It is well established that the aerodynamic efficiency of a sail assembly for sail craft generally is dependent on the airfoil section formed by the sail. An airfoil section, as viewed in profile, is determined by the camber, thickness and thickness distribution. Many known arrangements of sail assembly are constructed using a single layer of material allowing for camber but not

thickness of the airfoil section. There are a number of known arrangements for aerodynamically shaping the mast and the immediately adjacent section of the sail, which provide for some degree of thickness but not in the most efficient distribution. Other known arrangements provide for thickness in an efficient distribution but generally are complicated structurally and difficult in use.

It is an object of the present invention to provide a sail assembly for a sail craft which alleviates the disadvantages of known sail craft described in the preceding paragraph.

According to the present invention there is provided a sail assembly for a sail craft having a mast, comprising; (a) a main load-bearing membrane adapted to be coupled to the mast;

(b) a sail of flexible material adapted to cover at least partially both sides of the main load-bearing membrane; • and (c) a means to support the sail in an aerodynamic profile as described herein which is responsive to the direction and speed of ind which contacts the sail.

The term "aerodynamic profile" is understood herein to mean a profile which encourages airfoil lift and laminar rather than turbulent air flow past the sail assembly.

It is preferred that the support means for the sail comprises a framework of battens connected to the sail to support the sail in an elongated tear-drop like profile in transverse section with the mast forming the nose of the tear-drop when there is no wind contacting the sail assembly, whereby in use when wind contacts the sail the weather side of the sail deforms towards the main load-bearing membrane to define a generally flat configuration and the lee side of the sail undergoes a

deformation away from the load-bearing membrane to define a generally arcuate configuration thereby to form the aerodynamic profile.

It is preferred that the framework comprises, a plurality of pairs of vertically spaced battens connected to the sail with one batten of each pair on the lee side of the sail and the other batten of each pair on the weather side of the sail.

It is preferred that the framework further comprises at least one spar interconnecting the battens in one or more pairs of battens.

The battens may be constructed in a conventional manner of a flexible material, such as fibreglass. Alternatively, each batten may comprise a plurality of sections coupled together, for example by hinges, so that, in use, when the batten is on the weather side of the sail the batten is generally flat and when the batten is on the lee side of the sail the batten is generally arcuate, thereby to conform closely to the aerodynamic profile of the sail. It is particularly preferred that the sections are curved so that when the batten is on the lee side of the sail the batten conforms closely to the arcuate configuration which defines the aerodynamic profile on the lee side of the sail. It is also particularly preferred that, whilst the coupling means allows movement of the batten from the generally flat configuration on the weather side of the sail to the generally arcuate configuration on the lee-side of the sail and vice versa, the coupling means does not allow further outward movement of the batten beyond the arcuate configuration which defines the aerodynamic profile on the lee side of the sail.

It is preferred that each spar comprises, a length of tubing passing through openings in both sides of the sail and the main load-bearing membrane and

extending beyond the batten on each side, and an elastic cord extending through the tubing and connected to the respective batten on each side of the sail to tension the battens into the aerodynamic profile. It is preferred that the support means for the sail further comprises, means to couple each trailing edge of the sail to a respective side of the main load-bearing membrane for movement toward and away from the mast. In one arrangement the coupling means comprises a plurality of vertically spaced tracks connected to the main load-bearing membrane and a plurality of corresponding runners connected to the sail and coupled _^ to the tracks for sliding movement therealong. In another preferred arrangement the coupling means comprises a plurality of vertically spaced pulleys connected to the main load-bearing membrane and to the sail.

It is preferred that the sail covers at least 40%, and typically at least 50%, of both sides of the main load-bearing membrane. With such an arrangement it is preferred that at least the section of the main load—bearing membrane not covered by the sail comprises flexible material thereby to define an additional sail area of the sail assembly. The section of the main load-bearing membrane covered by the sail may comprise a framework of battens . In one particularly preferred arrangement the said section also includes a flexible sail material covering the framework. In one preferred arrangement the sail craft is a sail board comprising a buoyant board and a mast pivotally coupled to the board.

According to the present invention there is also provided a sail craft comprising: (a) a buoyant member;

(b) a mast extending from the buoyant member; and

(c) the sail assembly according to the present invention as described herein. The following is a description of a preferred embodiment of a sail assembly formed in accordance with the present invention as shown in the accompanying drawings, in which:

Figure 1 is a perspective view of the sail assembly coupled to a sailboard;

Figure 2 is a transverse section along the line A-A in Figure 1;

Figures 3 and 4 are transverse sections along the line A-A in Figure 1 which illustrate the effect of wind moving towards the sail assembly in the direction of the arrows X and Y;

Figure 5 is a side view of an alternative construction of a batten for the sail assembly when the batten is on the lee side of the sail; Figure 6 is a detailed view of the hinge connection of a part of the batten shown in Figure 5;

Figure 7 is a detailed view of the hinge connection of a part of the batten shown in Figure 5 when the batten is on the weather side of the sail; Figure 8 is a transverse section along the line

A-A in Figure 1 with an alternative construction of spar interconnecting the battens; and

Figure 9 is a section along the line B-B in Figure 8. The sail board shown in Figure 1 comprises a buoyant board 3, in the form of a surfboard, a mast 5 pivotally coupled to the board 3 and extending generally upwardly therefrom, a sail assembly generally identified by the number 7, and a boom 9 coupled to the mast 5. In

the embodiment shown in Figure 1 the boom 9 is a wishbone boom having two arms on opposite sides of the sail assembly 7.

The sail assembly 7 comprises a load-bearing membrane 15 coupled to the mast 5, a sail 17 fixed to the mast 5 and extending at least partially over both sides of the load-bearing membrane 15 and terminating in trailing edges 33, and a framework to support the sail 17. The load-bearing membrane 15 comprises an area

21 of sail material and a plurality of battens 23 to stiffen the sail material. The load-bearing membrane 15 is coupled to the mast 5 in any conventional manner.

In an alternative embodiment, not shown, the area 21 of sail material comprises only that area of the load-bearing membrane 15 which is not covered by the sail 17. The remaining area of the load-bearing membrane 15 comprises a framework of battens.

The sail 17 is formed from any conventional sail material. In the embodiment shown in the figures the sail 17 extends over approximately 80% of both sides of the load-bearing membrane 15. It should be appreciated that in other embodiments the area of coverage may be as low as 20 to 30%. It should also be appreciated that in other embodiments the sail 17 may extend beyond the load-bearing membrane 15.

The framework supports the sail 17 in an elongated tear-drop like profile, in transverse section, with the mast 5 defining the nose of the tear drop, when there is no wind acting on the sail 17. The framework comprises a plurality of pairs of vertically spaced battens 29a, 29b formed from flexible material, such as fibreglass, connected to the sail 17 and a plurality of spars 31 extending between the battens 29a, 29b in each pair of battens. In order to simplify the drawings only

one spar 31 is shown interconnecting each pair of battens. However, it can readily be appreciated that any suitable number of spars could be used.

The sail assembly further comprises means to couple the trailing edges 33 of the sail 17 to the load-bearing membrane 15. The coupling means is shown schematically in Figure 2 and comprises a plurality of vertically spaced tracks 37 connected to the load-bearing membrane 15 and a plurality of corresponding runners 39 connected to the sail movement therealong toward and away from the mast 5.

The framework supports the sail 17 so that the sail is responsive to the direction and speed of wind which in use contacts the sail 17 and can form an aerodynamic profile. This feature of the sail assembly is best illustrated in Figures 3 and 4.

In Figure 3, wind moving towards the sail assembly in the direction indicated by the arrows X contacts and deforms the weather side 41 of the sail 17 against the load-bearing membrane 15. The load-bearing membrane 15 shown in Figure 3 has a slightly curved profile in response to the wind, and such a profile is consistent with the profile assumed by a conventional sailboard under the influence of the wind. The inward deformation of the weather side 41 of the sail 17 causes a corresponding outward deformation of the lee side 43 of the sail 17 since the battens 29a, 29b on the weather and lee sides 41, 43 of the sail 17 are interconnected by the spars 31. Such movement of the sail 17 is also contributed to by the fact that a partial vacuum is created on the lee side 43 of the sail 17. As a consequence the sail 17 adopts a wing-shaped aerodynamic profile and the air flow over the weather and lee sides 41, 43 of the sail 17 will be essentially laminar.

It can be appreciated that the extent of the deformation and thus the form of the wing will depend on the speed and direction of the wind. Thus, for example, if the wind changes direction and moves towards the sail assembly in the direction of the arrows Y in Figure 4 there will be a re-positioning of the sail 17 as shown. In the embodiment described above in relation to Figures 1 to 4 the battens 29a, 29b are formed from a continuous length of flexible material, such as fibreglass. In an alternative embodiment, shown in Figures 5 to 7, each batten 29a, 29b is formed from a plurality of sections 61 of flexible material, such as fibreglass, which are interconnected by hinges 63. Conveniently, the hinges 63 may be formed from plastics material, for example by moulding, and are inserted onto the ends of adjacent ones of the sections 61.

The sections 61 of the battens 29a, 29b are curved so that-when in use the battens on the lee side 43 of the sail 17 define .the arcuate profile as is shown in Figures 5 and 6. However, nevertheless, the arrangement of the sections 61 is such that when in use the sail 17 is on the weather side 41 the battens define the generally flat profile shown in Figure 7. As a consequence, the combined profile on the weather and lee sides of the sail generally is the wing-shaped aerodynamic profile shown in Figures 3 and 4.

The hinges 63 are formed to allow relative movement of the sections 61 between the positions shown in Figures 6 and 7. The hinges 63 are also formed to prevent further relative movement of the sections 61 beyond the arcuate profile shown in Figures 5 and 6 towards the lee side 43. Thus, the hinges 63 are formed to prevent the sail 17 bellowing outwardly beyond the optimum aerodynamic profile on the lee side 43 of the sail 17.

It is envisaged that the construction of the battens 29a, 29b shown in Figures 5 to 7 may obviate altogether the need for spars 31 to be used to interconnect the pairs of battens 29a, 29b. Nevertheless, it is considered that spars 31 interconnecting the lowest pair of battens 29a, 29b is probably desirable from the view point of maintaining stability and form of the sail 17.

In the embodiment described above in relation to Figures 1 to 4 the battens 29a, 29b are interconnected by spars 31 so that, for example, movement or deformation of the lee side 43 of the sail 17 causes a direct corresponding movement or deformation of the weather side 41 of the sail 17. In an alternative embodiment, shown in Figures

8 and 9, the spars 31 are formed from tubular elements 71 which extend through openings in the load bearing membrane 15 and the sail 17 and extend beyond the sail 17 so that the open ends 73 of the tubular elements 71 are positioned outwardly of the battens 29a, 29b. The spars 31 further comprise elastic cords 75 extending through the tubular elements 71 and connected to the respective battens 29a, 29b. The arrangement is such that in use the elastic cords 75 will allow some independence of movement between the lee side 43 and the weather side 41 of the sail 17.

Many modifications to the preferred embodiments described herein may be made without departing from the spirit and scope of the invention. In this regard, it can be. readily appreciated that whilst the preferred embodiments are described in relation to a sail board the invention is not so limited and is equally applicable to other sail craft such as yachts. Furthermore, whilst in the preferred embodiments the trailing edges 33 of the sail 17 are coupled to and not fixed directly to the load bearing membrane 15, the

present invention is not so limited and extends to such arrangements, although one requirement for such arrangements is that the trailing edges 33 of the sail 17 are fixed directly to the load bearing membrane 15 so that the sail can fold or otherwise accommodate changes in the sail 17 as a consequence of movement of the sail 17 in response to wind.