WO1993007046A1

WO1993007046A1 - Naturally-aspirated surface-effect watercraft

Info

Publication number: WO1993007046A1
Application number: PCT/GB1992/001834
Authority: WO
Inventors: Trevor Goldsmith
Original assignee: Nasec Sa
Priority date: 1991-10-07
Filing date: 1992-10-08
Publication date: 1993-04-15
Also published as: AU2675992A; GB9409178D0

Abstract

A naturally-aspirated surface-effect watercraft (10) is driven by a stern drive (28) and has three hulls (32, 34, 36) defining tunnels (42) therebetween through which air is inducted by the forward motion of the craft (10) to create a lifting effect to raise the hulls (32, 34, 36) substantially clear of the water and thus minimise drag. The bow wave from the centre hull (32) is arranged to impinge on the opposing lateral surface (64) of each outer hull (34, 36) to cause a flow of aerated water in the tunnels (42), which reduces the surface friction of immersed areas over which the aerated water passes. Aerodynamic effects are utilized after the initial stage of acceleration from rest to lift the bow out of the water, and to utilize air lubrication effects to reduce skin resistance. At a higher, constant speed the craft (10) cruises substantially on a cushion of air.

Description

NATURALLY-ASPIRATED SURFACE-EFFECT WATERCRAFT

This invention relates to the hull configuration of naturally-aspirated surface-effect watercraft.

According to the present invention there is provided a hull configuration for a naturally-aspirated surface-effect watercraft designed to be power driven and having at least two, and preferably three, hulls defining tunnels therebetween through which air is inducted by the forward motion of the craft to create a lifting effect to raise the hulls substantially clear of the water and thus minimise drag/ characterised in that the bow wave from one of the hulls is arranged to impinge on the opposing lateral surface of another of the hulls to cause a flow of aerated water between the hulls, which reduces the surface friction of immersed areas of the hulls over which the aerated water passes.

According to a preferred form of the present invention there is provided a hull configuration for a naturally-aspirated surface effect watercraft designed to be power driven and having at least two, and preferably three, hulls defining tunnels therebetween through which air is inducted by the forward motion of the craft to create a lifting effect to raise the hulls substantially clear of the water and thus minimise drag, characterised in that the hulls are shaped such as to create an aerated

SUBSTITUTE SHEET flow of water therebetween during an initial stage of acceleration from rest, to utilise aerodynamic effects during a subsequent stage of acceleration to lift the bow out of the water, and to utilise air lubrication effects to reduce skin resistance, and at a higher, constant speed to allow the craft to cruise on a cushion of air with immersion only of an aft portion of at least one hull, with a trim in the fore-and-aft direction corresponding to that when stationary.

Preferably the aeration of water passing between the hulls is increased by the introduction thereinto of spray thrown forward by the stem of at least one of the hulls.

Preferably, again, the lower faces of the floors of the outer hulls are inclined upwardly and outwardly in their forward sections, transforming to substantially horizontal faces in the midships regions, such that aerated water is caused to flow under the after portions of the outer hulls, which are preferably fluted.

Desirably the outer hulls are so shaped as to cause the formation of tip vortices which run along the outer walls of the outer hulls and minimise stern drag, especially at lower speeds.

It has been found that the length to beam ratio is a critical consideration in the design according to the invention, and that a ratio in the region of 1:2.35 is particularly advantageous. This ratio corresponds to a

^SUB^STITUTE SHEET tunnel air volume to displacement relationship of approximately 23 cubic feet per ton displacement.

Tunnel extension means may be provided at the stern of the craft to increase the effective length of each tunnel. Preferably the tunnel extension means are mounted for movement between a raised position in which the effective tunnel length is increased and a lowered position in which the means obstruct the tunnel end to create a braking effect.

Although the invention may be carried out in a variety of ways, two particular embodiments thereof will now be described, by way of example, with reference to the accompanying drawings in which

Figure 1 is a side elevation of a naturally-aspirated surface effect craft according to the invention;

Figure 2 is a combination of bow and stern half-elevations of the craft of Fig. 1;

Figure 3 is a conventional combination of views showing the lines of the hull of the craft of Figs. 1 and 2;

Figure 4 is a fragmentary bow elevation of one of the hulls showing the aft fluted section;

Figure 5 is a fragmentary stern elevation of a modification of the craft of Figs. 1 and 2 having a surplus thrust vector unit fitted thereto; and

SUBSTITUTE SHEET Figure 6 is a view on the line VI-VI in Fig. 5

As shown in Figs. 1 and 2 a naturally-aspirated surface effect craft 10 has a hull 12 provided with a deck

14 surrounded by bulwarks 16 and a rail 18, on which is mounted conventional superstructure including a deckhouse

20, a gantry 22 supporting an aerial array 24, and a seat

26; projecting from the stern are a platform 28 and stern drive 30.

The hull 12 is of the cathedral type and comprises a centre hull 32, a port outer hull 34, and a starboard outer hull 36 which are each connected thereto by respective spans 40; the hull 12 thus defines two tunnels

42.

Referring now mainly to Fig. 3, it will be seen that the centre hull 32 consists essentially of inclined side walls 46 which are straight-sided when viewed in section and merge into a rounded upper stem 50, and a floor 52 which has a forward V-section 54 which continues the upper stem portion 50 downwardly along the line of intersection as a cutwater 56 and fairing into a convexly curved midships section 58 and similar aft section 60, the floor

52 being joined to the walls 46 at hard chines 62. The side walls 46 have an initial inclination to the vertical of 24.5° which reduces to 11.25 at station 5 and is maintained at a constant angle of 13.5 after station 6.

The V-section portions 54 of the floor 52 have an

SUBSTITUTE SHE inclination of 28° to the horizontal at station 2 which reduces to 9.5° at station 3. Viewed in fore-and-aft dimension the centre hull 32 exhibits a progressive curve from station 0.5 as indicated by line 56 in Fig. 3, the curvature being compounded curvature, particularly between stations 2.5 to 5.

The outer hulls 34,36 are each formed by an inboard wall 64 which has a constant inclination to the vertical along its length of 11.00 and is joined to the inboard edge of a floor 65, the join being made along the forward part of the hull 12 through a separation strip 66 which tapers forwardly, and along the aft part through a hard chine 67. The outboard edge of the floor 65 is similarly joined to a generally vertical outboard wall 68 through separation strip 70 and hard chine 71. The floor 65 is inclined outwardly and upwardly at an angle of 37 at station 1, 36.5° at station 2, and 12 at station 6; aft of station 6 the floor is fluted to form three channels 72, and becomes progressively horizontal.

The outer hull floors 65 each exhibit a curve in the fore-and-aft direction indicated by the line 74 in Fig. 3 and corresponding to the curvature of the centre hull 32. It will be noted that in the region of station 4 the chines 62,67 reach their lowest points, the chine 67 being lower than chine 62, and that between stations 1 and 5 the floors 65 exhibit compound curvature.

SUBSTITUTE SHEET The spans 40 each exhibit a rectilinear section which is horizontal except for the extreme after section abaft station 10 where the inclination is 1 . However, the spans 40 exhibit reverse curvature in the fore-and-aft direction having an upward inclination of 2 at the bow which changes by station 1 to a downward inclination of 11.5°, and which decreases to 3.5 at station 5. The junction between the lateral edges of the spans 40 and the adjacent hull plates is radiused for constructional reasons.

The centre hull 32 and the outer hulls 34,36, together with the common upper portion of the hull 12 are closed by a transom 76 which has a vertical upper portion 78, an inclined lower portion 80 and is curved athwartships.

The effect of driving the hull forward through the water will now be described with reference to three definable ranges of speed. ' Stage 1 (0-9 knots).

As forward thrust is applied by operation of the stern drive 30 the hull 12 is accelerated from rest. It will be noted that the spans 40 are above the recommended maximum load waterline so that there is always an air-space in the tunnels 42 thus ensuring an airflow through the tunnels 42 from the outset: without an airspace much greater power would be required to lift the

SUBSTITUTE SHEET hull 12 sufficiently out of the water to establish an air-flow through the tunnels 42. The airflow is initially important in Stage I to enhance aeration effects; as will be apparent its importance is even greater in Stages II and III.

One hydrodynamic effect of the forward motion is the formation of a bow wave by the curved floor 52 of the centre hull 32 in the region of station 3. This wave is thrown upwardly on each side at an angle of 14.5 and strikes the respective inner walls 64 of the outer hulls 34,36 to create turbulence and an aerated water stream in each of the tunnels 42, the aerated streams serving to reduce skin friction.

The flow of water along the inner side of each outer hull 34,36 and through the tunnels 42 is laminar, and is caused by the inward inclination of the inner walls 64 of the outer hulls 34, 36 to interact with the deflected bow wave, thus beneficially enhancing the turbulence and aeration of the water passing through each tunnel 42, and inducing an aerated lubrication effect on the hydrodynamically supported hull surfaces. The slightly lower position of the outer hull chines 67 as compared with the centre hull chines 62 not only prevents lateral bleed out of turbulent, aerated water at high speeds but also intercepts the bow wave at low speeds.

It should be noted that the stagnation point of the

SUBSTITUTE SHEET water ahead of the point of entry of all three hulls 32,34,36 is widened by the configuration of the hull section at that point, namely by the flat curvature of the floor 58 of the centre hull 32 and the separation strips 66 in the case of the outer hulls 34,36. The effect of the widening of the stagnation points is to increase the amount of spray thrown forward, which further enhances the aeration of the water entering the tunnels 42.

The separation strips 66 of the outer hulls 34,36 are very finely raked and the deadrise angle of the floor 65 from the bow until around station 4.5 is very steep. This configuration has the following consequences.

Firstly, it assists the outer hulls 34,36 to induct air at the correct angle, and to direct aerated flow under the floors 65 to the entrance to the fluted lengths of the outer hulls 34,36. The flutes 72 then retain the aerated water, preventing its lateral escape, as far as the transom 76, where it has a further beneficial effect to be described later. (The centre hull 32 is not similarly fluted, otherwise the stern drive 30 would be operated in turbulent water and its efficiency impaired.)

Secondly, the sharp deadrise over the initial lengths of the outer hull floors 65 reduces direct upward lift on the forward portions of the outer hulls 34,36 which is advantageous for maintaining fore-and-aft trim more closely to the horizontal. Thirdly, the progressive reduction in the deadrise angle causes roll-out vortices to form at a point, determined by the speed, between stations 1 and 4. The vortices pass down the length of the outer hull outboard walls 68 to about station 10.5 (abeam of the transom 76) where they block the formation of the normal divergent stern waves and turbulent water, thus greatly reducing drag.

Stern drag is also blocked by the aerated flow along the tunnels 42.

The progressive curve of the outer hulls 34,36 in the fore-and-aft direction as indicated by line 74 enables the craft 10 always to ride safely behind the surf line, which is formed forward of mid-ships, in following seas. Stage II (9 - 20 knots).

In this stage aerodynamic lift and air lubrication are instigated, and it is characterised by the development of the effects described in Stage I. As the craft 10 accelerates through this speed range the bow progressively lifts until at about 19 knots the craft 10 starts to climb up onto a cushion of air after which, over an approximate distance of up to 100 yards, momentum change takes effect, and the craft 10 moves more fully on air as it enters into Stage III. Two particular points should be noted. Firstly, the athwartships curvature of the transom 76 extends the aftermost part of centre hull 32 further aft

SUBSTITUTE SHEET than the corresponding parts of the outer hulls 34,36, which further reduces the drag co-efficient; secondly, as the propeller of stern drive 30 is thus placed further aft in more undisturbed water, the efficiency of the propeller is increased, particularly at higher speeds. An added advantage may be gained by the use of an extension frame, for example Gill brackets to locate the propeller further aft of the transom.

Stage III (20 knots upwards) .

In Stage III the craft 10 regains its original horizontal trim and is substantially supported by the stream of air flowing through the tunnels 42. The immersed portion of the hull 12 is thus somewhat triangular in section, and the immersed portion of the centre hull 32 moves from station 8.25 to station 10 at a depth of about \ " - The length of each tunnel 42 from stations 0 to 3 constitutes an induction section of reducing cross-sectional area; from 3 to 6, a compression section of further reducing cross-sectional area; and from 6 to 10.5 a lift section. Not only does the fluidised mix of air and water passing through the tunnels 42 support the craft 10 but also, by virtue of its profile as it issues from the stern, reduces drag in the manner previously described.

The air cushion effect ahead of station 6 dissipates impact loading on the spans 40 during rough weather, while

SUBSTITUTE SHEET such loading on the floors 52,65 is resisted by the compound curvature of the floors. The curvature of the vertical walls 46,64,68 of the hulls is not compounded in order to prevent "plate spring-out".

As shown in Figs. 5 and 6 each surplus thrust vector unit 100 comprises a base plate 102 secured to the transom 76 above a respective tunnel 42 and has a vector plate assembly 104 pivotally mounted to its lower edge about axis 105, the assembly 104 comprising a top plate 106 and two dependent side plates 108. Hydraulic rams 110 by means of which the assembly 104 may be raised and lowered are mounted between an upper bracket 112 secured to the top of the base plate 102 and a lower bracket 114 secured to the upper surface of the top plate 106.

In the fully raised position of the assembly 104 as shown in Figs. 5 and 6 the top plate 106 is aligned with the span 40, thus effectively increasing the waterline length of the craft and hence the speed to length ratio. On the other hand the side plates 108 are inclined to the tunnel walls 46,64 in the raised position but are aligned therewith in the lowered position so that the assembly 104 closes the end of the tunnel 42.

By appropriate settings of the angle of the top plate 106 of the thrust vector units 100 the following effects may be achieved:-

1. More rapid attainment of Stages II and III,

SUBSTITUTE SHEET previously described.

2. Improved manoeuvrability at higher speeds without loss of engine power, enabling controlled 90 turns to be made, both when the craft is going ahead and astern.

3. An instantly available trim and ride control system which does not involve loss of velocity in roll conditions; the inherent self-levelling characteristics of the craft are thus further enhanced.

4. Pronounced keel curvature in the fore and aft direction enhances the sea-keeping qualities of the craft when travelling at high speeds in following seas, but at the same time decreases the efficiency of air induction into the tunnels as compared with a hull having a straight keel profile; the thrust vector units render the efficiency of air induction of a curved keel profile hull equivalent to that of a straight keel profile hull.

5. In their fully lowered positions the vector plate assemblies 104 act as powerful brakes capable of stopping the craft dead in its tracks; braking does not involve the use of engine power which is thus left available for use in executing other manoeuvres. An important characteristic of the above-described

SUB hull 12 is its ability to assume the optimum orientation for any particular speed, whether loaded by the head or stern; moreover the design of the hull also compensates for loads which impart a heavy list.

In modifications of ^• the invention a broad range of diesel or petrol driven power units whether inboard or outboard may be used in conjunction with a broad range of propulsion systems including water jet drives, and surface piercing or standard propellers.

SUBSTITUTE SHEET

Claims

CLAIMS :

1. A hull configuration for a naturally-aspirated surface-effect watercraft (10) designed to be power driven and having at least two hulls (32,34,36) defining tunnels (42) therebetween through which air is inducted by the forward motion of the craft (10) to create a lifting effect to raise the hulls (32,34,36) substantially clear of the water and thus minimise drag, characterised in that the bow wave from one of the hulls (32) is arranged to impinge on the opposing lateral surface (64) of another of the hulls (34,36) to cause a flow of aerated water between the hulls (42), which reduces the surface friction of immersed areas of the hulls (32,34,36) over which the aerated water passes.

2. A hull configuration as claimed in claim 1 wherein the hulls (32,34,36) are shaped such as to create an aerated flow of water therebetween during an initial stage of acceleration from rest, to utilise aerodynamic effects during a subsequent stage of acceleration to lift the bow out of the water, and to utilise air lubrication effects to reduce skin resistance, and at a higher, constant speed to allow the craft (10) to cruise on a cushion of air with immersion only of an aft portion of at least one hull (32,34,36), with a trim in the fore-and-aft direction corresponding to that when stationary.

3. A hull configuration as claimed in claim 1 or 2,

SHEET wherein the aeration of water passing between (42) the hulls is increased by the introduction thereinto of spray thrown forward by the stem (58,66) of at least one of the hulls (32,34,36).

4. A hull configuration as claimed in any preceding claim, wherein the lower faces of the floors (65) of the outer hulls (34,36) are inclined upwardly and outwardly in their forward sections, and transforming to substantially horizontal faces in the midships regions, such that aerated water is caused to flow under the after portions of the outer hulls (34,36) .

5. A hull configuration as claimed in any preceding claim, wherein the outer hulls (34,36) are so shaped as to cause the formation of tip vortices which run along the outer walls (68) of the outer hulls (34,36) and minimise stern drag, especially at lower speeds.

6. A hull configuration as claimed in any preceding claim wherein the length to beam ratio is in the region of 1 :2.35.

7. A hull configuration as claimed in any preceding claim and additionally comprising tunnel extension means (100) provided at the stern (76) of the craft (10) to increase the effective length of each tunnel (42).

8. A hull configuration as claimed in any preceding claim, wherein the tunnel extension means are mounted for movement between a raised position in which the effective tunnel (42) length is increased and a lowered position in

SUBSTITUTE SHEET which the means obstruct the tunnel (42) end to create a braking effect.

9. A hull configuration as claimed in any preceding claim, wherein the after portions of the outer hulls are fluted.

10. A hull configuration as claimed in any preceding claim and having three hulls (32,34,36).

SUBSTITUTE S