GB2184408A - Improvements in or relating to sidewall gas-cushion vehicles - Google Patents

Abstract

A sidewall gas-cushion vehicle 1 is provided with a central keel structure 25 disposed within the vehicle-supporting cushion 3 and which depends downwardly to a level higher than the bottom edges of the sidewalls 5, whereby, should the vehicle heel over, the keel structure 25 contacts the water over which the vehicle travels, so as to divide the vehicle-supporting cushion into separate compartments 30, 31. <IMAGE>

Description

SPECIFICATION Improvements in or relating to sidewall gas-cushion vehicles BACKGROUND TO THE INVENTION This invention relates to sidewall gas-cushion vehicles, that is to say, to vehicles for travelling over a water surface wherein, in operation, they are supported above that surface, at least in part, by a cushion of pressurised gas, for example, air, formed and contained between the vehicle body and the surface, and wherein the sides of the vehicle-supporting cushion are contained by a pair of laterally-spaced sidewalls extending longitudinally along the sides of the vehicle body in substantially parallel array and depending therefrom so as to dip into the water and form a cushion-gas seal.

The ends of the vehicle-supporting cushion may be contained, at least in part, by flexible wall structure or skirt attached to and depending from the vehicle body.

Known forms of sidewall gas-cushion vehicles derive all their stability from the buoyancy of the sidewalls. Deeper cushion proportions therefore generally require more buoyant sidewalls which respond more to wave action.

It is difficult, if not impossible, therefore, to achieve increased cushion depth and smallwidth sidewalls necessary for improved seakeeping unless the size of the vehicle is sufficiently large to enable the "Deep Cushion Concept", (see U.K. Patent No. 1,184,062), to apply.

In addition, with sidewall vehicles having conventional flat bottoms, should wave impacting start to occur, a flat bottom is inherently subjected to large forces over a large area, which forces require considerable strengthening to resist and which cause a marked increase in accelerations experienced by those on board. The design and construction of the flat bottom hull structure becomes one of the dominant factors affecting the overall design.

It is generally necessary to fit twin water screws to a conventional sidewall gas-cushion vehicle, one water screw beneath each sidewall. Whilst giving excellent low speed manoeuvring, twin engine/shaft outfits are, of course, heavier and more expensive to install than one outfit of the same total power. The wide separation of the screws usually results in the craft being unable to operate properly on one engine, so that any advantages of redundancy are not realised.

The weight of the hull structure (which is very dependent on cushion area) effectively prohibits the use of low cushion loadings, which is the most effective way of reducing the vehicle resistance. The penalty in structural weight and cost exceeds the benefits obtained.

SUMMARY OF THE INVENTION According to the present invention, a sidewall gas-cushion vehicle is provided with a keel structure disposed within the vehicle-supporting cushion and extending substantially parallel to the sidewalls, along the longitudinal axis of the vehicle, the keel structure depending downwardly to a level higher than the bottom edges of the sidewalls, whereby, should the vehicle heel over, the keel structure contacts the water over which the vehicle travels, so as to divide the vehicle-supporting cushion into separate compartments.

BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a front or bow-end view, in section, of a sidewall gas-cushion vehicle, according to the invention, Figure 2 is a similar end view, and illustrates the various forces acting on the vehicle when heeling, and Figure 3 is a front or bow-end view, in section, of a known form of gas-cushion vehicle.

In the figures, like reference numerals refer to like components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to Figure 1, a sidewall gascushion vehicle 1 is shown travelling over a water surface 2 and is supported thereover by a cushion 3 of pressurised air formed between the vehicle body 4 and the water 2.

The sides of the vehicle-supporting air-cushion 3 are contained by a laterally-spaced pair of sidewalls 5 of rigid (solid) construction extending longitudinally along the sides of the vehicle body 4 in substantially parallel array and depending therefrom to dip into the water 2 and effect a cushion-gas seal. Due to the effect of the cushion 3, the level of water within the cushion space is substantially lower than the level of water outside the sidewalls 5. The front or bow end of the cushion 3 is contained by a multi-part flexible wall structure or skirt (not shown) and the stern or rear end thereof by a one-piece flexible skirt in the form of an inflated bag 6. The front and rear skirts are demountably attached to and depend from the vehicle body 4 and are in light contact with the water surface 2. See U.K.

Patent No. 2,022,040 for details of the form of flexible skirts used by the vehicle 1.

The air forming the cushion 3 is drawn in from the atmosphere, through side intakes (not shown), by a pair of laterally-spaced compressors 7 housed within the vehicle body 4 and driven by variable-speed engines (not shown), also housed within the body 4. The pressurised air is discharged to the space (the cushion space) occupied by the cushion 3 by way of ducts 8. The hollow, inflatable bag 6 is inflated by air tapped off the ducts 8 by branch ducts. (Not shown). The vehicle 1 is propelled by a single water-screw propeller driven by a steerable outboard motor mounted on the stern end of the vehicle body 4. The sidewalls 5, which are of substantially uniform cross-section, when viewed from the front, extend beneath the vehicle body 4 on opposite sides of the longitudinal axis 15 of the vehicle 1. The longitudinal axis 15 is didposed in a substantially vertical plane 20.

Each sidewall 5 has inner and outer side surfaces 21, 22 disposed substantially parallel to each other. The inner side surface 21 of a sidewall 5 extends downwardly below the outer side surface 22 thereof. Each sidewall 5 also has a bottom edge 23 which presents a downwardly and inwardly sloping face disposed at substantially 60O to the horizontal.

The vehicle 1 is provided with a keel structure 25 disposed within the vehicle-supporting cushion 3 and extending substantially parallel to the sidewalls 5. The keel structure 25 is of substantially uniform cross-section, when viewed from the front. As the keel structure 25 is part of the vehicle body 4, it is of rigid construction.

The keel structure 25 is symmetrical about the substantially vertical plane 20. As shown in Figure 1, the central keel structure 25 has symmetrical lowermost surface parts 25a which rise outwardly and upwardly, to join with symmetrical uppermost surface parts 25b which rise outwardly and upwardly in curves.

Parts 25b then curve downwardly to join with the inner side surfaces 21 of the sidewalls 5.

The keel structure 25 depends downwardly to a level 48 substantially higher than the bottom edges 23 of the sidewalls 5, whereby, with reference to Figure 2, should the vehicle 1 heel over, the keel structure 25 contacts the water surface 2 over which the vehicle 1 travels, so as to divide the vehicle-supporting cushion 3 into laterally-spaced, separate cushion compartments 30, 31. The vertical distance between the level 48 and the water level beneath is preferably no more than 5% of the width 47 of the vehicle-supporting cushion. Figure 1 shows this vertical distance to be about 5% of the cushion width 47.

As separate cushion-supply systems (7) are provided, one for each compartment 30, 31, the sealed compartment 30 now causes a powerful restoring moment to be generated.

This restoring moment is represented by arrows 32 (buoyancy of sidewall 5) and 33 (upward thrust of cushion air in compartment 30). The downwardly acting weight of the vehicle 1 is represented by arrow 34.

With reference once more to Figure 1, when the vehicle 1 is in an upright position, the keel structure 25 is well clear of the water. Drag forces which would otherwise be created by contact between the water and the structure 25 are thus avoided.

Figure 3 illustrates a conventional sidewall gas-cushion vehicle 41, (of the form disclosed by U.K. Patent No. 2,022,040), and the forces 32b, 33b and 34b, (corresponding to forces 32, 33 and 34 of Figure 2), generated when the vehicle heels over. It will be appreciated that the vehicle 41 is substantially less stable than vehicle 1, when the former vehicle heels over.

The vehicle of Figure 1 has sidewalls 5 of less width than the corresponding sidewalls 5b of Figure 3. This is because the cushion forces are used in the Figure 1 vehicle to stabilise the craft should it heel, instead of producing destabilising forces as illustrated by Figure 3. Roll amplitudes can be expected therefore, to be reduced substantially. The vehicle 41 relies on the buoyancy of the sidewalls 5b to provide stabilising forces. The vehicle 1 (Figure 1) does not. Consequently, its sidewalls 5 can be made of substantially less width.

Preferably the width 45 of a sidewall 5, taken at the water level outside the sidewall, is no more than 5% of the width 47 of the vehicle-supporting cushion. Figure 1 shows the sidewall width 45 to be 5% of the cushion width 47.

Preferably too, the width 45 of a sidewall 5, again taken at the outside water level, is no greater than 20% of the width 46 of the keel structure 25, taken at a level joining the two junctions of keel structure parts 25a, 25b. Figure 1 shows the width 45 to be about 16% of the width 46.

The trimaran-like vehicle 1 enables the body 4 structure thereof to absorb the effects of wave impact progressively, and to enable the impact loads to be taken by tension in curved body panels rather than by lateral loading over large areas. This is expected to substantially reduce the complexity of the structure by eliminating much of the local stiffening that is so expensive to install. Effectively splitting the body 4 into two halves further reduces bending moments and together with the thin sidewalls 5 is expected to enable a significantly larger cushion area to be achieved with the same structural weight, thus improving the resistance and power requirements.

The central keel structure 25 enables a single propulsion engine to be fitted with consequent savings in weight and purchase and installation costs. The keel structure 25 can be used to house passengers, crew, and an inboard propulsion engine.

In calm water the resistance characteristics of the vehicle 1 will undoubtedly be improved by virtue of the increased cushion area, slim sidewalls 5 and reduced appendage drag.

Provision of a trimaran-like gas-cushion vehicle rather than one of catamaran-like form is expected to provide the following advan tages: 1) Improved ride comfort in modest sea conditions.

2) Increased waveheight capability.

3) Reduced resistance in calm conditions.

4) Greatly simplified structural design.

5) Simpler and cheaper machinery installation.

Means may be provided whereby a downwardly-extending air (or other form of fluid) curtain is formed which issues from the bottom of the keel structure 25 to contact the water surface 2 below, whereby the vehiclesupporting cushion 3 is divided into separate cushion compartments at all times.

The present structure disposed between the sidewalls 5 and the keel 25 could be replaced by flexible membranes and crossbeams as disclosed bv U.K. Patent No. 1,184,062. This modification will provide, for a given weight, a larger cushion area.

Claims (11)

1. A sidewall gas-cushion vehicle provided with a keel structure disposed within the vehicle-supporting cushion and extending substantially parallel to the sidewalls, along the longitudinal axis of the vehicle, the keel structure depending downwardly to a level higher than the bottom edges of the sidewalls, whereby, should the vehicle heel over, the keel structure contacts the water over which the vehicle travels, so as to divide the vehiclesupporting cushion into separate compartments.

2. A vehicle as claimed in Claim 1, wherein separate cushion air supplies are provided, one for each compartment.

3. A vehicle as claimed in Claim 1 or 2, wherein the keel structure has symmetrical lowermost surface parts which rise outwardly and upwardly, to join with symmetrical uppermost surface parts which rise outwardly and upwardly, and then downwardly in curves, to join with the inner side surfaces of the sidewalls.

4. A vehicle as claimed in Claim 1, 2 or 3, wherein, in operation, the width of a sidewall, taken at the water level outside the sidewall, is no more than 5% of the width of the vehicle-supporting cushion.

5. A vehicle as claimed in Claim 3, wherein, in operation, the width of a sidewall, taken at the water level outside the sidewall, is no greater than 20% of the width of the keel structure, taken at a level joining the two junctions of the lowermost and uppermost surface parts.

6. A vehicle as claimed in any one of Claims 1 to 5, provided with means whereby, in operation, a downwardly-extending fluid curtain is formed which issues from the bottom of the keel structure to contact the water surface below.

7. A vehicle as claimed in any one of Claims 1 to 6, wherein the keel structure is used to house passengers, crew and an inboard propulsion engine.

8. A vehicle as claimed in any one of Claims 1 to 7, wherein the structure disposed between the sidewalls and the keel structure comprises flexible membranes and crossbeams.

9. A vehicle as claimed in any one of Claims 1 to 8, wherein, in operation, the vertical distance between the bottom of the keel structure and the water level below is no more than 5% of the width of the vehiclesupporting cushion.

10. A sidewall gas-cushion vehicle substantially as hereinbefore described with reference to the accompanying drawings.

11. Any novel subject matter or combination including novel subject matter herein disclosed, whether or not within the scope of or relating to the same invention as any of the preceding claims.