NZ294656A - Aerofoil sail: outer membrane on either side of an inner membrane creating two inflatable pockets and an inflatable forward compartment - Google Patents

Description

New Zealand No. 294656 International No. PCT/GB95/02554 Priority D*te{s): ;Complsto SpeoHfcaKion FjM: ;Claw: (6) ;Publication Dtf»: S&r. &.1 ;P.O. Joum«l No: IbtP.P. ;NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION ;Title of Invention: -no-title as-yet Pv ;Name, address and nationality of applicant(s) as in international application form: ;ALTOSAIL LIMITED, a British company of The Old School House, Laverton, Bath BA3 6Qz, United Kingdom ;294656 ;O ;A Sail ;This invention relates to a sail for a sailing vessel. More particularly, the invention relates to a sail having 5 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 10 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. ;15 ;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 20 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 25 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 30 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. ;British patent application No. 2231854A describes a baffled 35 sail having a base sail and outer baffles connected to the base sail by cord limiters. In use, one of the baffles is pressed against the base sail and the other baffle extends outwardly therefrom. ;NEW ZEALAND ;2 2 »A*J 1997 PATENT OFFICE AMENDED SHEET 2946 56 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 5 comprising an inner membrane and, on either side thereof, an outer membrane attached to the inner membrane, each said outer membrane forming with said inner membrane an inflatable pocket into and from which air may flow, characterised by the arrangement being such that, during 10 use, the pressure difference across the sail causes said leeward pocket to be substantially inflated and said windward pocket to be substantially deflated, the camber of the inner membrane is less than that of the outer membranes whereby, during use, the inner membrane at least partially 15 supports the windward outer membrane, and an inflatable compartment is provided adjacent the luff edge of the sail, said inflatable compartment being inflated during use, thereby increasing the thickness of the sail adjacent the luff edge.

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 25 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 30 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-3 5 mance of the sail is thereby considerably improved.

The inflatable compartment increases the thickness of the NEW ZEALAND 2 2 JAM 1997 PATENT OFFICE I A\;E,',0ED SHEET 294656 3 sail adjacent its luff edge, so improving the aerodynamic efficiency of the sail.

An inflatable compartment may be located between the inner 5 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 10 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 15 the effective length of the windward side of the sail.

According to a preferred embodiment of the invention, the inner membrane is substantially impermeable to air and each pocket includes a vent arranged to permit the flow of air 20 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 25 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 30 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 3 5 tube. When a pocket is flattened by wind incident on the sail, the tube passing through the pocket also collapses.

NEW ZEALAND 2 2 JAN 1997 PATENT OFFICE 2946 56 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 may be 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 15 sail's weight and distorting its shape.

Means may be 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 20 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 embodiment of the invention, the outer 25 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 30 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 may include a sealable vent 35 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 NEW ZEALAND 2 2 JAN 1997 PATENT OFFICE into the leeward pocket. The sail thus adopts an aerofoil cross section without the need for external air vents.

The sail may include one or more flexible ribs that extend 5 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 10 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 m?.de of an air 15 permeable material, or a mesh or net-like material.

The ribs may extend-diagonally downwards from the luff edge to the leach edge of the sail and a drain hole may be provided at the lowermost corner of each associated 20 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 25 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 35 inflatable pocket, the sail further including an inflatable compartment adjacent the luff edge, the method comprising inflating the compartment before use to increase the NEW ZtAL 2 2 JAN PATENT Of 294656 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.

According to a preferred embodiment of the invention, each pocket includes an air vent, and the leeward pocket is inflated by air flowing into the pocket through the air vent owing to tho pressure difference created during use across the sail.

In an alternative embodiment of the invention, 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 15 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.

In the accompanying drawings, Figs. 1 to 3b and 8a to 8c 20 illustrate examples of earlier sails invented by the present inventor. Those figures, which do not illustrate embodiments of the invention, are included to provide a full understanding of the invention. Where the earlier sails illustrated in Figs. 1 to 3b and 8a to 8c share 25 similar features with the sails embodying the invention and shown in Figs. 13 to 17, like reference numerals have been used.

Certain examples of earlier sails and sails embodying the 30 invention will now be described with reference to the accompanying drawings, of which: Fig. la is a cross sectional view of an earlier mainsail; Figs, lb and lc are partial side views of the mainsail shown in Fig. la; 294656 7 Fig. 2a is a side view of a hoisted prior art 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 earlier sail; Fig. 8a is a side view of an earlier sailboard rig; Figs. 8b and 8c are alternative sectional views of the sailboard rig shown in Fig. 8a; 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 earlier sail shown in Figs, la and lb comprises a inner 3 5 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 substan' ... • peripheries to 294656 8 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 10 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 20 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 25 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 3 0 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 3 5 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 NEW ZEALAND 2 2 JAN 1997 PATENT OFFICE .i.-r 294656 9 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 an earlier 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 earlier 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 l, the concertina folds allowing the tube 7 to collapse and lie flat against the inner membrane 1 wit: lout obfjfrirfin^ thrr vent 6, NEW .0 2 2 JAN 1997 PATENT OFFICE 294656 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 5 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 10 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 surfaca defined by the shape of the inner membrane 1 and a convex leeward surface defined by 15 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 20 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-25 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 30 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 35 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.

L NEW ZEALAND 2 2 JAN 1997 PATENT OFFICE 294656 11 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.

Figs. 8a, 8b and 8c show a further examples of an earlier 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 15 made of a mesh or net-like material. As in the earlier sail described above, the width of the outer membranes 2,3 as measured from tne luff edge of the sail to the leach edge is greater at any height than the width of the inner membrane l. In this instance, the sail is depicted in a 20 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 25 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 35 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 NEW ZEALAND 2 2 JAN 1997 PATENT OFFICE ^ rV 294656 provided for each compartment.

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

The s^i.'. operates as shown in Figs. 8b and 8c. When the 10 sailfccird 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 15 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 20 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 35 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 NEW 2 2 JAW 1997 PATENT OFFICE 294656 13 cross section on either tack.

Because the arrangement shown in Figs. 8a to 8c is completely sealed and does not have any open external vents 5 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 10 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 15 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 20 cross section.

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 25 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 30 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 35 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 NEW ZiLAuANO 2 2 JAM 1997 PATENT OFFICE 294656 14 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 10 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 20 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 25 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 3 5 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 NEW ZLAL.-mmu 2 2 JAN 1997 PATENT OFFICE Avr:,," 7 294656 W 15 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 10 increases as the clew outhaul 15 is eased, allowing the mast to straighten. The luff pockets 41 have a width equal to 0.3CX and a radius of curvature of 0.2Ci, where Cx 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, 20 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 25 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 0.33Ca and a radius of 0.22Ca, where Cs 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 3 5 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 NEW ZEALAND 2 2 JAN 1997 PATENT OFFICE 294656 16 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 5 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 10 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 15 to form a single, central luff pocket 41. An inflatable luff tube 45 of an impervious material is inserted into the luff pocket 41 anct inflated to produce a rounded bulge adjacent the leading edge of the sail.

As in the 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 the windward surface is equal to or 25 very slightly greater than that of the leeward surface, thus ensuring that there is no slack in the windward outer member.

Certain possible modifications to the embodiments shown in 30 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 35 directly to the pockets. Both of these methods reduce the total amount of material in the sail. 2 2 JAN 1997 PATENT OFFICE 17 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 earlier sails shown in Figs. 1 to 3b and 8a to 8c, and such sails are intended to be within the scope of the present invention.

NEW ZEALAND 2 2 JAN 1997 PATENT OFFICE • „ 294656

Claims (13)

1. A sail comprising an inner membrane (1) and, on either 5 side thereof, an outer membrane (2,3) attached to the inner membrane (1), each said outer membrane (2,3) forming with said inner membrane (1) an inflatable pocket (4,5) into and from which air may flow, characterised by the arrangement being such that, during use, the pressure difference across 10 the sail causes said leeward pocket (5) to be substantially inflated and said windward pocket (4) to be substantially deflated, the camber of the inner membrane (1) is less than that of the outer membranes (2,3) whereby, during use, the inner membrane (1) at least partially supports the windward 15 outer membrane (3), and an inflatable compartment (41) is provided adjacent the luff edge of the sail, said inflatable compartment (41) being inflated during use, thereby increasing the thickness of the sail adjacent the luff edge (23a). 20

2. A sail according to claim 1, wherein an inflatable compartment (41) is provided on each side of the inner membrane (1). 25

3. A sail according to claim 1 or claim 2, in which the inflatable compartment (41) comprises a pocket (41) containing an inflatable tube (45).

4. A sail according to any one of claims 1 to 3, in which 3 0 the width of the inflatable compartment (41) is approximately 30% of the chord of the sail.

5. A sail according to any one of claims 1 to 4, in which the inflatable compartment (41) is arranged to increase the 35 thickness of the sail on the windward side.

6. A sail according to any one of claims l to 5, in which NEW ZcALAND 2 2 JAN 1997 PATENT OFFICE 294656 w 19 the inflatable compartment (41) is arranged to modify the shape of the leeward pocket (4).

7. A sail according to any one of claims 1 to 6, in which 5 the inflatable compartment (41) is arranged to increase the effective length of the windward side of the sail.

8. A sail according to any one of claims 1 to 7, in which the inflatable compartment (41) includes a sealable vent 10 means.

9. A sail as claimed in any one of claims 1 to 8, in which the inner membrane (1) is substantially impermeable to air and each pocket (4,5) includes a vent (6,9) arranged 15 to permit the flow of air into and out of the pocket.

10. A sail according to any one of claims 1 to 8, in which the outer membranes (2,3) are substantially impermeable to air and the inner membrane (1) is adapted to allow air to 20 flow through it from one pocket (4,5) to the other.

11. A method of shaping a sail comprising an inner membrane (1) and, on each side thereof, an outer membrane (2,3) attached the inner membrane (1), each outer membrane 25 (2,3) forming, with the inner membrane (1), an inflatable pocket (4,5), the sail further including a inflatable compartment (41) adjacent the luff edge (23A), the method comprising inflating the inflatable compartment (41) before use to increase the thickness of the sail adjacent the luff 30 edge (23A) and inflating the pocket (5) on the leeward side of the sail by means of the pressure difference across the sail to increase the thickness of the sail.

12. A method according to claim 11, in which each pocket 3 5 (4,5) includes an air vent (6,9), and the leeward pocket (5) is inflated by air flowing into the pocket through the air vent (6) owing to the pressure difference created 294656 20 during use across the sail.

13. A method according to claim 11, in which the outer membranes (2,3) of the sail are substantially impermeable 5 to air and the inner membrane (1) is adapted to allow air to flow through it from one pocket (4,5) to the other, the sail being inflated before use and air being transferred during use from the windward pocket (4) to the leeward pocket (5) owing to the pressure difference created across 10 the sail. END OF CLAIMS