TECHNICAL FIELD

The present disclosure relates to a hydrofoil arrangement for a watercraft.

More particularly, the present disclosure relates to a hydrofoil arrangement for a watercraft and to a watercraft provided with such a hydrofoil arrangement. The watercraft is typically used for recreational or sporting purposes and can be a stand-up paddleboard, a canoe, a kayak or a sailboard whether powered by human energy or natural energy such as wind or waves, or by light electrical motors.

BACKGROUND It is well known that various “light” watercraft are used for recreational or sporting purposes and that these light watercraft, amongst others, can take the form of stand-up paddleboards, canoes, kayaks, surfboards or sailboards whether powered by human energy, natural energy such as wind or waves, or by light electrical motors.

Single-hulled (monohull) watercraft, such as kayaks and stand-up paddleboards have their main flotation in their centre and are therefore generally more stable along their length but they do display some lateral instability, i.e. the boards are prone to rolling side to side but are not prone to pitching over their bow or stern. Larger boats may be provided with a centreboard to reduce this lateral instability. Typically such monohull watercraft have a large surface contacting the water because they sit or float deeper in the water - this results in a lot of drag when moving through the water, which requires more power to increase their speed of movement. The main drawbacks to monohulls in active small watercraft water sports are excessive board weight, slow speed through the water and poor upwind performance.

Sometimes the monohulled watercraft are provided with outriggers or pontoons attached on either side of the watercraft to improve the stability thereof (similar to providing training-wheels on a bicycle).

One manner to overcome the above problems is to provide multi-hulled watercraft, e.g. catamarans, which have smaller spaced apart hulls of equal size arranged side-by-side parallel to each other. Catamarans have improved lateral stability that is derived from their wide beam. Catamarans also typically have smaller hulls, resulting in smaller displacement and shallower draft than monohulls. The two hulls combined also often have a smaller surface contacting the water - thus experiencing less drag and resulting in higher speeds from the same power input when compared to monohulls.

In larger (boat-sized) catamarans, which are typically used as passenger ferries and normally having a length exceeding about six meters, the hull length is sufficient to provide good pitch stability and thus pitching in these catamarans is not greatly impacted by movement of a person along the length of the catamaran. This is due to a combination of the floatation volume and hull length of such larger catamarans, wherein the longer and wider hulls define a greater floatation volume so that movement of a person on-board has little effect.

However, when smaller catamarans are designed to cater for single person applications, e.g. being used as stand-up paddleboards (sometimes known as standamarans) that are only about three to four meters in length, their operative centre of gravity is raised above the water surface and any fore-aft movement of the person, either by stepping forward or backward along the standamaran or simply leaning forward or backward, causes exaggerated longitudinal pitching of the watercraft. The pitching instability is largely caused by the narrow, pointy hulls of the catamaran providing very little floatation at its bow and stern. So, the slightest weight shift results in a disturbingly high-frequency pitching wobble that is quite disconcerting and causes a significant feeling of instability for the rider of the watercraft. This high-frequency wobble is not experienced in the larger (boat-sized) catamarans discussed above. Typically a small watercraft designer thus has to find a suitable trade-off between making a catamaran having narrower hulls that is faster but is more prone to pitch- wobble, or making a catamaran having wider hulls that is slower but less prone to pitching.

One method of reducing the drag experienced by watercraft is to provide hydrofoils that are configured to lift the watercraft out of the water. These hydrofoils are used on both large and small watercraft. The hydrofoil typically has a strut depending from the watercraft with the strut then supporting a straight or curved transverse wing. The wing has a wingspan dimension (i.e. being its transverse width in the port-starboard direction of the watercraft), a chord dimension (i.e. being its longitudinal length in the bow-stern direction of the watercraft), a thickness in the operative vertical dimension, and a profile curvature designed to generate the lift force.

For example, US 8,820,260 discloses a large wing-shaped SWATH (Small Waterline Area Twin Hull) watercraft that is designed to reach high speeds of over 100 knots (about 185km/h). This watercraft has two sets of fore and aft hydrofoils which are configured to rotate between an active position (shown in its Fig 3a) and an inactive position (shown in its Fig 3c). The hydrofoils are only used at high speeds to reduce drag and are not intended to provide pitching stability. Similarly, US 6,435,123 discloses another large watercraft typically exceeding 100m in length and having a full load displacement of well in excess of 1000 metric tonnes.

In contrast to the above disclosures, small watercraft are often only provided with a single hydrofoil. These hydrofoils are usually based around the traditional ‘aircraft’ foil arrangement consisting of a relatively large forward wing and a smaller tail wing that are connected via a fuselage - see for example US 10,160,525. The main forward wing is designed to provide the requisite lift, while the tail wing is designed to trim and stabilize the front wing but does not provide much lift. Both these wings are situated quite close together, generally only about 600mm apart and a central main vertical mast connects the fuselage to the watercraft (e.g. surfboard). As a result, riders generally stand on the surfboard with their feet straddling but centred around the main front wing. So effectively, there is one point of upward lift which means that there is virtually no pitching stability provided by the wing, rather pitch stability needs to be controlled by the rider maintaining their balance on the board. This layout could essentially be called a single fulcrum hydrofoil because the main wing and tail wing are so close to each other. An experienced rider can voluntarily cause pitching of the watercraft in a ‘sine wave’ motion through the water to ‘pump’ the board forward if no waves or sail/motor power is present. However, this ‘pumping’ can easily result in pitching instability if not controlled - as will often be felt by inexperienced riders. If the pitching is not adequately controlled, the watercraft may nose dive in an instant and the result can be catastrophic - foiling windsurfers travelling at full speed are extremely susceptible to this.

In addition to the above, the shape of the hydrofoil wing also impacts on the stability provided to the watercraft. One type of hydrofoil wing has as constant chord dimension. Another type of hydrofoil wing is shaped similar to an aircraft wing (or fish fins) in that its chord dimension is widest at the middle where the wing attaches to the fuselage with the chord dimension tapering towards the outer tips of the wing.

The above references to the background art and any prior art citations do not constitute an admission the art forms part of the common general knowledge of a person of ordinary skill in the art.

SUMMARY OF THE DISCLOSURE

According to a first aspect of the disclosure, there is provided a hydrofoil arrangement for a multi-hulled recreational or sports watercraft having two lateral hulls arranged substantially parallel to each other, the hydrofoil arrangement comprising a first fore hydrofoil extending fully across between the hulls and being provided at or near to a bow of the watercraft; and a second aft hydrofoil extending fully across between the hulls and being provided at or near to a stern of the watercraft.

During use, the fore hydrofoil is configured to be located and operatively remain fore of the watercraft’s centre of gravity and the aft hydrofoil is configured to be located and operatively remain aft of the watercraft’s centre of gravity. The fore and aft hydrofoils may be spaced apart from each other by a distance equivalent to at least 50% of the watercraft’s length.

The fore and aft hydrofoils may be substantially U-shaped, wherein each fore and aft hydrofoil comprises two opposed struts that respectively depend from each of the hulls, and a wing being supported by the opposed struts. The wing may be configured to be substantially horizontally orientated during use when the watercraft is floating stationary in water.

The fore and aft hydrofoils may be arranged in a canard configuration with the wing of the aft hydrofoil having a larger wingspan than the wing of the fore hydrofoil. In some embodiments the wing of the aft hydrofoil extends outwardly beyond its opposed struts.

Each wing may be spaced below the hulls by a preselected offset distance that is dependent on a chord dimension of the wing. The offset distance is selected to selectively reduce the lift force produced by the hydrofoils during use if the hulls rise too high above the water surface. In one example the offset distance is less than triple the average chord dimension of the wing. In another example the offset distance is less than double the average chord dimension of the wing.

The or each wing may have a central portion located between two opposed outer portions, wherein the central portion has a smaller chord dimension compared to the chord dimension of the two outer portions. The wing may be the wing of the aft hydrofoil. In one example the central portion of the wing is located in between the opposed struts and extends fully across between the struts, with the respective outer portions being located outwardly of the struts. In another example, the central portion of the wing is smaller and located in between the opposed struts but does not extend to the struts, so that the respective outer portions traverse the struts to extend both inwardly and outwardly of the struts. In another example, the central portion of the wing is larger and traverses and extends beyond the opposed struts, whereby the outer portions are located outwardly of and spaced apart from the struts. The central portion may comprise about 50-60% of the wingspan of the wing. In one example the central portion has a chord dimension being about 60-80% of the chord dimension of the outer portions. In one example the total area of the central portion is a factor of about 0.4-0.6 of the total area of the wing.

The fore and aft hydrofoils may be joined to the watercraft with an adjustable mounting permitting an angle-of-attack of each hydrofoil to be selectively adjusted.

The adjustable mounting may include support housings joined to each of the hulls, which housings each define a partially arcuate J-shaped channel, whereby each channel extends linearly for a first part of its length and subsequently extends through an arc for a second part of its length, and projecting members extending laterally from each hydrofoil, each projecting member being configured to be movably located within one of the channels, wherein moving the projecting members along the channels selectively adjusts the angle-of-attack of each hydrofoil. The projecting members may include two pairs of spaced apart pins, one pair of pins extending from each hydrofoil, whereby one pin of each pair of pins is configured to be located within the first part of its channel while the other pin of each pair of pins is configured to be located within either the first part or the second part of its channel.

The adjustable mounting may include a locking member being configured to immovably secure the hydrofoil in position after the selected angle-of-attack of the hydrofoil has been set.

According to a second aspect of the disclosure, there is provided a multi-hulled recreational or sports watercraft hydrofoil arrangement, wherein the watercraft has two lateral hulls arranged substantially parallel to each other, the hydrofoil arrangement comprising a first fore hydrofoil extending fully across between the hulls and being provided at or near to a bow of the watercraft; and a second aft hydrofoil extending fully across between the hulls and being provided at or near to a stern of the watercraft.

The multi-hulled recreational or sports watercraft hydrofoil arrangement may comprise a hydrofoil arrangement according to the first aspect of the disclosure.

According to a third aspect of the disclosure, there is provided a hydrofoil arrangement for a recreational or sports watercraft, the hydrofoil arrangement comprising a wing having a central portion located between two outer portions, wherein the central portion has a smaller chord dimension compared to the chord dimension of the two outer portions.

The hydrofoil arrangement may include a strut being configured to be attached to the watercraft and a fuselage connected to the strut, wherein the wing is joined to the fuselage to extend laterally outwardly on opposed sides of the fuselage.

The central portion may comprise about 50-60% of the wingspan of the wing. In one example the central portion has a chord dimension being about 60-80% of the chord dimension of the outer portions. In one example the total area of the central portion is a factor of about 0.4-0.6 of the total area of the wing.

The wing may define opposed centre-of-area points on the opposite sides of the fuselage that are spaced laterally further away from the fuselage compared to equivalent centre-of-area points of conventional wings having a constant width chord or a tapering chord.

BRIEF DESCRIPTION OF DRAWINGS

The above and other features will become more apparent from the following description with reference to the accompanying schematic drawings. In the drawings, which are given for purpose of illustration only and are not intended to be in any way limiting:

Figure 1 is a perspective view of a watercraft provided with a first embodiment of a hydrofoil arrangement, namely a catamaran style stand-up paddle board;

Figure 2 is a top plan view of the watercraft shown in Figure 1; Figure 3 is a front end view of the watercraft shown in Figure 1 ;

Figures 4a-e are a sectional side views seen along arrows IV-IV in Figure 3 showing an adjustable mounting for attaching the hydrofoil arrangement to the watercraft, with the hydrofoil being shown in various stages of attachment and/or set orientation; Figures 5a is a top plan view of a wing of the hydrofoil arrangement, which wing is shown as the rear wing in Figures 1 to 3;

Figures 5b is a top plan view of a conventional tapering wing known in the prior art and being shown for comparative purposes;

Figures 5c is a top plan view of an alternative shaped wing that is similar to the wing shown in Figure 5a;

Figure 6 a perspective view of a watercraft provided with a second embodiment of a hydrofoil arrangement;

Figure 7 is a perspective view of a third embodiment of a hydrofoil arrangement for a watercraft; and Figure 8 is a front view of a further example of the third embodiment of the hydrofoil arrangement shown in Figure 7.

DETAILED DESCRIPTION

Figures 1 to 3 shows a first embodiment of a watercraft 10 being a multi-hulled recreational or sports watercraft that has two lateral hulls 12, 14 being joined to each other by a central deck 16. The hulls 12, 14 are arranged substantially parallel to each other, wherein each hull 12 has a longitudinal axis 18 leading from its bow 20 to its stern 22. A second embodiment of a watercraft 110 is shown in Figure 6 - the watercraft 110 is substantially similar to the watercraft 10 and therefore the same reference numerals are used to indicate like parts.

The watercraft 10 is provided with a hydrofoil arrangement 24 comprising a first or fore hydrofoil 26 provided at or near to its bow and a second or aft hydrofoil 28 provided at or near to its stern. In the embodiment shown, the fore hydrofoil 26 is located forward of the deck 16 while the aft hydrofoil 28 is located rear of the deck 16. In one example, the fore and aft hydrofoils 26, 28 are spaced apart from each other by a distance equivalent to at least 50% of the full length of the watercraft 10. Typically, in watercraft of <5m in length, the hydrofoils 26, 28 are spaced apart from each other by a distance of about two meters or more.

Providing two spaced apart hydrofoils 26, 28 at either end of the watercraft 10 greatly improves the pitching stability of the watercraft 10. Without the hydrofoils 26, 28, the narrow bow and stern ends of the hull provide little flotation and tend to pitch/sink with very little longitudinal shifting of the weight of a rider, resulting in the high-frequency wobble previously described. However, with the hydrofoils 26, 28 provided at or near both ends, rider weight shifting has much less effect. This is basically because the hydrofoils 26, 28 are essentially horizontal flat plates of significant area that provide great resistance and are extremely difficult to move up and down (orthogonally to the major plate area) through the water. Rather, the hydrofoils 26, 28 preferably want to slip sideways (parallel with the major plate area) and move through the water horizontally.

Both hydrofoils 26, 28 extend fully across between and are attached to the hulls 12, 14. This configuration provides the further beneficial effect of the hydrofoils 26, 28 improving the structural integrity to the watercraft 10 by reducing any torsional twisting within the deck 16, e.g. as may occur if opposite directed forces are imposed on the respective hulls 12, 14 - such as may be encountered if the watercraft 10 diagonally traverses a wave with one hull descending into a wave trough while the other hull is being raised by a wave crest.

Accordingly, it will be appreciated that when a person riding the watercraft 10 is standing on the deck 16, the person will be positioned between the fore hydrofoil 26 and the aft hydrofoil 28. Generally, the watercraft 10 will be relatively lightweight, e.g. weighing < 30kg, and should be able to be carried by one or two people with the centre of gravity of the watercraft being located in the vicinity of the deck 16. In some examples the watercraft 10 can weigh only about 10kg. During use the fore hydrofoil 26 is configured to be located and operatively remain fore of the watercraft’s centre of gravity and the aft hydrofoil 28 is configured to be located and operatively remain aft of the watercraft’s centre of gravity. Also, when a person is riding on or moving about on the deck 16, the combined centre of gravity of the watercraft 10 and the person will still be located between the fore hydrofoil 26 and the aft hydrofoil 28.

A carry handle recess 30 is provided in the deck 16.

As can be seen more clearly in Figures 1 and 3, the fore hydrofoil 26 is substantially U-shaped when seen in end view (Figure 3) and comprises two opposed struts 32, each depending from one of the hulls 12, 14. A wing 34 (fore wing) is supported by the struts 32 wherein the fore wing 34 extends transversely to the longitudinal axes 18.

The fore wing 34 terminates flush at its joint to the struts 32 so that the fore wing 34 does not project beyond the struts 32. In some embodiments the struts 32 can be orientated substantially parallel to each other, whereby the fore wing 34 will have a wingspan being largely equivalent to the distance between the longitudinal axes 18 of the hulls 12, 14 (as shown in Figure 6). In other embodiments, the struts 32 can be angled with respect to each other to either converge or to diverge (as shown in Figures 1 to 3) as they extend further away from the hulls 12, 14, whereby the fore wing 34 will have a wingspan being smaller or larger than the distance between the longitudinal axes 18 of the hulls 12, 14. In some examples the fore wing 34 can project outwardly beyond the hulls 12, 14.

The aft hydrofoil 28 can be similarly shaped to that of the fore hydrofoil 26 and thus also be U-shaped when seen in end view. The aft hydrofoil 28 comprises two opposed struts 36, each depending from one of the hulls 12, 14, with a wing 38 (aft wing) being supported by the struts 36 - the aft wing 38 also extends transversely to the longitudinal axes 18. In some examples, the aft wing 38 can terminate flush with the struts 36 so that the aft wing 38 does not project beyond the struts 36 (see Figure 6).

In some examples the struts 36 can be orientated substantially parallel to each other, whereby the aft wing 38 will have a wingspan being largely equivalent to the distance between the longitudinal axes 18 of the hulls 12, 14. In other embodiments, the struts 36 can be angled with respect to each other to either converge or diverge as they extend further away from the hulls 12, 14, whereby the aft wing 38 will have a wingspan being smaller or larger than the distance between the longitudinal axes 18 of the hulls 12, 14.

Both the fore wing 34 and the aft wing 38 are configured to be substantially horizontally orientated during use when the watercraft is floating stationary in water. Accordingly, the wings 34, 38 are normally aligned along planes being parallel to the plane of the deck 16.

In the embodiment shown in Figures 1 to 3, the struts 36 are arranged substantially parallel to each other with the aft wing 38 extending beyond the struts 36 to be generally in an inverted T-shape (or inverted tt-shape) when seen in end view. In this embodiment the hydrofoil arrangement 24 has a canard configuration with the aft hydrofoil 28 having a larger wingspan than the fore hydrofoil 26. In one example the wingspan of the aft wing 38 is about 30%-300% larger than the wingspan of the fore wing 34; in another example the wingspan of the aft wing 38 is about 40%-60% larger than the wingspan of the fore wing 34; and in another example the wingspan of the aft wing 38 is about 50%-55% larger than the wingspan of the fore wing 34. Accordingly, the wingspan of the aft wing 38 is generally about 1.5 times larger than the wingspan of the fore wing 34, although this can vary to the order of being 1-3 times larger. The lift force generated by the wings 34, 38 increases as the wingspan increases. In the canard configuration, the larger aft wing 38 is configured to provide a greater amount of lift force than the fore wing 34 and this allows the centre of gravity of the watercraft 10 to be moved further aft, e.g. by locating the deck 16 further towards the stern 22 and/or allowing a rider to stand on the deck 16 closer to the stern. Standing slightly aft of centre is the most natural, intuitive position for a rider as it correlates to a position of a surfer on a surfboard. Also, moving the rider towards the aft initially causes the stern 22 to submerge slightly and consequently lifts the bow out of the water, thereby increasing the angle-of-attack of both hydrofoils 26, 28, which helps generate initial lift, whereafter the rider then levels out the watercraft 10. Therefore, a canard arrangement with smaller fore wing 34 and larger aft wing 38 is one of the preferred embodiments. The fore and aft wings 34, 38 are spaced below the hulls 12, 14 by a preselected offset distance that is dependent on a chord dimension of the wings 34, 38, wherein the offset distance is selected to selectively reduce the lift force produced by the hydrofoils 26, 28 during use if the hulls 12, 14 rise too high above the water surface. For a hydrofoil to produce its theoretical full lift force, the hydrofoil’s wing should be located at a depth of at least 1.5 times the average chord dimension of the wing (being the average axial width of the wing). Once the wing rises to a depth less than 1.5 times its average chord dimension, the wing is too close to the water surface and the lift force imparted by the wing begins to reduce (a characteristic known as the “submergence factor”).

For example, at depths of half the average chord dimension the lift force produced is reduced to about 80% of its full lift force. Thus positioning the fore and aft wings 34, 38 closer to the hulls 12, 14 so that they ride shallower below the water surface can reduce the lift force produced, whereas positioning the fore and aft wings 34, 38 further from the hulls 12, 14 so that they ride deeper below the water surface allows the full lift force to be produced. In one example the offset distance is less than triple the average chord dimension of the wings 34, 38 and in another example the offset distance is less than double the average chord dimension. In the exemplary embodiment each wing 34, 38 has an average chord dimension of about 120 mm and each wing 34, 38, is located about 200 mm below the bottom of the hulls 12, 14 so that in use the wings 34, 38 will be operatively located about 100-300 mm below the water surface.

A skilled addressee will appreciate that the depth of the wings 34, 38 is shallower than is common in conventional hydrofoil arrangements - conventional hydrofoils normally extend a long way down below the water surface so that the watercraft can be lifted well clear above the water surface and still have their wing(s) relatively deep in the water. In the exemplary embodiment, having the wings 34, 38 located close to the hulls 12, 14 utilises this reduction in lift force as an automatic trimming mechanism that keeps the bottom of the hulls 12, 14 trimmed close to or in contact with the water surface so that the hulls 12, 14 are not lifted too high - this further assists in reducing pitching problems that are characteristic of conventional hydrofoil arrangements.

Instead of attempting to lift the watercraft 10 fully clear of the water, the hydrofoils 26, 28 are configured to lift the watercraft 10 upwards enough to reduce the overall drag of the hulls 12, 14 but to not necessarily lift the watercraft 10 completely out of the water. The aim is to significantly increase speed because of reduced hull drag while avoiding pitching instability. The watercraft 10 generally maintains contact with the water or is lifted only slightly clear of the water surface to provide stability and avoid unwanted pitching.

Referring now to Figure 4, the hydrofoils 26, 28 are removably joined to the watercraft by an adjustable mounting 40 being configured to permit the angle-of-attack of each wing 34, 38 to be selectively adjusted, either with respect to each other or with respect to the watercraft 10. The adjustable mounting 40 also allows the hydrofoils 26, 28 to be easily removed for storage and transport. When used in relation to watercraft 10 that are surfboards or stand-up paddleboards, the adjustable mounting is often also referred to as a fin box and/or fin plug.

The adjustable mounting 40 comprises support housings 42 that are either joined to or integrally formed within each of the hulls 12, 14. The support housings 42 each define a partially arcuate J-shaped channel 44 that is arranged in an operative substantially vertical plane. Each channel 44 extends linearly for a first part 46 of its length and subsequently curves through an arc for a second part 48 of its length. The adjustable mounting 40 further comprises projecting members 50 extending laterally from each hydrofoil 26, 28, wherein each projecting member 50 is configured to be movably located within one of the channels 44, whereby moving the projecting members 50 along the channels 44 selectively adjusts the angle-of-attack of each hydrofoil. In the exemplary embodiment, the projecting members 50 comprise two pairs of spaced apart pins 50, wherein each pair of pins 50 is joined to and extends laterally from each of the struts 32, 36 of each hydrofoil 26, 28. The pins 50 are configured to be movably located within their complementary channels 44, whereby one pin 50 of each pair is configured to be located within the first part 46 of its channel 44 while the other pin 50 of each pair is configured to be located within either the first part 46 or the second part 48 of its channel 44.

Two spaced apart inlet passages 52 extend through the support housings 42 to permit relatively easy insertion or removal of the pins 50 from the channel 44. However, it will be appreciated that in other embodiments only a single passage 52 may be required to enable sequential insertion of the pins 50 into the channel 44. Yet further, in other embodiments the pins 50 may be laterally extendable or telescopic so that they can be projected directly into the channel 44 so that no passage 52 need be provided.

The adjustable mounting 40 further comprises a suitable locking member (not shown) being configured to immovably secure the hydrofoil 26, 28 in position after the desired angle-of-attack of the wings 34, 38 has been set.

The various procedural steps of attaching the hydrofoils 26, 28 to the watercraft 10 are shown in the various views of Figure 4. In Figure 4a the hydrofoil 26, 28 is shown separate from the watercraft 10 before being attached thereto. Figure 4b shows the hydrofoil 26, 28 being brought towards the watercraft 10 so that the pins 50 enter and slide through the passages 52. Subsequently, in Figure 4c, the hydrofoil 26, 28 is slid longitudinally into the channel 44 so that the pins are spaced away from the passages 52 - wherein both of the pins 50 are located in the linear first part 46 of the channel 44. In this position the angle-of-attack is about 0°. In Figure 4d, the hydrofoil 26, 28 is slid longitudinally further along the channel 44 so that one pin 50.1 remains located in the linear first part 46 of the channel 44, while the other pin 50.2 enters the curved second part 48 of the channel 44. The curvature causes a pivotal change in the orientation of the struts 32, 36, which accordingly causes a change in the orientation of the wing 34, 38, e.g. causing its angle-of-attack to be vary between 0°-1°. In Figure 4e, the hydrofoil 26, 28 is slid even further along the channel 44 so that the pin 50.2 reaches the end extent of the curved second part 48 of the channel 44 - this results in an even greater change in the orientation of the wings 34, 38, e.g. causing the angle-of-attack to be vary between 1 °-3°. It will be appreciated that increasing the extent of the curvature of the channel 44 or its radius will allow even greater changes to the angle-of-attack.

Referring now again to Figures 1 and 2 and also to Figure 5a, the aft wing 38 has a central portion 54 located between two opposed outer portions 56, wherein the two outer portions 56 are mirror images of each other, i.e. the aft wing 38 is reciprocal around its central axis 58. The central portion 54 has a smaller chord dimension compared to the chord dimension of the two outer portions 56. In the exemplary embodiment of the watercraft 10 shown in Figure 1, the central portion 54 is located in between the opposed struts 36 and the outer portions 56 are located outwardly of the struts 36. However, in other examples, the outer portions 56 can traverse the struts 36 to project inwardly and outwardly of the struts 36 to thereby reduce the wingspan ratio of the central portion 54 to the outer portions 56. Alternatively, the central portion 54 can traverse and extend beyond of the struts 36, whereby the outer portions 56 are located outwardly of and spaced apart from the struts 36, to thereby increase the wingspan ratio of the central portion 54 to the outer portions 56.

As shown in Figures 5a, the wing 38 is generally triangular when seen in top view. An alternative more curved shaped wing is shown in Figure 5c. It will be appreciated that there are many more designs of such wings that can be made using the same design principles, that comprise a central portion having a smaller chord dimension compared to the chord dimension of the two outer portions.

In one example the wing 38 has wingspan dimensions wherein the central portion 54 comprises about 50-60% of the wingspan of the wing 38. In one embodiment the central portion 54 has a chord dimension being about 60-80% of the chord dimension of the outer portions 56.

It will be appreciated that altering either of or both the wingspan and chord dimensions of the central portion 54 and/or the outer portions 56 will cause a change in the total surface area of the wing 38 and also allow a change to the position of a centre-of-area point 60 defined with the surface area of each wing-half on the respective sides of the central axis 58. Enlarging the surface area of the outer portions 56 with respect to the surface area of the central portion 54 will result in the centre-of-area point 60 being moved further away from the central axis 58. In one example the total area of the central portion 54 is a factor of about 0.4-0.6 of the total area of the wing 38. As is illustrated in Figure 5a and 5b, when compared to a structure of a conventional wing 62 that has a constant width chord dimension or a tapering chord dimension, the opposed centre-of-area points 60 of the wing 38 are spaced laterally further away from the central axis 58 than the equivalent centre-of-area points 60’ of the conventional wing 62. Accordingly, the configuration of the wing 38 has the effect of producing more lift force further out from the central axis 58, i.e. in use further outwards from the centre of the watercraft 10 and a rider’s centre of weight/balance. In turn this effect increases rotational inertia which reduces angular or rotational acceleration of the watercraft 10 and making the watercraft 10 more stable.

As an example, it is possible to compare two hydrofoils having an identical wing area and wingspan (the exemplary wing 38 shown in Figure 5a and the conventional wing 62 shown in the Figure 5b), wherein it is assumed that the centre-of-area points 60 are spaced apart from the central axis 58 by 287mm while the centre-of-area points 60’ are spaced apart from the central axis 58 by 215mm. It is further assumed that identical lift coefficients exist etc., so that the key factor is simply the centre-of-area of each wing 38, 62 and further assumed that half the rider’s weight (e.g. a rider weighing 80kg) will lift up at each centre-of-area point 60, 60’, i.e. half the rider’s weight lifts up on each half of each wing 38, 62 through the centre-of-area points 60, 60’ either side of the centre of rotational inertia.

Applying these values in the formula for rotational inertia / = — — , where

I = Inertia in kg.m ² M = mass/weight in kg and r = radius in meters for the wing 38, the rotational inertia is 1.647 kg. m ² whereas for the conventional wing 62, the rotational inertia is

40. (0.215) ² 0.924 kg. m ²

The above example results show that the rotational inertia of the wing 38 is about 78% higher than an equivalent conventional foil and this is the reason for providing a rider improved control. By further altering the configuration of the wing 38, the centre-of-area points 60 can be moved further outwardly away from the central axis 58, which would further increase its rotational inertia.

It is noted that the larger outer portions 56 of the wing 38 may potentially result in higher wing tip vortices and cause increased drag. Bending off these tip areas into winglets 76 (see Figure 7) can reduce the vortices formed and thus the drag and also provide additional control advantages. Although these winglets 76 could be bent upwardly, it is considered preferable to bend them downwardly to increase the depth that they extend below the water surface. This further reduces the likelihood of the winglets 76 projecting out of the water if the watercraft 10 is sharply banked in a turn that could potentially result in the wing 38 projecting out of the water on the outside of the turn. Referring now to Figure 7, the above distinctive central portion 54 and outer portions 56 configuration of the wing 38 can also be applied to a conventional surfboard hydrofoil arrangement 64.

The hydrofoil arrangement 64 comprises a strut 66 having an attachment plate 68 at its operative upper end being configured to be attached to a watercraft (e.g. to a surfboard, a sailboard or a stand-up paddleboard). At the opposed end of the strut 66 is a fuselage 70 that is orientated to extend substantially parallel to the longitudinal axis of the watercraft. The fuselage 70 carries at least one wing 72 that extends laterally outwardly on opposed sides of the fuselage 70. Typically, the wing 72 will be a main wing 72 provided towards the front of the fuselage 70, while a further tail wing 74 will be provided towards the rear of the fuselage 70. The main wing 72 shown in Figure 7 is substantially similar in shape to the aft wing 38 shown in Figure 5a, wherein the main wing 72 also has a central portion 54 located between two opposed outer portions 56 and wherein the two outer portions 56 are mirror images of each other, i.e. the main wing 72 is reciprocal around the fuselage 70. The central portion 54 has a smaller chord dimension compared to the chord dimension of the two outer portions 56. As discussed previously, the proportion of the wingspan of the outer portions 56 relative to the central portion 54 can be altered to thereby reduce or increase the wingspan ratio of the central portion 54 to the outer portions 56. In some examples the position of the main wing 72 and the tail wing 74 can be reversed so that the main wing 72 is aft of the fuselage 70 and the tail wing 74 is fore of the fuselage 70 so that the hydrofoil arrangement 64 is in a canard arrangement. The opposed outer edges of the outer portions 56 are downwardly bent off into the winglets 76.

It should be appreciated that either or both the fore and aft wings 34, 38 or the main wing 72 need not necessarily be planar as shown in Figures 1 to 7, but can be curved as is common in the art. Accordingly, Figure 8 shows an example of the main wing 72 that is curved in an anhedral shape. Further, the main wing 72 comprises outwardly projecting planar fences 78, which are substantially parallel to each other. The fences 78 are shown located at the transition boundaries between of the central portion 54 and the outer portions 56. In other examples, additional fences 78 can be discretely located along the main wing 78. The fences 78 assist in reducing vortices in the water during use.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the watercraft and/or hydrofoil arrangement as shown in the specific embodiments without departing from the spirit or scope of the disclosure as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. For example, by providing a simple clip-on seat, e.g. being configured to clip into the carrying handle recess 30 or into another suitable connection, the watercraft 10 can be converted from the stand-up paddle board to a kayak and/or surf-ski.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word

“comprise” or variations such as “comprises” or “comprising” is used in a non-limiting and an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in the various embodiments. A reference to an element by the indefinite article "a" does not exclude the possibility that more than one of the elements is present, unless the context clearly requires that there be one and only one of the elements.

Reference numerals

10, 110 watercraft 46 first part

12 hull 48 second part

14 hull 50 projecting members / pins

16 deck 52 passages

18 longitudinal axis 54 central portion

20 bow 56 outer portions

22 stern 58 central axis

24 hydrofoil arrangement 60, 60’ centre-of-area point

26 first/ fore hydrofoil 62 conventional tapered wing

28 second / aft hydrofoil 64 hydrofoil arrangement

30 carry handle recess 66 strut

32 struts 68 attachment plate

34 fore wing 70 fuselage

36 struts 72 main wing

38 aft wing 74 tail wing

40 adjustable mounting 76 winglets

42 support housings 78 fences

44 channel(s)