GB2518134A

GB2518134A - Hovercraft skirts

Info

Publication number: GB2518134A
Application number: GB201311979A
Authority: GB
Inventors: Malcolm Cox
Original assignee: GRIFFON HOVERWORK Ltd
Current assignee: GRIFFON HOVERWORK Ltd
Priority date: 2013-07-04
Filing date: 2013-07-04
Publication date: 2015-03-18
Anticipated expiration: 2033-07-04
Also published as: GB2518134B; GB201311979D0; WO2015001346A1

Abstract

A flexible skirt 10 for a hovercraft includes a skirt wall and a mesh diaphragm 30 extending across the interior of the skirt to be placed under tension when the skirt is inflated. The shape of the mesh and the orientation of the diaphragm within the skirt are configured such that in use when a lower edge of the inflated skirt wall encounters an obstacle causing a local reduction in vertical tension in the mesh diaphragm, the portion of the mesh above the obstacle lifts and a resultant change in shape of the mesh causes immediately adjacent portions of the mesh to lower, lowering the skirt wall to either side of the obstacle. The mesh may comprise a plurality of hexagonal or octagonal shapes which may be supplemented with diamond shapes. The mesh may extend across the bag part of the skirt. A further mesh may extend from the tails 24 of finger portions 22 to the underside of the sidebody 6.

Description

HOVERCRAFT SKIRTS

Field of the Invention

The invention relates to hovercraft skirts and in particular to hovercraft skirts including an internal diaphragm.

Background

Hovercratt (known also as air-cushion vehicles) are craft that can travel over water and land on a cushion of air. Known hovercraft include a hull and a flexible skirt that extends around and downwards from the periphery of the hull. A lift fan is used to create a pressurised cushion of air between the hull and the surface over which the craft is travelling, contained by the skirt. The craft is propelled forwards by one or more propulsors mounted on top of the hull. The hull typically also carries a cabin for the pilot/crew/passengers and, depending on the size of craft and intended use, may include further structures for holding equipment and/or cargo for example.

During operation, the lift fan must supply a sufficient volume of air to maintain the cushion, replacing air that leaks from gaps between the lower edge of the skirt and the surface over which the craft is travelling. Generally speaking, the rougher the surface over which the craft is travelling, the bigger the gaps between the skirt and the surface, the greater the leakage and therefore the greater volume of air must be supplied by the fan. If air leakage from the cushion can be minimised, the efficiency of the operation of the hovercraft can be improved either by a direct saving in lift power or by raising the whole craft slightly higher on its cushion. This improved clearance offers a more global reduction in skirt contact drag.

To better conform to the surface over which the hovercraft is travelling, and thus minimise the loss of cushion air and contact drag, various designs of flexible skirt have been devised.

The main types of hovercraft skirt include bag skirts (a tubular skirt operated at a pressure greater than the main cushion space), finger skirts (sometime referred to as segmented skirts) and combined bag and finger skirts. If the combination employs a bag which is operated at the same pressure as the main cushion, this is generally referred to as a loop and segment configuration. Finger skirts offer less resistance during travel over an uneven surface and are more able to conform to rough surface shapes and obstacles being traversed but can result in less craft stability than bag skirts and require distributive air feed arrangements within the hull structure. Combined bag and finger skirts offer a compromise between the low resistance and conformability of the finger skirt and the stability of the bag skirt. Combined bag and finger skirts or loop and segment skirts are commonly used today.

It is known to provide a diaphragm within a hovercraft skirt, especially within a bag or loop skirt (or those parts of a combined skirt), to further improve stability and in particular to help prevent the skirt bouncing. When tully inflated, the design of the skirt is such that the diaphiagn operates under considerable tension that prevents vertical oscillation of the skiit system. These diaphragms typically extend generally vertically within the skirt bag, across the bag or loop space from top to bottom. Although these diaphragms are beneficial to counteract skirt bounce, they have the disadvantage that they make the skirt less able to confoim to iough ground oi watei. In particulai, as a poition of skiit tiaverses over a local obstacle immediately adjacent portions of the skirt are discouraged from dropping into the space either side of the obstacle by the tensions in the internal diaphragm. This creates undesirable aft leakage and excess splay. Additionally, the portion of skirt positioned ovei the obstacle is held in place and prevented from rising in response to the obstacle. This causes extra skirt contact drag.

Conventionally, internal skirt diaphragms are constructed from elastomer-coated fabrics of similar construction to the bag or loop material. The diaphragms typically also need holes punched in them to allow free flow of air between the bag (or loop) volumes. In cold weather regions, the holes may have to be enlarged to prevent the build-up of blown snow inside the skirt. This can present a design problem as removing more material eventually leads to strength issues.

Summary of Invention

A general aim of the present invention is to provide a hovercraft skirt that uses an internal diaphragm (or diaphragms) for improved stability but that is better able to conform to a rough surface over which the hovercraft is traversing.

Accordingly, in a first aspect the invention provides a flexible skirt for a hovercraft, the skirt comprising: a skirt wall; and a mesh diaphragm extending across the interior of the skirt to be placed under tension when the skirt is inflated; wherein the shape of the mesh and the orientation of the diaphragm within the skirt are configured such that in use when a lower edge of the inflated skirt wall encounters an obstacle causing a local reduction in vertical tension in the mesh diaphragm, the portion of the mesh above the obstacle lifts and a resultant change in shape of the mesh causes immediately adjacent portions of the mesh to lower, lowering the skirt wall to either side of the obstacle.

This mesh diaphragm can be referred to as a "responsive mesh diaphragm".

The term "obstacle" is used to refer to any change in the height of the surface over which the craft is travelling. It may be part of the surface or a separate object. It may be solid or liquid (e.g. as is the case over rough water).

The skirt wall preferably defines an upper skirt section and a lower skirt section. The mesh diaphragm preferably extends across the interior of the upper skirt section.

The upper skirt section may be a bag section or a loop section. The lower skirt section may be a bag section, loop section, a plurality of fingers (or segments) extending downwardly from the upper skirt section, or fingers (or segments) in combination with a bag or loop section.

The mesh diaphragm can extend from an upper portion of the skid to a lower portion of the skirt being either attached to the lower part of the upper skirt section (e.g. bag or loop section), for example where it joins the lower bag section, or directly to the lower skirt section (e.g. the fingers themselves). Alternatively, all or part of the upper edge of the diaphragm may be connected to the hovercraft structure at or towards the top of the skirt rather than to the skirt itself.

The diaphragm preferably extends in a vertical plane or at an angle with a vertical component (preferably a significant vertical component, e.g. with an angle to the vertical of no more than 45 degrees. The inflated geometry of the skirt is designed such that the diaphragm is under tension when the skid is fully or partially inflated.

Appropriate responsive mesh configurations will not have direct (i.e. straight) load paths which provide a triangulated fixation from the lower edge of the net to the fixed upper edge when the lower edge is evenly loaded. This prerequisite eliminates any net with uniform triangular, or rectangular cells (either conventionally orientated or set at an angle to produce a diamond configuration). Whilst netting has been used as a skirt diaphragm before, this has been of rectangular mesh format and configured to provide a direct and essentially inextensible restraining diaphragm between the lower parts to the skirt and the upper structure.

The cells of the mesh are preferably arranged in a repeating pattern along the full length of the diaphragm to give a similar response in the diaphragm to local variation of skid load at any point along its length. The cells may be of the same size and shape as one another or a repeating pattern of different shaped cells.

A mesh of uniform cells that has the necessary response characteristic is of a hexagon form.

Similar characteristics can be achieved by combining two or more cell shapes, such as octagons combined with rectangles.

The mesh preferably relies on local rearrangement of its cell geometry to achieve response rather than elastic extension of its constituent elements. Such elastic behaviour is prone to global skirt movements rather than desirable local response and leads to changes in skid geometry with changes in craft weight. Elastic skirt elements can also promote undesirable skirt bounce.

For instance, to provide the desired movement of the mesh, the shape of the mesh (i.e. the shape of the open cells in the mesh defined by the mesh links) and the orientation of the mesh are preferably configured so that as the bottom (with respect to the intended orientation of the skirt in use) of a mesh cell is relieved of downward tension, the sides of the cell expand laterally outwardly, so that the height:width ratio of the cell shape reduces.

Conversely, if the sides of the cell are pushed closer together, the height:width ratio increases. In use, as the overall diaphragm is laterally constrained by its attachment to the bag section of the skirt, the tension on the bottom of mesh cells above an obstacle that the skirt is passing is reduced, causing lateral expansion of these cells. This in turn tends to relieve lateral tension on adjacent cells, causing an increase in height:width ratio which allows a lowering of the bottom of the cells and the portions of the skirt which are attached thereto. In this way, the lower parts of the skirt more closely conform to the profile of the obstacle to minimise leakage of cushion air and reduce skid contact drag.

Since the skid diaphragms are not required to hold pressure and further, must allow airflow around the bag or loop, the use of a net structure provides load carrying combined with the necessary permeability without the cost and weight of a proofed fabric. However, this does not preclude the construction of mesh geometries using sheets of fabric (proofed or otherwise) which are perforated to achieve the desired mesh geometry described above.

The diaphragm preferably extends within the bag or loop part of the skirt along a majority of the length of the skirt at the side of the hovercraft when installed. The arrangement may also be installed around the remainder of the craft periphery. However, there are certain loads and drag characteristics that arise in the bow skirt, particularly on a high speed marine hovercraft, that may make the use of a responsive net inadvisable in this area.

In some embodiments of the invention, the fingers of the skirt have a generally U-shape cross-section when viewed from above. The base of the U' at the top of the finger is joined to the bottom of the bag section. The tails of the U' extend back towards the hull of the craft, the inner end of each tail being connected to the hull either directly or by ties. The fingers are not connected to one another.

Where the inner ends of the fingers are spaced outwardly trom the hull and are connected to the hull by ties, these ties may be individual links or may take the form of a diaphragm. If a substantially horizontal response to local variation in skirt inflation loads is desired, these ties (or diaphragm) may be replaced with mesh of the form described above. The inner ends of the finger tails are preferably also connected to the structure of the craft above the inner ends of the fingers by further ties, these further ties taking load in the vertical direction or with a significant vertical component. The structure may be an inboard portion of the bag part of the skirt or, where present, sidebodies extending laterally outwards from the hull sides. These further ties preferably take the form of a mesh, most preferably a mesh that functions in the same manner as the mesh described above, to help the fingers of the skirt or, in the absence of fingers, the lower bag portions, better conform to obstacles they are traversing. These mesh ties may be connected to any part of the finger system, provided they carry some of the finger inflation loads to the craft structure as defined above.

Suitable meshes for use in embodiments of the invention should be composed of elements that are substantially inextensible at the load levels experienced by the skirt system. These elements will typically be made from commonly used rope or net fibres such as polyamide, polyester or aramid. In some configurations, the mesh may be constructed from multiple stiff links which are connected at nodal points to allow the necessary hinging movement Although the size of the mesh may be conveniently sized to coincide with the pitch of attached fingers, the response mechanism will occur with cells of any size unless the number of cells is so small that there is insufficient compliance available in the height of the diaphragm. The mesh can be very fine such that some configurations of knitted fabric will provide the necessary mechanism. For optimum performance, such knitted fabrics should be unconstrained by any form of proofing.

In some embodiments, to further improve the responsiveness of the skirt to obstacles and variations in the height of the surface over which the hovercraft is travelling, some or all of the inflated wall of the skirt may itself be constructed from a responsive mesh that can respond in the manner described above. In this case, however, as the skid wall must be substantially impermeable to air to retain the air cushion, the cells of the mesh are not open.

Rather, the cells are closed with panes of a flexible material. By using a mesh made from substantially inextensible elements, with the mesh opening sealed with a substantially air impermeable, flexible material, it is possible to carry the inflation loads through the mesh and thus achieve the benefits of the responsive mesh describe above, whilst the skirt wall is still able to contain the air cushion.

Where the skirt is of the bag and finger or loop and segment type, the wall of the bag/loop portion may be formed with a mesh wall in the above manner. Where the skirt is a bag skid, the entirety of the bag wall may be a mesh wall.

In some cases it may be appropriate to use a skid with this type of mesh wall without using an internal mesh diaphragm.

Accordingly, in a second aspect the present invention provides a flexible skirt for a hovercraft, the skirt comprising a skirt wall, at least a portion of the skirt wall being formed from a mesh with mesh cells covered with panes of an elastic material or dome-shaped flexible material, wherein the shape of the mesh and its orientation within the skirt wall are configured such that in use when a lower edge of the inflated skirt wall encounters an obstacle causing a local reduction in hoop tension in the mesh in the wall (i.e. tension around the wall between a lower portion of the wall and an upper podion of the wall), the portion of the mesh above the obstacle lifts and a resultant change in shape of the mesh causes immediately adjacent podions of the mesh to lower, lowering the skirt wall to either side of the obstacle.

The mesh wall of the second aspect may be configured in the same way as the mesh diaphragm of the first aspect above.

In a third aspect the invention provides a hovercraft including a hull and a flexible skid extending around and downwardly from the hull to form a plenum for an air cushion, wherein the skid is in accordance with the first aspect above.

Brief Description of the Figures

An embodiment of the invention is described below by way of example with reference to the accompanying figures! in which: Fig. 1 is a schematic side view of a hovercraft having a skirt according to an embodiment of the present invention; Fig. 2 shows a section from the skirt of the hovercraft of fig. 1, which in this example is a loop and segment skirt; Fig. 3 illustrates the manner in which the skid traverses an uneven surface, showing a comparison between a skid with a conventional diaphragm and a skid with a diaphragm in accordance with an embodiment of the present invention; Fig. 4 shows a diagram of a suitable hexagon mesh which has responded to a non-uniform load pattern at its lower edge. This response does not rely on elastic extension of the mesh elements but rather depends on local rearrangement of its cell geometry; Fig. 5 shows alternative possible mesh configurations; Fig. 6 shows another embodiment of the present invention applied to a bag skirt; and Fig 7 shows another loop and segment skirt in accordance with an embodiment of the present invention including a hexagon mesh bag/loop inflated wall.

Description of Embodiment

Fig. 1 shows a hovercraft having a central hull that carries a passenger cabin 4. In the example there are sidebodies 6 extending from either side of the hull 2, one on the left and one on the right. A skirt 10 is attached to the outboard sides of the sidebodies and to the bow and stern of the hull 2. Between them the hull 2, sidebodies 6 and skirt 10 form a plenum for a cushion of air on which the hovercraft rides. Air is supplied to the plenum by a lift fan (not shown). Propulsion for the hovercraft is provided by one or more motor driven fans or propellers 12 on the hull 2, usually mounted towards the stern. In other examples the skirt 10 may be connected directly to the hull 2 around its entire perimeter so that the sidebodies (where present) lie across the top of an upper part of the skid 10.

As best seen in fig. 2, the skirt 10 is of the combined loop and segment type, including a loop part 20 and, below the loop part 20, a plurality of segments or fingers 22. Lift air supplied by the lift fan inflates the loop part 20 and segments 22.

The loop part 20 is connected at a top edge to the sidebody 6. The loop part 20 extends around the whole craft. At the bow and the stern of the craft there is no sidebody and the top of the loop part 20 is connected directly to the hull 2.

As is conventional, the fingers 22 each have a generally U-shape cross-section when viewed from above. The base of the U is outermost and is connected to the bottom edge of the loop part 20. The inner ends, or tails' 24, of the U-shaped fingers are connected to the hull 2 by ties 26.

In accordance with the present invention, there is a mesh diaphragm 30 within the bag part of the skid 10. A top edge of the mesh is fixed to an upper edge portion of the loop part and a bottom edge of the mesh is fixed to a bottom edge portion of the loop pad, where it connects to the fingers 22. In an alternative embodiment, the mesh can be connected along its lower edge to upper edge portions of the fingers instead.

The diaphragm 30 extends along the full length of the straight portions of the bag part 20 of the skirt 10 at the sides of the craft and may extend around the aft end of the craft.

A similar mesh diaphragm 32 connects the tails 24 of the fingers 22 to the underside of the sidebody 6. This mesh 32 may extend around the whole peripheral skirt.

When the skirt is inflated the diaphragms 30, 32 are placed under tension. The mesh used for the diaphragms 30,32 in this example has a lattice of tessellated hexagonal cells and are shown under uniform vertical tension as would be the case if the skirt is inflated over a flat surface. The mesh is oriented in this case with one apex of the hexagon as a lowest point of the cell, with two sides of the hexagon cell upwardly diverging from this point. Similarly, the top of each hexagon is an apex, with two sides of the hexagon upwardly converging to this top point. The other two sides of the hexagonal cells are parallel with one another and join the top ends of the diverging sides to the bottom ends of the converging sides.

With this configuration of mesh, as illustrated schematically in figs. 3 and 4, when the fingers 22 of a portion of the skirt 10 are lifted as they pass over an obstacle, the tension in the meshes above these lifted fingers is partially relieved and this part of the mesh contracts, bringing the top and bottom points of the hexagon cells closer to one another. At the same time, the sides of the hexagon spread apart to increase the width of the cell. This in turn allows the sides of adjacent cells that are not lifted by the obstacle to contract and these adjacent cells to lengthen (i.e. the top and bottom points move further apart), which has the effect of lowering the bottom edge of the bag pad 20 of the skirt 10, and the connected fingers 22, so that the fingers better conform to the shape of the obstacle. In this way, leakage of air from below the skirt around the obstacle can be minimised. This offers either, a direct saving in lift power, or a general increase in cushion depth leading to lower skid drag.

Fig. 5 shows alternative mesh configurations that will also perform in the desired manner.

The responsive mesh diaphragm described above can also be used with good effect in bag or loop skirts which have no fingers oi segments attached, wheie the lower part of the skirt consists of a diaphragm or diaphragms which make the contact seal with the operating surface. Fig. 6 shows an example of such a bag skirt 10, with an internal mesh diaphragm 30.

In some embodiments it may further be desirable to construct the skirt wall, or at least portions of it, also with a mesh constiuction, as seen in fig. 7. In this example, the loop section 40 of a loop and segment skirt has an inflated wall that is formed from a mesh having a plurality of hexagonal cells 42. The mesh elements are formed of substantially inextensible cords and the cells of the mesh are covered with panes of a flexible material which do not inhibit the geometrical response of the mesh. If the panes are constructed from essentially flat panels, they should be an elastic sheet such as unreintoiced rubber or polyurethane. If the panels are flexible but relatively non-elastic, such as proofed skirt material, they should be constructed in a domed shape to allow the mesh to change shape without restriction.

Various variations and modifications to the specifically described example are possible within the scope of the invention. For example, although the example illustrated in fig. 7 includes mesh walls in combination with internal mesh diaphragms, other embodiments of the invention can use the mesh walls without using mesh diaphragms. Similarly, whilst fig. 6 shows an inflated bag constructed from conventional proofed fabric, it may also be constructed from a mesh with panes from a flexible material as described above.

Claims

Claims: 1. A flexible skirt for a hovercraft, the skirt comprising: a skirt wall; and a mesh diaphragm extending across the interior of the skid to be placed under tension when the skirt is inflated; wherein the shape of the mesh and the orientation of the diaphragm within the skirt are configured such that in use when a lower edge of the inflated skirt wall encounters an obstacle causing a local reduction in vertical tension in the mesh diaphragm, the portion of the mesh above the obstacle lifts and a resultant change in shape of the mesh causes immediately adjacent portions of the mesh to lower, lowering the skirt wall to either side of the obstacle.
2. A flexible skirt according to claim 1, wherein the skirt comprises an upper skirt section and a lower skirt section and the mesh diaphragm extends across the upper skirt section.
3. A flexible skirt according to claim 2, wherein the upper skid section comprises a bag section or a loop section.4. A flexible skirt according to claim 2 or claim 3, wherein the lower skirt section comprises one or more of a lower bag section, a lower loop section or a plurality of fingers or segments extending downwardly Thom the upper skirt section.
4. A flexible skirt according to any one of claims 2 to 3, wherein: the upper edge of the mesh diaphragm is attached to an upper podion of the upper skirt section or to a pad of the hovercraft structure at the top of the skirt; and the lower edge of the mesh diaphragm is attached to a lower podion of the upper skirt section or to an upper portion of the lower skid section.
5. A flexible skirt according to any one of the preceding claims, wherein the cells, when uniformly loaded, have no straight load paths from the lower edge of the mesh to the upper edge of the mesh.
6. A flexible skirt according to any one of the preceding claims, wherein said change in shape of the mesh results from a local rearrangement of its cell geometry.
7. A flexible skirt according to claim 6, wherein the shape of the mesh and the orientation of the mesh are configured so that as the bottom of a mesh cell is relieved of downward tension, the sides of the cell expand laterally outwardly, so that the height:width ratio of the cell shape reduces, whereby in use, the tension on the bottom of mesh cells above an obstacle that the skirt is passing is reduced, causing lateral expansion of these cells to relieve lateral tension on adjacent cells, causing an increase in the height:width ratio of the adjacent cells, which allows a lowering of the bottom of the adjacent cells and portions of the skirt attached thereto.
8. A flexible skirt according to any one of the preceding claims, wherein the cells are hexagonal.
9. A flexible skirt according to any one of claims ito 8, wherein the cell pattern is a combination of regular shapes that when uniformly loaded exhibit no direct load lines from a lower edge of the mesh to an upper edge of the mesh.
10. A flexible skirt according to claim 9, wherein the cell pattern is a regularly repeating pattern of octagons and rectangles.
11. A flexible skirt according to any one of the preceding claims, wherein cells of the mesh are arranged in a repeating pattern along the full length of the diaphragm.
12. A flexible skirt according to any one of the preceding claims, wherein cells of the mesh are of the same size and shape as one another.
13. A flexible skirt according to any one of claims 2 to 12, wherein the diaphragm extends within the upper skirt section of the skirt along a majority of the length of the skirt at the side of the hovercraft when installed.
14. A flexible skirt according to any one of claims 2 to 13, wherein the lower skirt section comprises a plurality of fingers, the fingers having a generally U-shape cross-section a base of the U' at the top of the finger being joined to the bottom of the upper skirt section and tails of the U' extend back towards the hull of the craft, the inner end of each tail being connected to the hull either directly, by ties or by a diaphragm.
15. A flexible skirt according to claim 14, wherein the inner ends of the finger tails are spaced outwardly from the hull and are connected to the hull by ties and the inner ends of the finger tails are also connected to the structure of the craft above the inner ends of the fingers by further ties.
16. A flexible skirt according to claim 15, wherein the ties take the form of a mesh.
17. A flexible skirt according to any one of the preceding claims, wherein the material forming the mesh elements is substantially inextensible.
18. A flexible skid for a hovercraft, the skid comprising a pressure-retaining skirt wall, at least a portion of the skid wall being formed from a mesh with mesh cells covered with panes of material, wherein the shape of the mesh and its orientation within the skirt wall are configured such that in use when a lower edge of the inflated skid wall encounters an obstacle causing a local reduction in hoop tension in the mesh in the wall, the portion of the mesh above the obstacle lifts and a resultant change in shape of the mesh causes immediately adjacent portions of the mesh to lower, lowering the skirt wall to either side of the obstacle.
19. A flexible skirt for a hovercraft, wherein the panes of material are elastic material or dome-shaped flexible material.
20. A flexible skid according to claim 18 or claim 19, wherein the skirt comprises an upper skirt section and a lower skirt section and the mesh is incorporated in the wall of the upper skirt section.
21. A flexible skirt according to claim 20, wherein the upper skirt section comprises a bag section or a loop section.
22. A flexible skid according to any one of claims 18 to 21, wherein the cells, when uniformly loaded, have no straight load paths around the wall of the skirt.
23. A flexible skid according to any one of claims 18 to 22, wherein said change in shape of the mesh results from a local rearrangement of its cell geometry.
24. A flexible skid according to claim 23, wherein the shape of the mesh and the orientation of the mesh are configured so that as the bottom of a mesh cell is relieved of hoop tension, the sides of the cell expand laterally outwardly, so that the height:width ratio of the cell shape reduces, whereby in use, the tension on the bottom of mesh cells above an obstacle that the skirt is passing is reduced, causing lateral expansion of these cells to relieve lateral tension on adjacent cells, causing an increase in the height:width ratio of the adjacent cells, which allows a lowering of the bottom of the adjacent cells and associated portions of the skirt wall.
25. A flexible skid according to any one of claims 18 to 24, wherein the cells are hexagonal.
26. A flexible skirt according to any one of claims 18 to 24, wherein the cell pattern is a combination of regular shapes that when uniformly loaded exhibit no direct load lines from a lower edge of the mesh to an upper edge of the mesh.
27. A flexible skirt according to claim 26, wherein the cell pattern is a regularly repeating pattern of octagons and rectangles.
28. A flexible skirt according to any one of claims 18 to 27, wherein cells of the mesh are arranged in a repeating pattern along the full length of the skirt wall.
29. A flexible skirt according to any one of claims 18 to 28, wherein cells of the mesh are of the same size and shape as one another.
30. A flexible skirt according to any one of of claims 18 to 29, wherein the material forming the mesh elements is substantially inextensible.
31. A flexible skirt according to any one of of claims 18 to 30, wherein the skirt is also in accordance with any one of claims 1 to 17.
32. A hovercraft including a hull and a flexible skirt extending around and downwardly from the hull to form a plenum for an air cushion, wherein the skirt is a skirt in accordance with any one of claims ito 17 or any one of claims 18 to 31.