GB2285027A

GB2285027A - A sail

Info

Publication number: GB2285027A
Application number: GB9326059A
Authority: GB
Inventors: Robert Hill
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-12-21
Filing date: 1993-12-21
Publication date: 1995-06-28
Anticipated expiration: 2013-12-21
Also published as: GB2285027B; GB9326059D0

Abstract

A sail comprises an inner membrane 1 and two outer membranes 2, 3, each attached substantially around the periphery of a respective side of the sail to form a respective pocket 4, 5. The width of the inner membrane 1 is less than that of the outer sail membranes so that, during use, the sail adopts an aerofoil cross section, resulting in improved aerodynamic performance. Air vents 6, 9 are provided in the outer membranes 3, 2, the vents being connected to the opposing pocket by means of collapsible tubes 7, 10 and respective holes 8, 11 in the inner membrane. The pockets may be divided into a number of separate compartment (18, Fig 4a) by a plurality of fabric ribs (17) that connect the inner membrane to the outer membranes. <IMAGE>

Description

A Sail This invention relates to a sail for a sailing vessel. More particularly, the invention relates to a sail having improved aerodynamic characteristics, and a method of shaping a sail.

The performance of a sailing boat is determined largely by the aerodynamic efficiency of the sail. A conventional sail comprising a single membrane of sail material adopts, in use, a curved shape similar to the shape of an aerofoil. However, the efficiency of such a sail is limited by the fact that it has no significant thickness (or chord height) and so does not correspond to a full aerofoil section.

Attempts have been made to provide a more efficient sail having an aerofoil cross section, by constructing the sail in the form of a rigid or semi-rigid wing comprising a covering stretched over a framework.

However, because an aerofoil cross section is asymmetric, such arrangements have had to include either some kind of articulation that allows the sail to change shape, or a pivot mechanism that allows the sail to be inverted, to permit the sail to be used efficiently on both tacks.

The resulting mechanisms have tended to be structurally complex, heavy and expensive.

A further known arrangement attempts to provide a sail having an approximate aerofoil cross section by using a wide mast and a single or double membrane sail.

The arrangement is, however, complicated and not very efficient.

It is an object of the invention to provide a sail having improved aerodynamic performance.

According to the present invention there is provided a sail comprising an inner membrane and, on either side thereof, an outer membrane attached to the inner membrane, each outer membrane forming, with the inner membrane, an inflatable pocket, the depth of the inner membrane being less than that of the outer membranes.

The depth of a membrane as referred to herein is defined as the ratio of the depth of curvature to the length of the chord, where the chord is the imaginary line extending from the leach edge of the sail to the luff edge, and the depth of curvature is the greatest perpendicular distance between the chord and the membrane at full extension.

As a result of the invention, when both the inner membrane and an outer membrane are at their fullest extension, the sail adopts an aerofoil cross section with the depth of the leeward surface being greater than the depth of the windward surface. In other words, the camber of the leeward surface is greater than the camber of the windward surface, where camber is an expression of the curvature of an aerofoil, equal to the maximum height of the aerofoil divided by its chord. The aerodynamic performance of the sail is thereby considerably improved.

Advantageously, the inner membrane is substantially impermeable to air and each pocket includes a vent arranged to permit the flow of air into and out of the pocket. The pockets may thus be inflated or deflated as appropriate during use, or deflated for convenient storage of the sail.

Each vent preferably comprises a hole in one of the outer membranes of the sail and means connecting the hole with the pocket formed by the inner membrane and the other outer membrane of the sail. As a result, the pocket on the leeward side of the sail may be filled and the pocket on the windward side may be evacuated by the pressure of the wind on the sail.

Preferably, the connecting means extends between the hole in the outer membrane and a corresponding hole in the inner membrane. The connecting means may comprise a collapsible tube. Thus, when a pocket is flattened by wind incident on the sail, the tube passing through the pocket also collapses.

The vents are preferably offset from one another.

As a result, the pockets are isolated from one another and air cannot flow from one pocket into the other pocket.

Each vent is preferably situated approximately one third of the width of the sail from the luff edge of the sail. Because the pressure difference created across the sail during use is greatest in this region, the respective pockets are filled and evacuated quickly and efficiently.

There is preferably associated with each hole in the outer membranes means for preventing water from entering the pockets. Spray and rain is thereby prevented from entering the sail and reducing its performance by increasing the sail's weight and distorting its shape.

Means are preferably provided for draining away water entering the pockets. The draining means may comprise a drain hole situated at the leach edge of the sail. Any water that does enter the sail is thus quickly removed. Positioning the drain hole at the leach edge ensures that the draining water is blown clear of the sail.

Alternatively, the outer membranes are substantially impermeable to air and the inner membrane is adapted to allow air to flow through it from one pocket to the other. Such an arrangement does not require any exterior vents and it is therefore suitable for use on sailboards and in other situations where the sail is likely to be immersed in the water.

The inner membrane may be made of an air permeable material, or may comprise a net or mesh, or may include one or more air vents. The sail preferably further includes a sealable vent for adjusting the quantity of air in the sail. When wind is incident on the sail, the windward pocket is flattened and the air in the sail passes through the inner membrane into the leeward pocket. The sail thus adopts an aerofoil cross section without the need for external air vents.

The sail preferably includes one or more flexible ribs that extend between the inner membrane and each outer membrane. The ribs may be shaped to hold the sail in a desired shape. The ribs preferably have the shape of an aerofoil.

The ribs may be impermeable and divide the pockets into a plurality of compartments, each compartment including an air vent.

Alternatively, the ribs may divide the pockets into a plurality of compartments, the ribs being adapted to allow air to flow from one compartment to the next. Each rib may include one or more air vents or be made of an air permeable material, or a mesh or net-like material.

A drain hole may be provided in each compartment.

Advantageously, the ribs extend diagonally downwards from the luff edge to the leach edge of the sail and the drain holes are provided at the lowermost corner of each associated compartment. Water will tend to flow downwards to that corner, and drainage will thus be improved.

The outer membranes are preferably made of a lighter weight material than the inner membrane. The outer membrane will thus be easily lifted away from the inner membrane and the sail will adopt a shape having a significant thickness more readily.

The sail may further include means for stiffening the sail. The stiffening means may be provided in the inner membrane.

The present invention further provides a method of shaping a sail comprising an inner membrane and, on each side thereof, an outer membrane attached the inner membrane, each outer membrane forming, with the inner membrane, an inflatable pocket, the method comprising inflating the pocket on the leeward side of the sail to increase the thickness of the sail.As a result of that arrangement the shape of the sail can varied to produce a sail having a desired set of characteristics for specific conditions.

Advantageously, each pocket includes an air vent, and the leeward pocket is inflated by air flowing into the pocket through the air vent owing to the pressure difference created during use across the sail.

Alternatively, the outer membranes of the sail are substantially impermeable to air and the inner membrane is adapted to allow air to flow through it from one pocket to the other, the sail being inflated before use and air being transferred during use from the windward pocket to the leeward pocket owing to the pressure difference created across the sail.

Certain illustrative embodiments of the invention will now be described with reference to the accompanying drawings, of which: Fig. la is a cross sectional view of a mainsail according to the present invention; Fig. ib is a partial side view of the mainsail shown in Fig. la; Fig.Ic is a partial side view of a sail; Fig. 2a is a side view of a hoisted sail; Fig. 2b is a cross section on line II-II of Fig. 2a; Figs. 3a and 3b are cross sectional views illustrating the operation of the sail; Fig. 4a is a side view of a sail according to a second embodiment of the invention; Fig. 4b is a sectional view on line IV-IV of Fig. 4a; Fig. 5 is a partial side view of a sail, illustrating its internal construction; Fig. 6a is a side view of a jib or staysail according to the present invention; Fig. 6b is a sectional view on line VI-VI of Fig. 6a, Fig. 7 is a side view of part of a boat's rig comprising a mainsail and a jib, each according to the present invention; Fig. 8a is a side view of a sailboard rig including a sail according to the present invention; Figs. 8b and 8c are alternative sectional views of the sail shown in Fig. 8a;; Fig. 9 is a side view of a portion of a staysail according to a further embodiment of the invention; Figs. 10a, lOb and lOc are cross-sectional views along line X-X of Fig. 9; Fig. 11 is a side view of a portion of a staysail according to a further embodiment of the invention, and Figs. 12a and 12b are cross-sectional views along line XII-XII of Fig. 11.

The sail shown in Figs. la and lb comprises a inner membrane 1, a starboard outer membrane 2 and a port outer membrane 3. The outer membranes 2, 3 are attached to the inner membrane 1 substantially around their peripheries to form pockets 4, 5 on either side of the inner membrane 1. The inner membrane 1 provides most of the strength of the sail and may be made of a standard sail material.

The outer membranes 2, 3 may be made of lighter weight material.

The width of each of the outer membranes 2, 3 from the luff attachment point to the leach attachment point is greater at any height of the sail than the width of the inner membrane 1 between those attachment points.

This allows the outer membranes 2, 3 to be spaced from the inner membrane 1, thereby increasing the thickness of the sail. The sail includes a bolt rope 12 at its luff edge for attachment in a conventional manner to a mast.

An air vent 6 in the port outer membrane 3 is connected te the starboard pocket 5 by means of a collapsible tube 7 of sail material which extends through the port pocket 5 to a hole 8 in the inner membrane 1.

Similarly, an air vent 9 in the starboard outer membrane 2 is connected to the port pocket 4 by a collapsible tube 10 which extends through the starboard pocket 5 to a second hole 11 in the inner membrane.

As shown in Fig. lb, the air vents 6, 9 are situated approximately one third of the distance from the luff edge to the leach edge of the sail, where the difference in air pressure across the sail is greatest during use.

The holes 8, 11 in the inner membrane 1 are offset vertically from one another and the port and starboard pockets are thus isolated from one another, which prevents air flowing through the sail from one side to the other.

The collapsible tubes 7, 10 may have concertina folds (not shown) and, as shown in Fig. lc, are larger in cross section than the associated vents 6, 9. The tubes 7, 10 are thus able to collapse without obstructing the associated air vents. The air vents 6, 9 may be elongated, as shown in Fig. lc, and may be provided with cowls of fabric or a rigid material, to prevent spray or rain from entering the pockets.

Figs. 2a and 2b show a mainsail having, on each side, a single pocket which extends over a substantial area of the sail. A single air vent 6, 9 is provided on each side of the sail. The vents 6, 9 are vertically offset from one another and the pockets are thus isolated from one another.

The inner membrane of the sail is attached to the mast 12a in a conventional manner by a bolt rope or slides. A drain hole 13 is provided at a lower corner of each pocket to allow drainage of any water (for example, rain or spray) that has entered the pocket through the air vents 6, 9. The halyard 14, outhaul 15 and tack tackle 16 are attached to the inner membrane of the sail in a conventional manner.

Operation of the sail will be explained with reference to Figs. 3a and 3b. In Fig. 3a, the boat is on the port tack with the wind blowing in the direction indicated by arrow A. The starboard outer membrane 2 is thus located on the leeward (low pressure) side of the sail and the port outer membrane 3 is located on the windward (high pressure) side of the sail.

The pressure difference across the sail causes air to flow through the port air vent 6 and the associated tube 7 and hole 8 into the starboard pocket 5. The starboard pocket 5 is thus filled with air. The flow of air into the starboard pocket 5 is indicated by the arrow X. At the same time, air is evacuated from the port pocket 4 through the starboard air vent 9 and the tube 10 and hole 11, as indicated by the arrow Y. The port outer membrane 3 thus lies in folds against the inner membrane 1, the concertina folds allowing the tube 7 to collapse and lie flat against the inner membrane 1 without obstructing the vent 6.

Because the width of the outer membranes 2, 3 as measured from the luff attachment point to the leach attachment point is greater at any height of the sail than the width of the inner membrane 1 between those points, when the leeward pocket is filled with air, the depth of the leeward outer membrane 2 is greater than the depth of the inner membrane 1. In other words, the leeward outer 2 has a smaller radius of curvature than the inner membrane 1. The windward outer membrane 3, on the other hand, is pressed against the inner membrane 1, the windward pocket 4 having been evacuated. The sail therefore adopts a thick aerofoil cross section with a concave windward surface defined by the shape of the inner membrane 1 and a convex leeward surface defined by the shape of the leeward outer membrane 2.

Fig. 3b shows the reverse situation in which the vessel is on the starboard tack with the wind blowing in the direction shown by arrow B. The port pocket 4 is now to leeward and is filled with air and the starboard pocket 5 is evacuated. The convex leeward surface is thus defined by the shape of the port outer membrane 3 and the concave windward surface is defined by the shape of the inner membrane 1. The sail is thus able to adopt a thick asymmetric aerofoil cross section on either tack.

The shape adopted by each of the membranes 1, 2, 3 during use is determined by its material and cut (for example the panel shapes, seam locations and so forth), and by the positioning within the membrane of battens and other stiffening devices. The shape of the concave windward side of the sail is determined by the shape of the inner membrane 1, as the windward outer membrane is pressed against the inner membrane. The shape of the convex leeward side of the sail is determined by the cut of the leeward outer membrane 2, 3, which forms the other side of the air filled pocket.

When air enters the sail, the inner membrane and the leeward outer membrane adopt their full extension giving the sail an aerofoil cross section of significant thickness. The aerodynamic performance of the sail is thereby considerably improved. Further, the sail adopts an aerofoil cross section regardless of the wind direction relative to the sail and without the need for complex articulation. The sail can also be folded for easy storage and can be attached to a conventional mast.

A further embodiment of the invention is shown in Figs. 4a and 4b. The drawing shows a hoisted mainsail attached to a mast 12a. The sail comprises an inner membrane 1 and starboard and port outer membranes 2, 3 (only the port outer membrane 3 being shown), which fcrm pockets on either side of the inner membrane 1.

The pockets are divided into a number of separate compartments 18 by a plurality of fabric ribs 17 that connect the inner membrane 1 to the outer membranes 2, 3 and extend horizontally from the luff edge of the sail to the leach edge. Each rib 17 has an aerofoil shape matching the desired aerofoil cross section of the leeward pocket at the height of that rib. The ribs 17 thus help to control the cross sectional shape of the leeward pocket. On the other hand, because the ribs are made of fabric, they collapse and lie substantially flat against the inner membrane 1 when they are located on the windward side of the sail.

The ribs are substantially impermeable and thus isolate the compartments 18 from one another. Separate air vents 6, 9 are therefore provided for each of the compartments 18. Operation of the sail, including the filling and evacuation of the pockets, is substantially as described above with reference to Figs. 3a and 3b.

The air vents 6, 9 in the outer membranes 2, 3 are situated approximately one third of the distance along the sail from the luff edge 20, in order to benefit from the high pressure difference across the sail in that region, which allows the pockets to be inflated and evacuated quickly and efficiently in use.

The ribs may be made of sail material or another air impervious material. A drain hole 13 is provided at a lower corner of each compartment 18 to allow water to drain from the compartment. The drain holes 13 are sufficiently small to ensure that the air pressure in the compartments is not significantly reduced by air escaping through the drain holes. The inner membrane 1 and/or the outer membranes 2, 3 may be provided with battens or other stiffening means. One such batten is indicated at 19.

A variant of the arrangement shown in Figs. 4a and 4b is shown in Fig. 5. The pockets are again divided by ribs 17 into a plurality of compartments 18 but, in this arrangement, the ribs are provided with air holes 20 which allow air to flow from one compartment to the next.

Only one air vent 6, 9 is therefore required on each side of the sail to fill all the compartments in the pocket.

Alternatively, the ribs may be made of an air permeable material or a mesh or net. As in the arrangement of Figs. 4a and 4b, the ribs serve to control the shape of the aerofoil cross section.

Figs. 6a and 6b show a jib or staysail in which the pockets are divided by ribs 17 into a plurality of compartments 18, each having separate vents 6, 9. The construction of the sail corresponds to that of the mainsail shown in Figs. 4a and 4b. The sail is attached to a forestay 21 and controlled by sheets 22 in a conventional manner.

Fig. 7 shows the rig of a sailing boat comprising a mainsail 23 and a jib 24. The sails are similar to those described above with reference to Figs. 4 and 6, except that in this embodiment the ribs slope downwards from the luff edge 20 of the sail to the leach edge 24 to improve drainage of water towards the drain holes 13. The slope of the ribs is exaggerated in the drawing for the sake of clarity.

Figs. 8a, 8b and 8c show a further embodiment of the invention, in which the sail comprises two air impermeable outer membranes 2, 3 (only the port outer membrane 3 being shown) and an air permeable inner membrane 1, which may be made of an air permeable material, or have one or more air holes, or be made of a mesh or net-like material. As in the previous embodiments, the width of the outer membranes 2, 3 as measured from the luff edge of the sail to the leach edge is greater at any height than the width of the inner membrane 1.

The outer membranes 2, 3 do not have any open external air vents and thus form a sealed envelope in which a quantity of air is trapped. The amount of air trapped in the sail may be adjusted by means of sealable air inlet 27, comprising an inlet tube haing a valve or closure. The trapped air is able to flow relatively freely from one pocket to the other through the permeable inner membrane.

The sail is divided into a number of compartments by ribs 17 which connect the inner membrane to the two outer membranes and extend from the luff edge of the sail to the leach edge. The ribs are made of a permeable or netlike material or include one or more holes to allow air to flow from one compartment to the next. Alternatively, the ribs are impermeable and separate sealable air inlets are provided for each compartment.

The sail is inflated, before use, through the inlet 27 and the inlet is then sealed. The amount of air let into the sail depends on the sailing conditions and the desired characteristics of the sail, but generally will be approximately the quantity needed to half fill the sail.

The sail operates as shown in Figs. 8b and 8c.

When the vessel in on the port tack with the wind blowing in the direction of arrow C (as shown in Fig.

8b), the pressure difference across the sail causes the air trapped within the sail to flow through the permeable inner membrane 1 from the windward port pocket 4 to the leeward starboard pocket 5. The port outer membrane 3 thus collapses against the inner membrane 1 and the starboard pocket 5 expands to adopt an aerofoil cross section. The shape of the aerofoil cross section is thus determined by the inner membrane 1, which defines the shape of the concave windward side of the sail, and the starboard outer membrane 2, which defines the shape of the convex leeward side of the sail. The shape of the aerofoil cross section is controlled additionally by the ribs. Battens or other stiffening devices may also be provided.

The shape of the sail may be varied in use by adjusting the amount of air trapped in the sail. As a result, the sail may be tuned for optimum performance in different weather conditions.

If the vessel changes onto the starboard tack with the wind blowing in the direction of arrow D (as shown in Fig. 8c), the air trapped in the sail is forced back through the permeable inner membrane 1 from the windward starboard pocket 5 into the leeward port pocket 4. The starboard outer membrane 2 then lies against the inner membrane 1. The sail is thus able to adopt an asymmetric aerofoil cross section on both tacks.

Because the arrangement shown in Figs. 8a to 8c is completely sealed and does not have any open external vents through which water can enter the sail, it is particulary suitable for sailboard sails, which are frequently immersed in the water. The sail might also be used on small dinghies, which are liable to capsize, or in conditions of heavy rain or spray. The sealed construction obviates the need for drain holes.

A number of variations and modifications of the sails described above are possible. For example, the outer membranes may extend over only part of the surface area of the inner membrane, such as the forward part of the sail, where the increased thickness has the most effect, or only the lower part of the sail. The outer membranes will always, however, have a greater depth than the inner membrane, so that the sail adopts, in use, a thick aerofoil cross section.

Further, in a modification of the construction shown in Figs 8a and 8b, the sealable air inlet 27 is omitted and the pockets in the sail are filled during use by air leaking through the outer membranes, owing to the natural porosity of the sail material. The arrangement could also include a mechanical pump to inflate the pocket on the leeward side of the sail and evacuate the windward pocket.

A further embodiment of the invention is shown in Figs. 9 and 10. In this embodiment, the sail has air vents 30,31 adjacent its luff edge, at or close to the region of maximum static pressure. This arrangement is suited particularly to jibs and staysails, where there is no mast to obstruct or shield the air vents from this region of high pressure.

The pockets 4,5 extend from the luff edge to approximately half the chord width of the sail, the maximum thickness of the aerofoil section being located towards the leading edge of the sail.

When the vessel is on the port tack, as shown in Fig. 10b, the starboard pocket 5 is to leeward and is completely inflated, owing to the difference in air pressure between the leeward vent 31 and the leeward surface of the sail. The windward port pocket 4 may also be partially inflated, giving an additional beneficial shaping to the leading edge of the sail. When the vessel is on the starboard tack, as shown in Fig. 10c, the situation is reversed, the leeward port pocket 4 being fully inflated and the windward starboard pocket 5 being partially inflated.

A further embodiment, shown in Figs. 11 and 12, combines the features of the sail shown in Figs. 9 and 10 with those of the sail shown in Fig. 1. In this embodiment, each pocket 4,5 is divided into forward and rearward portions 32,33,34,35 by a flexible inpervicus membrane 36,37. The forward part 32,33 of each pocket is inflated via an air vent 30,31 provided adjacent the leading edge of the sail, and the rearward portion 34,35 is inflated via a collapsible tube 7,10 that extends through the sail to a vent 6,9 located on the opposite side of the sail.

When the vessel is on the port tack, as shown in Fig. 12b, both portions 33,35 of the leeward starboard pocket are fully inflated and the forward portion 32 of the windward port pocket is partially inflated, thereby increasing the thickness of the sail's leading edge. The rearward portion 34 of the windward pocket is almost completely evacuated, owing to the pressure difference between the two sides of the sail. The port membrane 3 lies against the inner membrane 1 towards the rear of the sail and provides a smooth transition to the partially inflated forward portion 32 of the windward port pocket.

The situation is reversed when the vessel is on the starboard tack.

Claims

1. A sail comprising an inner membrane and, on either side thereof, an outer membrane attached the inner membrane, each outer membrane forming, with the inner membrane, an inflatable pocket, the depth of the inner membrane being less than that of the outer membranes.

2. A sail as claimed in claim 1 in which the inner membrane is substantially impermeable to air and each pocket includes a vent arranged to permit the flow of air into and out of the pocket.

3. A sail as claimed in claim 2 in which each vent comprises a hole in one of the outer membranes of the sail and means connecting the hole with the pocket formed by the inner membrane and the other outer membrane of the sail.

4. A sail as claimed in claim 3 in which the connecting means extends between the hole in the outer membrane and a corresponding hole in the inner membrane.

5. A sail as claimed in claim 3 or claim 4 in which the connecting means comprises a collapsible tube.

6. A sail as claimed in any one of claims 2 to 5 in which the vents are offset from one another.

7. A sail as claimed in any one of claims 2 to 6 in which each vent is situated approximately one third of the width of the sail from the luff edge of the sail.

8. A sail as claimed in any one of claims 3 to 7 in which there is associated with each hole in the outer membranes means for preventing water from entering the pockets.

9. A sail as claimed in any preceding claim including means for draining away water entering the pockets.

10. A sail as claimed in claim 9 in which the draining means comprises a drain hole situated at the leach edge of the sail.

11. A sail according to claim 1 in which the outer membranes are substantially impermeable to air and the inner membrane is adapted to allow air to flow through it from one pocket to the other.

12. A sail as claimed in claim 11 in which the inner membrane is made of an air permeable material.

13. A sail as claimed in claim 11 in which the inner membrane comprises a net or mesh.

14. A sail as claimed in claim 11 in which the inner membrane includes one or more air vents.

15. A sail as claimed in any one of claims 11 to 14 including a sealable vent for adjusting the quantity of air in the sail.

16. A sail as claimed in any one of the preceding claims, including one or more flexible ribs that extend between the inner membrane and each outer membrane.

17. A sail as claimed in claim 16 in which the ribs have the shape of an aerofoil.

18. A sail as claimed in claim 16 or claim 17 in which the ribs are impermeable and divide the pockets into a plurality of compartments, each compartment including an air vent.

19. A sail as claimed in claim 16 or claim 17 in which the ribs divide the pockets into a plurality of compartments, the ribs being adapted to allow air to flow from one compartment to the next.

20. A sail as claimed in claim 19 in which each rib includes one or more air vents.

21. A sail as claimed in claim 19 in which the ribs are made of an air permeable material, or a mesh or net-like material.

22. A sail as claimed in any one of claims 18 to 21 in which a drain hole is provided in each compartment.

23. A sail as claimed in claim 22 in which the ribs extend diagonally downwards from the luff edge to the leach edge of the sail and the drain holes are provided at the lowermost corner of each associated compartment.

24. A sail as claimed in any one of the preceding claims, in which the outer membranes are made of a lighter weight material than the inner membrane.

25. A sail as claimed in any one of the preceding claims, including means for stiffening the sail.

26. A sail as claimed in claim 25 in which the stiffening means are provided in the inner membrane of the sail.

27. A sail as claimed in any one of the preceding claims, in which each pocket includes an air vent positioned adjacent the luff edge of the sail.

28. A sail according to claim 27, in which each pocket is divided into two portions, the forward portion including the air vent positioned adjacent the luff edge of the sail

29. A method of shaping a sail comprising an inner membrane and, on each side thereof, an outer membrane attached the inner membrane, each outer membrane forming, with the inner membrane, an inflatable pocket, the method comprising inflating the pocket on the leeward side of the sail to increase the thickness of the sail.

30. A method according to claim 29, in which each pocket includes an air vent, and the leeward pocket is inflated by air flowing into the pocket through the air vent owing to the pressure difference created during use across the sail.

31. A method according to claim 29, in which the outer membranes of the sail are substantially impermeable to air and the inner membrane is adapted to allow air to flow through it from one pocket to the other, the sail being inflated before use and air being transferred during use from the windward pocket to the leeward pocket owing to the pressure difference created across the sail.

32. A sail substantially as herein described with reference to and as illustrated by Figs. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 of the accompanying drawings.

33. A method of shaping a sail, the method being substantially as described herein with reference to and as illustrated hy Figs. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 of the accompanying drawings.