GB2200598A - Multi-hull water borne craft - Google Patents

Abstract

A multi-hull water borne craft provided with hulls 10 having a high length to beam ratio and means at the stern for producing a curtain of fluid 15 extending between the hulls 10 and the undersurface of a covering 13 extending between the hulls 10 to provide in use of the craft increased aerodynamic lift forces generated by restricting or reducing the velocity of the ram air flowing between the hulls. <IMAGE>

Description

MULTI-HULL WATER BORNE CRAFT This invention relates to a- multi-hull water borne carft and particularly, but not essentially to a multi-hull craft which is intended to operate over areas of relatively sheltered and shallow water, such as rivers or lakes where disturbance of the water by the wash of crafts has often to be avoided in order not to erode banks or cause nuisance to other users of the water.

Known types of high-speed surface craft, having a Froude number greater than 0.75, engaged in passenger or freight transportation are well recognised as causes of substantial wash and such types include hydrofoils, hovercraft and planing craft when they are operating in their displacement mode. Conventional high-speed catamaran and mono-hull craft also generate wash to a noticeable extent.

The resistance of a hull travelling through water primarily consists of frictional resistance due to the viscosity of the water and wave-making resistance due to the pressure distribution formed around the three-dimensional hull. The frictional resistance is proportional to the wetted area of the hull. In order to minimise the wave-making resistance it is neccesary to minimise the velocity changes around the hull due to its shape which leads to consideration being given to a hull of very high length-to-beam ratio (say 15 to 1 up to 30 to 1) and with very fine bow lines so that the flow diverges with the lowest possible angle as it passes the bow.

In mono-hull form such hulls have insufficient volume for machinery, fuel and payload and in addition would lack stability in roll.

According to the present invention in one aspect there is provided a multi-hull water borne craft in which each hull has a length-to-beam ratio of not less than 15 to 1. Preferably the hulls extend beyond the payload area of the craft and the area between the hulls beyond the payload area is covered to enable increased aerodynamic lift forces to be generated arising from ram air flowing between the hulls.

According to the present invention in another aspect there is provided a multi-hull water borne craft provided at its stern region with means for producing a curtain of fluid extending between the hulls and the under surface of a covering extending between the hulls in order to provide increased aerodynamic lift forces to be generated by reducing the velocity of the ram air flowing between the hulls. The fluid may be water or air.

An embodiment of the invention will now be described, by way of an example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic perspective view showing the hulls and the payload area of a craft according to the present invention; Figure 2 is a diagrammatic perspective view showing the covering extending between the hulls; Figure 3 is a longitudinal section showing air flow over the craft; Figure 4 is a sectional view of the means for producing a water curtain; Figure 5 is a perspective view of the stern end of the craft; and Figure 6 is a side elevation showing a propulsion unit.

The craft is provided with two hulls 10 which have a length-to-beam ration of not less than 15 to 1. It will be appreciated that more than two hulls 10 may be provided. The hulls 10 are interconnected by a platform 11 which carries payload accommodation. The propulsion units 12 are preferably mounted in the stern of the hulls 10 as shown in Figure 6.

Because the hulls 10 are of considerable length the payload platform 11 need not extend to the full length of the hulls 10. the area between the hulls 10 ahead of and behind the platform 11 is covered by a lightweight rigid or flexible membrane structure 13 so that full advantage may be taken of upper surface and lower surface aerodynamic lift forces over the resulting relatively large area. The upper surface will achieve a useful lifting force at normal operating speeds by careful shaping of the longitudinal contours 14 as shown in Figure 3. The under surface of the platform 11 between the hulls 10 and the structure 13 is subjected to a percentage of the dynamic head ram air pressure when the craft is in forward motion relative to the airstream.If the airstream is allowed to pass freely beneath the platform 11 and structure 13 and between the hulls 10 then, with a suitably shaped upper contour 14 to the platform and cabin 16, there is potential for achieving aerodynamic lift from the overall form of the structure, enhanced by ground effect due to its proximity to the water surface.

The underside surface of the platform 11 and structures 13 between the hulls 10 may be shaped and angled in the longitudinal direction to maximise this result. This lift will be significant since the ratio of vessel weight to craft length will be relatively low. In head wind conditions an appreciable increase in craft speed will be achieved due to relief of displacement and hence reduction in hydrodynamic resistance.

A further enhancement of craft lift can be obtained if the ram air passing beneath the platform 11 and structure 13 is not allowed to pass freely beneath the vessel. With existing air-cushion vehicles of the sidewall or surface effect ship type the power-generated air cushion is contained at the stern by a flexible membrane seal, i.e. a skirt, and rigid materials have been proposed. In either case, such seals if used to restrict the flow beneath the vessel of the present invention, in order to gain a larger percentage recovery of dynamic head as lifting pressure, they would inevitably impose a drag force on the vessel and a maintenance cost penalty, but could be used.

However, because the ram air pressure will be low relative to typical air cushion pressures of hovercraft, say 2 to Slbs/ft2 as against 30 to 120lbs/ft2, it is possible to use, with little power expenditure, a water curtain 15 as shown in Figure 4 to reduce the velocity of the ram air flow beneath the vessel and thus recover static pressure from the free stream air dynamic head and such a curtain 15 will not be able to transmit a drag load to the craft as would occur if the exit area for the ram flow were to be covered by a flexible or rigid material seal. The source of water can be taken from that taken on board for engine cooling, or if waterjet propulsion is used from a tapping of the waterjet ducting. In either case, the water can be boosted if necessary by a pump in order to provide a water curtain 15 of sufficient strength. The water curtain 15 spans the area between the hulls 10, the water surface over which the vessel is travelling and the underside of the vessel. It is directed downwards and may be angled aft if its efficiency is maintained so that a small forward thrust force can be obtained. By using water already taken on board for other purposes there is no momentum drag penalty associated with the provision of the water curtain 15, but there would be a very small amount of thrust loss if some of the flow of a water-jet propulsion system were to be used.

The provision of aerodynamic lift is very significant in a craft according to the present invention where the craft is light in weight in relation to its plan area, such lift being proportional to this area.

Therefore, the percentage of total support provided by aerodynamic lift will be usually large, thereby relieving the buoyancy support required of the long slender hulls 10. With the growing development of lightweight fibre-reinforced materials and composite structures, the aerodynamic lift can become an even larger percentage of craft output if these materials are used for part or all of the structure of the vessel. As a result of this lift potential the wave drag component that does exist is even further reduced since it can be shown that wave-making drag is proportional to displacement divided by waterline length cubed. When aerodynamic lift forces are generated on the vessel according to the invention the hulls 10 will run at slightly less average immersion and hence there will, in addition, be a reduction in skin friction drag.

The resistance curve characteristic for a craft according to the present invention will show a "drag proportional to speed squared" form since the wave-making drag is negligible. As a consequence of this characteristic the margin of thrust over drag will not suffer from the usual high hump drag condition associated with high-speed craft and so the time required to reach a high operating speed will be unusually short, thereby providing in ferry application a high and therefore economic (cruise) speed.

Because the wave-making drag of the craft according to the present invention is so small the craft is particularly attractive in shallow water applications where wave-making drag is greatly increased in critical water-depth conditions; this occurs at around speeds equal to g x water depth. While such speeds are not likely to coincide with highspeed craft operation, it is interesting to note that a low-speed version of a vessel according to the present invention shows very low resistance in all water depths in comparison with other contemporary types of craft.

Because the resistance of a craft according to the present invention may well be over 90% skin friction resistance, it follows that as the craft length increases the resistance coefficient will fall due to Reynolds Number effects on a skin friction drag. Hence greater hydrodynamic efficiency will be achieved with size increase according to well-established viscous flow characteristics.

Since the craft of the invention is primarily aimed at sheltered water applications, the freeboard can be quite small and this assists in the provision of a vessel with very low overall height. This feature is particuarly important in river appliction where low bridges may occur.

The length of a craft according to the present invention, perhaps some two-thirds longer than contemporary craft possessing the same payload-carrying ability, will provide the craft with less pitching motion and hence greater passenger comfort in given sheltered-water wave conditions.

The stern mounting of the power units 12 in the hulls 10 extended well aft of the passenger cabin 16 will minimise noise and vibration in the cabin 16.

The hulls 10 may be symmetrical form assisting in low cost production. The vessel according to the invention has abnormally low draught which is an advantage in very shallow water. In one embodiment, water-jet propulsion is used, taking full advantage of the inherently low draught and, in addition, in being applied to a hull where resistance is nearby all due to the boundary layer, much of the boundary layer can be taken into the water-jet inlet flow, enhancing the overall efficiency of propulsion as that inlet flow increment is accelerated and restored to free steam velocity.

A further increase of the air lift beneath the hull can be obtained by allowing the engine exhausts to exhaust upstream of the water curtain 15, their pressurized flow slightly raising the resulting pressure of the air/gas mixture. In addition the noise from exhaust discharge will then be masked to some extent by the water curtain 15.

The vessel may be built in sections so facilitaing overland transportation.

Instead of the curtain 15 comprising water, it is possible for it to comprise air under pressure. Such air may be taken from air for cooling the engine or a fan may be provided.

Claims (13)

1. A multi-hull water borne craft comprising hulls having a high length to beam ratio and means located at the stern region for producing a curtain of fluid extending between the hulls and the under surface of a covering structure extending between the hulls to provide in use of the craft increased aerodynamic lift forces generated by restricting or reducing the velocity of the ram air flowing between the hulls.

2. A multi-hull craft as claimed in clairn 1, in which each hull has a length to beam ratio of not less than 15 to 1.

3. A multi-hull craft as claimed in claim 1 or claim 2, in which the hulls extend beyond the payload area of the craft and the area between the hulls beyond the payload area is covered.

4. A multi-hull craft as claimed in any preceding claim, in which the fluid is water or air.

5. A multi-hull craft as claimed in any preceding claim, in which the undersurface of structure extending between the hulls is shaped and/or angled in the longitudinal direction to produce areodynamic lift forces on the craft.

6. A multi-hull craft as claimed in any preceding claim, in which the fluid is water taken from the propulsion unit or units of the craft.

7. A multi-hull craft as claimed in claim 6, in which the water is that used for cooling a propulsion engine or engines.

8. A multi-hull craft as claimed in claim 7, in which the exhaust gases from the or each propulsion engine exhaust into the area between the hull upstream of the water curtain.

9. A multi-hull craft as claimed in any one of claims 1 to 5, in which the fluid is air under pressure.

10. A multi-hull craft as claimed in claim 9, in which the air is taken from air used for cooling the or a propulsion engine or from a fan.

11. A multi-hull craft as claimed in any preceding claim, in which the curtain of fluid is angled downwardly.

12. A multi-hull craft as claimed in any preceding claim, in which the upper surface of the craft is shaped so as to produce aerodynamic lift forces.

13. A multi-hull craft substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.