WO2018234969A1

WO2018234969A1 - METHOD FOR CONTROLLING A NAUTICAL MACHINE AND NAUTICAL MACHINE

Info

Publication number: WO2018234969A1
Application number: PCT/IB2018/054466
Authority: WO
Inventors: Dmitry REPIN; Andrew BOGDANOV
Original assignee: Individual
Current assignee: Individual
Priority date: 2017-06-20
Filing date: 2018-06-18
Publication date: 2018-12-27
Anticipated expiration: 2019-12-20
Also published as: GB201709844D0; RU2744812C1

Abstract

A method of controlling a watercraft comprising at least one hydrofoil, the at least one hydrofoil comprising one or more apertures on a first and/or second surface thereof. An amount of fluid to expel from, or draw into, at least one of the one or more apertures is determined. Fluid is expelled or drawn through at least one of the one of more apertures in accordance with the determined amount, thereby to alter and/or generate a lifting force, yaw, pitch and/or roll force on the hydrofoil so as control the watercraft. A corresponding watercraft having a hydrofoil, apertures and control means is also disclosed. In an embodiment, the stability, energy efficiency and/or turning efficiency of a personal watercraft can be improved. Other embodiments include wind-powered watercraft in which a rolling force caused by the action of wind on, e.g., one or more sails or kites is counteracted by way of a hydrofoil.

Description

METHOD OF CONTROLLING A WATERCRAFT AND A WATERCRAFT

TECHNICAL FIELD

The present disclosure relates to a watercraft comprising a hydrofoil.

Aspects of the invention relate to a method of controlling a watercraft, and a watercraft implementing the method.

BACKGROUND

Various watercraft are known. For example, surfboards can be used to surf on waves. Kite surfing involves using a kite to power a surfboard that is attached to a user's feet. A jetski is a motorised watercraft that uses a vectored waterjet to propel itself through the water and control direction. Each of these watercraft has its own advantages and disadvantages. For example, surfing and kite surfing have steep learning curves, and may require specific oceanic conditions before they can be performed. Jetskis are easier for a novice to use, but require powerful motors to enable them to attain reasonable speeds, due to their large size (relative to, say, a surfboard) and draft.

Is an object of the present invention to address disadvantages of the prior art, and/or to provide an alternative method, and watercraft as described.

SUMMARY OF THE INVENTION

Aspects and embodiments of the invention provide to a method of controlling a watercraft, and a watercraft implementing the method.

According to an aspect of the invention, there is provided a method of controlling a watercraft comprising at least one hydrofoil, the at least one hydrofoil comprising one or more apertures on a first and/or second surface thereof, the method comprising:

determining an amount of fluid to expel from, or draw into, at least one of the one or more apertures; and

expelling fluid from, or drawing fluid, into the at least one of the one or more apertures, thereby to alter and/or generate a yaw, pitch and/or roll force on the hydrofoil so as to control the watercraft. The first surface may be an upper surface and the second surface may be a lower surface.

The first surface may be a left surface and the second surface may be a right surface.

Expelling and/or drawing water through at least one of the one or more apertures may cause a change in pressure on the surface associated with the aperture(s), thereby to alter and/or generate the yaw, pitch and/or roll force. The hydrofoil may include hydrofoil portions that are offset from each other longitudinally, laterally, or both, relative to an intended direction of travel through water. By expelling or drawing the water through one or more of the apertures to change the net lift provided by each hydrofoil portion, a rotational force may be generated, thereby to generate the yaw, pitch and/or roll force.

The watercraft may have a longitudinal axis in an intended direction of travel through water, and the at least one hydrofoil may comprise:

a first hydrofoil portion including one or more of the apertures; and

a second hydrofoil portion angularly offset around the longitudinal axis from the first hydrofoil portion, the second hydrofoil portion including one or more of the interfaces;

wherein the expelling or drawing the fluid comprises expelling or drawing the fluid differently through the one or more apertures on the first hydrofoil portion relative to the one or more apertures on the second hydrofoil portion. The first hydrofoil portion may include the first surface in the form of an upper surface and the second surface in the form of a lower surface, and the second hydrofoil portion may include the first surface in the form of an upper surface and the second surface in the form of a lower surface, and each of the first and second hydrofoils portions' upper surfaces and lower surfaces may include at least one of the apertures. The method may comprise selectively expelling or drawing the fluid from some or all of the apertures, thereby to generate the force.

The watercraft may include at least one lateral axis, and the at least one hydrofoil may comprise a third hydrofoil portion including one or more of the apertures, and a fourth hydrofoil portion offset rearwardly, relative to an intended forward direction of travel of the watercraft, from the third hydrofoil, the fourth hydrofoil portion including one or more of the apertures. Expelling or drawing the fluid may comprise expelling or drawing the fluid differently through the one or more apertures on the third hydrofoil portion relative to the one or more apertures on the fourth hydrofoil portion.

The third hydrofoil portion may include the first surface in the form of an upper surface and the second surface in the form of a lower surface, and the fourth hydrofoil portion may include the first surface in the form of an upper surface and the second surface in the form of a lower surface. Each of the third and further hydrofoil portions' upper surfaces and lower surfaces may include at least one of the apertures. The method may comprise selectively expelling or drawing the fluid through some or all of the outlets, thereby to generate the force.

At least one of the hydrofoil portions includes a plurality of the apertures, one or more of the apertures comprising rearward apertures and one or more of the apertures comprising forward apertures, the one or more rearward apertures being offset rearwardly, relative to an intended forward direction of travel of the watercraft, from one or more of the forward apertures. Expelling or drawing the fluid may comprise expelling or drawing the fluid differently through the rearward outlets relative to the forward outlets, thereby to generate the yaw, pitch and/or roll force on the hydrofoil.

The method may comprise expelling or drawing the fluid at or adjacent a flow separation point of one or more of the hydrofoil portions. The method may comprise determining a rate at which to expel or draw the fluid through each aperture, based on at least one of:

an angle of attack of at least one of the hydrofoil portions;

a waterspeed of the hydrofoil;

a yaw angle of the watercraft;

a pitch angle of the watercraft;

a roll angle of the watercraft; and

a rate of change in any one or more of an angle of attack of at least one of the hydrofoil portions, a waterspeed of the hydrofoil, a yaw angle of the watercraft, a pitch angle of the watercraft, and a roll angle of the watercraft; and

expelling or drawing the fluid in accordance with the determined rate(s). The method may comprise expelling a portion of the fluid as a jet in order to propel the watercraft through the water. The watercraft may comprise an outlet separate from the one or more apertures, for expelling a fluid under pressure to propel the watercraft through water.

The method may comprise controlling the expelling or drawing of the fluid through the one or more apertures so as to improve any one or more of the following:

stability of the watercraft;

energy efficiency of the watercraft; and

turning efficiency of the watercraft.

The watercraft may include a platform for a human and/or a load. The at least one hydrofoil may be mounted beneath the platform such that, in use, the lift generated by the hydrofoil as it is propelled through the water lifts the platform off the water surface.

The watercraft may be a wind-powered. For example, the watercraft may be sail- or kite- powered. The watercraft may be a sailboard, windsurf board, kite-surf board, sailboat, yacht, skiff, or any other form of wind-powered watercraft.

The force may be directed to counteract an opposite roll force generated by wind acting on one or more sails, kites, and/or other means of wind-powering the watercraft. According to another aspect of the invention, there is provided a watercraft comprising:

at least one hydrofoil;

one or more apertures disposed on a first and/or second surface of the hydrofoil; means for determining an amount of fluid to expel from, or draw into, at least one of the one or more apertures; and

control means for controlling the expelling or drawing of the fluid through the at least one of the one or more apertures in accordance with the amount determined by the means for determining the amount of fluid, thereby to alter and/or generate a yaw, pitch and/or roll force on the hydrofoil so as to control the watercraft. The watercraft may have a longitudinal axis in an intended direction of travel through water, wherein:

the at least one hydrofoil comprises:

a first hydrofoil portion including one or more of the apertures; and

a second hydrofoil portion angularly offset around the longitudinal axis from the first hydrofoil portion, the second hydrofoil portion including one or more of the apertures; and the control means are configured to selectively expel or draw the fluid differently through the one or more apertures on the first hydrofoil portion relative to the one or more apertures on the second hydrofoil portion, thereby to generate the force.

The first hydrofoil portion may include the first surface in the form of an upper surface and the second surface in the form of a lower surface, and the second hydrofoil portion may include the first surface in the form of an upper surface and the second surface in the form of a lower surface, wherein each of the first and second hydrofoils portions' upper surfaces and lower surfaces includes at least one of the apertures, wherein the control means may be configured to selectively expel or draw the fluid from some or all of the apertures, thereby to generate the force.

The watercraft may have at least one lateral axis, and the at least one hydrofoil may comprise a third hydrofoil portion including one or more of the apertures and a fourth hydrofoil portion offset rearwardly, relative to an intended forward direction of travel of the watercraft, from the third hydrofoil, the fourth hydrofoil portion including one or more of the apertures, wherein the control means may be configured to selectively expel or draw the fluid differently through the one or more apertures on the third hydrofoil portion relative to the one or more apertures on the fourth hydrofoil portion, thereby to generate the force.

The third hydrofoil portion may include the first surface in the form of an upper surface and the second surface in the form of a lower surface, the fourth hydrofoil portion may include the first surface in the form of an upper surface and the second surface in the form of a lower surface, and each of the third and further hydrofoil portions' upper surfaces and lower surfaces may include at least one of the apertures, wherein the control means are configured to selectively expel or draw the fluid through some or all of the outlets, thereby to generate the force. The hydrofoil may include a plurality of the apertures, one or more of the apertures comprising rearward apertures and one or more of the apertures comprising forward apertures, the one or more rearward apertures being offset rearwardly, relative to an intended forward direction of travel of the watercraft, from one or more of the forward apertures, and the control means may be configured to selectively expel or draw the fluid differently through the rearward outlets relative to the forward outlets, thereby to generate the force.

At least one of the apertures may positioned at or adjacent a flow separation point.

The watercraft may comprise one or more sensors, comprising any one or more of:

one or more accelerometers for determining translational and/or rotational position and/or motion of the watercraft;

one or more water speed sensors for determining a speed of the hydrofoil and/or the watercraft relative to the water;

one or more angle sensors for determining an angle of attack of the hydrofoil relative to the water;

one or more angle sensors for determining a level of the watercraft relative to the horizontal;

one or more height sensors for determining a height of the platform above the water; one or more sensors for determining a direction of a flow over one or more surfaces of the hydrofoil;

one or more pressure sensors for determining a fluid pressure;

one or more GPS location units; and

one or more gyroscopes for determining translational and/or rotational position and/or motion of the watercraft;

wherein the means for determining an amount of fluid to expel from, or draw into, at least one of the one or more apertures is configured to receive signals from the one or more sensors.

The means for determining an amount of fluid to expel from, or draw into, at least one of the one or more apertures is configured to make the determination based on at least one of: an angle of attack of at least one of the hydrofoil portions;

a water speed of the hydrofoil;

a yaw angle of the watercraft; a pitch angle of the watercraft;

a roll angle of the watercraft; and

a rate of change in any one or more of an angle of attack of at least one of the hydrofoil portions, a waterspeed of the hydrofoil, a yaw angle of the watercraft, a pitch angle of the watercraft, and a roll angle of the watercraft.

The watercraft may be configured to expel a portion of the fluid as a jet in order to propel the watercraft through the water. The watercraft may comprise an outlet separate from the one or more apertures, for expelling a fluid under pressure to propel the watercraft through water.

The means for determining an amount of fluid to expel from, or draw into, at least one of the one or more apertures determines the amount of fluid so as to improve any one or more of the following:

stability of the watercraft;

energy efficiency of the watercraft; and

turning efficiency of the watercraft. The means for determining an amount of fluid to expel from, or draw into, at least one of the one or more apertures comprises a digital processor.

The watercraft may comprise a platform for a human and/or a load, the at least one hydrofoil being mounted beneath the platform such that, in use, the lift generated by the hydrofoil as it is propelled through the water lifts the platform off the water surface.

The platform may be a surfboard or a sailboard, or take the physical shape and/or form of a surfboard or sailboard. The watercraft may be wind-powered. For example, the watercraft may be sail- or kite- powered. The watercraft may be a sailboard, windsurf board, kite-surf board, sailboat, yacht, skiff, or any other form of wind-powered watercraft.

The force may be directed to counteract an opposite roll force generated by wind acting on one or more sails, kites, and/or other means of wind-powering the watercraft. The watercraft may comprise one or more pumps for pressurising the fluid to cause it to selectively be expelled or drawn through the one or more apertures. The watercraft may be autonomous or semi-autonomous.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a watercraft according to an embodiment of the invention;

Fig. 2 is a plan view of the watercraft of Fig. 1 ; Fig. 3 is a vertical section through a simplified illustration of a hydrofoil for use with a watercraft according to an embodiment of the invention;

Fig. 4 is a plan view of the hydrofoil of Fig. 3; Fig. 5 is a vertical section through a simplified illustration of a hydrofoil for use with a watercraft according to a further embodiment of the invention;

Fig. 6 is a plan view of the hydrofoil of Fig. 5; Fig. 7 is a detailed sectional view showing an aperture for use with an embodiment of the invention;

Fig. 8 is a detailed sectional view showing an aperture for use with another embodiment of the invention;

Fig. 9 is a rear perspective view of a simplified illustration of a hydrofoil for use with a watercraft according to a further embodiment of the invention; Fig. 10 is a rear perspective view of a simplified illustration of a hydrofoil for use with a watercraft according to yet a further embodiment of the invention;

Fig. 1 1 is a schematic of a control system for use with an embodiment of the invention;

Fig. 12 is a vertical section through a hydrofoil according to a further embodiment of the invention;

Fig. 13 is a schematic of valves and a pump for use with an embodiment of the invention; and

Fig. 14 is a rear perspective view of a watercraft in the form of a sailboat, according to a further embodiment of the invention. DETAILED DESCRIPTION

Referring to the drawings, and Figs 1 and 2 in particular, a watercraft 100 comprises a hydrofoil in the form of hydrofoil unit 102 and a platform 104. In this embodiment, the platform 104 takes the physical shape and form of a sailboard. The platform 104 is mounted to the hydrofoil unit 102 by way of struts 106. In the embodiment illustrated, the struts 106 take the form of a scissor-like arrangement, which provides a relatively rigid attachment between the hydrofoil unit 102 and the platform 104 when in use, while allowing the struts 106 to fold flat for easy transport and storage. The watercraft 100 is designed such that, in use, lift generated by the hydrofoil unit 102 as it is propelled through the water lifts the platform 104 off the water surface. The hydrofoil unit 102 comprises a first hydrofoil portion 108, a second hydrofoil portion 110, a third hydrofoil portion 112, and a fourth hydrofoil portion 1 14, all of which extend laterally from a central body 116. First handful portion 108 is paired with second hydrofoil portion 110, and third hydrofoil portion 112 is paired with fourth hydrofoil portion 114.

Figs 3 and 4 show a schematic arrangement of the first and second hydrofoil portions 108 and 1 10. Referring to Fig. 3, the hydrofoil portions 108 and 1 10 each includes a first surface in the form of an upper surface 118, and a second surface in the form of a lower surface 120. In this context, "upper" and "lower" refer to the intended orientation of the hydrofoil unit 102 when the watercraft 100 is in operation in water. The upper surface 1 18 includes several apertures in the form of upper holes 122. Each of the upper holes 122 is effectively an outlet/inlet formed in or on the upper surface 1 18. Each of the upper holes 122 is in fluid communication with a corresponding upper duct 124. In the illustrated embodiment, the upper ducts 124 join an upper manifold 126, which in turn is in fluid communication with control means in the form of a control system 128.

It should be noted that where the hydrofoil is intended for use vertically (e.g., when used in an application such as a sailboat as described below), the "upper" and "lower" surfaces will be "left" and "right" surfaces of the hydrofoil, relative to a direction of travel of the watercraft. References to "upper" and "lower" in the following description also cover "left" and "right" surfaces where relevant to the particular application of the hydrofoil.

The lower surface 120 is not shown in plan, but is similar to the upper surface 1 18. The lower surface 120 similarly includes several apertures in the form of lower holes 130. Each of the lower holes 130 is effectively an outlet/inlet formed in or on the lower surface 120. Each of the lower holes 130 is in fluid communication with a corresponding lower duct 132. In the illustrated embodiment, the lower ducts 132 join a lower manifold 133, which in turn is in fluid communication with the control system 128. Although the control system 128 is shown as being within the hydrofoil unit 102, any or all of the components of the control system 128 may be positioned in any suitable part of the watercraft 100.

In the embodiment of Figs 1 to 4, the watercraft 100 has a longitudinal axis 134 in an intended direction of travel through water. The first hydrofoil portion 108 is angularly offset around the longitudinal axis from the second hydrofoil portion 1 10. In the illustrated embodiment, the angular offset is 180°, in that both the first hydrofoil portion 108 and the second hydrofoil portion 110 are aligned with each other and a horizontal axis through the hydrofoil unit 102. It will be appreciated however, that the angular offset may be greater or less than 180°, depending upon the particular performance and stability characteristics that are required in a particular embodiment.

The third and fourth hydrofoil portions 1 12 and 1 14 may include features corresponding to those described in relation to Figs 3 and 4, and the description of Figs 3 and 4 in relation to first and second hydrofoil portions 108 and 1 10 correspondingly apply to the third and fourth hydrofoil portions 112 and 1 14.

Turning to Figs 5 and 6, in which like reference signs are used to indicate common features from other embodiments, there is shown an alternative embodiment of a hydrofoil unit 102. In this case, the upper surface 1 18 of the first portion 108 and second portion 110 includes a first row 136 of the upper holes 122, a second row 138 of the upper holes 122, and a third row 140 of the upper holes 122, each of which is fed by a corresponding upper manifolds 126, 127 and 129. Similarly, the lower surface 120 of the first portion 108 and second portion 110 includes a first row 142 of the lower holes 130, a second row 144 of the lower holes 130, and a third row 146 of the lower holes 130, each of which is fed by a corresponding lower manifold (not shown).

The control system 128 is described in more detail below with reference to Figs. 1 1 and 13, but in short, control system 128 includes valves, actuators and actuator drivers that enable positive and negative fluid pressure to be supplied to the upper manifold(s) 126, 127 and 129 and the lower manifold(s) 133. The control system 128 is controlled by means in the form of control circuitry 162 (described in more detail below) programmed and configured to determine an amount of fluid to expel from, or draw through, the upper holes 122 and lower holes 130 (to "draw" the fluid, in this context, means to "suck" the fluid through the apertures/holes). The control circuitry 162 is also configured to control the valves via the actuators that are driven by actuator drivers, such that fluid is selectively expelled or drawn through the upper holes 122 and lower holes 130. By controlling which of the upper holes 122 and lower holes 130 are expelling or drawing fluid, a yaw, pitch and/or roll force may be generated by the hydrofoil so as to control the watercraft 100, and/or improve its efficiency.

In this context, "control" may include providing a control input. The skilled person will understand that a control input may not necessarily be capable of providing complete control in all circumstances. By way of analogy, a car steering wheel controls a car, but may be less effective on a dirt road, and wholly ineffective on an ice patch. Similarly, a force generated by the effect of the fluid being expelled or drawn through the aperture(s) of the described embodiments may not be sufficient to allow a user (or a control system) to maintain control. For example, at least speeds where water flows over the hydrofoil relatively slowly, water passing through the apertures may provide some force, but that force may not be sufficient to wholly stabilise the watercraft, for example. Nevertheless, the expelling and/or drawing of water through the aperture(s) still provides a form of control. In the embodiments shown, the fluid that is expelled or drawn through the apertures is the fluid within which the watercraft 100 is operating. For example, the fluid may be drawn through an orifice (not shown) that generally remains immersed in the water (e.g. seawater, or river water) through which the watercraft 100 is moving. The water is drawn through the orifice and then provided to the fluid control and/or thrust system, as described in more detail below. It will be appreciated, however, that other forms of fluid are contemplated. For example, exhaust gases from an internal combustion engine (not shown), and/or ambient air, may be mixed with water drawn through the orifice.

The upper holes 122 and lower holes 130 may take any suitable form. The shape (in plan) of the holes and/or the angle each upper duct 124 and lower duct 132 makes with the respective upper surface 118 and lower surface 120 may be selected to encourage any particular desired hydrodynamic behaviour(s). If the ducts 124/132 are circular in cross- section and interface with their respective surfaces 1 18/120, the holes 122/130 may be oval in plan, for example. Irrespective of the duct cross sectional shape, any one or more of the holes may be circular, oval (with a major axis at any suitable angle relative to a longitudinal axis of the watercraft), square, rectangular, polygonal, or any other suitable shape. Whatever the shape, it may have any suitable aspect ratio, from relatively compact through to an elongated slit. Formations such as vanes or guides may be provided within one or more of the ducts, and/or at or adjacent one or more of the apertures.

Each duct may have a portion of converging or diverging longitudinal cross section, especially near and/or at the corresponding aperture. This may assist in speeding up or slowing down fluid the is being expelled. If a particular upper hole 122 or lower hole 130 is only used for expelling fluid (i.e., it does not use suction), then the associated duct may join the hole 122/130 at an acute angle. Fig. 7 shows an example of such an upper duct 124. The upper duct 124 terminates in a nozzle 148, which accelerates the fluid 150 flowing through it from the upper duct 124. The acute angle at which the fluid 150 is expelled into the water 152 flowing over the upper surface 118 encourages maintenance of laminar flow over the upper surface 1 18. Alternatively, or in addition, the increased fluid flow over the surface caused by the injection may decrease pressure due to the Bernoulli effect, as known by those skilled in the art. Alternatively, or in addition, expelling fluid through any or the upper holes 122 and/or lower holes 130 may increase or decrease turbulence, which may decrease or increase pressure, as known by those skilled in the art. Any combination of these principles may allow an increased angle of attack and/or an improved response to stalling, for example. Corresponding principles may apply in embodiments where fluid is drawn through one or more of the upper holes 122 and/or lower holes 130.

If a particular upper hole 122 or lower hole 130 is used for both expelling and suction of fluid, then the associated duct may join the hole 122/130 at a less acute angle, at right angles or at a reverse angle. Fig. 8 shows an example of such an upper duct 124. The duct 124 and nozzle 148 in this case are normal to the upper surface 1 18. As such, fluid 150 that is expelled exits the nozzle 148 normally to the water flowing over the upper surface 118. Fluid 154 that is drawn through the nozzle 148 is similarly pulled from all directions around the nozzle, although water will tend to be preferentially drawn in from ahead of the nozzle 148 due to the movement of the water across the nozzle 148. The skilled person will appreciate that the angle that each individual duct 124/132 makes with its associated surface 118/120 may be optimised for its particular purpose. For example, some ducts 124/132 may only be used for expelling fluid, and may therefore make an acute angle with their associated surfaces 1 18/120. Other ducts 124/132 may only be used for drawing fluid, and may therefore make a different angle with their associated surfaces 1 18/120. Yet other ducts 124/132 may be used for both drawing and expelling fluid at different times, and their angles may therefore be a compromise between ideal performance in each of the two modes. Any suitable combination of ducts 124/132 may be provided, depending upon the application. In addition to the expelling and drawing of fluid through apertures, the watercraft 100 may include a mechanism for propelling the watercraft 100 through the water. For example, the watercraft 100 may include one or more external propellers (not shown) at the front and/or the rear of the hydrofoil. The one or more external propellers are driven by a motor, such as an electric motor (not shown) provided with power by a battery (not shown) and/or by an internal combustion motor (not shown). The watercraft may also be driven by wind power, such as by way of one or more sails or kite.

In other embodiments, propulsion of the watercraft 100 may be achieved by use of one or more outlets separate to the apertures. The outlets may form part of a separate waterjet assembly dedicated to propulsion, or propulsion may be wholly or partly provided by the fluid expelled by one or more of the apertures. For example, Fig. 9 shows an embodiment in which propulsion is provided by external means (such as a waterjet, propeller, or wind-power as described elsewhere in this description). Alternatively, propulsion may be provided by the expelling of fluid from at least some of the ducts 124/132. The expelling is sufficiently powerful that it causes the watercraft 100 to be propelled through the water. The watercraft 100 may in this case be at least partially controlled by the amount of thrust provided by fluid expelled from one or more of the ducts 124/132. This control may be in addition to control provided by the expelling and suction of fluid through the apertures such that the hydrodynamic response of the hydrofoil portions causes a yaw, roll and/or pitch force as described above. In other embodiments, the fluid expelled from ducts 124/132 for the purpose of propelling the watercraft 100 is kept relatively constant, and control is provided solely by the expelling and suction of fluid through the apertures such that the hydrodynamic response of the hydrofoil portions causes a yaw, roll and/or pitch force as described above.

Fig. 10 (and Fig. 1) shows an embodiment in which a separate propulsion duct 156 is integrated with the hydrofoil unit 102. In this case, the propulsion duct 156 is at the rear of the central body 116 but in other embodiments may be placed in any suitable position, including separate from the hydrofoil unit 102. The propulsion duct 156 may provide unvectored thrust, with control only being provided by the expelling and suction of fluid through the apertures such that the hydrodynamic response of the hydrofoil portions causes a yaw, roll and/or pitch force as described above. In other embodiments, the propulsion duct 156 may be vectored in a manner similar to, for example, a jetski waterjet. As described below, the control system may control vectoring of the propulsion duct in concert with the expelling and/or suction of fluid through the apertures, thereby to control and stabilise the watercraft 100.

In the embodiments described so far, the hydrofoil unit 102 includes a first pair of hydrofoil portions 108 and 110, and a second pair of hydrofoil portions 1 12 and 1 14. In this arrangement, the force may be generated by controlling the fluid expelled or drawn through one or more of the apertures such that more lift is provided on one side of the longitudinal axis 134 than the other. For example, for a particular speed and angle of attack, expelling fluid through one or more upper ducts 124 on the first hydrofoil portion 108 will result in less laminar separation on that surface, and hence greater lift. As a result, a rolling force will be generated about longitudinal axis 134, which can be used to either roll the watercraft 100 (e.g., into turn) or provide a counterforce to an undesirable roll (e.g., due to lack of user skill).

Pitch may be controlled in a similar manner. In the embodiments described so far, pitch may be controlled for a particular speed and angle of attack by increasing or decreasing the lift provided by the forward pair of hydrofoil portions 108 and 1 10, and/or decreasing or increasing the lift provided by the rearward pair of hydrofoil portions 1 12 and 1 14. The amount of lift provided by each of the forward pair and rearward pair of hydrofoil portions may be controlled in accordance with any of the mechanisms described above. It will be appreciated that pitch control relates to the difference in lift provided by the forward pair of hydrofoil portions 108 and 110 compared to that provided by the rearward pair of hide for portions 112 and 114. In most scenarios, both sets will be providing lift, and it is the relative amount of lift that is adjusted to provide forward and aft pitch control. Returning to the embodiment shown in Figs 5 and 6, pitch may be controlled for a particular speed and angle of attack by drawing or expelling fluid differentially through the first row 136, second row 138, and third row 140 of the upper holes 122, and the first row 142, second row 144, and third row 146 of the lower holes 130. For example, by expelling fluid from the first row 142 at a relatively high rate, while expelling fluid from the third row at a relatively low rate, a net forward pitch force may be generated by the hydrofoil portions. The high rate expelling of the fluid from the first row 142 provides a downward thrust at that point, which is in front of a centre of lift 158. Simultaneously, the low rate expelling of the fluid from the third row 146 improves laminar flow, and hence lift, at that point, which is behind the centre of lift 158. Since the first row 142 provides downward force and the third row 146 provides upward force, the net result is a rotational force in the direction of arrow 160, which provides forward pitch control.

The skilled person will appreciate that it is not necessary for the hydrofoil unit 102 to include four separate hydrofoil portions 102 as shown in the above embodiments. For example, the first hydrofoil portion 108 and second hydrofoil portion 110 may simply be opposite sides of a single hydrofoil. A similar structure may apply to the third hydrofoil portion 1 12 and fourth hydrofoil portion 1 14. In other embodiments, the hydrofoil may include only a first hydrofoil portion 108 and second hydrofoil portion 110. Depending upon the position and layout of the apertures on the top and/or bottom surface of the hydrofoil portions 108 and 1 10, such embodiments may allow control of pitch, roll, yaw, or any combination thereof. At least some of the apertures may be positioned at or adjacent a flow separation point of one or more of the hydrofoil portions. As described above, this may encourage laminar flow, resulting in lift being generated more efficiently, and improving stall characteristics.

Turning to Fig. 1 1 , there is shown a schematic of control circuitry 162 and associated components. The control system 162 may include a digital processor 164, which may take the form of a general-purpose microcontroller comprising a processor and memory, the memory storing software instructions that, when executed, implement a method of controlling a watercraft as described herein. One or more sensors may be included, each configured to provide signals to the digital processor 164. A non-exhaustive list of such sensors includes:

• One or more accelerometers for determining translational and/or rotational position and/or motion of the watercraft 100.

• One or more water speed sensors for determining a speed of the watercraft relative to the water.

• One or more angle sensors for determining an angle of attack of the hydrofoil unit 102 relative to the water.

• One or more angle sensors for determining a level of the watercraft relative to the horizontal. • One or more height sensors for determining a height of the platform 104 above the water.

• One or more sensors for determining a direction of a flow over one or more surfaces of the hydrofoil. The direction of the flow may be affected by, e.g., yaw. For example, if there is yaw to the right, the hydrofoil will move through the water at an angle, which may be measured by determining the angular direction of fluid flow over the hydrofoil (or one or more hydrofoil portions).

• One or more pressure sensors for determining a fluid pressure, including a fluid pressure at or adjacent a surface, within one or more of the ducts, and/or within any other component associated with the expelling or drawing of fluid from the ducts and or a thrust duct where provided.

• One or more GPS location units.

• One or more gyroscopes for determining translational and/or rotational position and/or motion of the watercraft 100.

Thrust parameters 166 may be provided from, for example, a throttle. The throttle may be provided on, for example, a separate controller that may be strapped to a user's wrist or hand and operated to control a speed of the watercraft 100. An example of such a controller is handle 168 in Fig. 1. The handle 168 is attached to the platform 104 by way of a strap 170, which the user may hold onto to stabilise themselves when using the watercraft 100. A set of control wires (not shown) are embedded within the strap 170, and pass control signals from a finger-operated throttle 171 to the digital processor 164. In alternative embodiments, the thrust control signals are sent wirelessly via a wireless interface (not shown). Thrust may also be wholly or partly controlled by user movement. For example, particularly when the watercraft 100 is under way, leaning forwards may be interpreted as a request for greater speed, while leaning backwards may be interpreted as a request for less speed. The amount and/or rate of forwards/backwards leaning may be interpreted to determine whether speed control is the likely intended purpose of the lean. For example, a rapid forwards lean followed by a backwards lean may be interpreted as the initiation of a trick, rather than an attempt to rapidly speed up and then slow down. Thrust may be controlled by any combination of controller and user movement. Other forms of thrust control will be apparent to those skilled in the art.

The digital processor 164 is connected to a control system 128. Components of the control system 128 are shown in Fig. 13. The control system 128 includes a pump 174 and valves 176. The valves 176 are opened and closed by actuators (not shown) that act under the control of the digital processor 164 (control connections not shown for clarity) to control the flow of fluid to and from manifolds (e.g., upper manifold 126) and ducts (e.g, upper ducts 124). The valves 176 may be digital, in the sense that they may only have an "open" or "closed" state. Alternatively, the valves may be control in a proportional manner, so as to modulate the fluid flow through them.

The pump may be a single pump that provides fluid under both positive and negative pressure, or separate pumps may be provided for positive and negative pressure. In yet other embodiments, only positive pressure or negative pressure is supplied.

In the illustrated embodiment, the valves 176 to the left of the pump 174 are configured to selectively be connected to the negative pressure by their associated valves 174. When the relevant valves are opened, the negative pressure draws water through the manifold 126, ducts 124 and open valves 176 towards the pump. This has the effect described elsewhere in this description. The valves to the right of the pump 174 are configured to be selectively connected to the positive pressure by their associated valves 176. When the relevant valves are opened, the positive pressure pushes water from the pump 174, through the manifold 126, ducts 124 and open valves.

In some embodiments, one or more valves may be arranged to allow the same ducts to be selectively connected to positive or negative pressure as required. In other embodiments, each duct may only selectively be provided with positive or negative pressure. In yet other embodiments, a mixture of these alternatives may be provided.

The illustrated valves are merely one example showing a subset of the valves for a particular embodiment. By way of example only, a particular valve in a particular embodiment may control supply of:

• positive pressure directly to a duct;

· positive pressure to a manifold;

• negative pressure directly to a duct;

• negative pressure to a manifold;

• positive and negative pressure directly to a duct;

• positive and negative pressure to a manifold. Particular embodiments may use any one or more such valve supply arrangements.

The pump may take any suitable form. For example, the pump may comprise a motor that drives an impeller. The motor may be an internal combustion engine, an electric motor, or a hybrid of the two. An electric motor may be powered by a battery. An electric motor may also be powered by a wind turbine.

The digital processor 164 is programmed to accept inputs from the various sensors and the thrust controller. From these inputs, the digital processor 164 determines a likely intention of the user, and translates this into a set of control signals for thrust and control. For example, if the inputs suggest that the watercraft 100 is presently at rest, and the user opens the throttle, the digital processor 164 will determine that the user wants the watercraft 100 to move forwards. In that case, the digital processor 164 will control the various actuators and the impeller so as to provide forward thrust. At the same time, fluid will be expelled and/or drawn from various apertures as required to ensure stability of the watercraft 100 as it starts moving. If, for example, the digital processor 164 determines from the inputs that the user is leaning back slightly, fluid may be expelled and/or drawn through one or more of the apertures in order to provide a forward pitch control that will tend to counteract the effect of the user leaning back.

Similarly, if a slight lean to one side is detected at low speed (e.g., when the watercraft 100 is just beginning to move), fluid may be expelled and/or drawn through one of the apertures so as to provide a roll force tending to counteract the lean. In contrast, if a slight lean to one side is detected at a higher speed, the digital process 164 may interpret this as initiation of a turn. In that case, fluid may be expelled and/or drawn through one or more of the apertures in order to stabilise the watercraft 100, and/or improve the efficiency of the turn. In this regard, principles that would apply to controlling yaw, pitch and/or roll in an aircraft may be applied, with modifications to take into account the differences due to the medium through which the hydrofoil is moving and the layout of the various hydrofoil portions 108-1 14.

The digital processor 164 may determine a rate at which to expel or draw the fluid through each aperture based on, for example, any one or more of:

• an angle of attack of at least one of the hydrofoil portions;

• a waterspeed of the hydrofoil;

· a pitch angle of the watercraft; • a roll angle of the watercraft;

• a yaw angle of the watercraft; and

• a rate of change in any one or more of an angle of attack of at least one of the hydrofoil portions, a waterspeed of the hydrofoil, a yaw angle of the watercraft, a pitch angle of the watercraft, and a roll angle of the watercraft.

The fluid may then be expelled and/or drawn in accordance with the determined rate(s).

The expelling and/or drawing of the fluid through the one or more apertures may be controlled so as to improve, for example, any one or more of the following:

• stability of the watercraft;

• energy efficiency of the watercraft; and

• turning efficiency of the watercraft. Examples of how expelling and drawing fluid through the apertures may be used to control a hydrofoil unit 102 will now be described with reference to Fig. 12, in which like reference signs are used to indicate common features from other embodiments. Upper surface 118 has a first row A, a second row B, a third row C, and a fourth row D, of the upper holes 122. Lower surface 120 has a first row E, a second row F, a third row G, and a fourth row H, of the lower holes 130. The hydrofoil unit 102 has an angle of attack a.

At low speed and moderate positive attack angle a, positive pressure is applied through row E (i.e., fluid is expelled through the holes of row E), while moderate negative pressure is applied through rows C and D (i.e., fluid is drawn through the holes of rows C and D), resulting in an increase of the overall lift.

At low speed and large positive attack angle a, large positive pressure is applied through rows E and F, and moderate positive pressure through rows C and D, which stabilises the watercraft.

At a cruising speed and neutral attack angle a, moderate negative pressure is applied through rows C and D, resulting in a decrease in overall drag.

At a fast speed and neutral attack angle a, large positive pressure is applied through rows C and D, and G and H, resulting in a decrease in overall lift. The skilled person will appreciate that other combinations of positive and/or negative pressures, resulting in corresponding expelling or suction of fluid through the relevant rows, will provide additional options for improving control, stability and/or efficiency of the hydrofoil unit 102.

In alternative embodiments, the hydrofoil may support a load platform for carrying loads heavier than a human being. For example, a large watercraft with a hydrofoil according to an embodiment of the invention may include a load platform for carrying, for example, large cargo items. A smaller watercraft with a hydrofoil according to an embodiment of the invention may include a load platform for carrying, for example, smaller items. In either case, the load platform may take the form of a simple flat platform upon which loads can be placed and restrained, or may take the form of a wholly or partly open enclosure within which cargo may be placed, or within which one or more humans may be transported. In yet other embodiments, the load platform may take the form of one or more mounts for equipment. For example, a camera mount may allow the mounting of a gimbal for a stabilised camera. This arrangement enables the camera to capture video or photographic imagery, either automatically or under the control of a camera operator. In either case, the camera may be operated by an operator who is also being carried by the watercraft, or may be operated remotely.

Turning to Fig. 14, there is shown a sail-powered watercraft in the form of a sailboat 200. The sailboat 200 comprises a hull 202, a mast 204, and sails 206. Typically, a sailboat includes at least one vertically extending hydrofoil to resist the lateral force applied to the hull 202 due to the action of wind on the sails 206. In the absence of some such hydrofoil, the hull 202 will tend to sideslip across the water, rather than moving forward through it.

The hydrofoil unit may take the form of any one or more of a keel 208, a dagger board 210, a rudder 212, and peripheral hydrofoils 214. Although the illustrated sailboat 200 includes all of these, in most cases a sailboat will have only one, two or three of these elements. The principles of the invention will now be described with reference to the keel 208. It will be appreciated, that similar principles may be applied to any or all of the other elements.

The keel 208 may include one or more apertures 216, formed in the keel's left and right surfaces (which correspond with the upper and lower surfaces of earlier-described embodiments). Each aperture may take the form of a hole 218, which may be similar to any of the holes 122/130 described above in relation to other embodiments. Each hole 218 is connected to a duct 124/132, which again may be similar to that described above in relation to other embodiments. Although the embodiment of Fig. 1 1 shows four holes 218, this is for the sake of clarity only. In practical applications, it is envisaged that a greater number of holes may be provided. The holes 218 may be positioned in rows, as described above in relation to other embodiments, or may be laid out in any other suitable fashion. The concentration of holes 218 may vary across the length of the keel 208. For example, due to the leverage effect, it may be desirable to provide more control over the lower portion 220 of the keel 208, as this will tend to provide a greater force than the same amount of pressure applied closer to the hull 202.

In use, a digital processor, which may be similar to that described above in relation to embodiment, is used to control how much fluid is expelled or drawn through each hole 218. Typically, the fluid will be expelled or drawn through each hole in such a way as to generate lift. In this context, "lift" means a force directed normally through the surface of the keel 208. Because the sailboat 200 and keel 208 are symmetric, no lift will be generated unless the direction the sailboat 200 is moving within the water is at an angle to the longitudinal direction of the hull 202. Lift may be generated by expelling and/or drawing fluid through the holes 218 as described above in relation to other embodiments. The lift may be generated for the purpose of counteracting a rotational force applied to the hull due to the action of wind on the sails 206.

Undesirable lift may also be generated during tacking, as the angle of attack of the keel 208 changes under the influence of the rudder 212. By expelling or drawing fluid through one or more of the holes 218 while turning, this undesirable lift may be reduced, which may improve the efficiency of the turn as well as reducing undesirable roll of the sailboat 200.

The skilled person will appreciate that similar functionality may be applied to any one or more of dagger board 210, a rudder 212, and peripheral hydrofoils 214, instead of or in addition to the keel 208. Where the hydrofoil is located fore and/or aft, it may be used to generate a yaw force. For example, the rudder 212 may take the form of a vertically extending hydrofoil. By controlling the amount of water expelled or drawn through one or more apertures on either side of the rudder 212, a yaw force may be used to provide a steering input. Depending upon to implementation, this may be a supplement to steering control provided by pivoting of the rudder in a known way.

The fluid pressure required to expel or draw fluid through the holes 218 may be provided in any suitable manner. For example, a motor may be used to power an impeller. The motor may be an internal combustion engine, an electric motor, or a hybrid of the two. An electric motor may be powered by a battery. An electric motor may also be powered by a wind turbine. This has the useful effect of providing more power when the wind is greater, which is when it may be more useful to control the lift provided by the keel 208.

Although a wind-powered watercraft in the form of a sailboat 200 has been described, other forms of wind power and wind-powered watercraft are envisaged. For example, instead of one or more sails, a wind-powered watercraft may be powered by one or more kites as an alternative, or in addition, to one or more sails. The wind-powered watercraft may take the form of a sailboard, windsurf board, kite-surf board, sailboat, yacht, skiff, or any other form of wind-powered watercraft. Such watercraft may include a hydrofoil for lifting a hull or platform above the water, and/or a horizontal hydrofoil as described above in relation to the sailboat 200. While various embodiments have been described with separate vertical and horizontal hydrofoils, it will be appreciated that a watercraft may include both vertical and horizontal hydrofoils. Also, an angled, curved and/or bent hydrofoil may generate both vertical and horizontal force components. In relation to all embodiments, the apertures may take any suitable form. For example, at least some of the apertures may take the form of a nozzle that extends from the surface of the hydrofoil unit 102. The aperture may open into one or more channels, and/or have its output directed by one or more vanes. One or more of the apertures may take the form of a slot, such as a lateral slot that extends along part of the hydrofoil portion.

Where more than one aperture is provided on a surface, they need not be laid out in rows as shown in some of the embodiments above. The apertures may be laid out in any suitable pattern that provides the required performance. For example, to improve roll control, it may be desirable to have a greater density of the apertures at the outer ends of the hydrofoil, as the improved leverage may increase the amount of control available. Longitudinally offset apertures may be laterally offset relative to each other to reduce their interference on each other, due to, for example, turbulence.

Optionally, additional control may be provided by way of movable control surfaces, which may include flaps, ailerons, elevators and/or spoilers, for example.

Optionally, the digital processor 164 may be operable in different modes to suit the skills and intentions of the user. For example, a "beginner" mode may maximise stability at the expense of efficiency and/or controllability. An "expert" mode, on the other hand, may allow more controllability at the expense of stability, which may be acceptable to an expert user.

In any embodiment, the watercraft may be autonomous or semi-autonomous. For example, a watercraft with a load platform may be able to navigate its way to a delivery destination at which its cargo is to be unloaded. A watercraft carrying a camera may follow a predetermined path, or determine its own path, in order to obtain the required video or photographs. An autonomous or semiautonomous watercraft will typically require some means by which to determine its position. This may take the form of, for example, a GPS unit, and/or some form of inertial guidance system, involving one or more gyroscopes and/or accelerometers, for example.

An "amount" of fluid referred to herein is to be understood as covering any suitable unit. For example, depending on the embodiment, an amount of fluid may include a pressure at which fluid is ejected or drawn through the aperture(s), a flow rate (e.g., in litres/second), a flow speed (e.g., in metres/second), and/or any combination of these units. The amount may be determined and implemented directly (e.g., a particular pressure is determined and generated) or indirectly (e.g., a flow rate is determined, and then converted to a pressure and/or valve opening percentage such that the determined flow rate is generated at the aperture(s)). Although the invention has been described reference to a number of specific non-exhaustive and non-limiting embodiments, the skilled person will appreciate that the invention may be embodied in many other forms.

Claims

1. A method of controlling a watercraft comprising at least one hydrofoil, the at least one hydrofoil comprising one or more apertures on a first and/or second surface thereof, the method comprising:

expelling or drawing the fluid through at least one of the one or more apertures based on the determined amount, thereby to alter and/or generate a yaw, pitch and/or roll force on the hydrofoil so as to control the watercraft.

2. The method of claim 1 , the watercraft having a longitudinal axis in an intended direction of travel through water, wherein the at least one hydrofoil comprises:

a first hydrofoil portion including one or more of the apertures; and

a second hydrofoil portion angularly offset around the longitudinal axis from the first hydrofoil portion, the second hydrofoil portion including one or more of the apertures;

wherein the expelling or drawing the fluid comprises expelling or drawing the fluid differently through the one or more apertures on the first hydrofoil portion relative to the one or more apertures on the second hydrofoil portion.

3. The method of claim 2, wherein the first hydrofoil portion includes the first surface in the form of an upper surface and the second surface in the form of a lower surface, and the second hydrofoil portion includes the first surface in the form of an upper surface and the second surface in the form of a lower surface, and wherein each of the first and second hydrofoils portions' upper surfaces and lower surfaces includes at least one of the apertures; the method comprising selectively expelling or drawing the fluid from some or all of the apertures, thereby to generate the force.

4. The method of any preceding claim, wherein the watercraft includes at least one lateral axis, and the at least one hydrofoil comprises:

a third hydrofoil portion including one or more of the apertures; and

a fourth hydrofoil portion offset rearwardly, relative to an intended forward direction of travel of the watercraft, from the third hydrofoil, the fourth hydrofoil portion including one or more of the apertures; wherein the expelling or drawing the fluid comprises expelling or drawing the fluid differently through the one or more apertures on the third hydrofoil portion relative to the one or more apertures on the fourth hydrofoil portion.

5. The method of claim 4, wherein:

the third hydrofoil portion includes the first surface in the form of an upper surface and the second surface in the form of a lower surface; and

the fourth hydrofoil portion includes the first surface in the form of an upper surface and the second surface in the form of a lower surface;

wherein each of the third and further hydrofoil portions' upper surfaces and lower surfaces includes at least one of the apertures; and

the method comprises selectively expelling or drawing the fluid through some or all of the outlets, thereby to generate the force.

6. The method of any preceding claims, wherein at least one of the hydrofoil portions includes a plurality of the apertures, one or more of the apertures comprising rearward apertures and one or more of the apertures comprising forward apertures, the one or more rearward apertures being offset rearwardly, relative to an intended forward direction of travel of the watercraft, from one or more of the forward apertures, wherein:

expelling or drawing the fluid comprises expelling or drawing the fluid differently through the rearward outlets relative to the forward outlets, thereby to generate the yaw, pitch and/or roll force on the hydrofoil.

7. The method of any preceding claim, comprising expelling or drawing the fluid at or adjacent a flow separation point of one or more of the hydrofoil portions.

8. The method of any preceding claim, comprising:

determining a rate at which to expel or draw the fluid through each aperture, based on at least one of:

an angle of attack of at least one of the hydrofoil portions;

a waterspeed of the hydrofoil;

a pitch angle of the watercraft;

a roll angle of the watercraft;

a yaw angle of the watercraft; and a rate of change in any one or more of an angle of attack of at least one of the hydrofoil portions, a waterspeed of the hydrofoil, a yaw angle of the watercraft, a pitch angle of the watercraft, and a roll angle of the watercraft; and

expelling or drawing the fluid in accordance with the determined rate(s).

9. The method of any preceding claim, comprising expelling a portion of the fluid as a jet in order to propel the watercraft through the water.

10. The method of any preceding claim, wherein the watercraft comprises an outlet separate from the one or more apertures, for expelling a fluid under pressure to propel the watercraft through water.

1 1. The method of any preceding claim, comprising controlling the expelling or drawing of the fluid through the one or more apertures so as to improve any one or more of the following:

stability of the watercraft;

energy efficiency of the watercraft; and

turning efficiency of the watercraft.

12. The method of any preceding claim, wherein the watercraft includes a platform for a human and/or a load, the at least one hydrofoil being mounted beneath the platform such that, in use, the lift generated by the hydrofoil as it is propelled through the water lifts the platform off the water surface.

13. The method of any one of claim 1 to 1 1 , wherein the watercraft is a sailboard, windsurf board, kite-surf board, sailboat, yacht, skiff, or any other form of wind-powered watercraft.

14. The method of claim 13, wherein the hydrofoil includes a vertical component, and the first surface is a left surface and the second surface is a right surface, whereby the force may be directed to counteract an opposite roll force generated by wind acting on one or more sails, kites, and/or other means of wind-powering the watercraft.

15. A watercraft comprising:

at least one hydrofoil; one or more apertures disposed on a first and/or second surface of the hydrofoil; means for determining an amount of fluid to expel from, or draw into, at least one of the one or more apertures; and

control means for controlling the expelling or drawing of the fluid through the at least one of the one or more apertures based on the amount determined by the means for determining the amount of fluid, thereby to alter and/or generate a yaw, pitch and/or roll force on the hydrofoil so as to control the watercraft.

16. The watercraft of claim 15, having a longitudinal axis in an intended direction of travel through water, wherein:

the at least one hydrofoil comprises:

a first hydrofoil portion including one or more of the apertures; and a second hydrofoil portion angularly offset around the longitudinal axis from the first hydrofoil portion, the second hydrofoil portion including one or more of the apertures; and

the control means are configured to selectively expel or draw the fluid differently through the one or more apertures on the first hydrofoil portion relative to the one or more apertures on the second hydrofoil portion, thereby to generate the force.

17. The watercraft of claim 16, wherein:

the first hydrofoil portion includes the first surface in the form of an upper surface and the second surface in the form of a lower surface; and

the second hydrofoil portion includes the first surface in the form of an upper surface and the second surface in the form of a lower surface, wherein each of the first and second hydrofoils portions' upper surfaces and lower surfaces includes at least one of the apertures; and

the control means are configured to selectively expel or draw the fluid from some or all of the apertures, thereby to generate the force.

18. The watercraft of any one of claims 15 to 17, having at least one lateral axis, wherein:

the at least one hydrofoil comprises:

a third hydrofoil portion including one or more of the apertures; and a fourth hydrofoil portion offset rearwardly, relative to an intended forward direction of travel of the watercraft, from the third hydrofoil, the fourth hydrofoil portion including one or more of the apertures;

the control means are configured to selectively expel or draw the fluid differently through the one or more apertures on the third hydrofoil portion relative to the one or more apertures on the fourth hydrofoil portion, thereby to generate the force.

19. The watercraft of claim 18, wherein:

the third hydrofoil portion includes the first surface in the form of an upper surface and the second surface in the form of a lower surface;

the fourth hydrofoil portion includes the first surface in the form of an upper surface and the second surface in the form of a lower surface, wherein each of the third and further hydrofoil portions' upper surfaces and lower surfaces includes at least one of the apertures; and

the control means are configured to selectively expel or draw the fluid through some or all of the outlets, thereby to generate the force.

20. The watercraft of any one of claims 15 to 19, wherein:

the hydrofoil includes a plurality of the apertures, one or more of the apertures comprising rearward apertures and one or more of the apertures comprising forward apertures, the one or more rearward apertures being offset rearwardly, relative to an intended forward direction of travel of the watercraft, from one or more of the forward apertures; and

the control means are configured to selectively expel or draw the fluid differently through the rearward outlets relative to the forward outlets, thereby to generate the force.

21. The watercraft of any one of claims 15 to 20, wherein at least one of the apertures is positioned at or adjacent a flow separation point.

22. The watercraft of any one of claims 15 to 21 , comprising one or more sensors, comprising any one or more of:

one or more water speed sensors for determining a speed of the hydrofoil and/or the watercraft relative to the water; one or more angle sensors for determining an angle of attack of the hydrofoil relative to the water;

one or more pressure sensors for determining a fluid pressure;

one or more GPS location units; and

23. The watercraft of any one of claims 15 to 22, wherein the means for determining an amount of fluid to expel from, or draw into, at least one of the one or more apertures is configured to make the determination based on at least one of:

an angle of attack of at least one of the hydrofoil portions;

a water speed of the hydrofoil;

a pitch angle of the watercraft;

a roll angle of the watercraft;

a yaw angle of the watercraft; and

24 The watercraft of any one of claims 15 to 23, configured to expel a portion of the fluid as a jet in order to propel the watercraft through the water.

25. The watercraft of any one of claims 15 to 24, wherein the watercraft comprises a waterjet outlet separate from the one or more apertures, for expelling a fluid under pressure to propel the watercraft through water.

26. The watercraft of any one of claims 15 to 25, wherein the means for determining an amount of fluid to expel from, or draw into, at least one of the one or more apertures determines the amount of fluid so as to improve any one or more of the following:

stability of the watercraft;

energy efficiency of the watercraft; and

turning efficiency of the watercraft.

27. The watercraft of any one of claims 15 to 26, wherein the means for determining an amount of fluid to expel from, or draw into, at least one of the one or more apertures comprises a digital processor.

28. The watercraft of any one of claims 15 to 27, comprising a platform for a human and/or a load, the at least one hydrofoil being mounted beneath the platform such that, in use, the lift generated by the hydrofoil as it is propelled through the water lifts the platform off the water surface.

29. The watercraft of claim 28, wherein the platform is a surfboard or a sailboard, or takes the physical shape and/or form of a surfboard or sailboard.

30. The watercraft of any one of claims 15 to 29, wherein the watercraft is a sailboard, windsurf board, kite-surf board, sailboat, yacht, skiff, or any other form of wind-powered watercraft.

31. The watercraft of claim 30, wherein the hydrofoil includes a vertical component, the first surface is a left surface and the second surface is a right surface, configured such that the force may be directed to counteract an opposite roll force generated by wind acting on one or more sails, kites, and/or other means of wind-powering the watercraft.

32. The watercraft of any one of claims 15 to 31 , comprising a pump for pressurising the fluid to cause it to selectively be expelled or drawn through the one or more apertures.

33. The watercraft of any one of claims 15 to 32, wherein the watercraft is autonomous or semi-autonomous.