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Background of the Invention

The vast majority of sail systems employed in the exploding field of boardsailing are characterized by two booms, one on each side of the sail. Such booms are often termed "a wishbone boom". The construction and mounting of the wishbone boom and associated sail are such that the lee

(downwind) part of the wishbone boom presses into, creases, and distorts the shape of the sail. This creasing and distortion causes air to flow such that i*t has a crosswise (up and down) component. The path of the air flowing along the sail is thereby lengthened, which creates- extra drag without at the same time creating extra lift. Sail efficiency is therefore reduced.

Referring particularly to the leeside (downwind side) of the sail, it is emphasized that a very large portion of total sail force is created by the lower pressure on such leeside, thus the lee is extremely important and anything that disturbs airflow along such side creates a dispro¬ portionately large loss of sail efficiency. Thus, the distortion caused by the lee component of the wishbone boom, as it contacts the sail, has a distinct adverse affect on sail efficiency.

It is further emphasized that the boom has a wake, namely a turbulent area behind its leeside. Such turbulent area extends downstream for several diameters of the boom. When a boom component is placed close to the sail on the leeside thereof, as is the situation relative to the lee component of a wishbone boom, there is turbulence and "consequent flow separation immediately behind the boom. Such separation combines with air separation from the lee¬ side of the sail caused by the distorted shape thereof. The separation is greater when -the sail is tilted, as is conventionally done during boardsailing, in that the boom

component (on the leeside) presents a maximum surface area across the airflow.

It is also pointed out that the boom itself creates drag, and that anything that reduces the overall size of the boom reduces such drag.

Summary of the Invention

In accordance with one important aspect of the present invention, there is no boom component on the leeside of the sail, there being instead only a single boom disposed on the windward side. The windward side is the most beneficial place to locate a boom, one reason being that the wake of the boom will not create flow separation (the wake being blown into the sail by the oncoming wind) . The single boom on the windward side does not effect any distortion of the sail but instead permits the sail to achieve a highly efficient, aerodynamic shape—a shape characterized by the complete absence of the severe crease caused by the lee component of a wishbone boom. In accordance with another major aspect of the present invention, the boom is shifted through the sail when the boat comes about, and relationships are created whereby there is no leakage of air through the sail in the region through which the boom shifts, or at any other region. Stated more definitely, the boom is caused to rotate through the sail, the rotation being about pivot points located near the ends of a curved boom. On both tacks, the boom curves toward the operator, who stands on the board and on the windward side of the boom. In accordance with another major aspect of the present invention, the sail has upper and lower components, and the foot of the upper laps over the head of the lower on the windward (upwind) side. Thus, particularly since the mast is conventionally inclined toward the wind during normal sailboarding operations, the wind flows down the upper component of the sail and then onto the lower component

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thereof without leaking therebetween. The no-leakage rela¬ tionship, with the foot of the upper sail component to wind¬ ward of the head of the lower sail component, is effected automatically on both tacks, in response to rotation of the boom.

In one embodiment, the boom is straight instead of curved, and need not rotate about its axis. Thus, reduced costs of manufacture may be achieved when the cost factor is paramount. Means, in the form of a generally vertical batten in the lower sail, are provided in order to insure that the upper edge of such sail will not droop when the winds are light.

The lower edge region of the upper sail component is curved (or otherwise extended) a substantial distance down¬ wardly, the distance being sufficient that such lower edge region is unsupported by outhaul tension, and therefore blows against the upper edge region of the lower sail component, for enhanced sealing. The invention relates further to the combination of the sail system with the board, to the method of sailing, to the method of rotating the boom and of achieving the no-leakage lapped relationship on both tacks, and to other important aspects.

Brief Description of the Drawings

FIG. 1 is a perspective view of a sailboard sailing on a port tack, the wind being directed towards the rear and from the port side of the boat; FIG. 2 is an enlarged, fragmentary elevational view of the intermediate sail portion, the view being seen from the side opposite that illustrated in FIG. 1 (namely, the view being the same as that seen by the operator shown in FIG.

1); FIGS. 3 through 7, inclusive, are fragmentary sectional views illustrating the boom and the overlapped sail portions, and showing how rotation of the boom creates the desired lapped relationship on either tack, each of the

views being what would be seen by a viewer looking rear- wardly in FIG. 1 from a position just in advance of the left hand of the illustrated operator;

FIGS. 8 through 12, inclusive, are views corresponding, respectively, to FIGS. 3 through 7, such FIGS. 8 through 12 being what would be seen when looking downwardly from a position somewhat above the boom,-

FIG. 13 is an isometric, fragmentary view illustrating only the forward end of the boom, the associated portion of the mast, and the associated handle;

FIG. 14 corresponds to FIG. 13, but shows the boom in a different rotated position and also illustrates adjacent sealing portions of the sail system;

FIG. 15 is a horizontal, sectional view on line 15-15 of FIG. 14;

FIG. 16 is a fragmentary, isometric view showing the outhaul means at the outer end of the boom;

FIG. 17 is a horizontal, sectional view on line 17-17 of FIG. 16; FIGS. 18 through 23, inclusive, illustrate an embodi¬ ment of the invention wherein there are connector means extending between the boom and the edge of one of the sail elements;

FIGS. 24 through 27, inclusive, illustrate an embodi- ment wherein the boom is straight, and a batten is mounted in the lower sail component; and

FIG. 28 illustrates an outhaul means which permits the upper and lower sail components to be so adjusted, relative to each other, as to achieve the optimum sealing relation- ship.

Detailed description of the Preferred Embodiments

Referring to FIG. 1, a sailboard is shown in operating condition,, on a port tack, the operator being in a balanced condition relative to the sail system. In other words, the operator is holding up the sail, while the sail is holding up the operator. The feet of the operator are then resting

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on the port side of the board 11 (hull) , which board is prevented from moving sideways by a daggerboard the upper portion of which is shown at 12.

The sail system comprises a mast 13, the lower end of which is associated with board 11 by a universal joint 14. Joint 14 permits the mast to rotate about its longitudinal axis and, additionally, permits the sail system to drop into the water so as to prevent propulsion by the wind when the operator falls off. A line, not shown, is provided to permit the operator to retrieve the sail system from the water in order to start sailing again.

A single boom, numbered 15, is connected at its forward end to mast 13 at an intermediate elevation on the mast. Boom 15 is curved, along at least a substantial portion of the length. As shown in FIG. 8, the boom has a large portion wh-ich is generally arcuate, such arcuate portion merging with a relatively straight portion toward the boom end remote from mast 13.. Boom 15 is strong, but light in weight, being preferably tubular aluminum of suitable wall thickness.

Sail means 17 are associated with mast 13 and with boom 15.. The boom and the sail means are so constructed, and so associated with each other, that the boom may shift through the sail means from one side of the boat to the other side thereof. Furthermore, there is no leakage of air through the sail means while the sailboard is in sailing condition on any tack.

Sail means 17 comprises an upper sail 18 and lower sail 19, such sails overlapping each other at an elevation generally the same as that of the ends of boom 15. The luff of each sail 18 and 19 is a sleeve which is mounted around mast 13, whereas the leech of each sail is connected to the outer end of boom 15. There will be described the apparatus and method for shifting boom 15 through sail means 17, namely between the lower edge (foot) of upper sail 18 and the upper edge (head) of lower sail 19, all without permit¬ ting leakage of wind between the lapped foot and head of the sails while sailing progresses.

Boom 15 is rotatably associated with the mast 13 and is also rotatably associated with the sail portions (leech portions) remote from the mast. Thus, the boom may rotate,- about an axis extending transverse to the mast and generally through or adjacent the ends of the boom, through an angle of at least 180 degrees and—very preferably—270 degrees or more. Furthermore, the foot (lower edge) of upper sail 18, which foot is numbered 21, extends downwardly in lapped relationship relative to the head of lower sail 19, such head being numbered 22. Foot 21 is curved, as best shown in FIG. 2, the curvature corresponding generally to that of boom 15 and being such that the boom may swing below foot 21 while the sailboard is coming about. As will be described subsequently, the amount of downward curving of foot 21 is sufficient to achieve an enhanced sealing relationship.

The illustrated foot 21 is shown as being stiffened by a batten 23 (FIGS. 3-5) which is curved longitudinally of the edge 21, and is sufficiently laterally flexible to permit the associated sail to bend in response to wind pressure. The foot may be otherwise suitably stiffened, if desired, as by cross-sewing of relatively stiff material or folded material. Such stiffening of foot 21 is not essential, but minimizes the tendency of the foot to flap during coming about. Head 22 is preferably straight, not curved, as viewed in elevation (FIG. 2) , it being understood that the head 22 curves laterally in response to the billowing action of the wind as shown in FIG. 1.

Particular reference will next be made to FIG. 1 and FIGS. 3 through 12, it being emphasized that FIG. 3 corresponds to FIG. 8, FIG. 4 to FIG. 9, and so forth.

When the sailboard is sailing on the port tack, as shown in FIG. 1, the left hand 25 of the operator is forward and the right hand 26 is rearward. When the operator desires to come about, he manipulates the sail system in such a manner as to turn the board 11 into the wind, namely to port. As the boat turns into the wind, the operator must

stand erect and support the mast 13 substantially vertical or else it will fall over. The operator is then standing substantially vertically on board 11, in balanced condition, instead of relying on the sail to support him. To support the mast, the operator releases his left hand 25 from the boom and grasps the mast" with such left hand at a handle means described hereinafter. Substantially simultaneously, as soon as the sails 18 and 19 start to luff, the operator uses his right hand 26 to lift (pivot) boom 15 upwardly (FIGS. 4-5 and 9-10) , in a sharp strong motion by which the curved region of the boom 15 strikes upper sail 18 and pushes it as shown in FIGS. 5 and 10.

It is to be remembered that, during this operation, the boat may be continuing to turn into the wind, so that the wind will "back" the sail, attempting to push the sail portions toward the port side of the boat. In fact, the above-described rotation of the boom causes backing of the sail, since the boom is pushed away from the operator.

However, because the boom is then engaged with upper sail 18, and is pushing such upper sail to starboard, the backing of the sail does not permit the lower region of upper sail

18 to move to port. This is not true of lower sail 19, which sags and is then blown to port, beneath the foot 21 of upper sail 18, as shown in-FIG. 5. The head of the lower sail thus shifts below the foot of the upper sail, and moves beyond the upper sail in the port direction.

After boom 15 has pivoted upwardly sufficiently far and also around to the FIG. 5 position and somewhat therepast, the operator releases the boom so that it drops by gravity downwardly to the position of FIGS. 6 and 11. In other words, the boom is hanging in a generally vertical plane, being suspended from its ends. When thus hanging, the boom prevents the lower sail from flapping back and getting on the wrong side of the upper sail. Because of the relation- ship of the shape of boom 15 to that of foot 21, the boom is hanging below the foot 21 as shown in FIG. 6.

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The operator next releases his left hand 25 from the mast and grasps the mast with his right hand 26, while walk¬ ing around the mast forwardly thereof. He then grasps the hanging boom 15 (FIGS. 6 and 11) ^" with his left hand 25, and _ pivots the boom upwardly from the FIG. 6 to the FIG. 7 position, while also releasing his right hand from the mast and grasping the boom to ^" achieve the position shown in FIG. 12. The operator 10 then pulls the boom 15 toward himself with a hard, quick pull, which fills the sails to thus achieve a fast start on the starboard tack.

The described operation occurs very rapidly, just as fast as the operator can move his hands and walk around the mast, at the appropriate ^' time.while the sails are luffing. Because of the factors described at the first portion of this specification, the sail means assumes a single, *un- creased, aerodynamic shape and this, in combination with the fact that there is only one boom and it is on the windward side of the sail, creates high speed of the board through the water. Expert sailors can vary the above-described procedure as desired. For example, it is often possible to come about without ever grasping the mast. The expert (when the boat and mast are in proper positions) rapidly "throws" the boom up and over, pirouettes about the mast, and grabs the boom on the other side of the sail.

Very importantly, there is no leakage through the region between the sails, on either tack. On both tacks, because of the above-described manually-operated means (the boom) , and method, the foot of upper sail 18 is on the wind- ward side of the head of lower sail 19, and the foot and head are lapped relative to each other. Thus, and because mast 13 is tilted toward the operator, there is a downward compo¬ nent of wind along the sail which flows from the upper sail 18 and then spills downwardly over the foot 21 onto lower sail 19. To leak between the sails, the wind would have to reverse direction and flow upwardly, and there is no tendency for this to occur. (If the mast were tilted in the

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wrong direction, namely away from the operator, it would be desirable to spill the wind from the sail since the operator would be tending to fall over forward.) Furthermore, the sails are so cut that there is a snug, engaged relationship between the lapped regions of the upper and lower sails. Preferably, the curvature (in a generally horizontal plane) of the foot of upper sail 21 is slightly greater than that of the head of lower sail 19, creating a tendency which causes the upper sail to press itself against the lower sail at the lapped regions for maximized prevention of leakage between the sails.

Referring again to FIG. 2, it is pointed out that the edge 21 curves down so far that—if the head of sail 19 were not behind it, edge 21 would flap in the wind. Thus, the curvature of edge 21 is preferably much greater than that employed on a conventional sail. In the present system, the downwardly-extended foot region of upper sail 18 is blown, by the wind, into close surface engagement with the head of sail 19, thus enhancing the seal therebetween. The boat will still sail, though greatly less satis¬ factorily, if the sail edges are in incorrect positions (namely, with the upper edge of the lower sail nearest the operator.)

There will next be described the preferred form of apparatus for rotatably associating boom 15 with mast 13, particular reference- being made to FIGS. 13 through 15.

This comprises a combination bearing, connector, and handle element, numbered 27. The illustrated handle is a ring 28 which is mounted around mast 13, one inner portion of the ring being tightly clamped against the mast by means of a ϋ- shaped element 29 held in place by screws 30 (FIG. 15)

■which extends through the ring. Thus, in the illustrated form there is such a tight clamping of the element 27 to mast 13 that movement of boom 15 from side to side effects rotation of the mast 13 about its longitudinal axis.

Ring 28 connects through a relatively thin (in vertical dimension) neck portion 31 to a bearing sleeve or socket 32.

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As ^" shown in FIG. 15, sleeve 32 contains a bushing 33 having a flange 34 at the end thereof remote from the mast. The material of the bushing and flange are suitably selected to achieve the desired amount of friction, and may vary from Teflon to nylon or other suitable substance. A collar 36, which is fixedly mounted on boom 15, bears against flange 34 to prevent penetration of the boom 15 to the bottom of sleeve or socket 32. There is thus present a combined rotational and thrust bearing, in combination with the clamp means and the handle.

It is a feature of ^' the described apparatus that, merely by loosening screws 30, the elevation of the boom relative to mast 13 may be adjusted. This is done in order to compen¬ sate for use of the craft by persons of different height, so that the boom 15 will be at the most comfortable elevation for the particular operator. The upper and lower sails 18 and 19 are moved upwardly and downwardly with the boom, there being adjustable apparatus (not shown) for connecting the upper end of the luff of the sail 18 to the top of mast 14, and there being a downhaul for connecting the lower end of the luff of sail 19 to' the bottom of the mast.

Means are provided to prevent leakage of air between sails 18 and 19 at the region closely adjacent mast 13. Referring particularly to FIG. 14, these comprise sealing elements 37 and 38 respectively associated with the luff and adjacent portions of upper and lower portions of sails 18 and 19. Elements 37 and 38 are formed, preferably, of the type of foam material employed for wet suits, and have outwardly-extending, abutting flange portions which fit snugly against the upper and lower surfaces of the neck portion 31 of element 27. As shown in FIG. 14, seals are

-.thereby created preventing air leakage between the sails relatively adjacent the mast. When the sailboard comes about, the neck 31 merely slides between the sealing elements 37 from one side of the sails to the other, the flange portions of the sealing elements permitting such sliding operation.

It is to be understood that means may be provided to effect rotation of the boom 15, about the axis extending generally through the ends thereof, by means of a crank or handle disposed forwardly of the mast and connected to the boom 15. However, it is presently preferred that no such crank or handle be provided, and that rotation of the boom be effected by direct engagement therewith by the hands of the operator 10.

Referring next to FIGS. 16 and 17, there is shown the' outhaul means 40 for connecting the leeches of sails 18 and 19 to the outer end of boom 15. Such apparatus has the important advantage of permitting a change in the degree of tension on each sail, without at the same time permitting any sail to wrap around the boom. An oversize ring 41 is mounted loosely around boom 15 and has ears or lugs 42 through which suitable fastening means 43 are p ^' ro ided to secure such ring to the leeches of sails 18 and 19. U-shaped connectors 44 and 45 are provided on opposite sides of ring 41, each spaced 90 degrees from the ears 42.

An elongated bushing 46, which may be formed of the same material as that of bushing 33, is extended into the outer end of boom 15. The bushing has a flange 47 at its outer end, which flange is engaged by a collar 48 which is fixedly secured to a journal element 49 telescoped into the bushing. Outwardly from collar 48, journal element 49 has two pulleys 51 and 52 at its extreme end, and has a cleat 53 on one side thereof.

A line 54 is tied at one end to connector 44, then extended around pulley 52 to connector 45, then extended around connector 45 back around pulley 51, then extended

-around connector 44 to the cleat 53. The described relationship permits a powerful outhaul operation to be effected, securing ring 41 in any desired position along the boom, in accordance with the wind and the desires of the operator. It is to be understood that the outhaul is not tightened sufficiently far to prevent rotation of boom 15

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about its axis, during coming about, and that the curvature of boom 15 is so related to the characteristics of the sail that there will always be sufficient clearance to permit rotation of the boom through the sail. In an extreme condition, when there is a very tight condition at the outhaul 40, and the boom 15 has a relatively large curvature, the line 54 may be released or loosened temporarily when the board sailer comes about.

The line 54 may be extended through the boom, longi- tudinally thereof, to a cleat located near the mast. This facilitates operation of the outhaul.

The described outhaul 40 has the distinct advantage that the boom 15 rotates relative to the sails, without causing the sails to wrap therearound. The journal element 49 tends to remain relatively stationary, not rotate, being held against rotation to a substantial degree by the tension of line 54 and by the fact that there are two line portions on each side of the outhaul means.

Referring next to FIG. 28 (last sheet of drawings) , the construction of the outhaul is identical to that described in FIGS. 16 and 17, except as follows. Each fastening means 43a is not connected directly to ring 41a, there instead being interposed separate slotted connectors 60 and 61. Threaded fasteners 62 and 63, having suitable lockwashers, are employed to connect connectors 60 and 61 to the ring ^• 41a. Thus, by loosening (and then re-tightening) one or both fasteners 62 and 63, the positions of the two fastening means 43a, relative to each other, may be adjusted longi¬ tudinally of the boom. Such adjustment is effected to cause optimized, surface, sealing engagement between the head and foot regions of the sails when line 54 is tightened and the -craft is sailing.

There have been described above various surprising and unexpected results, including (without limitation) the automatic prevention of leakage between the sails, the cooperation between the boom and sails in such manner as to reverse the position of the boom while achieving correct

positioning of the sails, etc. A further surprising and unexpected result is that the frictional resistance of the boom 15 to rotation about the axis through the ends thereof varies in proportion to the force of the wind and the position of the sail. Thus, when the boat is coming about, and the sails are luffing, there is no tension in the sails and consequently no compressive (column) force .on the boom and associated bearing means. The collar 48 is then not pressing against flange 47 (FIG. 17) nor is collar 36 pressing against flange 34 (FIG. 15) . Thus, the boom 15 can rotate with the desired degree of freedom, as determined by the characteristics of the material forming the bushings surrounding the boom ends. However, when the boat is sailing on either tack, and the wind is strong, the tension in the sails creates a very substantial compressive (column) load -in the boom 15, forcing the collars against the associated flanges and thus creating substantially increased friction tending to prevent rotation of the boom about its axis extending through the ends of the boom. This prevents the boom from flapping about in an undesired manner, particularly when the wind is strong. Embodiment of Figures 18 through 23

In accordance with the present embodiment, fail-safe means are provided to ensure that the edges of the sails are in the correct, lapped relationship at all times, even in the event of some highly unusual wind, wave, or other conditions which might otherwise tend to disturb the correct relationship.

Means are provided to connect the boom 15 ^" to the edge of one of the sails, namely to the lower edge (foot) of upper sail 18. In the illustrated embodiment, there are

-four straps, each an endless loop, and each numbered 56.

The lengths of the straps depend upon the positions where they are located along the boom, and means (not shown) may be provided to prevent any movement, longitudinally of the boom, of the strap ends contacting the same.

Referring to FIG. 20, which corresponds to FIG. 3 except that one of the straps 56 is shown, the relationship is shown during which the board is sailing on the port tack. When the operator 10 decides to come about, he follows the same procedure described above relative to FIGS. 3 through 12, inclusive, starting by pivoting boom 15 upwardly until it engages upper sail 18. This relationship is shown in FIG. 21. Then, when the boom drops downwardly, as indicated in FIG. 22, the straps 56 provide additional barriers preventing the upper edge 22 (head) of lower sail 19 from moving backward beneath the upper sail. Thus, both during coming about and when the board is sailing on either tack (the port tack represented by FIG. 20, or the starboard tack represented by FIG. 23) , the upper edge (head) 22 of lower sail 19 will not be able to move upwardly on the wrong side (namely, the windward or upwind side) of upper sail 18.

Embodiment of Figures 24 through 27

Except as specifically stated, the embodiment of FIGS. 24 through 27 is identical to that described relative to FIGS. 1 through 17 and 28.

The boom 70 of the present embodiment is straight, not curved. The use of a straight boom reduces costs in that it eliminates the need for bearings. It also reduces, soroe- what, the amount of aluminum required for the boom. A further benefit is achieved, in light winds, in that ^' the operator can hold the sail somewhat closer to his body with a straight boom than he can with a curved boom, thus permit¬ ting the boat to sail slightly closer to the wind. Further- more, when the boom is straight, it is not necessary to teach a beginner how to rotate the boom.

On the other hand, the advantages of a curved, rotat- able boom are great. Primarily, the advantage is that control of overlapping is effected positively and auto- matically, even when the sails are flapping (which often occurs when a beginner is sailing the boat) . An expert is far better able, than is a beginner, to sail with a straight

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boom since the expert can often compensate for the tendency of the sails to flap during coming about. It will be recalled that, as described relative to FIGS. 3 through 7, that the downwardly-hanging boom (FIG. 6) prevents the upper region of lower sail 19 from flapping back to incorrect position.

In heavy winds, the curved boom permits the operator to extend his body further away from the sail, thus increasing the righting torque necessary to compensate for the heavy wind.

The degree of curvature of the boom may be adjusted as desired, and (particularly when cost factors are paramount) the boom may be straight instead of curved. It is also

-possible to employ two booms relative to the same set of sails, one boom for heavy winds and one for light winds.

The booms are readily interchangable.

Referring to FIG. 24, and to FIG. 25, which is a horizontal section—looking downwardly—taken on line 25-25 of FIG. 24, it will be seen that the boat is illustrated as being on the port tack. The operator (not shown) is stand¬ ing on the board, on the side of straight boom 70 remote from the sail components 18 and 19. As is greatly prefer¬ red, the foot of upper sail 18 is closer to the operator than is the head of lower sail 19. The described relation- ship is also shown in FIG. 26, which is a vertical section on line 26-26 of FIG. 24, looking rearwardly.

When the operator desires to come about, he manipulates the sail system in such manner as to turn the craft into the wind. He also pushes the boom away from his body, so as to cause backing of the sail. The wind on the back side of the sail will then blow lower sail 19 below boom 70, from the

-position shown in FIG. 26 to that shown in FIG. 27. The upper sail 18 will drag on boom 70, as shown in phantom lines in FIG. 27, and therefore will be blown to the FIG. 27 (solid lines) position after the lower sail 19 will be blown thereto. Therefore, as is desired, the foot of upper sail

18 will be closer to the operator than will the head of

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lower sail 19. However, as indicated above, the straight boom does not provide compensation for flapping of the sails.

The connector means, described relative to the embodi- ment of FIGS. 18 through 23, may also be employed with the embodiment of FIGS. 24 through 27. Thus, the straight boom 70 is connected to the lower edge of upper sail 18, by straps such as are shown at 56 in the embodiment of FIGS. 18 through 23. In FIG. 24, a generally vertical batten 71 is illus¬ trated as mounted in the lower sail 19, extending to the upper edge of such sail. Batten 71 may be provided in the lower sail of all embodiments described in this application. The use of such batten 71 is presently preferred when the winds are light, because it prevents the upper edge (head edge 22, FIG. 2) of lower sail 19 from drooping or sagging during sailing under light-wind conditions. The batten 71 is removable, and may be removed when the winds are medium or heavy. It is pointed out that, when the winds are light and there is no batten, the head edge of lower sail 19 tends to sag, but this condition immediately corrects itself when the winds pick up. The head edge of the lower sail then elevates due to billowing of the lower sail, and the correct surface-engagement seal between ^' the overlapped sail edges is effectively maintained.

It is emphasized that the amount of outhaul tension and the outhaul position are so controlled that the adjacent edges of the upper and lower sails are spaced a substantial distance away from teh boom (even when the boom is straight, as shown in FIG. 25)—free from interference from the boo — so that the composite sail is free to achieve an optimum aerodynamic shape and a high ratio of lift to drag.

The foregoing detailed description is to be clearly understood as given by way of illustration and example only, the spirit and scope of this invention being limited solely by the appended claims.