portion of the upper surface of the foil 200 will be in direct contact with the atmosphere so will be at atmospheric pressure. An example of a suitable diameter of the bleed holes 205 for distributed suction is in the range of approximately 0.05 mm to approximately 0.5 mm and more preferably in the range of approximately 0.05 mm to approximately 0.1 mm. Based on experiments performed on airfoils, it is estimated that the suction volume flow coefficient C_Q required to completely stabilize the most unstable portion of a laminar boundary layer on a flat plate in water at any Reynold's number when the fluid and plate are at the same temperature is C_Q = suction flow rate/unS

= Q UoS

= 1.2 x 10^ wherein U_o is the free stream velocity and S is the plate's wetted area. It can be shown that

C_Q = (a/S) C_L so that a/S * .00012 /C_L wherein a is the total area of the bleed holes and C_L is the lift coefficient of the foil 200, which is defined as weight supported by the foil

Therefore, assuming a value of C_L in the range of approximately 0.1 to approximately .01 , the total bleed hole area a for producing distributed suction is preferably in the range of approximately 0.0004 to approximately 0.001 times the total plan area S (including the area of the bleed holes 205) of the lower surface of the foil 200. The bleed holes 205 are preferably formed over substantially the entire span of the foil 200 and over the entire region of the lower surface in the chordwise direction where a turbulent boundary layer would be expected to occur at the normal cruising speed of the foil 200 in the absence of distributed suction. The boundary layer is typically laminar near the leading edge of the foil 200, so bleed holes 205 can be omitted from this area. If other means are also used to stabilize the boundary layer, such as heating, the region in which bleed holes are formed can be further reduced. When viewed in plan, the bleed holes 205 are preferably staggered in the spanwise direction of the foil 200 so that the suction is more even over the bottom surface of the foil 200. For example, as shown in Figure 4, which is a view of the bottom surface of the foil 200 of Figure 3, each row of bleed holes 205 may be staggered by a half pitch with respect to the bleed holes in an adjoining row. In this figure, the arrows indicate the direction of water flow over the bottom surface of the foil 200.

The bleed holes 205 need not have any particular shape or orientation. For example, as shown in Figure 3, the bleed holes 205 may extend in straight lines through the foil 200 with their ends roughly perpendicular to the surfaces of the foil 200. However, preferably the angle between the upper ends of the bleed holes 205 and the upper surface of the foil 200 is less than 90 degrees so as to give the bleed water some rearward momentum as it leaves the bleed holes 205 and enters the air-filled cavity 202 above the foil 200. As shown in Figure 5, which shows a modification of the region illustrated in Figure 3, the upper ends of the bleed holes 205 may be curved towards the trailing edge to give the bleed water some rearward momentum. Alternatively, as shown in Figure 6, small elbow-shaped tubes 206 each having a curved inner bore can be attached to the upper surface of the foil 200 atop each of the bleed holes 205 to rearwardly change the direction of movement of the bleed water so that it is substantially parallel to the upper surface of the foil 200. Similarly, the lower ends of the bleed holes 205 may be angled towards the leading edge of the foil 200 to reduce momentum loss along the lower surface of the foil 200. Thus, the bleed holes 205 may be drilled straight through the foil 200 such that their lower ends are sloped towards the leading edge and their upper ends are sloped towards the trailing edge of the foil 200, as shown in Figure 7.

Instead of having bleed holes, a region of the foil 200 in which distributed suction is desired can be formed from a porous material which is permeable to water, such as a porous sintered metal of suitable porosity, as shown in Figure 8. When the foil 200 is made of such a material, water can seep through the pores of the foil 200 from the lower surface to the upper surface to provide distributed suction on the lower surface. The porosity of the foil can be selected using the same formula as used to determine the total surface area of bleed holes 205.

The bleed holes 205 in the lower surface of the foil 200 need not extend all the way to the foil's upper surface. For example, as shown in Figure 9, a hollow plenum 225 can be formed inside the foil 200. The plenum 225 is connected with the lower surface of the foil 200 by a plurality of bleed holes 226 having dimensions and spacing similar to the bleed holes 205 of Figure 3, and it is connected with the upper surface of the foil 200 through one or more exhaust passages 227, the outer ends of which communicate with the inside of the air-filled cavity 202. A plenum provides the advantage that the bleed holes 226 can be drilled through the thin skin of the foil 200 along its bottom surface into the plenum 225 rather than having to be drilled through the entire thickness of the foil 200, so formation of the bleed holes is much easier. The plenum 225 need not have any particular shape or structure. For example, it may be a discrete container installed within the foil 200, or it may be defined by the walls of the foil 200. Another advantage of a plenum is that it makes it easy to clean out the bleed holes 226. With the passage of time, the bleed holes 226 may become clogged by algae or debris present in the water. The embodiment of Figure 9 is equipped with a cleaning mechanism for the bleed holes 226. The cleaning mechanism includes a pump 229 disposed within the hull 110 or in any other convenient location of the hydrofoil craft, and it is connected by an internal passage 228 within the support arm 210 and a flexible hose 230 between an unillustrated source of a cleaning fluid, such as water, and the plenum 225 within the foil 200. A valve 231 may be installed between the pump 229 and the plenum 225. During normal operation of the hydrofoil craft, the valve 231 is closed and the cleaning mechanism has no effect on the distributed suction produced by the bleed holes 226. Namely, bleed water flows from the lower surface to the top surface of the foil 200 through the bleed holes 226, the plenum 225, and the exhaust passage 227. When it is desired to clean the bleed holes 226, the valve 231 is opened, and the cleaning fluid is forced by the pump 229 under high pressure into the plenum 225, from which the cleaning fluid flows out through the bleed holes 226 and the exhaust passage 227, carrying any accumulated algae or debris with it and thereby cleaning the bleed holes 226 and the exhaust passage 227. Cleaning may be performed when the hydrofoil craft is stationary, such as when tied to a dock, so the pump 229 and the source of cleaning fluid can be installed outside the hydrofoil craft, if desired. Alternatively, cleaning can be performed while the hydrofoil craft is foil-borne by momentarily pumping cleaning fluid into the plenum 225 at predetermined intervals. In the embodiment of Figure 9, air is introduced from the atmosphere to the upper surface of the foil 200 via an internal passage 213 formed in the illustrated support arm 210 for the foil 200, although air may be introduced in any other desired manner. A valve 216 (which may be operated manually or automatically by a controller 218) is connected to the internal passage 213 by flexible tubing 217, for example, to regulate the flow rate of air through the internal passage 213. The controller 218 can control the valve 216 based on the speed of the hydrofoil craft, for example, to maintain the pressure in the air-filled cavity 202 close to atmospheric pressure or to shut off the supply of air to the upper surface of the foil 200 when additional lift is desired. Each support arm 210 is pivotally supported by the hull 110 for pivoting about a transverse axis 122. The upper end of at least one of the support arms 210 extends into the hull 110 and is connected to a shock strut 123 or other biasing member which exerts a downwards biasing force on the support arm 210 to prevent it from collapsing against the hull 110 under the weight of the hull 110 while enabling the support arm 210 to pivot about the transverse axis 122 so that the foil 200 can move up and down in concert with upgusts and downgusts of water velocity contacting the foil 200. Distributed suction can be applied to the lower surface of a totally submerged foil

200, but it can also be effectively applied to the lower surface of a planing foil having an upper surface exposed to the atmosphere, or to the immersed portion of a surface- piercing foil. For example, in the case of a planing foil, bleed holes can be drilled between the lower surface of the foil, which contacts water to generate lift, and the upper surface of the foil, which will usually be above the water surface and in contact with the atmosphere.

If the hydrofoil craft is intended for use at fairly low speeds (such as below 50 knots) at which cavitation of the foil is unlikely to occur, an unventilated foil may be preferable to a ventilated foil. In this case, distributed suction may be applied to both the upper and lower surfaces of the foil in order to maintain a laminar boundary layer along both surfaces. Figure 10 illustrates an embodiment of such a hydrofoil craft. Separate bleed holes 233 for producing distributed suction are formed in the foil 200 in its upper and lower surfaces. Separate fluid passages 234 and 235 are respectively connected to the upper and lower sets of bleed holes 233 and extend through the illustrated support arm 210 from the foil 200 to inside the hull 110. The lower surface of the foil 200 is at higher than atmospheric pressure, so the fluid passage 235 for the lower set of bleed holes 233 can be connected directly to the atmosphere, and the naturally occurring difference in pressure will be sufficient to suck water away from the lower surface of the foil 200 without the need for a pump. In contrast, the upper surface of the foil 200 is commonly at below atmospheric pressure at normal cruising speeds, so a suction device, such as a pump 236, is connected to the fluid passage 234 for the upper bleed holes 233 by a flexible hose 237, for example. The bleed water drawn from the bleed holes 233 and through the two fluid passages 234 and 235 can be dumped overboard. Alternatively, the discharge from the pump 236 may be directed aft in the form of a propulsive jet. The efficiency is a maximum if the velocity of the propulsive jet is equal to the speed of the hydrofoil craft. The size of the bleed holes 233 and the total surface area of the bleed holes 233 relative to the area of the surfaces of the foil 200 in which they are formed can be the same as for the embodiment of Figures 2 - 9.

If the foil 200 is equipped with more than one support arm 210, one or both of passages 213 and 228 of Figure 9 and one or both passages 234 and 235 of Figure 10 may be formed in any one or more of the support arms 210. For example, one support arm 210 may have no internal passages while another support arm 210 has two of the internal passages, or a plurality of support arms may have one or more of the internal passages. Thus, the support arms 210 need not be identical in structure.

In the embodiment of Figure 10, performing distributed suction on the upper surface of a foil 200 involves power input to a pump 236. An alternative method of reducing the drag on a foil 200 which does not require the input of power is to allow the boundary layer on the upper surface of the foil 200 to become turbulent, and to introduce small diameter gas bubbles (referred to as microbubbles) into the turbulent boundary layer. The skin friction in a turbulent boundary layer is proportional to the mass density of the fluid in the boundary layer. By introducing microbubbles into the boundary layer, a mixture of water and gas bubbles is formed which has a lower mass density and produces less skin friction than a boundary layer containing water alone. An embodiment in which microbubbles are introduced into the turbulent boundary layer on the top surface of a foil 200 of a hydrofoil craft is shown in Figure 11. The foil 200 has an upper surface in which small-diameter holes 239 (preferably approximately 100 - 500 micrometers in diameter) are formed. The holes 239 are connected with the atmosphere by a passage 234 extending through a support arm 210 from the foil 200 to above the water surface, such as to inside the hull 110. The subatmospheric pressure along the upper surface of the foil 200 when the hydrofoil craft is moving sucks air from the atmosphere and through the passage 234 and the holes 239 to form microbubbles which mix with the boundary layer water to reduce the mass density of the boundary layer. As in the embodiment of Figure 10, bleed holes 233 may be formed in the lower surface of the foil 200 and connected to the atmosphere via another passage 235 in the support arm 210 to produce distributed suction along the lower surface. Thus, in this embodiment, a laminar boundary layer stabilized by distributed suction through the bleed holes 233 exists along the lower surface of the foil 200, while a turbulent boundary layer producing a reduced skin friction due to the introduction of the microbubbles exists along the upper surface of the foil 200. As in the embodiments of Figures 9 and 10, the one or more support arms 210 for the foil 200 are pivotably supported by the hull 110 for pivoting about a transverse axis 122 and are biased away from the hull 110 by a biasing member such as a shock strut 123. Figure 11 shows only a single support arm 210 for the foil 200. If the foil 200 is equipped with more than one support arm 210, passages 234 and/or 235 may be formed in any one or more of the support arms 210. For example, a passage 234 for air can be formed in one of the support arms 210, and a passage 235 for bleed water can be formed in another of the support arms 210, or both types of passages 234 and 235 may be formed in each of the support arms 210.

The introduction of microbubbles is effective to reduce the skin friction of the upper surface of the foil when the foil is operating at subcavitating speeds. If it is also desired to operate the foil at a Reynold's number at which cavitation would be expected to occur on the upper surface, the upper surface may be ventilated by atmospheric air introduced along the support arm 210 (such as down the trailing edge of the support arm 210) once cavitating speeds are reached. When the foil 200 is being ventilated, the holes 239 in the upper surface are not needed for generating microbubbles. If the holes 239 are connected at this time with the bleed holes 233 by unillustrated valves which can be opened and closed by remote control, the holes 239 may be made to communicate with the bleed holes 233 once an air-filled cavity is formed atop the upper surface of the foil 200 to allow water to bleed directly from the lower surface to the upper surface. Therefore, it is possible to have the holes 239 serve two different functions, depending on the operating conditions of the foil 200.

Another method of maintaining a laminar boundary layer on a surface of a foil of a hydrofoil craft at high Reynold's numbers is to heat the surface to a temperature above that of the fluid contacting the surface. Heating can produce a significant increase in the Reynold's number at which the laminar boundary layer first becomes turbulent. In order to stabilize the boundary layer, the wetted surface of the foil is preferably heated to a temperature in the range of approximately 10 to approximately 80 degrees C above the ambient water temperature, and more preferably in the range of approximately 20 to approximately 65 degrees C above the ambient water temperature. No substantial effect can be expected if the difference between the surface temperature and the ambient water temperature is less than approximately 10 degrees C. On the other hand, if the surface of the foil 200 is hot enough to cause steam bubbles to form, the favorable effect of heating is totally lost.

Heating the wetted surface of a foil of a hydrofoil craft can be performed in any manner. When the hydrofoil craft is powered by a liquid-cooled engine, an efficient way of heating the foil is to use heated coolant from the engine cooling system. Figure 12 illustrates an embodiment of the present invention in which a foil 200 is heated in this manner. The foil 200 is connected to a hull 110 of the hydrofoil craft by one or more support arms 210 in the same manner as described with respect to Figure 9, although a hydrofoil craft having a heated foil need not be of the illustrated structure. The hydrofoil craft of this embodiment has a liquid cooled engine 240 disposed in the hull 110 and cooled by an engine cooling system 241. Heated coolant from the cooling system 241 is circulated through the foil 200 to heat the lower surface of the foil 200 to a temperature higher than the ambient temperature of the water. One or more passages 242 for the engine coolant are formed in the foil 200 and are connected to the engine cooling system 241 by passages 243 formed in the support arm 210 and by flexible hoses 244 extending between the support arm 210 and the engine cooling system 241. A temperature sensor 245 may be installed in a location on the foil 200, such as on an interior surface, where it can sense the temperature of the lower surface without disrupting the laminar boundary layer, and a controller 246 may be connected to the temperature sensor 245 and the cooling system 241 to control the operation of the cooling system 241 to maintain the temperature of the lower surface of the foil 200 in a suitable range. Instead of using a temperature sensor 245 on the foil 200, the temperature of the engine coolant can be measured at the inlet and outlet of the engine cooling system 241, and the surface temperature of the foil 200 can be calculated based on the ambient water temperature, which can be readily measured, and the drop in the coolant temperature as the coolant travels to and from the foil 200. In Figure 12, the upper surface of the foil 200 is in contact with an air-filled cavity 202 formed by air passing through an internal passage 213 in the support arm 210, so it is sufficient to heat only the lower surface of the foil 200. However, when the foil 200 is unventilated and is wetted along both its upper and lower surfaces, it may be desirable to heat both the upper and lower surfaces to stabilize the laminar boundary layer along both surfaces.

Using the engine cooling system 241 to heat the foil 200 is an efficient method not only of maintaining a laminar boundary layer but also of dissipating the heat of the engine 240. However, other means can be used to heat the foil 200. For example, as shown in Figure 13, an electric heating coil 250 connected by wires 252 to an electric power supply 251 in the hull 110 can be installed in the foil 200. A temperature sensor 253 is mounted in the foil 200 for sensing the temperature of the lower surface of the foil 200, and the power supply 251 is controlled by a controller 254 in response to the output of the temperature sensor 253 so as to maintain the temperature of the lower surface in a suitable range.

It is possible to employ a combination of two or more of ventilation of the upper surface of a foil, distributed suction, heating a surface of the foil, and the introduction of microbubbles into a boundary layer of the foil of a hydrofoil craft. Figure 14 illustrates an example of a foil 200 equipped with a plurality of mechanisms to stabilize the boundary layer along the lower surface. In a first region 207 adjoining the leading edge of the foil 200 along its lower surface, provided that the surface of the foil 200 is mirror smooth, the Reynold's number is sufficiently low (less than approximately 5 x IO⁶) that the boundary layer will be laminar in the absence of any flow stabilizing device, so in this region 207, neither heating nor distributed suction is employed. In a second region 208 disposed aft of the first region 207, the lower surface of the foil is heated by suitable means (such as the arrangement illustrated in Figure 13) to raise the critical Reynold's number for the boundary layer. The second region 208 begins no further aft than the point at which the Reynold's number is such that the boundary layer would become turbulent in the absence of any stabilizing mechanism (greater than approximately 5 x IO⁶). In a third region 209 extending aft from the second region 208 to the trailing edge of the foil, distributed suction is applied to the lower surface using any of the arrangements described above, for example. The third region 209 begins no further aft than the point where the Reynold's number reaches the critical Reynold's number for the heated second region 208 (approximately 5 x 10⁷). The second and third regions 208 and 209 may overlap each other in the chordwise direction of the foil 200, or they may even coincide so that the heating can reduce the amount of distributed suction required to maintain a laminar boundary layer. However, the momentum change in the water passing through the bleed holes entails a drag loss which is not experienced with surface heating of the foil 200. Therefore, when minimizing drag is a major consideration, there is minimal or no overlap between the third region 209 and the second region 208, and the third region 209 is made as small as possible while still being sufficiently large that there are no areas of the lower surface of the foil 200 where a turbulent boundary layer exists.

Figure 1 shows a hydrofoil craft according to the present invention operating with its foils in a submerged state, but it is also possible for the hydrofoil craft to be operated with one or more of its foils in a semisubmerged (surface piercing) state, or in a planing state in which the foil has an equilibrium condition in which it planes atop the water surface in calm water with its upper surface exposed to the atmosphere or only slightly submerged (with the upper surface less than an inch, such as one-tenth of an inch below the surface of the water, for example).

Figure 15 illustrates an embodiment of a hydrofoil craft according to the present invention operating with its forward foil 120 in a planing mode. The structure of this embodiment may be the same as that of any of the other embodiments of the present invention, such as the embodiment of Figure 1. The aft foils 130 are shown operating in a fully submerged state, but may instead be operated in a semisubmerged or a planing state. Furthermore, if the hydrofoil craft has more than two sets of foils, any one of the foil sets may be in a planing, a semisubmerged, or a fully submerged state.

A planing forward foil 120 produces excellent heave stability without the need for active control surfaces on the forward foil 120 for height control, while the submerged aft foils 130 give the hydrofoil craft pitch stability. With the forward foil 120 operating on or just below the surface of the water in its equilibrium state, the upper surface of the forward foil 120 does not generate any significant lift. When the forward foil 120 moves deeper in the water than its equilibrium depth, the flow of water over the upper surface of the forward foil 120 generates an increased lift which automatically returns the forward foil 120 to its equilibrium depth. Another method of giving the forward foil 120 heave stability is to select its dimensions such that the total lift-generating area on its lower surface is larger than the area necessary to support the hull 110 above the water surface at cruising speed. In this case, in its equilibrium state, the forward foil 120 will plane with only a portion of its lower surface in contact with the water. When a downward force pushes the forward foil 120 deeper into the water than its equilibrium depth, the area of the lower surface of the forward foil 120 in contact with the water and generating lift will increase, thereby returning the forward foil 120 to its equilibrium depth. The equilibrium depth of submergence of the forward foil 120 can be set to a desired value by appropriately selecting the surface area of the forward foil 120 and by adjusting the mean angle of incidence of the forward foil 120, which can be done by adjusting the angle of the support arm(s) 121 for the forward foil 120 with respect to the horizontal.

If the forward foil 120 is to be operated in a planing state, a swept shape for the foil 120, i.e., a shape in which the spanwise ends of the foil 120 are located aft of the spanwise center, can be advantageous. With a swept shape, at higher speeds, the forward foil 120 planes on the two spanwise ends while the spanwise center section of the forward foil 120 is clear of the water. Operation in this manner gives excellent roll stability. If the hydrofoil craft rolls to either side, more of one spanwise tip of the forward foil 120 (on the side to which the craft has rolled) and less of the opposite spanwise tip will be in planing contact with the water, resulting in a strong restoring moment which counteracts the roll. Another attractive feature of this arrangement is that the faster the hydrofoil craft runs, the smaller is the area of the forward foil 120 planing on the water, so the skin friction drag is proportionally reduced and the resistance is roughly independent of speed. Figures 17 through 19 are respectively a top plan view, a front elevation, and a side elevation of the 120 forward foil of the embodiment of Figure 15. As shown in Figures 18 and 19, the lower surface of the foil 120 may be equipped with one or more downwardly extending fins 120a. Such fins 120a can serve a number of useful functions. One function is to apply a side force to the foil 120 for turning. Another function is to act as bumpers and protect the lower surface of the foil 120 from collisions with submerged objects, such as logs, which will strike the fins 120a before contacting the other portions of the foil 120. The operation of a hydrofoil craft according to the present invention running in a planing mode is as follows. In calm water or in long, gradual waves, the forward foil 120 planes on the surface of the water with its upper surface exposed to the atmosphere or only slightly submerged, as schematically illustrated in Figure 15. When the hydrofoil craft encounters steeper waves, the forward foil 120 will plane along the surface of the troughs of the waves, but will plunge through the crests, entering on one side of a crest and emerging from the opposite side. The trajectory of the forward foil 120 in waves is schematically illustrated in Figure 16. The reason for such a trajectory is thought to be as follows. Along the trough of a wave, the support arms 121 pivot downwards with respect to the hull 110 from their position in calm water. As they do so, the angle of incidence of the forward foil 120 increases, so the lift acting on the forward foil 120 is sufficient to keep it planing along the surface of the trough. In contrast, when the forward foil 120 is traveling upwards towards the crest of a wave, the support arms 121 pivot upwards with respect to the hull 110 from their calm water position. As they pivot upward, the angle of incidence of the forward foil 120 decreases, resulting in a decrease in lift. At some point, the angle of incidence is such that the forward foil 120 will no longer plane along the surface of the wave, so it plunges into the wave.

When the forward foil 120 enters a wave in this manner, it creates a hole in the wave which is larger than the forward foil 120. This hole extends to the surface of the wave, so the upper surface of the forward foil 120 is in contact with the atmosphere through this hole. However, if the forward foil 120 goes deep into a wave, the hole in the wave will eventually collapse around the forward foil 120. In order to prevent cavitation as well as to prevent a large change in the lift acting on the forward foil 120 should the hole in the wave collapse, the hydrofoil craft may be equipped with an arrangement for ventilating the upper surface of the forward foil 120, with air being introduced through or along the support arm, as described in U.S. Patent No. 5,469,801 or U.S. Patent Application No. 08/481,628. As stated earlier, the lower surface of a planing foil may be subjected to distributed suction by forming bleed holes in the lower surface. In addition, the lower surface may be heated to stabilize its boundary layer. If the forward foil 120 is operated such that its upper surface is in contact with air substantially all the time, i.e., the forward foil 120 is either planing or its upper surface is ventilated, then it may be advantageous to camber the lower surface of the forward foil 120.

Operating a foil in a planing mode has a number of significant advantages. As stated above, a planing foil gives a hydrofoil craft both heave stability and roll stability without the need for any active control surfaces and the complicated automatic control systems which such control surfaces require. In addition, with a foil operating on or just below the surface, there is less likelihood of its striking against submerged objects invisible to the helmsman of the hydrofoil craft, such as rocks, pilings, or marine life. Furthermore, since the upper surface of the foil is ventilated by the atmosphere at substantially all times, a steady lift can be generated by the foil without the occurrence of cavitation.

Figure 20 is a front elevation of an example of a surface-piercing foil 150 which can be used in the present invention. The illustrated foil has generally the shape of a W and is connected to an unillustrated hull by two support arms 121 secured to two lower vertices of the W. The support arms 121 are pivotably connected to the hull in the manner described in U.S. Patent No. 5,469,801 to enable the foil 150 to move vertically with respect to the hull. The lower surface of the foil 150 may be equipped with one or more downwardly extending fins 151 corresponding to the fins 120a of Figure 19. Ventilation-preventing plates 152 may be secured to the upper and/or lower surfaces of the foil 150 to prevent atmospheric air from traveling down the length of the foil 150 from portions of the foil 150 disposed above the water's surface. The use of a surface- piercing foil can increase the vertical stability of a hydrofoil craft according to the present invention and prevent a large loss in lift of the foils which could cause the hull of the hydrofoil craft to return to the water surface from a foil-borne state. In a conventional hydrofoil craft, the use of surface-piercing foils results in an extremely rough ride in waves. However, by connecting the support arms for a surface-piercing foil to the hull in a manner which enables the foil to move up and down with respect to the hull in response to forces acting on it, in the manner described in U.S. Patent No. 5,469,801 , a smooth ride can be obtained. As described above, a foil of a hydrofoil craft according to the present invention may be ventilated with an air-filled cavity on its upper surface so as to prevent cavitation. However, in some instances, the drag resulting from the air-filled cavity may be undesirable. An alternative to ventilating the upper surface of the foil is to design the shape of the upper surface to generate very low lift, i.e., very little suction so as to inhibit cavitation.

Figure 21 schematically illustrates the flow field around a shallow ly submerged foil section having a ventilated, blunt base 161 at its trailing edge. In this figure, dimensions and angles have been exaggerated for clarity. The ventilated base 161 makes it possible to treat the upper and lower flows as separate flows with the curve O-O in the figure as the dividing streamline. This streamline has an angle θ with respect to the horizontal at the leading edge of the foil 160. The lower flow along the lower surface of the foil 160 is deflected downwards at an angle τ_XL with respect to the horizontal, and a corresponding lift force is developed on the lower surface of the foil 160. At the same time, the upper flow along the upper surface of the foil 160 is at an angle τ_xu with respect to the horizontal at the trailing edge. The mass flow of water dm/dt over the upper surface of the foil 160 is given by dm dt = δuo where p = the mass density of water u₀ = the speed of the flow upstream of the foil δ = the thickness of the upper flow From Newton's momentum theory, the lift on the upper surface is equal to the change in vertical momentum which the foil 160 imparts to the flow, i.e., lift = (_/oδuo) x o(θ - τ_xu) or

C_L = lift = 2δ (θ - τ_xu) (1) fcpur c c

Therefore, the lift generated by the upper surface is zero if τ_XL = θ.

The upper flow angle θ with respect to the horizontal in front of the foil 160 (called "upwash" in aerodynamics) is caused by the high static pressure under the foil 160. The condition τ_xu = θ can be satisfied by defining the line O-A coinciding with the upper surface of the foil 160 as a straight line, in which case the static pressure (measured with respect to the ambient pressure) will be zero over the entire upper surface. However, a straight line for the curve O-A may result in a foil section which is too thin to carry the bending loads imparted to the foil. Therefore, it may be more practical from the standpoint of strength if the line O-A is defined by a curve having tangents at the leading and trailing edge of the upper surface which are parallel to each other but nonaligned, with both tangents at an angle θ to the horizontal when the lower surface of the foil 160 is at its design angle of attack.

A curved surface will experience higher than ambient pressure (shown by + + + marks in Figure 21) where it is accelerating the flow upward, and it will experience negative pressure (shown by — marks) where it is pulling or sucking the flow back to its original direction. These pressures are best described by the static pressure coefficient

C_p = Δp (2)

where Δp = the local static pressure above ambient pressure If p_^ is the ambient static pressure, then

Δp = p-p_∞ or p = Δp + p_∞. If Δp = -p.,,,, a vacuum (zero absolute pressure) exists. Cold water will boil at a pressure slightly greater than this, so an approximate equation for the speed at which cavitation (boiling) will occur is, from Equation (2),

or

(feet/ second)

*27.3{-C_p (knots) (3)

A typical minimum value of C_p for a conventional foil operating at a high speed and hence a low angle of attack (small lift coefficient C_L) is C_p = -0.25. Substitution of this value into equation (3) gives 55 knots for the speed at which zero pressure will occur. Cavitation will occur at a slightly lower speed than this.

A shape for the upper surface of the foil 160 which has been found to give a very low absolute value of C_p and at the same time give good structural strength to the foil 160 is a generally S-shaped curve having an inflection between the leading and trailing edges of the foil 160, with the tangent to the curve at the leading edge being substantially parallel to the tangent to the curve at the trailing edge. An example of such a curve is one approximated by the formula y = ax² - bx⁴. For such a shape, calculations have shown C_p to have a maximum absolute value of about 0.05, i.e., C_p >_ -0.05 with the foil in a deeply submerged state, with C_p being closer to 0 when the foil is shallowly submerged. With C_p equal to about -0.05, equation (3) indicates that cavitation will not take place on the upper surface until the speed of the foil is about 122 knots. For more shallow submergences, the speed at which cavitation begins is even higher. Therefore, such a foil exhibits vast improvements over conventional foils in its ability to operate at high speeds without cavitation. The lower surface of the foil 160 in Figure 21 is preferably shaped so as to generate lift while minimizing pressure drag, which is the horizontal component of the lift acting on the lower surface. A wide variety of shapes having very low pressure drag have been proposed, and any such shapes may be used for the lower surface. One example of a suitable shape is that proposed by Gurevich in 1937, which is a simple arc of a circle. Another suitable shape, shown in exaggerated form in Figure 21, is generally that of an S with an inflection between the leading and trailing edges of the lower surface and with the trailing edge sloping downwards to deflect water downwards to generate lift. On the forward portions of the lower surface, the lift has a forward horizontal component which cancels out the rearward horizontal component of the lift in the rear portions of the lower surface, resulting in a substantially zero pressure drag.

The base 161 of the foil 160 at its trailing edge can be ventilated in a variety of manners via an unillustrated support arm for connecting the foil 160 to the hull of a hydrofoil craft, either along the interior or the exterior of the support arm. For example, air may pass through a conduit formed in the support arm and then be supplied to the base 161 of the foil 160 through another conduit formed in the foil.

A foil having the shape characteristics described with respect to Figure 21 may be employed in any of the other embodiments of the present invention, although the present invention may employ foils having any other desired shape.

When a hydrofoil craft like that shown in Figure 1 with pivotable support arms 121 biased away from a hull 110 is operating in a foil-borne state in waves, a rolling moment may be applied to the forward foil 120 and transmitted to the hull 110 through the support arms 121. The rolling moment can be alleviated by having the support arms 121 individually biased away from the hull 110 and by making the support arms 121 and/or the connection between the support arms 121 and the foil 120 sufficiently flexible that the support arms 121 can be sloped with respect to the horizontal by different angles from each other, permitting the foil 120 to be sloped with respect to the horizontal in the transverse direction of the hull 110 while the hull 110 remains level without rolling. Figure 22 is a perspective view of an example of a foil arrangement for a hydrofoil craft which can alleviate rolling moments. The overall structure of this hydrofoil craft may be similar to that illustrated in Figure 1, for example. The foil arrangement shown here includes a foil 120 connected to the hull 110 of the hydrofoil craft by first and second support arms 121, each of which is connected to the hull 110 for pivoting about a transverse axis 122 passing through a pivot point. Each of the support arms 121 is connected to a separate biasing mechanism 123, such as an adjustable shock strut, which biases the support arm 121 in a direction tending to pivot the foil 120 away from the hull. The two biasing mechanisms 123 are operable independently of each other and so can apply different forces to the support arms 121. The foil arrangement is sufficiently flexible that the foil 120 can twist so that its spanwise ends are at different heights when the pivot points of the support arms 121 are at the same height. In Figure 22, the solid line shows a state in which the foil 120 is level and the two support arms 121 are at the same angle with respect to the horizontal, while the dashed lines show a state in which the foil arrangement has flexed so that one spanwise end of the foil 120 is higher than the other spanwise end and the supports arms 121 are at different angles with respect to the horizontal, with the pivot points 122 at the same height as each other.

Figure 23 is a schematic rear view of a hydrofoil craft equipped with the foil arrangement of Figure 22 when operating in calm water in a foil-borne state. The foil 120 is shown planing on the surface of the water, but it may instead be operating in a partially or totally submerged state. In this state, the lift acting on the foil 120 is substantially symmetrical with respect to the transverse centerline of the hull, so there are no rolling forces acting on the hydrofoil craft. The foil 120 is therefore level, and the two support arms 121 have the same angle with respect to the horizontal.

Figure 24 is a rear view of the hydrofoil craft of Figure 23 when operating in waves. In this figure, the righthand side (the starboard side) of the foil 120 is in contact with the water, while a portion of the lefthand side (the port side) of the foil 120 is broaching the water surface. Because more of the righthand side of the foil 120 contacts the water than the lefthand side, the lift forces acting on the righthand side of the foil 120 are greater than those acting on the lefthand side of the foil 120, so a rolling moment is generated which tends to roll the hull 110 to port (counterclockwise in the figure). Because the foil arrangement is flexible, the foil 120 is capable of twisting with respect to the horizontal so that the lift force acting on the foil 120 is nonvertical. In the situation shown in Figure 24 in which the lift force on the righthand side of the foil 120 is greater than the lift force on the lefthand side, the lefthand side of the foil 120 moves downward with respect to the righthand side so that the lift force is directed to the left of the center of gravity of the hull 110. The lift force therefore exerts a clockwise torque on the hull 110 about the center of gravity which is opposite in direction to the rolling moment, so the net rolling moment acting on the hull 110 is reduced.

When the hydrofoil craft is operating in waves such that the lefthand side of the foil 120 generates greater lift than the righthand side, the righthand side of the foil 120 will drop with respect to the lefthand side of the foil 120, i.e., the attitude of the hydrofoil craft will be the mirror image of that illustrated in Figure 24. Therefore, the lift force will be directed to the right of the center of gravity of the hull 110 and will produce a moment tending to counteract the rolling moment. Figure 25 schematically illustrates another example of a foil arrangement according to the present invention. This arrangement differs from the arrangement of Figures 22 - 24 in that the single foil 120 of Figure 22 is replaced by a plurality of foils which are disposed side by side and which can move independently of each other. In Figure 25, there are two foils 120a and 120b, each connected to an unillustrated hull 110 by a support arm 121, with each support arm 121 equipped with a separate biasing mechanism 123. However, there may be more than two foils disposed side by side, and each foil may have more than one support arm 121. Figure 26 is a schematic rear view of a hydrofoil craft equipped with the foil arrangement of Figure 25 when operating in calm water in a foil-borne state. In this state, the forces acting on the two foils 120a and 120b are the same, so the foils move in synchrony and function in substantially the same manner as a single, continuous foil. Figure 27 is a schematic rear view of the hydrofoil craft when operating in a sea state in which the height of the water surface varies in the lateral direction of the hydrofoil craft. Specifically, the water surface is higher under the righthand (starboard) foil 120b than under the lefthand (port) foil 120a. Because the two foils can move independently of each other, the hull 110 can be maintained substantially level without rolling.

If the water beneath the righthand foil 120b is moving upwards more than the water beneath the lefthand foil 120a, the movement of the water will exert a rolling moment on the hull 110. However, as the support arms 121 for the foils pivot with respect to the hull 110, the angle of incidence of the foils varies. As a support arm 121 pivots towards the hull 110, the angle of incidence of the corresponding foil increases, and as the support arm 121 pivots away from the hull 110, the angle of incidence decreases. Therefore, the lefthand foil 120a in Figure 27 will have a greater angle of incidence than the righthand foil 120b. Assuming that both foils in Figure 27 are in contact with the water surface to the same extent and that the slope of the water surface in the fore and aft direction of the hull 110 is the same for both foils, the lefthand foil 120a will generate greater lift than the righthand foil 120b due to its greater angle of incidence, and the difference in the amounts of lift will exert a clockwise moment about the center of gravity of the hull 110 which tends to counteract the rolling moment.

The foil arrangements shown in Figures 22 - 27 are not restricted to use at any particular location along the hull of a hydrofoil craft, and a hydrofoil craft equipped with a plurality of sets of foils may use such foil arrangements for each set.

Figure 28 schematically illustrates the suspension for a foil 312 of a hydrofoil craft of the type disclosed in U.S. Patent No. 5,469,801 in which one or more support arms 311 for a foil 312 of a hydrofoil craft are pivotably connected to a hull 310 of the hydrofoil craft at a pivot point 314 and are biased away from the hull 310 by a suitable biasing mechanism 315 such as an adjustable shock strut. In a hydrofoil craft of this type, the position of the pivot point 314 for the support arms 311 remains constant with respect to the hull 310 at all times.

Figure 29 schematically illustrates a portion of an embodiment of a hydrofoil craft according to the present invention in which one or more support arms for a foil 332 of the hydrofoil craft are pivotable about a pivot point 334 which can vary in position with respect to the hull 310. In this embodiment, the hull 310 is supported atop a foil 332 by an upper suspension 320 and a lower suspension 30 situated beneath the upper suspension 320.

The upper suspension 320 can have any structure which enables it to expand and contract in response to vertical forces acting on it to vary the position with respect to the hull of a pivot point 334 of the lower suspension 30. The upper suspension 30 is schematically illustrated as comprising three links 321 - 323 and a biasing member 324. The first and third links 321 and 323 each have an upper end pivotably connected to the hull 310, while the second link 322 is pivotably connected at its ends to the lower ends of the first and third links 321 and 323. The biasing member 324 can be any mechanism capable of exerting a variable force on the links 321 - 323 to maintain them in a desired position in which they can support the weight of the hull 310. In Figure 29, the biasing member 324 is schematically illustrated as a compression spring connected between the upper end of the third link 323 and the lower end of the first link 321. Examples of devices that can be used as the biasing member 324 include but are not limited to mechanical springs, pneumatic springs, and hydraulic shock struts.

The lower suspension 30 includes one or more support arms 331 each having an upper end pivotably connected to the upper suspension 320 at a pivot point 334 and a lower end connected to the foil 332 of the hydrofoil craft. The pivot point 334 is shown in Figure 29 as coinciding with the joint between the first and second links 321 and 322 of the upper suspension 320, but the pivot point 334 may be in any location on the upper suspension 320 that experiences vertical movement. A biasing member 33, schematically illustrated as a compression spring, is provided for biasing the support arms 331 away from the hull 310. The biasing member 333 can be any device capable of applying a variable force on the support arms 31, and it can be the same or different in structure from the biasing member 324 for the upper suspension 320.

The foil 332 is illustrated as operating in a planing mode, but it may instead by a submerged or semisubmerged (surface-piercing) foil 32. In addition to the illustrated foil 32, the hydrofoil craft may include other foils which also generate lift for supporting the hull 310. The hydrofoil craft will typically be equipped with a propulsion system, which has been omitted from the drawings for simplicity, but the hydrofoil craft may also be towed through the water. For simplicity, the upper suspension 320 has been schematically illustrated as comprising a single set of links 321 - 323. However, when the lower suspension 30 includes a plurality of support arms 331 spaced in the transverse direction of the hull 310, the upper suspension 320 may include a plurality of sets of links 321 - 323 also spaced in the transverse direction of the hull 310. The different sets of links may be interconnected so as to move as a single unit, or they may be arranged so as to move independently of each other.

In Figure 29, the first and third links 321 and 323 for the upper suspension 320 pivot with respect to the hull 310 about axes extending in the transverse direction of the hull 310 to vary the position of the pivot point 34, but the distance of the pivot point 334 for the support arms 331 may be varied by different types of motion. For example, Figure 30 shows an example of a hydrofoil craft in which a pivot point 354 performs one dimensional movement with respect to a hull 310, e.g., if the hull 310 is horizontal, then the pivot point 354 moves vertically with respect to the hull 310 without horizontal translation with respect to the hull 310. The hydrofoil craft has an upper suspension 340 including a platform 341 which is connected to the hull 310 by a plurality of biasing members 342 which permit the platform 341 to move along a straight line (e.g., vertically) with respect to the hull 310. The biasing members 342 may be of any type, such as the types described with respect to Figure 29. The upper suspension 340 may include guide members, such as telescoping rods, for guiding the movement of the platform 341 with respect to the hull 310. A lower suspension 350 is connected to the upper suspension 340 and includes one or more support arms 351 , each of which has a lower end connected to a foil 352 of the hydrofoil craft and an upper end pivotably connected to the platform 341 at a pivot point 354. The support arms 351 are biased away from the hull 310 by one or more biasing members 353 schematically illustrated as a compression spring and which can be any of the devices described above with respect to biasing member 324.

The upper suspensions 320 and 340 in Figures 29 and 30 permit vertical movement of the foil to be decoupled from changes in the angle of attack of the foil, in contrast to the arrangement shown in Figure 28 in which any vertical movement of the foil 312 by pivoting of the support arms 311 automatically produces a change in the angle of attack of the foil 312. As a result of the decoupling, variation in the angle of attack of the foil is reduced. Therefore, when the foil is operating in a planing mode, it can be maintained on the surface of the water in waves rather than plunging into the peaks of the waves.

Suspensions like those shown in Figures 29 and 30 can be installed at any desired location along a hull 310. For example, if the hydrofoil craft has a plurality of sets of foils spaced along the hull, each of the foil and can be equipped with such a suspension. Figure 31 is a graph showing the drag as a function of speed of a typical subcavitating foil supporting a constant weight. The foil is assumed to have a chord of 15 feet and a constant span of 500 feet and to be supporting a weight of 10,000 tons. The total resistance of the foil includes induced drag and skin friction. Induced drag, which is the resistance penalty of developing dynamic lift by pushing water downwards, varies inversely with the square of the speed. Skin friction, which is caused by the water's viscosity, is approximately directly proportional to the square of the speed. Thus, as the induced drag decreases, the skin friction increases. From Figure 31, it can be seen that for a given foil, there is a speed at which the resistance of the foil is a minimum. This speed is the optimal speed for the particular foil. At this speed, the foil is operating at its optimal lift coefficient C_L, which is defined as

lift coefficient = C_L = weight supported by the foil

1/2 ) V²S

where V is the speed of the foil,

S is the plan area of the foil, and p is the mass density of the water. The very high resistance of the foil in the upper speed range above the optimal speed in Figure 31 is due to the fact that the lift coefficient in this range is much smaller than the optimal value. If the span of the foil can be somehow reduced once the foil reaches the speed corresponding to minimum drag, the lift coefficient of the foil can be maintained at about its optimal value at any speed, resulting in a great reduction in drag at high speeds.

Figure 34 shows various ways in which the span of a foil can be varied based on the speed of the foil. If the span of the foil is varied in accordance with speed in the manner shown by the solid line in Figure 34 so as to maintain the lift coefficient of the foil constant once the speed of minimum drag is reached, the drag of the foil becomes as shown in Figure 32. If the span is reduced with increasing speed so as to maintain the lift coefficient constant, the plan area of the foil is reduced inversely with the span of the speed, so skin friction remains constant. However, reducing the span reduces the aspect ratio of the foil, so the induced drag rises and the total drag increases with speed. Nevertheless, the total drag on the foil is far less than if the span of the foil were constant. The drag at 100 knots for the foil of Figure 32 is only 71 % of the drag of the foil of Figure 31, and at 200 knots, the drag is only 57% .

If the span of the foil is varied in the manner shown by the dashed line in Figure 34 so that the lift coefficient is the optimal value at every speed above the speed of minimum drag, the drag of the foil becomes as shown in Figure 33. At 100 knots, the drag of the foil of Figure 33 is only 66% of the drag of the foil of Figure 31, while at 200 knots, the drag falls to 28% that of the foil of Figure 31. It can be seen that dramatic decreases in drag and increases in efficiency can be obtained by varying the span of a foil in accordance with the speed of the foil. The benefits of varying the span of a foil with speed can be obtained with planing foils, submerged foils, and surface- piercing foils. Furthermore, these benefits can be obtained when the span of a single foil is varied as well as when the total span of a plurality of foils is varied by selectively raising or lowering the foils so that different numbers of foils are in contact with the water at different speeds.

Figure 35 illustrates an embodiment of a variable span foil 400 having a plurality of telescoping sections 401 and 402. The foil 400 can be elongated or contracted in the span-wise direction of the foil to set the span of the foil to any value between a fully elongated span and a fully contracted span. The foil 400 may have any number of sections, which can be telescoped with respect to each other by any suitable mechanism, such as hydraulic or pneumatic cylinders or electric motors, disposed either inside or outside the foil. In this embodiment, the foil 400 includes a center section 401 and two end sections 402 telescoped inside the center section 401. Two racks 404 extending in the spanwise direction are disposed inside the foil 400. One end of each rack 404 is connected to one of the end sections 402 of the foil 400, while the other end of the rack 404 is engaged with a pinion 405 driven by an electric motor 406. When the pinion 405 is rotated in a first direction, the outer ends of the racks 404 are driven away from each other so as to increase the span of the foil 400, and when the pinion 405 is rotated in the opposite direction, the outer ends of the racks 404 are pulled towards each other so as to decrease the span of the foil 400. For structural simplicity, the foil 400 preferably includes at least one section (the center section 401 in the case of Figure 35) which does not undergo widthwise movement with respect to the hull of the hydrofoil craft when the span of the foil 400 is being adjusted. Support arms 403 for connecting the foil 400 to the hull of the hydrofoil craft can conveniently be connected to the stationary center sections 401, but it is also possible to connect support arms to one or more of the movable end sections 402. Figures 36 - 38 illustrates another embodiment of the present invention in which a telescoping foil is replaced by a plurality of foils 410 disposed side-by-side in alignment. Each foil 410 is connected to an unillustrated hull of a hydrofoil craft by one or more support arms 411 which are pivotable about a transverse axis 412 extending in the widthwise direction of the hull. The support arms 411 of at least one of the foils 410 and preferably of a plurality of the foils 410 are connected to the hull in a manner such that the corresponding foils can be selectively raised above the water surface when desired during foil-borne operation while other of the foils are still contacting the water to generate lift, thereby enabling the total span of the plurality of foils 410 to be varied. In this example, each of the foils 410 has substantially the same chord, although the chord may vary from foil to foil, and the chord of a single foil may vary over its span. The spans of the foils may also vary among the foils or they may be uniform. As shown in Figure 38, the foils 410 are aligned when viewed in plan as well as when viewed in elevation.

When the total span of a plurality of foils 410 is varied by raising one or more foils above the water surface, the foils may be raised in any desired manner. When the support arms 411 for the foils are pivotable about a transverse axis 412 as in this example, it is convenient to lift the foils 410 by moving them backwards along an arc about the transverse axis 412, but the foils may be lifted in other manners. For example, they may be swung laterally along an arc about a fore and aft axis, or they may be lifted above the water surface along a straight line.

It is possible to interconnect adjoining foils 410 by a releasable interlock mechanism so that when adjoining foils are in a lowered position, they move as a unit and behave as a single foil. However, when the support arms 411 are pivotably connected to the hull in a manner permitting the foils 410 to move with respect to the hull in response to water forces acting on the foils, it is advantageous for the foils 410 to be able to move independently of each other, as described earlier. The total induced drag on the plurality of foils 410 is lower when adjoining foils

410 are closely spaced from each other than when they are widely spaced. When the foils 410 are abutting, the total drag will be closest to that of a single foil having the same overall span. If the support arms 411 of the foils 410 are infinitely stiff against lateral forces in the spanwise direction of the foils 410, the ends of adjoining foils 410 can be virtually abutting each other when at the same height. However, actual support arms will usually have some flexibility in the lateral direction and will undergo some lateral movement during foil-borne operation. Therefore, it may be necessary to separate adjoining foils from each other by a spanwise gap δ which is large enough to prevent the adjoining foils 410 from overlapping each other in the spanwise direction in response to lateral forces so that adjoining foils will not strike against each other, particularly when the support arms 411 for adjoining foils are pivoting about the transverse axis 412 in different directions or at different speeds in response to vertical fluid forces acting on the foils 410.

The gap δ is preferably as small as possible, such as at most 1 times the chord of adjoining foils, with particularly good results being obtained when the gap δ is at most about 0.2 times the chord, since the total induced drag on a plurality of foils arranged side by side changes significantly as the gap changes from 0 to about 0.2 times the chord. However, even when the gap ό is greater than 1 chord, such as in the range of about 1 to 10 chords, adjoining foils can interact to produce a higher lift/drag ratio for the two foils than if the foils were operating totally independently of each other.

Figures 39A - 39D and 40 illustrate a support arm structure having a high lateral stiffness which enables the gap between adjoining foils to be minimized. As shown in these figures, which are respectively a front elevation and a side elevation, two support arms 421 are secured at their lower ends to the tips of a foil 420, while their upper ends are secured to a sleeve 423 which is pivotably connected to an unillustrated hull for pivoting about a transverse axis. Each support arm 421 may have a substantially vertical portion 422 near the tips of the foil 420 which can act as an end plate to minimize the strength of residual trailing vortices shed at each tip. Figure 39B is a transverse cross section taken through the vertical portions 422 of Figure 39 A. The arrows in this figure indicate the direction of water flow. This effect of the vertical portions 422 can be enhanced by yawing the support arms 421, as shown in cross section in Figure 39C, or cambering the support arms 421, as shown in cross section in Figure 39D, near the foil 420 so as to introduce a swirl in the opposite direction of the vortices.

Another way of preventing adjoining foils from interfering with each other when moving in different directions or at different speeds is shown in Figure 46, which is a plan view of the foils 450 of another embodiment. In this embodiment, the support arms 451 of adjoining foils 450 have the same length but pivot about transverse axes 452 which are offset from each other in the lengthwise direction of the hydrofoil craft so that adjoining foils 450 will move along paths which do not intersect in the normal range of movement of each foil 450. Therefore, adjoining foils 450 will not interfere with each other even if they overlap in the spanwise direction of the foils 450. In fact, spanwise overlap of adjoining foils 450 is advantageous because the vortices shed by the tip of one of the foils 450 tend to cancel the vortices shed by the tip of an adjoining foil 450 located behind it, thereby reducing the downwash and the induced drag of the adjoining foil 450.

The tendency of the gaps between adjoining foils to increase induced drag can be reduced by the provision of small upturned wings 413 on the opposing ends of adjoining foils 410, as shown in elevation in Figures 41 and 42. With such wings 413, not only are the losses less for a given gap between adjoining foils 410, but the gaps can also be reduced in size since adjoining foils 410 overlapping in the spanwise direction will tend to brush each other aside, probably without making physical contact because of the water between them, rather than squarely impacting each other.

It is not necessary for all the foils in a set to lie in a single plane or along a straight line. For example, as shown in elevation in Figure 43, a plurality of foils 420 may be aligned along two intersecting lines to form a dihedral shape, or as shown in Figure 44, a plurality of foils 430 may have a swept shape as viewed in plan. Many other arrangements of adjoining foils are possible.

As stated above, the foil chord need not be constant. It may vary among the foils in a set, and any one foil may have a varying chord. Figure 45 is a plan view of an example in which the chord of a plurality of foils 440 decreases from the outer ends of the outermost foils 440 towards the centermost foils 440. Because of the decreasing chord, the centermost foil on which the hydrofoil craft is supported at cruising speed can have a higher aspect ratio than if all the foils had a constant chord, resulting in a lower induced drag.

The span of a single variable-span foil, such as the foil 400 shown in Figure 35, can be varied continuously, so it is possible for the span of the foil 400 to closely approximate the span for minimum total drag shown by the dashed line in Figure 34. However, when a hydrofoil craft has a plurality of foils of fixed span, the total span of the foils can only be varied in a stepwise manner. An example of stepwise variation of the span is illustrated in Figures 47 - 49, which are schematic front elevations of an embodiment of a hydrofoil craft 460 according to the present invention having a set of six foils 462 disposed side by side. At least the outer two foils 462 on each side can be selectively raised and lowered with respect to the hull 461 of the hydrofoil craft. At take-off, as shown in Figure 47, all of the foils 462 are down and in contact with the water. At some medium speed, as shown in Figure 48, the two outermost foils 462 are raised above the water surface and the four inboard foils 462 are maintained in a lowered position. At a still higher cruising speed of the hydrofoil craft, as shown in Figure 49, two more of the foils 462 are raised above the water surface so only the two centermost foils 462 of the set remain in contact with the water. Due to the stepwise variation in the total span of the foils, at some speeds, the total drag on the foils will be higher than the minimum total drag given by the dashed line in Figure 34. However, since the hydrofoil craft will spend most of its time at its cruising speed, by selecting the span of the two center foils 462 to equal the span of minimum total drag corresponding to the cruising speed, the overall efficiency of the plurality of foils 462 can be made nearly the same as that of a single variable-span foil. From a strength standpoint, a plurality of foils each having a relatively small aspect ratio but having a large overall aspect ratio is superior to a single foil having an aspect ratio equal to the overall aspect ratio of the plurality of foils since the bending stresses in each of the smaller foils can be far lower than those in a single foil of the same aspect ratio. Therefore, the overall weight of the plurality of foils can be much less than that of a single foil of the same total span.

All of the foils in a set may have the same profile, but it is preferable if each foil has a shape selected for the principal speed range in which it is to operate. Thus, in the example of Figures 47 - 48, the two outermost foils 462 preferably have a shape which gives them maximum efficiency at low speeds, the two foils 462 inboard of these preferably have a shape producing maximum efficiency at medium speeds, and the center foils 462 preferably have a shape which gives them maximum efficiency at the cruising speed of the hydrofoil craft.

When the total span of the foils is adjusted by raising the foils above the water surface in a step-wise manner, the foils need not be raised in any particular order. However, it is generally preferable to lift the outermost foils first so that the remaining foils will be closely spaced and obtain the benefits of having a large total aspect ratio.

As stated above, a hydrofoil craft according to the present invention will typically have more than one set of foils spaced in the longitudinal direction of the hull, e.g., a forward and an aft set of foils. The wake behind a first or forward foil 470 is schematically illustrated in Figure 50. If a second or aft foil 471 is placed at a point on the wake where the wake has a downwards slope, the drag of the aft foil 471 will be higher than that of the forward foil 470 because it is "climbing uphill" with respect to the wake. An aft foil 471 placed at a point where the wake is substantially horizontal, will have roughly the same drag as the forward foil 470. However, an aft foil 471 placed at a point where the wake has an upwards slope will have much less drag than the forward foil 470 because it will be "coasting downhill", i.e. , the resulting force vector will be inclined forward. This phenomenon, which is referred to as favorable interference, occurs whether the forward and aft foils are planing or submerged, so long as the aft foil 471 is situated on the wake of the forward foil 470. Therefore, the total drag on a hydrofoil craft having a forward foil and an aft foil positioned at a location where the wake has an upwards slope (an upwards velocity component) will be greatly reduced. Ideally, in two-dimension nonviscous flow, the spacing between the forward and aft foils 470, 471 can be arranged such that the combined wave drag of the two foils is zero. While it is not possible to completely cancel the total wave drag of the two foils in a real, three dimensional flow because of laterally propagated waves, it is nevertheless possible to significantly reduce its magnitude.

In hydrodynamics, it is known that in the mathematical theory of two-dimensional flow, a planing or slightly submerged foil will leave sinusoidal waves in its wake, so that the elevation (η) of the water surface behind the foil is approximately given by

where K is a constant which depends upon the lift, speed and submergence depth of the foil, g is the acceleration due to gravity, χ is the horizontal distance behind the foil's trailing edge, and V is the speed of the vessel.

Thus, the most favorable location for an aft foil, where it will cancel all the wave drag of the forward foil, is a location at which the elevation JJ is 0, i.e. , where sin(gχ/V²) = 0. The smallest nonzero value of x at which this equation is satisfied is a value which satisfies

Solving for x gives

π V²

X =

8 For a hydrofoil craft traveling at 100 knots, this most favorable location is 2,778 (0.53 miles) behind the trailing edge of the forward foil, an unpractically large distance.

In the much more complicated case of three-dimensional flow which occurs in the real world, the transverse downwash waves in the wake propagate inwards from the region behind the spanwise ends of the forward foil, and this effect moves the most favorable region for the aft foil much closer to the forward foil. For a hydrofoil craft with a forward foil and an aft foil, the optimum distance of the aft foil behind the forward foil for three-dimensional flow is approximately

2.225 b g

where b is the span of the forward foil.

If the forward foil has a fixed span, the location of the region where the wake from the forward foil has an upwards velocity component will move further and further behind the forward foil as the speed of the hydrofoil craft increases, making it impossible to maintain the aft foil at the optimal distance behind the forward foil, and possibly resulting in the aft foil being in an unfavorable region of the wake where the wake has a downwards velocity component.

However, if the span of the forward foil is decreased as the speed of the hydrofoil craft increases, the effect upon the wavelength of the wake of the forward foil will work against the effect of increasing speed so as to enable the aft foil to be maintained in a region of upwash. The span of the forward foil can be varied in order to attain favorable interference in any of the ways described with respect to the preceding embodiments.

In addition to varying the span of a forward foil to control the region where favorable interference by the aft foil takes place, it is possible to vary the position in a longitudinal direction of an aft foil disposed behind a forward foil so that the aft foil is located where the wake has an upwards velocity component. Figures 51 show an arrangement having three sets of foils 481 - 483 spaced in the fore and aft directions of a vessel 480, each set including one or more foils. At least the second and third sets of foils 482 and 483 are capable of being moved between a lowered position contacting the water and a raised position above the water surface. The rear sets of foils 482, 483 can be raised or lowered independently of each so that one set is always located in a region where the wake from the forward foil set 481 has an upwards velocity component. During takeoff and low speed operation, the wake behind the forward foil set 481 is such that the second foil set 482, if lowered, would be in a region having an upwards velocity component and the third foil 483, if lowered, would be in a region having a downwards velocity component. Therefore, in this speed range, the second foil set 482 is lowered, the third foil set 483 is raised, and the vessel 480 is supported on only the front and second foil sets 481 and 482. As the speed increases, the wavelength of the wake increases such that both of the second and third foil sets 482 and 483 can be in a region with an upwards velocity component, so both of the second and third foil sets 482 and 483 are lowered to generate lift. As the speed further increases, the wavelength of the wake increases so that the second foil set 482, if lowered, would be in a region with a downwards velocity component and the third foil set 483, if lowered, would be a region with an upwards velocity component. Therefore, the second foil set 482 is raised above the water so that the vessel 480 is supported on only the first and third foil sets 481 and 483. If the vessel 480 is equipped with more than three foil sets, the various sets can be raised and lowered in accordance with the speed of the vessel in a manner similar to that described above so that the aft foil sets contacting the water are always in a region with an upwards velocity component. The wake produced behind a straight foil (with zero sweep) with a finite span has a three-dimensional shape. At a given distance behind the foil, the height and vertical velocity of the wake will vary in the widthwise direction of the foil. Figure 52 shows an example of the water height contour behind a straight foil as calculated by theory. Each contour represents a constant wake height. In order to capture as much energy as possible from the wake, an aft foil is preferably at a location at which as much of its area as possible is in a region where the wake has an upwards velocity component, with the contour of zero height having the maximum upwards velocity. In order for as much of the aft foil to be near the zero height contour as possible, a swept shape may be used, as shown by the dashed lines in the figure. At a given distance behind the forward foil, there will be locations in which the wake has an upwards velocity component and locations in which the wake has a downwards velocity component. For this reason, it may be desirable to have a plurality of aft foils which can be raised and lowered as desired. In this way, the aft foils can be selectively raised and lowered so that only those foils located in a region where the wake from the forward foil has an upwards velocity component are made to contact the water, thereby enabling the foil to take advantage of favorable interference. Many types of maritime vessels are driven through the water by a propeller connected to an engine by a rotating propeller shaft. In some vessels, the propeller shaft is quite long. For example, in a hydrofoil craft, in which the engine is normally inside the hull of the vessel and the propeller is well below the keel, the length of the propeller shaft may be 1/3 to 1/2 of the overall length of the hull. In order to avoid "whipping" (also referred to as "shaft whirl") of the shaft as it rotates, it is necessary to support the shaft in a manner such that the fundamental natural frequency in bending of a portion of the shaft between two support points is greater than the rotational frequency of the shaft. It is possible to avoid shaft whirl by supporting a shaft at frequent intervals along its length by bearings secured to the hull by rigid struts, for example. However, when the propeller shaft is disposed on the exterior of a hull, the presence of rigid struts extending between the hull and the propeller shaft may be undesirable because they can greatly increase the drag of the vessel and because they make it difficult to support the propeller shaft in a manner such that the propeller shaft can be raised as necessary, such as when the vessel is operating in shallow water. An alternative to supporting a propeller shaft along its length by bearings mounted on struts is to support the shaft only at its extreme ends (at the end adjoining the hull and at the end adjoining the propeller), and to form the shaft of a large- diameter, thin-walled tube capable of having a large unsupported length. Since there are no rigid struts connected to the shaft between its two ends, it is possible to support the shaft such that it can be easily raised and lowered as desired. However, when a large- diameter propeller shaft is rotating in water at an angle to the flow of water, the shaft can develop extremely large lateral forces (normal to the longitudinal axis of the propeller shaft) due to the Magnus effect. When the vessel has a single propeller shaft, these lateral forces may produce large rolling forces which can cause the vessel to roll to considerable angles even in calm water. Even when rolling forces are not generated, the lateral forces frequently produce uneven wear on the bearings for the propeller shaft and thereby reduce the useful life of the bearings, the replacement of which can be extremely expensive. Aside from the problem of lateral forces acting on the shaft, a shaft of high bending stiffness can be expensive to manufacture.

Difficulties occur when installing a propeller shaft on the interior of a hull as well. With conventional shaft support arrangements, it is considered necessary for each of the bearings to be secured to a support which is completely immobile in all directions. For example, the bearings for the portion of a propeller shaft within a hull are typically mounted on the transverse frames of the hull. The bearings must be aligned with one another with high precision, such as within a thousandth of an inch. Since the bearings are mounted on essentially immovable supports, the process of aligning the bearings is time-consuming and enormously expensive. Similar problems occur within the installation of rotating shafts on land, such as in factories where rotating shafts are used in e transmission of power.

The present inventor has found that it is unnecessary to attach the bearings for a rotating shaft directly to an immovable support, such as rigid struts or the framing of the hull of a vessel, and that it is unnecessary to restrain the bearings with the same degree of stiffness in all lateral directions of the shaft in order to prevent shaft whirl. Rather, it is possible to attach the bearings to a relatively flexible elongated support member, such as a beam. If the beam or other support member has a natural frequency higher than the maximum expected rotational frequency of the shaft about at least one transverse axis, the natural frequency about other axes may be significantly lower than the maximum expected rotational frequency of the shaft without shaft whirl occurring. For example, the beam may be unrestrained against vibration about the other axes. Preferably, the natural frequency of bending of the beam or other support member about the at least one axis is at least twice the higher expected rotational frequency of the shaft. Figure 53 illustrates an embodiment of a vessel employing a shaft support arrangement which is able to support a propeller shaft so as to prevent shaft whirl while employing a lightweight structure. While the illustrated vessel is a hydrofoil craft, a shaft support arrangement according to the present invention can be employed with any type of maritime vessel driven by a propeller. The illustrated hydrofoil craft is similar to that shown in Figure 1 and may incorporate one or more features of any of the other embodiments of the present invention. Like the hydrofoil craft of Figure 1, it includes a hull 510 supported above a water surface by a forward foil 520 and one or more aft foils 522 located to the rear of the forward foil 520. In this embodiment, the forward foil 520 supports the majority of the weight of the hull 510 (90% or more, for example), while the aft foils 522 primarily provide stability to the craft in pitch, although the weight of the hull 510 can be distributed among the foils in a different ratio. The forward foil 520 may be operated in either a fully submerged mode, a semisubmerged mode, or in a planing mode, as shown in Figure 53. The aft foils 522 are illustrated as operating in a fully submerged state, although like the forward foil 520, they may instead be operated in a planing or a semisubmerged mode. The support arms 521 for the forward foil 520 are pivotably connected to the hull 510 for pivoting about a transverse axis to enable the forward foil 520 to move up and down with respect to the hull 510 in response to water forces acting on it.

The hydrofoil craft is propelled by a propeller 530 connected to an unillustrated engine disposed within the hull 510 by a propeller shaft 533. The propeller 530 is supported by a propeller support 531 extending downwards from the stern of the hull

510. Preferably, the propeller support 531 can be raised and lowered with respect to the hull 510 by a suitable mechanism, such as a hydraulic cylinder 532, to enable the depth of the propeller 530 to be adjusted.

The propeller shaft 533 has a forward end adjoining the hull 510 and a rear end connected to the propeller 530. Between these two ends, the propeller shaft 533 is laterally supported by an axially-extending support member in the form of a fairing 540 surrounding the propeller shaft 533 and extending generally parallel to the propeller shaft 533 between the hull 510 and the propeller 530, and by one or more bearings 544 disposed inside and laterally supported by the fairing 540. The fairing 540 need not have any particular shape, but preferably it is streamlined so as to minimize drag as it passes through the water. The inside of the fairing 540 may be either sealed with respect to the surrounding water and filled with air, for example, or it may have holes which allow water to enter the fairing 540 to equalize the static pressure on the inside and outside of the fairing 540 as well as to lubricate the bearings 544. The fairing 540 can be made of a wide variety of materials, including but not being limited to metals, carbon fibers, fiberglass, plastics, and wood. Carbon fibers are particularly suitable because of their high strength-to- weight ratio. Figure 54 is a transverse cross-sectional view of the fairing 540 taken along line II-II of Figure 53. The fairing 540 comprises two arcuate, thin- walled shells 541 formed, in this embodiment, from carbon fibers. The transverse cross-sectional shape of the fairing 540 is generally that of an ellipse with a major axis extending parallel to the centerline plane of the hull 510 and a minor axis extending at right angles to the major axis. In order to reduce drag, the leading edge of the fairing 540 (the left end in Figure 54) is preferably somewhat rounded while the trailing edge is preferably sharp. A sharp trailing edge also prevents atmospheric air from flowing down the outside of the fairing 540 to ventilate the propeller 530. The wall of each shell 541 is thickened in a portion 542 between the leading and trailing edges of the fairing 540 to provide a support for the bearings 544. Each thickened portion 542 has an arcuate recess 542a for receiving a thin- walled cylindrical positioning tube 543 which contains the bearings 544. If desired, longitudinal stiffeners can be installed inside the fairing 540 to increase the bending stiffness about one of its axes. The bearings 544 for rotatably receiving the shaft 533 may be of any type able to withstand the operating conditions of the hydrofoil craft. The bearings 544 in this embodiment are water-lubricated Cutlass bearings mounted inside opposite ends of each positioning tube 543. Other examples of suitable bearings are ball bearings, roller bearings, and pin bearings. The number of bearings 544 is not critical, and there may be a single bearing 544 inside the fairing 540. The separation between the bearings 544 is selected so that the fundamental frequency in bending about some transverse axis of a portion of the shaft 533 between any two adjacent bearings 544 is higher than the rotational frequency at which the shaft 533 is expected to operate. This separation will depend upon the stiffness of the shaft, i.e. , the more flexible the shaft 533, the smaller will be the separation between adjacent bearings 544. Generally, the lower the bending stiffness of the shaft 533, the more economical it is to manufacture. Therefore, the propeller shaft 533 may be designed with the minimum dimensions required to transmit the desired torque to the propeller 530, and the bearings 544 can be disposed at whatever spacing is required to prevent whirling of the propeller shaft 533. The bearing spacing may also be such that the fundamental frequency in bending about a transverse axis of a portion of the shaft between two non-adjacent bearings 544 is lower than the expected rotational frequency of the shaft 533. The propeller shaft 533 can be constructed in a variety of manners. It can be a one-piece member extending over the entire length of the fairing 540, or it can be divided into a plurality of sections connected with one another by suitable joints. In the present embodiment, as shown in Figure 55, the propeller shaft 533 comprises two sections 533a coaxially connected by a splined joint 534 which permits axial movement of the two sections 533a relative to each other.

The weight of the fairing 540 and the propeller shaft 533 can be supported in various ways. For example, the fairing 540 may be supported by the hull 510 through support members such as struts connected between the hull 510 and the fairing 540, or it may be supported in whole or in part by the propeller shaft 533, which can in turn be supported by the hull 510 at its ends. As another alternative, the fairing 540 may be cantilever supported by the hull 510, with the weight of the propeller shaft 533 and the propeller 530 supported by the fairing 540. Preferably, the number of struts or other support members connected between the hull 510 and the fairing 540 and/or the propeller shaft 533 is as small as possible in order to reduce appendage drag as well as to make it easier to raise and lower the propeller shaft 533 when desired. In the present embodiment, the forward end of the propeller shaft 533 is supported by a drive shaft projecting from the hull 510, and the rear end of the propeller shaft 533 and the propeller 530 are supported by the vertical propeller support 531 , while the weight of the fairing 540 is supported primarily by the propeller shaft 533. Although additional supports may be connected between the hull 510 and the fairing 540, preferably the fairing 540 is sufficiently stiff in bending that it does not require any external supports between its ends.

The fairing 540 performs several functions. One function is to laterally restrain the bearings 544 of the propeller shaft 533 so that the fundamental frequency of the shaft 533 in bending about a transverse axis between adjacent bearings 544 will be higher than the expected rotational frequency of the shaft during operation. A second function which the fairing 540 may perform is to isolate the rotating propeller shaft 533 from the free stream of water beneath the hull 510 to prevent the occurrence of the Magnus effect. To accomplish this function, the fairing 540 preferably extends over as much of the submerged length of the propeller shaft 533 as possible. For example, the fairing 540 may have a length so that substantially no portion of the shaft 533 is exposed to the free stream of water beneath the hull 510. Because the fairing can prevent the occurrence of the Magnus effect, the rotation of the propeller shaft 533 does not generate lateral forces. Therefore, the vessel does not develop rolling forces due to such lateral forces, so the vessel is more stable, and the wear of the bearings 544 becomes substantially uniform, increasing their useful life. Another function of the fairing 540 is to decrease the drag of the hydrofoil craft. If the fairing 540 has a streamlined shape, its drag ends up being less than that of a propeller shaft 533 directly exposed to the water at an identical angle, even though the fairing 540 is usually larger in cross section than a typical propeller shaft 533. The fairing 540 can be used as a convenient location for depth sensors, speed measuring transducers, and other sensors because they are in a flow of water undisturbed by the hull 510. Because the fairing 540 is partially submerged, it can also be used to transport cooling water to the engine of the hydrofoil craft. In this embodiment, an inlet 545 for cooling water is formed in a lower portion of the fairing 540 which is submerged when the hydrofoil craft is running at its normal height above the water. The inlet 545 is connected to the rear end of a cooling pipe 546 disposed within the fairing 540. The forward end of the cooling 546 pipe is connected to the cooling system for the engine by suitable means, such as by a flexible hose 547.

In order to enable the propeller 530 to be raised and lowered with respect to the hull 510, the ends of the propeller shaft 533 are preferably equipped with universal joints 535 or other type of coupling which can drivingly connect the propeller shaft 533 to a drive shaft driven by the engine and to the propeller 530 at various angles, depending on the height of the propeller 530 with respect to the hull 510.

A stationary hood 511 may be mounted on the underside of the hull 510 surrounding the forward end of the fairing 540 to reduce drag and to prevent objects from catching on the forward end of the fairing 540. At one or both of its ends, the fairing 540 is preferably restrained against rotation about its longitudinal axis. For example, as shown in Figure 57, the aft end of the fairing 540 in this embodiment has a slit 540a which loosely engages with the propeller support 531, thereby permitting the angle between the fairing 540 and the propeller support 531 to vary as the propeller 530 is raised and lowered while preventing the fairing 540 from rotating about its longitudinal axis. The aft end of the fairing 540 in this embodiment has a transition section in which its cross-sectional shape changes from a streamlined ellipse to a shape (such as circular) matching the shape of a pod at the lower end of the propeller support 531 so as to reduce drag where the fairing 540 meets the propeller support 531. Figure 58 illustrates the shape of the periphery of the fairing 540 in locations A and B of Figure 57 and shows how the shape of the fairing 540 changes.

A streamlined fairing 540 will generally be stiffer with respect to bending about the minor axis of its transverse cross section than about its major axis (the horizontal dashed line in Figure 54) because the bending modulus of the transverse cross section is greater about the minor axis than about the major axis. Therefore, the fairing 540 will present greater resistance to movement of the bearings 544 in a direction parallel to the major axis than to movement parallel to the minor axis. However, it is not necessary for the fairing 540 to have equal stiffness in all directions to stabilize the propeller shaft 533 against shaft whirl. It is sufficient for the fairing 540 to restrain the bearings against lateral movement with respect to the longitudinal axis of the shaft 533 in a single plane containing the axis of the shaft 533 such that the fundamental frequency of bending in that plane of a portion of the shaft 533 between adjacent bearings 544 is higher than the expected rotational frequency of the shaft 533, and it is not necessary for the fairing 540 to restrain the bearings 544 against lateral movement in other planes containing the axis of the shaft 533. Thus, the fairing 540 can be elongated along its major axis to give bending stiffness in the plane containing the major axis, while it can be made extremely narrow measured along its minor axis to reduce its drag. Preferably, the natural frequency of bending of the fairing 540 about its minor axis (the axis about which it is stiffest) is at least twice the highest expected rotational frequency of the shaft 533, in which case the natural frequency of bending of the fairing 540 about other axes, such as the major axis, can be significantly less than the highest expected rotational frequency of the shaft 533.

The embodiment of Figures 53 - 58 is used to support a propeller shaft 533 on the exterior of the hull 510 of a vessel, but a shaft support arrangement according to the present invention can be used to support a propeller shaft on the interior of a hull. In this case, since there is no need for streamlining, the fairing 540 which supports the propeller shaft 533 can be replaced by a different type of axially-extending support member, such as an open-sided frame, a rod, or any of the shapes illustrated in Figures 62 - 65, described below.

An axially-extending support member for restraining a rotating shaft need not be a stationary member and can in fact be a second shaft which rotates as it supports the first shaft. Figures 59 - 61 illustrate an embodiment of a shaft support arrangement for a vessel equipped with twin propeller shafts. As shown in these figures, the vessel includes a hull 550 having two engines 551 connected to two propeller shafts 552 disposed in parallel. A propeller 553 is mounted on the aft end of each propeller shaft 552. Each propeller shaft 552 is rotatably supported near its aft end by a thrust bearing 554 secured to the hull 550. In this embodiment, each shaft 552 functions as an axially- extending support for the other shaft 552. Bearing assemblies 560 are installed on the shafts 552 at intervals determined by the stiffness of each shaft 552 and the maximum expected rotational frequency of the shafts 552, the intervals between bearing assemblies 560 being such that the natural frequency in bending in a plane containing both shaft 552 of the portion of a shaft 552 between two adjacent bearing assemblies 560 is higher than the maximum expected rotational frequency of either shaft 552, thereby preventing shaft whirl. Each bearing assembly 560 includes two bearings 561 each of which rotatably receives one of the shafts 552. The bearings 561 are connected with each other by a rigid bearing holder 562. The bearing assemblies 560 themselves may be supported entirely by the shafts 552, or they may be supported by unillustrated external supports mounted on the hull 550 to prevent sagging of the shafts 552 in the space between the engines 551 and the thrust bearings 554. However, such external supports can be employed simply to bear the static weight of the shafts 552 and the bearing assemblies 560 and need not be capable of rigidly restraining the bearings 560 against movement in all directions, so the external supports can be light-weight members which can be readily attached to a frame or other structural portion of the hull. Thus, the procedure of installing the bearing assemblies 560 is far easier than with conventional bearings which are rigidly mounted on frames so as to be essentially immobile in all directions.

As stated above, an axially-extending support member for a shaft need not be in the shape of a fairing surrounding the shaft. Figures 62 - 65 illustrate other embodiments of support arrangements for a rotating shaft according to the present invention employing different types of axially-extending support members. In the embodiment of Figure 62, a rotating shaft 570 is rotatably received by one or more bearings 571 spaced along the length of the shaft 570 and disposed in a spacer tube 572 extending in the axial direction of the shaft 570. The bearings 571 are positioned along the shaft 570 such that such that the fundamental frequency in bending about a transverse axis of a portion of the shaft 570 between any two adjacent bearings 571, or between a bearing 571 and an adjacent support point for the shaft 570 other than a bearing, is higher than the rotational frequency at which the shaft 570 is expected to operate. The bearings 571 are secured to the inside of the spacer tube 572 in a suitable manner, such as by a press fit. The spacer tube 572 is reinforced by an axially- extending support member 575 comprising two flat plates 576 extending from the outer surface of the spacer tube 576 in the same plane as each other and substantially radially with respect to the center of the shaft 570. The plates 576 are rigidly secured to the spacer tube 572 by a suitable method in accordance with the materials of which the plates 576 and the spacer tube 572 are formed. The spacer tube 572 serves primarily as a means of securing the bearings 571 to the plates 576, and if the plates 576 can be attached directly to the bearings 571, the spacer tube 572 may be omitted. The plates 576 may be supported at their lengthwise ends, or they may be secured to a base or other stationary member between their ends.

The embodiment of Figure 63 is similar to the embodiment of Figure 62 except that an axially-extending support member 580 is in the form of a T-shaped beam extending in the axial direction of the rotating shaft 570. One or more bearings 571 are disposed in a spacer tube 572 which is rigidly secured to an end of the web 581 of the beam 580. The beam 580 may be supported at or between its lengthwise ends. For example, the flange of the beam 580 may be secured to an unillustrated base. In the embodiment of Figure 64, an axially-extending support member 585 comprises two diametrically opposed T-shaped beams 586, each having a web 587 rigidly secured to the outer surface of a spacer tube 572. One or more bearings 571 are disposed inside the tube 571 and spaced from each other in the lengthwise direction of the shaft 570. The support member 585 may be supported at or between its lengthwise ends.

In the embodiments of Figures 62 - 64, the axially-extending support members 575, 580, and 585 provide resistance against lateral movement of the bearings 571 for the shaft 570 in all radial directions with respect to the longitudinal axis of the shaft 570, although the level of resistance depends upon the direction. When these three embodiments are oriented as shown in the figures, the axially-extending support members provide a high degree of resistance against movement of the bearings 571 in the vertical direction and a smaller degree of resistance against lateral movement in the horizontal radial direction. Figure 65 illustrates an embodiment in which one or more bearings 571 for shaft 570 are substantially unrestrained against lateral movement in a certain radial direction. In this embodiment, an axially-extending support member 590 comprises a box-shaped beam surrounding a spacer tube 572 in which a shaft 570 and one or more bearings 571 are disposed. The inner dimensions of the beam 590 are selected such that the bearings 571 are restrained against lateral movement in the widthwise direction of the beam 590 (the horizontal direction in the figure) but are unrestrained in the height direction of the beam 590 (the vertical direction in the figure). As long as the shaft bearings 571 are restrained against lateral movement in one direction transverse to the axis of the shaft 570, the shaft 570 can be prevented from undergoing shaft whirl.

An axially-extending support member extending between two or more bearings for a shaft is convenient when it is difficult or impossible to secure the individual bearings to an immovable member, such as a base, a floor, or a hull of a ship. However, when a shaft support arrangement according to the present invention is used in a factory, for example, or inside the hull of a ship, it may be possible to secure each of the bearings for a shaft to an immovable member, in which case an axially-extending support member can be replaced by individual stiffeners for individual bearings, with each of the stiffeners being secured to an immovable member. Figures 66 - 69 illustrate embodiments of shaft support arrangements according to the present invention which do not employ an axially-extending support member interconnecting bearings. In each embodiment, a rotating shaft 600 is rotatably supported by one or more bearings 601 (only one of which is shown in each embodiment) spaced in the lengthwise direction of the shaft 600. Each bearing 601 is supported by one or more stiffeners, each of which is connected between the bearing 601 and an unillustrated base or other immovable member. The stiffeners in Figures 66 - 69 have the same cross-sectional shape as the axially-extending support members in Figures 62 - 65, respectively, but each stiffener supports only a single bearing 601 rather than extending between two or more bearings. The stiffeners can be secured to the bearings 601 in any desired manner. In these figures, each stiffener is secured to a retainer in the form of a cylindrical tube 602 which holds the bearing 601, but depending upon the structure of the bearing 601, it may be secured directly to the stiffener. The embodiment of Figure 66 includes two plate-shaped stiffeners 605 extending radially from the outer surface of the tube 602 in the same plane. The embodiment of Figure 67 includes a T-shaped stiffener 610 having a web 611 extending radially from the outer surface of the tube 602 and a flange 612 at the end of the web 611 remote from the tube 602. The embodiment of Figure 68 has two T-shaped stiffeners 615, each having a web 616 extending radially from the tube 602 in the same plane as each other and a flange 617 at the end of the web 616 remote from the tube 602. The embodiment of Figure 69 has a box-shaped stiffener 620 which prevents lateral movement of the bearing 601 and the tube 602 in the widthwise direction of the stiffener 620 (the horizontal direction in the figure) but allows lateral movement in the height direction (the vertical direction). The one or more stiffeners for each bearing 601 provide greater resistance against lateral movement of the corresponding bearing 601 in one radial direction than in another radial direction with respect to the axis of the shaft 600. For example, when the stiffeners are oriented in the manner shown in the figures, the stiffeners of Figures 66 - 68 provide greater resistance against lateral movement of the bearing 601 in the vertical radial direction than in the horizontal radial direction, and the box-shaped stiffener 620 of Figure 69 provides resistance against lateral movement in the horizontal direction but substantially no resistance against lateral movement of the bearing 601 in the vertical direction. The stiffeners can be secured to a base of other immovable member in any suitable manner. For example, in the embodiment of Figure 66, one or both of the stiffeners 605 can be secured to a unillustrated base, and in the embodiments of Figures 67 and 68, the flanges of one or both stiffeners can be secured to a base. If the shaft 600 is equipped with more than one bearing 601 spaced from the illustrated bearing in the lengthwise direction of the shaft 600, the other bearings 601 may be supported by other stiffeners having a structure the same as or different from the illustrated stiffeners.

The shaft support arrangement of the present invention provides a number of significant advantages over a conventional support arrangement. When the shaft support arrangement is used with a propeller shaft of a maritime vessel, individual bearings for the shaft need not be secured to the hull by rigid support struts, and the shaft can be unconnected to the hull between its ends, so the propeller shaft can be easily raised and lowered with respect to the hull. Bearings for the shaft can be disposed along the shaft at frequent intervals, so the shaft can have a small diameter, making it inexpensive to manufacture and easy to handle. Furthermore, the shaft and the bearings for the shaft form a preassembled unit which can be installed on a vessel, so the time-consuming process of aligning bearings on rigid struts or on the frame of the hull of a vessel is avoided. A shaft support arrangement according to the present invention has a wide range of applications and can be used to support any type of shaft or other rotating body for the transmission of torque. For example, it can support drive shafts of rotating industrial machinery, drive shafts of automotive vehicles, and rotating drill strings for oil wells. Various types of maritime vessels are capable of being beached on a gently sloping shore to enable cargo or passengers to be loaded or unloaded from the vessel without the need for a dock. Examples of such vessels include conventional landing craft, long used by the military for amphibious operations, and air cushion vehicles, which can be built so as to be capable of traveling both on water and on land. Figure 70 is a schematic perspective view of a hull 710 of an embodiment of a maritime vessel according to the present invention which is capable of being easily beached and unbeached. For simplicity, the hull 710 is schematically illustrated as being prismatic with a constant beam and a constant depth over its length, but these dimensions may vary over the length, the exact shape of the hull 710 not being critical to the present invention.

The vessel may be of any type. For example, it may be a conventional surface vessel which travels primarily in a hull-borne state, a planing vessel, or a hydrofoil craft, particularly one incorporating features of one or more of the other embodiments of the present invention. When the vessel is a hydrofoil craft equipped with foils connected to the hull by support arms, the support arms and the foils may be retracted to above the bottom of the hull when the vessel is to be beached.

An open-bottomed chamber 711 is formed in the bottom of the hull 710 over at least a portion of the length of the hull 710. In the example of Figure 70, the chamber 711 extends over the entire length and over substantially the entire width of the bottom of the hull 710. The upper surface of the chamber 711, formed by the bottom surface of the hull 710, is intended to function in the present embodiment as a planing surface, so during hull-borne operation of the vessel, the chamber 711 is usually open at the forward end and aft ends of the hull 710. Figures 71 and 72 illustrate two examples of possible transverse cross-sectional shapes for the bottom of the hull 710. The shape shown in Figure 72 is particularly preferred because bottom pressures are reacted by the bottom of the hull 710 in tension. The vessel will typically be equipped with an unillustrated propulsion system, which may be of any desired type, such as one employing water propellers, water jets, air propellers, or sails. The vessel may also be towed. For simplicity, other conventional features with which the vessel may be equipped, such as a superstructure, have been omitted from the drawings. If one or both lengthwise ends of the chamber 711 extend to a lengthwise end of the hull 710, the vessel is preferably equipped with a mechanism which can close the lengthwise ends during a beaching operation and open the ends when it is desired to use the upper surface of the chamber 711 as a planing surface. An example of a closing mechanism is a door, a cover, a plate, or a flexible skit which is mounted on the hull 710 and can be moved down over an open end of the chamber 711 at low vessel speeds. As shown in Figure 73, the present embodiment is equipped with both a forward closing mechanism 712 and an aft closing mechanism 713 for this purpose.

As shown in Figure 76, the vessel is also equipped with a pump 715 which can pump a fluid (such as water) under pressure into the chamber 711 during a beaching operation. The fluid can be either stored on the vessel or can be pumped from outside the vessel. A convenient source of fluid is the body of water in which the vessel is operating.

Figure 74 is a close-up cross section of one chine of the hull 710 during a beaching operation. The chine is shown spaced from the beach by a clearance δ. If the pump 715 pressurizes the water in the chamber 711 to a pressure Δp higher than the pressure outside the hull 710 and the water in the chamber 711 flows through the clearance with a velocity U , then in accordance with Bernoulli's equation, - f∑p for water

where is the density of water. If it is assumed that the ground clearance δ is constant around the periphery of the hull 710, then the mass flow rate out of the chamber 711 will be

m = p(l + b)6u_j

wherein I is the length and b is the beam of the hull 710. The power required to achieve this mass flow rate is

(l+b)δ Δp for water

For a 10,000 ton ship, if I = 400 feet, b = 100 feet, δ = 2 feet, and Δp = 560 lb/ft², then

P = 13.25 x 106 lb-ft/sec

= 24,000 HP _* 35,000 BHP to the pump 715.

By comparison, if air were used to lift a hull 710 of the same size, as in an air cushion vehicle, the power requirements would be vastly higher (on the order of IO⁶ BHP).

The smaller the clearance δ through which water escapes from the chamber 711, the lower the power requirements. As shown in Figure 75, the hull 710 may be equipped with a skirt 714 which may be dropped during beaching so as to decrease the clearance δ while still maintaining the hull 710 far enough above the beach surface to clear rocks, bumps, and debris.

When the vessel is to be beached, the vessel will typically be moving in a hull- borne state at a low speed towards the beach or other surface on which the hull 710 is to be beached, either under power or coasting. If the vessel is a barge or other vessel without a propulsion system, it can be towed onto a beach from ashore as it floats on a cushion of water.

The hull 710 can be moved as far up a beach above the shoreline as the supply of water or other fluid to the pump 715 permits. If an inlet 716 for the pump 715 is installed near the stern of the hull 710, the entire region of the hull 710 forward of the inlet 716 can be beached to above the shoreline, as shown in Figure 76. If a conduit 717 (such as a hose) for water is connected to the pump inlet 716 and allowed to trail in the water behind the hull 710, the entire length of the hull 710 can be beached to above the shoreline, as shown in Figure 77. When the vessel has moved to a desired location on the beach, the pump 715 can be turned off to allow the hull 710 to settle onto the beach.

When it is desired to unbeach the vessel, the pump 715 can be operated to lift the hull 710 off the ground surface on which the vessel is beached. If the ground surface has sufficient slope, gravity will be sufficient to pull the vessel backwards into the water until the hull is refloated. If the ground surface is not sufficiently sloped, the vessel can be readily pushed or towed or moved under its own power back into the water (depending upon the type of propulsion system), since the vessel has little resistance to horizontal movement when floating on a cushion of water. If the ground surface on which the vessel is beached is extremely soft or unconsolidated, the force of pressurized water being forced from beneath the hull 710 may produce erosion of the ground surface beneath the hull 710. If the erosion is severe, it may form a cavity beneath the hull 710 into which the hull 710 may settle instead of sliding along the ground surface back into the water. Therefore, in situations in which there is a likelihood of erosion, a protective sheet 718 of a suitable material, such as plastic, can be laid on the ground surface beneath the hull 710. The sheet 718 can be stored on a drum 719 rotatably mounted near the bow of the hull 710, and during beaching of the vessel, the sheet 718 can be unrolled from the drum 719 and spread beneath the hull 710 as the vessel is moving. When it is time to unbeach the vessel, the pressurized water pumped out of the hull 710 will contact the protective sheet 718 rather than the soil forming the ground surface, thereby preventing erosion of the ground surface and enabling the vessel to easily slide back into the water.

When a conventional surface vessel is floating in calm water, the weight of the vessel is continuously supported by the water over the entire wetted length of the vessel. In contrast, when a hydrofoil craft is operating in a foil-borne state with its hull raised above the water surface, the weight of the vessel is supported only at discrete support points corresponding to the locations where support arms for the foils of the hydrofoil craft are connected to the hull. Thus, the load distribution of the hull of a hydrofoil craft in a foil-borne state is quite different from that of the hull of a conventional surface vessel operating in calm water.

Because of the manner in which loads are applied to the hull of a hydrofoil craft, the design of large hydrofoil craft capable of carrying heavy pay loads presents significant challenges. Heavyweight hull construction, such as used in tankers and other surface vessels, is capable of providing a hull with long unsupported lengths between support points, but such a hull would be impractical for most hydrofoil craft in which there is a premium on reducing the weight of the hull and maximizing the payload. As an alternative, it is possible to employ lightweight hull construction in which the hull has relatively short unsupported lengths. However, the shorter the unsupported lengths of the hull, the more foils are required to support the hull, and increasing the number of foils may lead to complicated control problems.

Figure 78 schematically illustrates an embodiment of a hydrofoil craft according to the present invention which can have a long yet light-weight hull, enabling the hydrofoil craft to carry a large payload. The hydrofoil craft includes a hull 810, one or more forward foil 820, and one or more aft foils 830 located aft of the forward foil 820. Each of the foils 820, 830 is connected to the hull 810 by one or more support arms 821 and 831, respectively, in any suitable manner, such as in the manner described with respect to any of the preceding embodiments or as described in U.S. Patent No.

5,469,801, for example. The foils 820, 830 may support the hull 810 in a partially submerged state or in a state in which the hull 810 is lifted entirely above the water surface. The embodiment of Figure 78 has foils 820, 830 disposed in only two locations along the length of the hull 810, but additional foils may be provided in other locations. The hydrofoil craft may be propelled in any suitable manner and may be equipped with an unillustrated propulsion mechanism. It is also possible for the hydrofoil craft to be towed through the water.

An upright 840 extends upwards from the hull 810, and a plurality of connecting members 841 are connected between the upright 840 and the hull 810 for transmitting at least a portion of the weight of the vessel to the upright 840. The connecting members 841 can be any members which are capable of acting in tension to transmit forces between the upright 840 and the hull 810. The connecting members 841 may be capable of transmitting compressive loads, or to minimize weight, they may be tension members, such as cables, which are substantially incapable of transmitting compressive loads and transmit only tensile loads.

The connecting members 841 may be in a variety of forms, including but not limited to wire rope, nonlaid rope, filaments, ribbons, chains, rods, beams, plates, and sheet-like members, depending upon the types of loads which the connecting members 841 are intended to transmit. There are no particular restrictions on the materials of which the connecting members 841 are made, suitable materials including but not being limited to metals, natural fibers, and synthetic polymers. The material can be selected in accordance with the desired strength and resistance to corrosion, abrasion, and shock required of the connecting members 841. There is also no restriction on the number of connecting members 841.

Depending upon the vibrational characteristics of the individual connecting members 841 and the frequency of the forces acting on them, it may be desirable to provide a mechanism for preventing resonance of the connecting members 841, such as dampers 842 or stays 843 connected between adjoining connecting members 841 to dampen vibrations or alter the natural frequency of the connecting members 841.

The upright 840 serves to transmit at least a portion of the weight of the vessel to one or more of the foils 820, 830. Therefore, the upright 840 should be capable of withstanding a compressive load. The upright 840 can have any shape which enables it to withstand the loads to which it is subjected. For example, as shown in Figure 82, it may be a mast-like structure with a single leg. Alternatively, it may have a plurality of legs connected to opposite beamwise sides of the hull 810, as shown in Figure 79, and/or a plurality of legs spaced in the lengthwise direction of the hull 810, as shown in Figure 80. In Figure 78, the upright 840 extends substantially vertically, but all or a portion of the upright 840 may instead extend at an angle to the vertical. The height of the upright 840 is not restricted and can be selected based on strength considerations. There may be a single upright 840, or a plurality of uprights 840 may be spaced in the longitudinal direction of the hull 810, as shown in Figure 81.

Preferably, the one or more uprights 840 collectively support at least 50% of the weight of the portion of the vessel above the support arms 821 and 831 for the foils (including the weight of the hull 810 and any equipment or payload on the hull 810), the weight being transmitted to the uprights 840 through the connecting members 841. More preferably, the uprights 840 collectively support at least 80% of this weight and still more preferably at least 90% of this weight. In this manner, most of the weight of the vessel above the support arms is transmitted to the foils through the connecting members 841 and then through the upright 840 rather than by bending stresses in the hull 810. Accordingly, the hull 810 can have a lightweight structure yet have adequate strength.

Preferably, each upright 840 is connected to the hull 810 as close as possible to a location where a support arm for one of the foils is connected to the hull 810 to minimize stresses in the hull 810. In Figure 78, the upright 840 is located directly above the pivot point 822 for the support arms 821 for the forward foil 820 and a structural member 811 within the hull 810 extends between the upright 840 and the pivot point 822 so that the load applied to the upright 840 can be transmitted directly to the pivot point 822. In the embodiment of Figure 80, the upright 840 has two legs 844 spaced from each other in the longitudinal direction of the hull 810 and linked by a connecting member 845. Support members 812 within the hull 810 are connected between the legs 844 of the upright 840 and a pivot point 822 for the support arms 821 for the forward foil 820 to transmit a load from the uprights 840 to the pivot point 822. The connecting members 841 can be connected to any desired portion of the hull 810. For example, they can be disposed substantially in the centerline plane of the hull 810, as shown in Figure 82, or they can extend to opposite beamwise sides of the hull 810 to leave an unobstructed space between the connecting members 841, as shown in Figure 83. They can be connected directly to some portion of the hull 810, either at or below the deck, or if there is a superstructure on the hull 810 which is sufficiently strong, the connecting members 841 can be connected to the superstructure and the weight of the vessel above the support arms can be transmitted to the connecting members 841 through the superstructure.

Preferably all of the connecting members 841 are sloped with respect to the horizontal so as to transmit some portion of the weight of the vessel above the support arms to the upright 840. The connecting members 841 can be oriented with respect to the vertical in a variety of manners. Two possible arrangements are shown in Figure 78. On the forward side of the upright 840 in the figure, a plurality of connecting members 841 are connected to the upright 840 at the same height, with the angle of the connecting members 841 with respect to the vertical varying along the length of the hull 810. On the aft side of the upright 840, the connecting members 841 all extend at approximately the same angle with respect to the vertical, and the height at which the connecting members 841 are attached to the upright 840 varies among the connecting members 841. Many other arrangements of the connecting members 841 are possible.

The connecting members 841 may be separate from one another, or two or more can be attached to one other. For example, a single cable can extend from an aft portion of the hull 810 upwards to the upright 840 and then downwards from the upright 840 to a forward portion of the hull 810 to define two connecting members 841.

A hydrofoil craft employing the hull structure shown in Figures 78 - 83 is not limited to any particular size or weight. The use of an upright 840 and connecting members 841 connecting a hull to the upright 840 is particularly advantageous for a large hydrofoil craft, but the same structure can also be used for a small hydrofoil craft. This structure can also be advantageously used for vessels other than hydrofoil crafts to transmit the weight of the vessel to desired locations.

Although the present invention has been described with respect to a number of preferred embodiments, the present invention is not restricted to the structure of these embodiments. Features of one embodiment may be combined with features of one or more different embodiments to obtain arrangements which are different from those illustrated in the drawings while still falling within the scope of the present invention.

Claims

What is claimed is:

1. A hydrofoil craft comprising: a hull; a foil disposed beneath the hull for generating lift to support the hull and having an upper and lower surface; and passages for fluid communicating beneath the lower surface of the foil and a region of the hydrofoil craft at a lower pressure than the lower surface during foil-borne operation of the hydrofoil craft.

2. A hydrofoil craft as claimed in claim 1 wherein the passages comprise bleed holes formed in the lower surface of the foil.

3. A hydrofoil craft as claimed in claim 2 wherein the bleed holes have a diameter of approximately 0.05 mm to approximately 0.5 mm.

4. A hydrofoil craft as claimed in claim 2 wherein the bleed holes extend between the upper and lower surfaces of the foil.

5. A hydrofoil craft as claimed in claim 1 wherein the foil has a plenum formed in an interior of the foil and communicating with an upper surface of the foil, and the bleed holes communicate between the lower surface and the plenum.

6. A hydrofoil craft as claimed in claim 5 including a cleaning passage for connecting the plenum with a source of cleaning fluid which can be injected into the plenum and discharged through the bleed holes to clean the bleed holes.

7. A method of operating a hydrofoil craft comprising: disposing a lower surface of a foil of a hydrofoil craft in contact with water to generate lift; and sucking water through the lower surface of the foil and discharging the water to a region of the hydrofoil craft at a lower pressure than the lower surface of the foil.

8. A method as claimed in claim 7 including ventilating an upper surface of the foil with an air filled cavity and discharging the water sucked through the lower surface into the cavity.

9. A method as claimed in claim 8 including ventilating the upper surface of the foil along a support arm connecting the foil to a hull of the hydrofoil craft.

10. A method as claimed in claim 7 including sucking the water through holes in the lower surface having a diameter of approximately 0.05 mm to approximately 0.5 mm.

11. A method as claimed in claim 10 wherein the holes communicate between the upper and lower surfaces of the foil.

12. A method as claimed in claim 7 including sucking the water through holes in the lower surface into a plenum within the foil.

13. A method as claimed in claim 12 including discharging a fluid under pressure from the plenum through the holes to clean the holes.

14. A hydrofoil craft comprising: a hull; a foil disposed below the hull for generating lift to support the hull and having a plurality of holes formed in a surface thereof; and a conduit connected to the holes for supplying air to the holes such that microbubbles are discharged from the holes into a boundary layer on the surface of the foil.

15. A hydrofoil craft as claimed in claim 14 wherein the holes have a diameter of approximately 100 to approximately 500 micrometers.

16. A hydrofoil craft as claimed in claim 14 including a support arm connecting the foil to the hull, wherein the conduit is formed in the support arm.

17. A hydrofoil craft as claimed in claim 16 wherein the conduit communicates with the atmosphere and air is sucked through the conduit by a difference between atmospheric pressure and a pressure in the boundary layer.

18. A method of operating a hydrofoil craft comprising: supporting a hull of a hydrofoil craft with a foil having a submerged upper surface; and discharging microbubbles from the upper surface into a boundary layer on the upper surface.

19. A method as claimed in claim 18 including discharging the microbubbles through holes having a diameter of approximately 100 to approximately 500 micrometers formed in the upper surface of the foil.

20. A method as claimed in claim 19 including supplying the holes with atmospheric air.

21. A method as claimed in claim 20 including supplying the air through a support arm connecting the foil with the hull.

22. A hydrofoil craft comprising: a hull; an engine disposed in the hull and having a cooling system; a foil disposed beneath the hull for generating lift to support the hull; and a conduit connected between the cooling system of the engine and the foil for circulating engine coolant through the foil to heat a surface of the foil to approximately 10 to approximately 80 degrees C above ambient water temperature to stabilize a boundary layer on the surface.

23. A method of operating a hydrofoil craft comprising: supporting a hull of a hydrofoil craft with a foil; and heating a surface of the foil to a temperature above an ambient water temperature to stabilize a boundary layer on the surface.

24. A method as claimed in claim 23 comprising heating the surface by circulating coolant for an engine of the hydrofoil craft through the foil.

25. A method as claimed in claim 23 comprising heating the surface of the foil to a temperature in the range of approximately 10 to approximately 80 degrees C above ambient water temperature.

26. A method of operating a hydrofoil craft comprising: supporting a hull of a hydrofoil craft with a foil; heating a first region of a lower surface of the foil to above an ambient water temperature to stabilize a boundary layer in the first region; and sucking water through a second region of the lower surface of the foil located behind the first region and discharging the water to a region of the hydrofoil craft at a lower pressure than the lower surface of the foil.

27. A method of operating a hydrofoil craft comprising: supporting a hull of a hydrofoil craft above a water surface with a foil planing on the water surface and connected to the hull by a support arm pivotably connected to the hull for pivoting about a transverse axis as the hull is moving and supported above the water surface.

28. A foil for use with a maritime vessel comprising: an upper surface having a leading edge, a trailing edge, and an inflection between the leading and trailing edge; a lower surface for generating lift; and a base which is arranged to be ventilated separating the trailing edge of the upper surface from a trailing edge of the lower surface.

29. A foil as claimed in claim 28 wherein a tangent to the upper surface at the leading edge is substantially parallel to a tangent to the upper surface at the trailing edge.

30. A foil as claimed in claim 29 wherein the upper surface has a shape approximately defined by the equation y = ax² - bx⁴.

31. A foil as claimed in claim 28 wherein the upper surface produces substantially no lift at a design angle of attack of the foil.

32. A foil as claimed in claim 28 wherein the lower surface has a shape producing substantially no pressure drag at a design angle of attack of the foil.

33. A foil as claimed in claim 32 wherein the lower surface has an inflection between a leading edge and a trailing edge of the lower surface.

34. A hydrofoil craft comprising: a hull; a foil for generating lift to support the hull; first and second support arms connected to the foil and pivotably connected to the hull for pivoting about a transverse axis extending in a widthwise direction of the hull; and first and second biasing mechanisms associated with the first and second support arms, respectively, for independently biasing the first and second support arms away from the hull.

35. A method of operating a hydrofoil craft comprising: supporting a hull with a foil disposed beneath the hull and connected to the hull by first and second support arms pivotably connected to the hull for pivoting about a transverse axis extending in a widthwise direction of the hull; and independently biasing the first and second support arms away from the hull.

36. A hydrofoil craft comprising: a hull; first and second foils disposed side by side in a widthwise direction of the hull for generating lift to supporting the hull; first and second support arms connected to the first and second foils, respectively, and each pivotably connected to the hull for pivoting about a transverse axis extending in a widthwise direction of the hull; and first and second biasing mechanisms associated with the first and second support arms, respectively, for independently biasing the first and second support arms away from the hull.

37. A method of operating a hydrofoil craft comprising: supporting a hull with first and second foils disposed side by side beneath the hull and connected to the hull by first and second support arms pivotably connected to the hull for pivoting about a transverse axis extending in a widthwise direction of the hull; and independently biasing the first and second support arms away from the hull.

38. A hydrofoil craft comprising: a hull; a foil disposed beneath the hull for generating lift to support the hull; a first suspension movably mounted on the hull and having a support point biased away from the hull; and a second suspension including a support arm connected to the foil and pivotably connected to the first suspension at the support point for pivoting about a transverse axis extending in a widthwise direction of the hull and biased away from the first suspension.

39. A method of operating a hydrofoil craft comprising: supporting a hull of a hydrofoil craft with a foil disposed beneath the hull and connected to the hull by a first suspension movable with respect to the hull and a second hull mounted on the first suspension and movable with respect to the second suspension.

40. A hydrofoil craft comprising: a hull; a first foil set having an adjustable total span for supporting generating lift to support the hull; and at least one support arm connecting the foil set to the hull.

41. A hydrofoil craft as claimed in claim 40 wherein the foil set comprises a foil having a plurality of telescoping sections which can be moved with respect to each other in a spanwise direction of the foil to vary the span of the foil.

42. A hydrofoil craft as claimed in claim 40 wherein the foil set comprises a plurality of adjoining foils each connected to the hull by a support arm and at least partially nonoverlapping an adjoining one of the foils in a spanwise direction, at least one of the support arms being connected to the hull such that the corresponding foil can be raised above a water surface while the hydrofoil craft is moving with the hull above the water surface with at least one other foil in the foil set contacting the water to generate lift.

43. A hydrofoil craft as claimed in claim 42 wherein the at least one of the support arms is pivotably connected to the hull for pivoting about a transverse to raise the corresponding foil above the water surface.

44. A hydrofoil craft as claimed in claim 42 wherein adjoining foils are nonoverlapping in the spanwise direction of the foils.

45. A hydrofoil craft as claimed in claim 44 wherein adjoining foils are aligned in the spanwise direction.

46. A hydrofoil craft as claimed in claim 45 wherein adjoining foils are spaced from each other in the spanwise direction by at most 10 times a chord of adjoining foils.

47. A hydrofoil craft as claimed in claim 42 wherein a chord of the foils decreases from spanwise ends towards a spanwise center of the foil set.

48. A hydrofoil craft as claimed in claim 46 wherein each foil has an upturned wing formed at an end thereof.

49. A hydrofoil craft as claimed in claim 42 wherein each foil partially overlaps an adjoining one of the foils in the spanwise direction of the foils.

50. A hydrofoil craft as claimed in claim 49 wherein each support arm is pivotably connected to the hull for pivoting about a transverse axis, the transverse axes for adjoining foils being staggered in a fore and aft direction of the hull.

51. A method of operating a hydrofoil craft comprising: supporting a hull of a hydrofoil craft above a water surface with a plurality of adjoining foils at least partially nonoverlapping in a spanwise direction; and decreasing a total span of the foils as a speed of the hydrofoil craft increases by lifting at least one of the foils above the water surface.

52. A foil arrangement for a hydrofoil craft comprising: a foil having first and second spanwise ends; and a pair of support arms each having a first end secured to one of the ends of the spanwise ends of the foil and a second end sloping towards the other support arm.

53. A foil arrangement as claimed in claim 52 including a sleeve rigidly interconnecting the second ends of the support arms.

54. A foil arrangement as claimed in claim 52 wherein the first ends of the support arms are substantially normal to an upper surface of the foil.

55. A foil arrangement as claimed in claim 52 wherein the first ends of the support arms have transverse cross sections which converge towards a spanwise centerline of the foil.

56. A foil arrangement as claimed in claim 52 wherein the first ends of the support arms have cambered transverse cross sections.

57. A hydrofoil craft comprising: a hull; a first foil disposed beneath the hull in contact with water to generate lift; and a second foil disposed beneath the hull behind the first foil to generate lift in a region where a wake formed by the first foil has an upwards velocity component.

58. A method of operating a hydrofoil craft comprising: supporting a hull of a hydrofoil craft with a first foil and a second foil disposed behind the first foil in a wake of the first foil where the wake has an upwards velocity component.

59. A method of operating a hydrofoil craft comprising: supporting a hull of a hydrofoil craft above a water surface on a first foil set and a second foil set disposed behind the first foil set; and varying a total span of the first foil set in accordance with a speed of the hydrofoil craft to maintain the second foil set in a region where a wake formed by the first foil set has an upwards velocity component.

60. A method as claimed in claim 59 wherein varying the total span comprises decreasing the total span as the speed of the hydrofoil craft increases.

61. A method as claimed in claim 60 wherein the first foil set includes a plurality of foils, and varying the total span comprises varying the number of the foils in the first foil set contacting the water to generate lift.

62. A method of operating a hydrofoil craft comprising: supporting a hydrofoil craft above a water surface in a first speed range with a first foil and a second foil located behind the first foil in a region where a wake formed by the first foil has an upwards velocity component, with a third foil located behind the second foil raised above the water surface; and raising the second foil above the water surface and lowering the third foil to contact the water in a region where the wake formed by the first foil has an upwards velocity component in a second speed range higher than the first speed range in which the second foil, if contacting the water, would be in a region where the wake has a downwards velocity component.

63. A maritime vessel comprising: a hull; an engine supported by the hull; a propeller shaft having a forward end driven by the engine and a rear end extending from the hull; a propeller mounted on the rear end of the propeller shaft; a rear support connected to the hull and supporting the propeller shaft aft of the front end; and an axial support extending in the axial direction of the propeller shaft outside the hull and including at least one bearing rotatably receiving the propeller shaft.

64. A shaft support arrangement comprising: a shaft; a bearing rotatably receiving the shaft; a stiffener secured to the bearing and providing a greater stiffness against movement of the bearing in a first radial direction than in a second radial direction with respect to an axis of the shaft.

65. A shaft support arrangement as claimed in claim 64 wherein the stiffener provides substantially no resistance against movement of the bearing in the second radial direction.

66. A maritime vessel comprising: a hull having a chamber in a lower portion thereof communicating with a bottom of the hull; and a pump for discharging water from the chamber to outside the hull with sufficient force to support the weight of the hull above a ground surface.

67. A method of beaching a maritime vessel comprising: discharging water from a chamber in a bottom of a hull of a vessel to lift the hull above a ground surface; and moving the hull along the ground surface.

68. A method of unbeaching a vessel comprising: discharging water from a chamber in a bottom of a hull of a vessel to lift the hull above a ground surface on which the hull is beached; and moving the hull along the ground surface into water until the hull floats in the water.

69. A method according to claim 68 wherein the hull is moved along the ground surface primarily under the force of gravity.

70. A maritime vessel comprising: a hull; an upright extending upwards from the hull; and a plurality of connecting members connected between the upright and the hull for transmitting a portion of the weight of the vessel to the upright.

71. A maritime vessel as claimed in claim 70 wherein the upright supports at least 50% of the weight of that portion of the vessel above a water surface.

72. A maritime vessel as claimed in claim 70 wherein the vessel is a hydrofoil craft.