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 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, for 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. Although potentially aerodynamically efficient, such an arrangement involves the expense and weight of a special mast and is applicable only to sails supported up the luff by a mast.

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 camber of the inner membrane being less than that of the outer membranes, wherein an inflatable compartment is provided adjacent the luff edge of the sail.

The camber of a membrane as referred to herein is defined as the greatest perpendicular distance from the chord to the membrane, where the chord is the imaginary straight line extending from the luff edge of the sail to the leach when the membrane is at full extension.

As a result of the invention, when both the leeward inner membrane and an outer membrane are at their fullest extension, the sail adopts an aerofoil cross section with the camber of the leeward surface being greater than the camber of the windward surface. In other words, the curvature of the leeward surface is greater than the curvature of the windward surface, so producing an aerofoil having a significant thickness. The aerodynamic perfor¬ mance of the sail is thereby considerably improved.

The inflatable compartment increases the thickness of the sail adjacent its luff edge, so improving the aerodynamic efficiency of the sail.

An inflatable compartment may be located between the inner membrane and each of the outer membranes.

The inflatable compartment may include a sealable vent means. This allows the amount of air in the compartment to be adjusted. The inflatable compartment may comprise a pocket containing an inflatable tube. The width of the inflatable pocket may be approximately 30% of the chord of the sail. The inflatable pocket may be arranged to increase the thickness of the sail on the windward side, to modify the shape of the leeward pocket and/or to increase

the effective length of the windward side of the sail.

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. 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 guickly 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.

In an alternative arrangement, 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.

Advantageously, the ribs extend diagonally downwards from the luff edge to the leach edge of the sail and a drain hole is 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 lift easily away from the inner membrane and the sail will adopt readily a shape having a significant thickness.

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 sail further including an inflatable compartment adjacent the luff edge, the method comprising inflating the compartment before use to increase the thickness of the sail adjacent the luff edge and inflating the pocket on the leeward side of the sail to increase the thickness of the sail.

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 examples of sails illustrating 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;

Fig. lb is a partial side view of the mainsail shown in Fig. la;

Fig. lc 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;

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;

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;

Fig. 8a is a side view of a sailboard rig;

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;

Figs. 10a, 10b and 10c are cross-sectional views along line X-X of Fig. 9;

Fig. 11 is a side view of a portion of a staysail;

Figs. 12a and 12b are cross-sectional views along line XII- XII of Fig. 11;

Fig. 13 is a side view of a mainsail according to the invention;

Figs. 14a, 14b and 14c are cross-sectional views along line XIV-XIV of Fig. 13, showing sections of the complete sail, the inner membrane and luff tubes, and an outer membrane respectively;

Figs. 15a, 15b and 15c are cross-sectional views along line XV-XV of Fig. 13, showing sections of the complete sail, the inner membrane and luff tubes, and an outer membrane respectively;

Figs. 16a, 16b and 16c are cross-sectional views along line XVI-XVI of Fig. 13, showing sections of the complete sail, the inner membrane and luff tubes, and an outer membrane respectively, and

Fig. 17 is a cross-sectional view of a sail according to a second embodiment of the invention.

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 to 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.

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 camber of the leeward outer membrane 2 is greater than the camber of the inner membrane 1. In other words, the leeward outer membrane 2 has a greater curvature (i.e., 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 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 asym¬ metric 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, 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 example of a sail 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, which form 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 19 or other stiffening means.

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 edges 23a,24a of the sails to the leach edges 23b,24b 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 examples of a sail, in which the sail comprises two air impermeable outer membranes 2,3 (only the port outer membrane 3 being shown) and an inner membrane 1, which may include a vent 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. In this instance, the sail is depicted in a form designed for use with a sailboard 26: it may, of course, be adapted for use with other sailing vessels.

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 having 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 net-like 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 half the quantity needed to fill the sail.

The sail operates as shown in Figs. 8b and 8c. When the sailboard is 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 sailboard 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 asymmetrical aerofoil cross section on either tack.

Because the arrangement shown in Figs. 8a to 8c is com¬ pletely sealed and does not have any open external vents through which water can enter the sail, it is particularly 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 camber than the inner membrane, so that the sail adopts, in use, a thick aerofoil cross section.

A further example of a sail is shown in Figs. 9 and 10. In this example, 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

compartments 32,33,34,35 by a flexible impervious membrane 36,37. The forward compartment 32,33 of each pocket is inflated via an air vent 30,31 provided adjacent the leading edge of the sail, and the rearward compartment 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 compartments 33,35 of the leeward starboard pocket are fully inflated and the forward compartment 32 of the windward port pocket is partially inflated, thereby increasing the thickness of the sail's leading edge. The rearward compartment 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 compartment 32 of the windward port pocket. The situation is reversed when the vessel is on the starboard tack.

An embodiment of the invention is shown in Figs. 13 to 16. In this embodiment, ribs are not provided and reliance is placed on the cut of the inner and outer membranes for the shape of the aerofoil section. The inner and outer membranes 1,2,3 are connected to one another around periphery of the sail 23. The port and starboard pockets 4,5 extend over the entire height of the sail. Three vents 6,7 are provided for each pocket, these being located at approximately , and sail height. In practice, only one vent may be required for each pocket.

The inner membrane is fully battened, six battens 19 being provided in this case. Vents 40 are provided along the leach edge of the sail at each batten position, to allow air to exhaust from the windward pocket. The halyard 14,

outhaul 15 and tack tackle 16 are attached to the inner membrane. The shape of the sail can be altered in conventional fashion by adjusting the tensions of the luff and the clew outhaul.

A pocket, hereinafter referred to as a luff pocket 41, is provided on each side of the sail, adjacent the luff edge. The luff pockets 41 are made of lightweight sail material and are attached to the inner membrane 1 along the luff edge 23a and along a seam 42 that extends over the entire height of the sail 23. The luff pockets 41 are located between the inner membrane 1 and the outer membranes 2,3. Each luff pocket 41 is open at the head and the foot of the sail, forming upper and lower apertures 43,44.

The luff pockets 41 are designed to hold inflatable luff tubes 45. The luff tubes 45 are made of an impervious material (e.g., polythene sheet) and include sealable vents 46 through which they may be inflated. The luff tubes 45 are inserted into the luff pockets 41 and inflated, thereby producing rounded bulges on either side of the inner membrane, as shown in Figs. 14a and 14b.

In the embodiment shown in the drawings, the luff pockets 41 have a uniform width of approximately 30% of the maximum sail chord from the clew height up to approximately half the height of the sail. Thereafter, the luff pockets taper uniformly towards the head of the sail. The lower ends of the luff pockets 41 taper towards the foot of the sail.

The purpose of the luff pockets is to increase the thickness of the sail adjacent its luff edge on the windward side, so improving the aerodynamic efficiency of the sail. The size and radius of the luff tubes are so designed that the effective length of the windward surface is equal to or very slightly greater than the effective length of the leeward surface at every section up the

height of the sail. This ensures that any slack in the windward outer membrane is taken up, thereby reducing drag.

The desired shape of the sail at three different heights is shown, for illustrative purposes, in Figs. 14, 15 and 16. Figs. 14a, b and c show a section through the sail at the height of the clew 47. When the sail is fully stretched to produce maximum mast bend as measured, the inner membrane 1 has 4% camber at 50% chord and the outer membrane 2 has 10% camber at 40% chord. The camber of both surfaces increases as the clew outhaul 15 is eased, allowing the mast to straighten. The luff pockets 41 have a width equal to 0.3C.. and a radius of curvature of 0.2C.., where C _v is the length of the chord at clew height.

Figs. 15a, b and c show a section through the sail at mid height, as measured from the clew to the head. The inner membrane 1 has 6% camber at 50% chord and the outer membrane 2 has 14% camber at 45% chord. The camber of both surfaces increases slightly as the clew outhaul is eased, but not by as much as at clew height. The width and the radius of the luff pockets are the same as at clew height.

Figs. 16a, b and c show sections at approximately 85% of the sail height, as measured from the clew 47. The inner membrane 1 has 8% camber and the outer membrane 2 has 16% camber, both at 50% chord. The luff pockets 41 have a width of 0.33C ₆ and a radius of 0.22C ₆ , where C ₆ is the length of the chord at that height.

The luff tubes 45 are inserted into the luff pockets 41 and inflated before use. During use, the pocket 5 formed by the inner membrane 1 and the leeward outer membrane 2 becomes filled with air owing to the pressure difference across the sail, as shown in Fig. 14a. The windward pocket 4 collapses and the outer membrane 3 is pressed against the inner membrane 1 and the windward luff pocket 41. The

configuration is, of course, reversed when the vessel goes onto the other tack.

The windward side of the sail thus adopts a shape having a convex forward part and a concave rear part, whilst the leeward side is completely convex. The leeward luff pocket improves the shape of the leeward membrane by causing the position of maximum camber to form forward of the 50% chord position where it would otherwise naturally occur. The combined effects of the luff pockets provide a very efficient aerofoil shape.

A second embodiment of the invention is shown in Fig. 17. In this embodiment, the inner membrane 1 is made double thickness over approximately the leading third of the chord to form a single, central luff pocket 41. An inflatable luff tube 45 of an impervious material is inserted into the luff pocket 41 and inflated to produce a rounded bulge adjacent the leading edge of the sail.

As in teh previous embodiment, the purpose of the luff tube is to increase the thickness of the sail adjacent its luff edge, so improving the aerodynamic efficiency of the sail. The size and radius of the luff tube 41 is so designed that the effective length of teh windward surface is equal to or very slightly greater than that of teh leeward surface, thus ensuring that there is no slack in the windward outer member.

Certain possible modifications to the embodiments shown in Figs. 13 to 17 will now be described. Instead of being attached to the inner membrane along the luff edge 23a and the seam 42, the luff pockets 41 may be attached at their forward edges directly to the outer membranes. Alternatively, the outer membranes 2,3 may be attached directly to the pockets. Both of these methods reduce the total amount of material in the sail.

The luff pockets 41 may themselves be made of impermeable material and provided with sealable air vents, thereby obviating the need for separate luff tubes.

It is to be understood that a luff tube or tubes substantially as described above with reference to Figs. 13 to 17 may be incorporated into any of the sails shown in Figs. 1 to 12, and such sails are intended to be within the scope of the present invention.