GB2522066A

GB2522066A - Boat hydrofoil

Info

Publication number: GB2522066A
Application number: GB1400558.1A
Authority: GB
Inventors: Michael Barron
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-01-14
Filing date: 2014-01-14
Publication date: 2015-07-15
Anticipated expiration: 2034-01-14
Also published as: GB201400558D0; EP2894090A1; GB2522066B

Abstract

A hydrofoil (100, Fig 1A) for a boat 110 comprises a first section 102 for providing a first hydrodynamic force and a second section 104 for providing a second hydrodynamic force. The hydrofoil is connectable to a boat such that the first section extends generally downwards away from a hull 108 of the boat and towards the centreline 112 of the boat and the second section 104 extends under the first section 102 and generally away from the centreline of the boat. The hydrofoil can provide stable boat ride height, good boat stability and increased lift for a given width of hydrofoil.

Description

BOAT HYDROFOIL

The present invention relates to a hydrofoil for a boat and to a boat having one or more hydrofoils.

Hydrofoils are increasingly being used on boats to achieve better performance by lifting the boat up out of the water so as to reduce drag.

However, current foil systems often fail to deliver the desired performance with ease of boat handling. It is a principal aim of this invention to provide a hydrofoil for a boat that overcomes the shortcomings of the current foil systems.

Thus, according to an aspect of this invention there is provided a hydrofoil for a boat, the hydrofoil comprising: a first section for providing a first hydrodynamic force; and a second section for providing a second hydrodynamic force; wherein the hydrofoil is connectable to a boat such that the first section extends generally downwards away from a hull of the boat and towards the centreline of the boat and the second section extends under the first section and generally away from the centreline of the boat.

It will be appreciated that a hydrodynamic force" as referred to above is a force (e.g. "lift") which is provided by a structure as a result of the flow of water around that structure. The hydrofoil of the present invention has a first section that provides a first hydrodynamic force and that extends generally downwards away from the hull of the boat and towards the centreline of the boat. Thus, the amount of the first section that is in the water varies gradually with boat ride height. As will be appreciated, the amount of lift produced by a hydrofoil is dependent on boat speed and the area of the hydrofoil that is in the water. When boat speed is higher, more lift is produced per area of a hydrofoil.

In the present invention, as boat speed increases, progressively less of the first section needs to be, and is, in the water. Since less of the first section is in the water at higher speeds, the amount of drag produced by the first section is reduced. Conversely, when boat speed decreases, less lift is produced per area of the first section. However, this is compensated for in the present invention because progressively more of the first section is in the water. Thus, despite changes in boat speed, a more constant amount of lift and a generally stable boat ride height is provided by the hydrofoil of the present invention.

Furthermore, the hydrofoil of the present invention has a second section that provides a second hydrodynamic force and that extends under the first section and generally away from the centreline of the boat. Thus, the location for some of the lift generation of the hydrofoil is further away from the centreline of the boat. Also, when the boat is generally level, the base for hydrofoiling can be very wide. This provides good boat stability.

Furthermore, including first and second sections for providing hydrodynamic forces that are above and below one another increases (e.g. doubles) the available hydrofoil area for a given width of hydrofoil. This may be useful where hydrofoil width is restricted, for example in A Class catamarans.

In use, the direction in which the second hydrodynamic force acts is preferably generally upwards. The direction in which the first hydrodynamic force acts is preferably generally upwards and towards the centreline of the boat. Thus, as the boat heels over, the first section preferably provides progressively more and more upward lift. As will be discussed in more detail below, the boat preferably comprises multiple hulls with one of the hydrofoils of the present invention being provided on each hull. Accordingly, when the boat is heeled over, only one hydrofoil is likely to be in the water. Conversely, when the boat is level, two (oppositely configured) hydrofoils are likely to be in the water.

Thus, for a same set of hydrodynamic conditions, the second hydrodynamic force of the hydrofoil (which tends to act in tandem with the second hydrodynamic force of another hydrofoil to a greater extent) need not be as great, and preferably is not as great, as the first hydrodynamic force (which tends to act in tandem with the first hydrodynamic force of another hydrofoil to a lesser extent). However, embodiments are contemplated in which, for a same set of hydrodynamic conditions, the first hydrodynamic force of the hydrofoil is not as great, or is the same, as the second hydrodynamic force.

In preferred embodiments, the hydrofoil is connectable to the boat at or near a first end of the first section. A second end of the first section may be directly or indirectly connected to a first end of the second section. The first section and second section may be formed as part of a unitary structure. The second section preferably has a free second end.

The angle between a) the direction in which the first hydrodynamic force acts and b) the direction in which the second hydrodynamic force acts is preferably between Q° and 80°, more preferably between 0° and 60°, for example approximately 45°. Thus, in some embodiments, the first and second hydrodynamic forces act in substantially the same direction. Similarly, the angle between a) the notional plane of the first section (which is generally perpendicular to the direction in which the first hydrodynamic force acts) and b) the notional plane of the second section (which is generally perpendicular to the direction in which the second hydrodynamic force acts) is preferably between 0° and 80°, more preferably between 0° and 60°, for example approximately 45°.

Thus, in some embodiments, the notional planes of the first and second section are substantially parallel. It will be appreciated that, in some embodiments, the second section may also extend upwards towards the hull of the boat.

It will be appreciated that the expression "notional plane" has been used because the first and second sections generally are not planar structures and will typically have irregular or curved cross-sections (so as to provide "lift"). The first and second sections may also curve from a first end of the section to a second end of the section.

The hydrofoil preferably comprises a third section between the first and second sections. This can, for example, mean that there is always a part of the hydrofoil that is in the water and provides lateral stability. The third section may allow the hydrofoil to extend deeper into the water thereby giving a higher ride height. The third section may also or instead be used to create other useful hydrodynamic forces, e.g. hydrodynamic forces towards the centreline of the boat and/or upwards. Thus, the third section preferably extends generally downwards away from the hull of the boat and/or preferably provides a third hydrodynamic force. In use, the direction in which the third hydrodynamic force acts is preferably generally towards the centreline of the boat and/or is preferably generally at an angle upwards and towards the centreline of the boat.

In preferred embodiments, the angle between a) the general direction of extension between first and second ends of the first section and b) the general direction of extension between first and second ends of the third section is between 200 and 80°, more preferably between 35° and 55°, for example approximately 45°. The angle between a) the general direction of extension between first and second ends of the third section and b) the general direction of extension between first and second ends of the second section is preferably between 80° and 135°, more preferably between 80° and 100°, for example approximately 90°.

In some embodiments, the first section, second section and/or third section of the hydrofoil has a static part and a moveable part that can be moved in use so as to alter the hydrodynamic properties of the section in question. The static part is preferably upstream of the moveable part in normal use, i.e. when the boat is moving forwards and the hydrodynamic forces are being produced.

In some embodiments, the moveable part is pivotable about an axis which is at or towards the upstream end of the moveable part.

In some embodiments, the first section, second section and/or third section of the hydrofoil has a static part and a deformable part which can be deformed in use so as to alter the hydrodynamic properties of that section. The static part is preferably upstream of the deformable part in normal use, i.e. when the boat is moving forwards and the hydrodynamic forces are being produced.

In some of these embodiments, the deformable part is flexibly connected to the static part, for example by a strut and/or by the outer casing of the section in question. In others of these embodiments, the deformable part comprises a cavity within the casing of the section in question, the cavity preferably being fluidly connected to an opening at or near the downstream end of the section in question.

The present invention also extends to a boat comprising one or more hydrofoils as described above. The boat preferably comprises multiple hulls, with at least one hydrofoil of the one or more hydrofoils being provided on each hull. The one or more hydrofoils may comprise at least two hydrofoils (e.g. one hydrofoil on each hull) that are oppositely configured to one another and/or that are mirror images of one another.

The one or more hydrofoils may each have a position that is adjustable, for example by one or more of: translating that hydrofoil in the fore to aft direction of the boat; rotation of that hydrofoil about an axis that runs in the fore to aft direction of the boat; pivoting that hydrofoil about an axis that runs athwartships; and/or retracting or deploying that hydrofoil in a vertical direction, for example up through the hull of the boat. In preferred embodiments, the one or more hydrofoils are each able to be partially or completely stowed by rotation of that hydrofoil, for example about an axis that runs in the fore to aft direction of the boat, and/or by retraction of that hydrofoil, for example up through the hull of the boat. The means for adjusting the position of the hydrofoil and the means for stowing the hydrofoil may be the same or different means.

By way of example only, embodiments of the invention will now be described in detail with reference being made to the accompanying drawings in which: Figures lA-iD are simplified end views of hydrofoils according to embodiments of the present invention; Figure 2 shows a simplified end view and cross-sectional views of a hydrofoil according to an embodiment of the present invention when provided on a hull of a multi-hulled boat; Figures 3A-3C show means for rotating and/or stowing hydrofoils according to embodiments of the present invention; Figures 4A-4B show alternative means for stowing a hydrofoil according to an embodiment of the present invention; Figure 5 shows means for pivoting a hydrofoil according to an embodiment of the present invention; Figure 6 shows in more detail a hydrofoil according to an embodiment of the present invention; Figures 7A-7B show in more detail a hydrofoil according to another embodiment of the present invention; Figure 8 shows in more detail a hydrofoil according to another embodiment of the present invention; and Figures 9A-9C show cross-sectional views of hydrofoils according to embodiments of the present invention in which the sections of the hydrofoils have movable or deformable parts.

Figures lA-iD show several profiles for hydrofoils 100 according to embodiments of the present invention. In these embodiments, each hydrofoil is connected by suitable fixings to a hull 108 of a boat. Each hydrofoil 100 has a first section 102 that provides a first hydrodynamic force, a second section 104 that provides a second hydrodynamic force, and a third section 106 that links the first section 102 to the second section 104. The third section 106 may or may not provide a third hydrodynamic force. The various sections of the hydrofoils 100 may be separate structures that are later connected together or may form part of a unitary structure. The hydrofoils 100 of Figures 1A and 1C are generally curved whereas the hydrofoils 100 of Figures lB and 1D are generally angular.

Figure 2 shows the hydrofoil 100 of Figure 1D when attached to a hull 108 of a multi-hulled boat 110. The hydrofoil 100 is connected to the hull 108 near a first end of the first section 102 such that the first section 102 extends generally downwards away from the hull 108 of the boat 110 and towards the centreline 112 of the boat 110. The third section 106 then extends generally downwards away from the hull 108 of the boat 110. The second section 104 then extends under the first section and generally away from the centreline 112 of the boat 110. Only half of the boat 110 is shown in Figure 2, but it will be appreciated that a second mirror image' hydrofoil, which is oppositely configured to the hydrofoil 100, is fixed to the second hull of the boat 110.

Figure 2 also shows the first section 102 and the second section 104 in cross-section. The first section 102 has a pressure side surface 102a that runs from a leading edge 102c to a trailing edge 102d, and a suction side surface 102b that runs from the leading edge 102c to the trailing edge 102d. The leading edge 102c points fore and the trailing edge 102d points aft. In normal use, when the boat 110 is moving forwards, the pressure side surface 1 02a and suction side surface 102b create the first hydrodynamic force. The direction in which the first hydrodynamic force acts is generally upwards and towards the centreline 112 of the boat.

The second section 104 also has a pressure side surface 1 04a that runs from a leading edge 104c to a trailing edge 104d, and a suction side surface 104b that runs from the leading edge 104c to the trailing edge 104d. The leading edge 104c again points fore and the trailing edge 104d points aft. In normal use, when the boat 110 is moving forwards, the pressure side surface 104a and suction side surface 104b create the second hydrodynamic force. The direction in which the second hydrodynamic force acts is generally upwards.

In the above embodiments, the position of the hydrofoil 100 is fixed.

However, in the following embodiments, the hydrofoils can be repositioned or stowed. Repositioning the hydrofoil 100 may be desirable so as to adjust the balance, e.g. pitch, of the boat 110. Stowing the hydrofoil 100 may be desirable in light wind conditions, i.e. when the hydrofoil 100 is unlikely to create sufficient lift and would merely create drag.

Figure 3A shows the hydrofoil 100 of figure 1A when fitted to a rotating mount 114 and figures 3B and 3C show the hydrofoil 100 of figure lB when fitted to a rotating mount 114. Figures 3A-C show that the hydrofoils 100 can be stowed, out of the water, by rotation of that hydrofoil 100 about an axis 116 that runs along the hull 108 in the fore to aft direction of the boat 110. In these embodiments, the rotating mount 114 can also be translated along the hull 108 so as to vary the fore-aft position of the hydrofoil 100 to alter the pitch of the boat 110 as desired.

Figures 4A and 4B show an alternative way to stow the hydrofoil 100 of Figure 1 B. As is shown in Figure 4A, the hydrofoil 100 is first rotated from its fully deployed position (shown with a dashed line) to a partially deployed position (shown with a solid line). As is shown in Figure 4B, the hydrofoil 100 is then retracted from its partially deployed position (shown with a dashed line) to a fully stowed position (shown with a solid line) up through the hull 108 of the boat. The opposite process is then used to redeploy the hydrofoil 100. As will be discussed below with reference to Figures 7A-7B, the fully deployed position may also be used as a "light wind" position for the hydrofoil and the partially deployed position may also be used as a "strong wind" position for the hydrofoil.

Figure 5 is a side view of the hydrofoil 100 of figure lAwhen mounted on the hull 108 of a boat at a pivot point 118. The attack angle of the hydrofoil 100 can be altered by pivoting the hydrofoil 100 about an axis that runs athwartships through the pivot point 118.

Figure 6 illustrates the angles between the various sections of a fixed hydrofoil 100. In this embodiment, the angle p between the general direction of extension of the first section 102 and the general direction of extension of the third section 106 is 40°, and the angle 9 between the general direction of extension of the third section 106 and the general direction of extension of the second section 104 is 110°. The angle between the directions in which the first and second hydrodynamic forces act, and also the angle between the notional planes of the first and second sections, is therefore 30°. These angles mean that the second section 104, in addition to extending away from the centreline of the boat, also extends slightly upwards towards the hull 108 of the boat. This provides the boat with good strong wind stability, i.e. when the majority of the first section 102 is lifted out of the water.

Figure 7A illustrates the angles between the various sections of a rotatable hydrofoil 100. In this Figure, the first section 102 is not as steep as the first section in the embodiment of Figure 6 and the second section 104 is more horizontal than the second section in the embodiment of Figure 6. In particular, in this embodiment, the angle p between the general direction of extension of the first section 102 and the general direction of extension of the third section 106 is 45° and the angle e between the general direction of extension of the third section 106 and the general direction of extension of the second section 104 is 90°. The angle between the directions in which the first and second hydrodynamic forces act, and also the angle between the notional planes of the first and second sections, is therefore 45°. This arrangement provides the boat with good light wind performance, i.e. when the majority of the first section is in the water (see Figure 7A), because the first and second sections can contribute significantly to the upward lift. However, in strong wind conditions, the hydrofoil can be rotated such that the first section 102 is steeper and the second section 104 extends slightly upwards towards the hull 108 of the boat (see Figure 7B).

This provides the boat with good strong wind stability.

Figure 8 illustrates the angles between the various sections of an alternative fixed hydrofoil 100. The angle p between the general direction of extension of the first section 102 and the general direction of extension of the third section 106 is 45° and the angle e between the direction of extension of the third section 106 and the direction of extension of the second section 104 is 135°. The angle between the directions in which the first and second hydrodynamic forces act, and also the angle between the notional planes of the first and second sections, is therefore 00. This means that the second section 104 extends upwards towards the hull 108 of the boat to the same degree that the first section 102 extends downwards away from the hull 108 of the boat.

This provides the boat having the fixed hydrofoil with excellent strong wind stability and allows the first and second sections to provide hydrodynamic forces which act in generally the same direction.

Figures 9A-9C show cross-sectional views of hydrofoil sections having movable or deformable parts. The features of these embodiments can be applied to the first section, second section and/or third section of the hydrofoils 100 described above.

In Figure 9A, the hydrofoil section shown has an upstream static part 120 and a downstream moveable part 122 that can be moved in use so as to alter the hydrodynamic properties of the section. The moveable part 122 is pivotable about an axis which is towards the upstream end of the moveable part 122.

In Figure 9B, the hydrofoil section shown has an upstream static part 120 and a downstream deformable part 124 which can be deformed in use so as to alter the hydrodynamic properties of the section. The deformable part 124 is flexibly connected to the static part 120 by a strut 125 and by the outer casing of the section.

In Figure 9C, the hydrofoil section shown also has an upstream static part 120 and a downstream deformable part 124 which can be deformed in use so as to alter the hydrodynamic properties of the section. However, in this embodiment, the deformable part 124 comprises a cavity 126 within the casing of the section. The cavity 126 is fluidly connected to an opening 128 at the downstream end of the section.

Claims

CLAIMS1. A hydrofoil for a boat, the hydrofoil comprising: a first section for providing a first hydrodynamic force; and a second section for providing a second hydrodynamic force; wherein the hydrofoil is connectable to a boat such that the first section extends generally downwards away from a hull of the boat and towards the centreline of the boat and the second section extends under the first section and generally away from the centreline of the boat.
2. A hydrofoil as claimed in claim 1 wherein, in use, the direction in which the first hydrodynamic force acts is generally upwards and towards the centreline of the boat.
3. A hydrofoil as claimed in claim 1 or 2 wherein, in use, the direction in which the second hydrodynamic force acts is generally upwards.
4. A hydrofoil as claimed in claim 1, 2 or 3 wherein, for a same set of hydrodynamic conditions, the first hydrodynamic force is greater than the second hydrodynamic force.
5. A hydrofoil as claimed in any one of the preceding claims wherein the hydrofoil is connectable to the boat at or near a first end of the first section.
6. A hydrofoil as claimed in any one of the preceding claims wherein a second end of the first section is directly or indirectly connected to a first end of the second section and/or wherein the first section and second section form part of the same unitary structure.
7. A hydrofoil as claimed in any one of the preceding claims wherein the second section has a free second end.
8. A hydrofoil as claimed in any one of the preceding claims wherein: the angle between a) the direction in which the first hydrodynamic force acts and b) the direction in which the second hydrodynamic force acts is between 00 and 800; and/or the angle between a) the notional plane of the first section and b) the notional plane of the second section is between QO and 800.
9. A hydrofoil as claimed in any one of the preceding claims wherein the second section also extends generally upwards towards the hull of the boat.
10. A hydrofoil as claimed in any one of the preceding claims comprising a third section between the first and second sections.
11. A hydrofoil as claimed in claim 10 wherein the third section extends generally downwards away from the hull of the boat.
12. A hydrofoil as claimed in claim 10 or 11 wherein the third section provides a third hydrodynamic force.
13. A hydrofoil as claimed in claim 12 wherein, in use, the direction in which the third hydrodynamic force acts is generally towards the centreline of the boat.
14. A hydrofoil as claimed in claim 12 or 13 wherein, in use, the direction in which the third hydrodynamic force acts is generally at an angle upwards and towards the centreline of the boat.
15. A hydrofoil as claimed in any one of claims 10-14 wherein the angle between a) the general direction of extension between first and second ends of the first section and b) the general direction of extension between first and second ends of the third section is between 20° and 80°.
16. A hydrofoil as claimed in any one of claims 10-15 wherein the angle between a) the general direction of extension between first and second ends of the third section and b) the general direction of extension between first and second ends of the second section is between 80° and 1350.
17. A hydrofoil as claimed in any one of the preceding claims wherein the first section, second section and/or third section has a moveable and/or deformable part.
18. A boat comprising one or more hydrofoils as claimed in any one of the preceding claims.
19. A boat as claimed in claim 18 wherein the boat comprises multiple hulls, at least one hydrofoil of said one or more hydrofoils being provided on each hull.
20. A boat as claimed in claim 18 or 19 wherein the one or more hydrofoils each have a position that is adjustable by translating that hydrofoil in the fore to aft direction of the boat.
21. A boat as claimed in claim 18, 19 or 20 wherein the one or more hydrofoils each have a position that is adjustable by rotation of that hydrofoil about an axis that runs in the fore to aft direction of the boat.
22. A boat as claimed in any one of claims 18-21 wherein the one or more hydrofoils each have a position that is adjustable by pivoting that hydrofoil about an axis that runs athwartships.
23. A boat as claimed in any one of claims 18-22 wherein the one or more hydrofoils each have a position that is adjustable by retracting or deploying that hydrofoil in a vertical direction.
24. A boat as claimed in any one of claims 18-23 wherein the one or more hydrofoils are each able to be partially or completely stowed by rotation of that hydrofoil, for example about an axis that runs in the fore to aft direction of the boat, and/or by retraction of that hydrofoil, for example up through the hull of the boat.
25. A boat as claimed in any one of claims 18-24 wherein the one or more hydrofoils comprises at least two hydrofoils that are oppositely configured to one another and/or that are mirror images of one another.
26. A hydrofoil for a boat substantially as hereinbefore described with reference to and as illustrated in figures 1-9.
27. A boat having one or more hydrofoils substantially as hereinbefore described with reference to and as illustrated in figures 1-9.Amendments to the claims have been made as follows:CLAIMS1. A hydrofoil for a boat, the hydrofoil comprising: a first section for providing a first hydrodynamic force; and a second section for providing a second hydrodynamic force; wherein the hydrofoil is connectable to a boat such that the first section extends generally downwards away from a hull of the boat and towards the centreline of the boat and the second section extends under the first section and generally away from the centreline of the boat.2. A hydrofoil as claimed in claim 1 wherein, in use, the direction in which the first hydrodynamic force acts is generally upwards and towards the IC) 10 centreline of the boat.3. A hydrofoil as claimed in claim 1 or 2 wherein, in use, the direction 0 in which the second hydrodynamic force acts is generally upwards.4. A hydrofoil as claimed in claim 1, 2 or 3 wherein, for a same set of hydrodynamic conditions, the first hydrodynamic force is greater than the second hydrodynamic force.5. A hydrofoil as claimed in any one of the preceding claims wherein the hydrofoil is connectable to the boat at or near a first end of the first section.6. A hydrofoil as claimed in claim 5 wherein a second end of the first section is directly or indirectly connected to a first end of the second section and/or wherein the first section and second section form part of the same unitary structure.7. A hydrofoil as claimed in claim 6 wherein the second section has a free second end.8. A hydrofoil as claimed in any one of the preceding claims wherein: the angle between a) the direction in which the first hydrodynamic force acts and b) the direction in which the second hydrodynamic force acts is between 00 and 800; and/or the angle between a) the notional plane of the first section and b) the notional plane of the second section is between QO and 800.9. A hydrofoil as claimed in any one of the preceding claims wherein, in use, the second section also extends generally upwards towards the hull of the boat.10. A hydrofoil as claimed in any one of the preceding claims IC) comprising a third section between the first and second sections.11. A hydrofoil as claimed in claim 10 wherein, in use, the third 0 section extends generally downwards away from the hull of the boat.LI) 12. A hydrofoil as claimed in claim 10 or 11 wherein the third section provides a third hydrodynamic force.13. A hydrofoil as claimed in claim 12 wherein, in use, the direction in which the third hydrodynamic force acts is generally towards the centreline of the boat.14. A hydrofoil as claimed in claim 12 or 13 wherein, in use, the direction in which the third hydrodynamic force acts is generally at an angle upwards and towards the centreline of the boat.15. A hydrofoil as claimed in any one of claims 10-14 wherein the angle between a) the general direction of extension between first and second ends of the first section and b) the general direction of extension between first and second ends of the third section is between 20° and 80°.16. A hydrofoil as claimed in any one of claims 10-15 wherein the angle between a) the general direction of extension between first and second ends of the third section and b) the general direction of extension between first and second ends of the second section is between 800 and 1350.17. A hydrofoil as claimed in any one of claims 10-16 wherein the first section, second section and/or third section has a moveable and/or deforrnable part.18. A boat comprising one or more hydrofoils as claimed in any one of IC) the preceding claims.19. A boat as claimed in claim 18 wherein the boat comprises multiple O hulls, at least one hydrofoil of said one or more hydrofoils being provided on LCD each hull.20. A boat as claimed in claim 18 or 19 wherein the one or more hydrofoils each have a position that is adjustable by translating that hydrofoil in the fore to aft direction of the boat.21. A boat as claimed in claim 18, 19 or 20 wherein the one or more hydrofoils each have a position that is adjustable by rotation of that hydrofoil about an axis that runs in the fore to aft direction of the boat.22. A boat as claimed in any one of claims 18-21 wherein the one or more hydrofoils each have a position that is adjustable by pivoting that hydrofoil about an axis that runs athwartships.23. A boat as claimed in any one of claims 18-22 wherein the one or more hydrofoils each have a position that is adjustable by retracting or deploying that hydrofoil in a vertical direction.24. A boat as claimed in any one of claims 18-23 wherein the one or more hydrofoils are each able to be partially or completely stowed by rotation of that hydrofoil, for example about an axis that runs in the fore to aft direction of the boat, and/or by retraction of that hydrofoil, for example up through the hull of the boat.25. A boat as claimed in any one of claims 18-24 wherein the one or more hydrofoils comprises at least two hydrofoils that are oppositely configured IC) to one another and/or that are mirror images of one another.26. A hydrofoil for a boat substantially as hereinbefore described with 0 reference to and as illustrated in figures 1-9.tX) 27. A boat having one or more hydrofoils substantially as hereinbefore described with reference to and as illustrated in figures 1-9.