GB2558181A

GB2558181A - Hydrofoil system for a watercraft

Info

Publication number: GB2558181A
Application number: GB1612559.3A
Authority: GB
Inventors: Sharon Perry John
Original assignee: Individual
Current assignee: Individual
Priority date: 2016-07-20
Filing date: 2016-07-20
Publication date: 2018-07-11
Also published as: GB201612559D0

Abstract

A hydrofoil system for a watercraft suitable for controlling the pitch and / or height of the water craft above the water surface. Comprising a hydrofoil supported by a nominally perpendicular strut, wherein the strut is pivotally mounted to allow the hydrofoil and strut to rotate about an axis that is nominally perpendicular to the plane of the water flow and lying ahead of the strut. Preferably the mounting means is a universal joint that allows an additional axis of rotation permitting the angle of attack of the hydrofoil to be adjusted and/or allow the angle of the strut to the water plane to be adjusted, and/or permit the hydrofoil and strut to be moved to a stowed position when not in use. Preferably the rotation of the hydrofoil and strut may be controlled to intentionally reduce a roll angle and/or control the heave of the craft, via a sensing system and pulleys or telescopic strut and the like. The hydrofoil may have an optional winglet extension on one end of the hydrofoil / wing to assist in controlling the height of the hydrofoil, should the hydrofoil break through the water surface.

Description

(54) Title of the Invention: Hydrofoil system for a watercraft Abstract Title: Hydrofoil system for a watercraft (57) A hydrofoil system for a watercraft suitable for controlling the pitch and / or height of the water craft above the water surface. Comprising a hydrofoil supported by a nominally perpendicular strut, wherein the strut is pivotally mounted to allow the hydrofoil and strut to rotate about an axis that is nominally perpendicular to the plane of the water flow and lying ahead of the strut. Preferably the mounting means is a universal joint that allows an additional axis of rotation permitting the angle of attack of the hydrofoil to be adjusted and/or allow the angle of the strut to the water plane to be adjusted, and/or permit the hydrofoil and strut to be moved to a stowed position when not in use. Preferably the rotation of the hydrofoil and strut may be controlled to intentionally reduce a roll angle and/or control the heave of the craft, via a sensing system and pulleys or telescopic strut and the like. The hydrofoil may have an optional winglet extension on one end of the hydrofoil / wing to assist in controlling the height of the hydrofoil, should the hydrofoil break through the water surface.

At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

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Hydrofoil system for a water craft

This invention relates to a hydrofoil system for a water craft. The system is described with reference to the trimaran configuration of sailing boat but could also be applicable to other configurations of sailing boats including, but not limited to, proas, catamarans and mono-hulls and also to powered watercraft.

The invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows an arrangement of hydrofoils that has been used with some sailing trimarans

Figure 2 shows an arrangement as figure 1 but with an immersed hull

Figure 3 shows an arrangement as figure 2 but with one hydrofoil stowed

Figure 4 shows an arrangement of curved hydrofoils that has been used with some sailing trimarans

Figure 5 shows an arrangement of two Tee-foil units on a sailing craft, one unit active, the other stowed

Figure 6 shows details of a Tee-foil unit

Figure 7 shows an arrangement of two Tee-foil units on a sailing craft, both units active

Figure 8 shows a view on a tee-foil unit looking in a direction perpendicular to the plane of symmetry of the unit.

Figure 9 shows an overall perspective view of a sailing boat sailing with one tee foil unit active and a second one stowed.

Figure 10 shows detail of the active Tee-foil unit included in Figure 9

Figure 11 shows detail of the stowed Tee-foil unit included in Figure 9

Hydrofoils, in the context of sailing boats, are underwater 'wings' that generate hydrodynamic forces when the boat is in motion. The forces generated by hydrofoils can wholly or partially support the weight of a boat and using hydrofoils to provide such support can potentially be more efficient than utilising buoyancy forces on an immersed hull for the same purpose (in this context efficiency is taken to mean lift to drag ratio). However, there will generally be a low speed range in which hydrofoils are less efficient than the use of buoyancy forces as a means to support the weight of a boat. The main reason for using hydrofoils on boats is to reduce drag over the medium to high speed range and so improve overall performance, but there may also be a benefit from increased motion damping in rough water.

Figure 1 shows an arrangement of hydrofoils that has been used with some sailing trimarans. This figure is a view down the longitudinal axis of the craft and it shows the craft sailing with all three hulls lifted clear of the water by hydrofoils. Hydrofoils hl and h2 would typically be on hinged mounts so that they can be retracted upwards when not in use, as shown in figure 1, or they would be fitted through trunkings built into the outer hulls so that they can be retracted into these trunkings when not in use.

With reference to figure 1, the arrow A represents the horizontal force vector resulting from wind on the rig, the arrows Hl and H2 represent the hydrodynamic forces on the hydrofoils hl and h2 respectively and the arrow W represents the force of gravity acting vertically downwards though the Centre of Gravity (COG) of the craft, these being the principle forces acting on the craft. In addition to the hydrofoils shown in figure 1, there would normally be a means to stabilise the craft in pitch, typically by a hydrofoil(s) mounted horizontally at the stern of the craft and there would be a means to steer the craft, typically by a rudder(s) mounted vertically at the stern of the craft. The rudder(s) and stern hydrofoil(s) are not shown in figure 1.

With reference to Figure 1, under steady state conditions the sum of the vertical components of the forces Hl and H2, together with any vertical force from the stern hydrofoil(s), balances the weight of the craft. Similarly the sum of the horizontal components of forces Hl and H2 balances force A, the aerodynamic force on the rig. Force A causes the craft to make leeway, i.e. its velocity includes not only a component aligned with the nominal geometrical centreline of the craft but also a lateral component of velocity. Leeway increases the angle of attack for hl and reduces it for h2. This in turn results in an increase in force Hl and a reduction in force H2. The geometry of the craft is arranged so that the force vectors Hl, H2, A and W, as seen in a view down the craft centreline, all pass through a single point, this point being marked CoE in figure 1. This means that there is no moment arm between the lines of action of these forces that would generate a moment to heel the craft, hence the craft is inherently stable in roll. There may be some heel perturbation caused by lateral accelerations of the CoG of the craft during course changes and also by forces that do not act through CoE, for example lateral forces on the rudder or the effect of movements of crew weight. Any such heel perturbation will be countered by a correcting couple generated by lateral displacement of CoG, and hence the line of action of W, relative to the point CoE.

Figure 1 shows the craft sailing with the hulls clear of the water, but the craft can also be stable in roll when one or more hulls are immersed, as in figure 2. In this case the force G is balanced not only by the vertical components of Hl and H2 but also by a buoyancy force B.

It is also possible to achieve roll stability with the configuration as shown in figure 1 but with one of the hydrofoils retracted clear of the water, as figure 3. With reference to figure 3, the upwind hydrofoil, h2, is retracted to a stowed position and hl, the downwind hydrofoil, is active. Having only the downwind hydrofoil active reduces the wetted surface and hence skin friction. Also, the force generated by the downwind hydrofoil will always have an upwards component, reducing the proportion of craft weight that needs to be carried by hull buoyancy and so reducing drag over the medium to high speed range, whereas the vertical component of force generated by the upwind hydrofoil deployed as in figure 2 may act either upwards or downwards, depending on leeway and the pitch angle of the craft.

With reference to figure 3, as the force of the wind on the rig increases so does the balancing horizontal component of force Hl. There is a corresponding increase in the vertical component of force Hl, this causing the craft to raise slightly relative to the water surface giving a compensating reduction in the buoyancy force B. Further increase in the force of the wind will cause all hulls to rise clear of the water surface in which case the buoyancy force B is no longer available to compensate for changes in vertical force from the hydrofoil hl and so it becomes necessary to actively control the magnitude of the force A by continuous adjustment of the rig in order to maintain stability in roll.

Figure 4 shows a variation to the hydrofoil arrangement shown in figures 1,2 and 3. In this case the straight hydrofoils are replaced by curved hydrofoils that fit through curved trunkings built into the outer hulls and which can retract into these trunkings when not deployed. In principle this configuration is similar to that shown in figures 1,2 and 3 since the overall hydrodynamic force vector(s) acting on the immersed part of the curved foil(s) can have a similar line(s) of action to the hydrodynamic force vector(s) acting on the non-curved foils shown in figures 1,2 and 3. This arrangement with curved foils is now widely used on high performance sailing trimarans, the hydrofoils usually being deployed as shown in figure 4 with the upwind hydrofoil retracted and inactive and a downwind hydrofoil serving both to carry part of the weight of the craft and to resist leeway. The design intent for a modern trimaran sailing boat with curved hydrofoils would typically be to carry around 70% of the total weight of the craft on the downwind curved hydrofoil when the craft reaches the designed maximum speed.

It is noted that for a given height of the point CoE above water, and hence rig height, the arrangement with curved hydrofoils as figure 4 has the advantage that the width of the craft measured over the outer hulls is less than for the arrangement shown in figures 1,2 and 3. It is desirable to minimise the width of the craft to reduce weight, air drag and space taken up by the craft when it is not in use. The curved hydrofoils do not project outside the outer extremity of the outer hulls when in the stowed position, so are not vulnerable to damage when the craft is docked.

With the arrangements shown in figures 1 to 4, the active hydrofoils penetrate the water surface. This has the advantage that as the hydrofoil rises relative to the water surface the immersed area of the hydrofoil reduces, reducing lift to restore a balance of vertical forces. This is a significant advantage for craft that are intended to regularly sail with all hulls clear of the water, as figure 1, but is not essential for a vertical balance of forces for craft sailing with an immersed hull(s) as figures 2,3 and 4.

Hydrofoils running close to the water surface generally have a lower efficiency than hydrofoils that are more deeply immersed. Hydrofoils operating close to the water surface generate surface waves with consequent dissipation of energy. Also, air pockets are liable to form on the low pressure surfaces of hydrofoils operating close to a water surface, especially if part of the hydrofoil actually penetrates the water surface. Air pockets forming on hydrofoil surfaces can greatly reduce lift and efficiency. Figures 1,2,3 and 4 show hydrofoils that penetrate the water surface and the regions of such hydrofoils close to the water surface, i.e within approximately a distance equal to the chord of the hydrofoil, will generate little lift but will generate drag.

The arrangement as shown in figure 5, in which the hydrofoils hl and h2 are mounted from the craft on struts si and s2, allows the hydrofoils to be deeper below the surface and hence potentially more efficient than with the arrangements shown in figures 1,2,3 and 4. The combination of a strut and a perpendicular hydrofoil, as shown in figure 5, is henceforth referred to as a 'tee-foil unit'.

With reference to figure 5, the strut si will generate drag that offsets at least some of the greater efficiency of the fully immersed hydrofoil. To minimise drag on the struts the cross sections of the struts could be streamlined but streamlined struts will themselves act as hydrofoils in the presence of leeway induced by wind force A and so will generate a hydrodynamic force SI. This force may be comparable in magnitude to Hl and since it does not act though point Ce it will de-stabilise the craft in roll, hence the arrangement shown in figure 5 is not inherently stable in roll if the tee-foil units are rigidly mounted to the craft.

In order to reduce, eliminate or control the hydrodynamic force acting on the strut(s) of a Tee-foil unit(s) and so reduce, eliminate or control the roll instability that may be caused by these forces, as discussed above, the present invention incorporates a tee-foil unit(s) having a streamlined strut(s), this tee-foil unit(s) being mounted on pivots with the axis of rotation of the pivots nominally perpendicular to the water flow and lying in the plane of the strut and ahead of the strut, as shown in figure 6.

Figure 6, shows a tee-foil unit with upper and lower mounts for attachment to a water craft. These mounts incorporate pivots that allow the strut and hydrofoil combination to rotate about the axis X-X. The mounting arrangement may also provide for additional axis of rotation to permit the angle of attack of the hydrofoil to be adjusted and/or to allow the angle of the strut to the water plane to be adjusted and/or to permit the tee-foil unit to be taken into a stowed position when not required to be deployed.

If the tee-foil unit as shown in figure 6 is allowed to freely rotate about the axis x—x then, ignoring the small effect of the weight and/or buoyancy of the tee-foil unit, the strut will align with the local water flow, minimising the hydrodynamic force, both lift and drag, that acts on the strut. With reference to figure 5, once the hydrodynamic force on the strut, force SI, has been minimised or eliminated, the combination of forces acting on the craft becomes similar to that shown in figures 3 or 4 and as previously discussed, those configurations are potentially stable in roll.

Provided the hydrodynamic loads on the strut are small, a tee-foil unit as shown in figure 6 is structurally preferable to the arrangements shown in figures 1,2,3 and 4 all of which include a hydrofoil(s) that are cantilevered from the main structure of a craft with the hydrodynamic loading applied over a region towards the free end of the hydrofoil. If a tee-foil unit as shown in figure 6 is operating with the hydrofoil well immersed below the water surface the hydrodynamic loading on the two arms of the hydrofoil will balance and little bending moment will be transferred to the strut. As the hydrofoil approaches and then breaks through the water surface the loading on the two arms of the hydrofoil will become unbalanced and bending moment will be transferred to the strut. It is noted that deflection of the strut resulting from such bending moment will alter the direction of the total hydrodynamic force on the attached hydrofoil such as to make the actual bending moment on the upper part of the strut less than it would be if the strut were totally ridged.

Figure 5 shows a craft with an immersed hull and with a single active tee-foil unit (an opposite side tee-foil unit is shown in a stowed position) but the present invention is also applicable to sailing with tee-foil units deployed on both sides of the craft, with or without an immersed hull(s), these configurations all being potentially stable in roll provided hydrodynamic forces on the struts of the tee-foil units are minimised or eliminated. For example, figure 7 shows a craft sailing with all hulls clear of the water and with a tee-foil unit deployed on each side of the craft. With the force directions indicated by the arrows in this figure, force Hl will always be positive (i.e. acting in the direction of the arrow), whereas force H2 may be positive or negative depending on the pitch angle of the craft and the leeway.

The present invention also allows for the possibility that application of some torque about the pivot axis of a tee-foil unit as shown in figure 6 could be used to intentionally induce a roll angle of the craft and/or to control the height of the craft above the water in the case that it is sailing with all hulls clear of the water surface. Hence, the use of some form of height/roll sensing system to apply appropriate torques to the struts could be a means to control the roll angle and/or the height of the craft above the water surface, although it is noted that control of height could alternatively be achieved by altering the angle of attack of hydrofoils hl and h2, as marked in figure 7, or by actuating trim tabs at the trailing edge of these hydrofoils.

The present invention also provides a possible way to control the height of a tee-foil unit relative to the water surface by utilising the difference in hydrodynamic forces acting on the two 'arms' of a hydrofoil that is inclined to the water surface as that hydrofoil approaches, then breaks through, the water surface. This gives a further possibility to control the roll angle of a craft or to control the height of a craft above the water in the case of a craft sailing with all hulls clear of the water. If a pivoted Tee-foil unit is operated with the strut inclined to the water surface, as strut si in figure 7, and with the hydrofoil surfaces well immersed, the hydrodynamic forces acting on the two 'arms' of the hydrofoil will balance and the hydrofoil will not generate significant torque about the pivot axis of the Tee-foil unit. However, as the hydrofoil at the lower end of a tee-foil unit approaches and then breaks through the water surface, the hydrodynamic drag force acting on the higher arm of the hydrofoil, labelled arm 'a' in figure 7, will become less than for the lower arm, arm 'b'. This effect will generate a torque about the pivot axis X—X, as shown in figure 6, this torque turning the strut so that the strut generates hydrodynamic force in a direction that will tend to correct the change in height of the tee-foil unit relative to the water surface.

There will also be an imbalance between the lift forces on the two arms of the hydrofoil as it approaches and then breaks through the water surface. If the axis X—X is arranged to be parallel to the hydrofoil lift forces then this imbalance of lift forces will generate no torque about axis X—X. However, if the axis X—X is tilted relative to the hydrofoil lift forces as shown by Angle Ά' in figure 8, then an imbalance of lift force between the two arms of the hydrofoil will augment the torque effect of the drag imbalance between the two arms, increasing the height correcting effect. The addition of a small perpendicular hydrofoil surface to the end of the lower hydrofoil arm could provide additional hydrofoil surface to augment the area of the immersed part of the strut, to increase the height correcting effect when the immersed strut area becomes small as the hydrofoil breaks through the water surface.

In rough water it could be the case that the higher arm of the hydrofoil on a tee-foil unit will be intermittently breaking the water surface and this could cause a rapid oscillation of the tee-foil unit about its pivot axis. To counter this it may be desirable to add damping to act on the rotation of the tee-foil unit, for example by means of a viscous damper. It may also be desirable to include a spring that is set up to apply a torque about the pivot axis of a tee-foil unit, this torque countering the weight/bouyancy of the tee foil unit so as to align the tee-foil unit with the water flow when the craft is moving slowly.

As an example of an embodiment of the invention, figure 9 shows the invention applied to a trimaran sailing craft that is sailing with the central hull immersed, a downwind tee-foil unit deployed and an upwind tee-foil unit retracted and stowed. For this craft the hydrofoils are intended for roll control and partial support of craft weight rather than to enable the craft to sail with all hulls clear of the water, hence a conventional rudder without lifting surfaces can be used to steer the craft.

Figure 10 shows a closer view of the deployed downwind tee-foil unit and figure 11 shows a closer view of the upwind tee-foil unit. These figures are diagrammatic and are not intended to represent details of an actual construction.

With reference to figure 10, the upper end of the strut of each tee-foil unit is attached to the main structure of the craft through a universal joint, 'a' which allows three degrees of rotational freedom. A cable, 'b', connects from near the bow of the outer hull to a mounting point on a short arm projecting from the leading edge of the strut of the tee-foil unit. An arrangement such as a pulley system is provided to adjust the length of this cable so as to adjust the angle of attack of the hydrofoil at the lower end of the strut. An alternative to this cable could be a rigid member, for example a telescopic strut with a screw arrangement to adjust length.

With reference to Figure 10, an arrangement of adjustable cables, 'c' and'd', together with an intermediate strut 'e', is arranged so that the angle of the tee-foil unit strut relative to the water surface can be set such that the line of action of the hydrodynamic force on the hydrofoil is at an angle of elevation relative to a horizontal plane such that the craft is stable in roll. This angle may need to be adjusted to take account of change to the height of the centre of effort of the rig as sail area is changed to cater for varying wind strength. A larger adjustment of the cables 'c' and'd' can be made to move the tee-foil unit from the active position to the stowed position shown in figure

11. There is the possibility of an additional cable between the lower mounting point on the tee-foil unit and the adjacent hull so as to hold the tee-foil unit down in the deployed location, particularly in rough water. Alternatively a small torque could be applied through a spring to the strut of the teefoil unit so as to bias the orientation of the strut to the waterflow such that there is a small hydrodynamic downforce on the strut, holding the strut down and keeping the cables (d) and (c) taught.

It is envisaged that for a small craft, adjustment to the length of cables 'c' and'd' could be by a manually operated pulley system arranged to make equal alterations simultaneously to the lengths of 'c' and'd'. For a larger craft these cables might be adjusted by a powered system.

Although the tee-foil unit is shown restrained and controlled by cables in figure 10 there are other mechanical arrangements that could take the place of these cables.

When both tee-foil units are in the stowed position it is noted that the overall width of the craft is significantly less than for the configurations shown in figure 3 and figure 4, for a given rig height and assuming that in all cases the geometry is such that the craft is stable in roll. This reduction in width, as measured over the outer hulls, allows a lighter structure with lower windage and/or a taller rig that is potentially aerodynamically more efficient.

It is noted that practical craft having the configurations shown in figures 3 or 4 do not normally achieve roll stability purely by means of the hydrofoil system. Such craft usually require to be sailed with at least two immersed hulls and/or with movements of crew weight or other ballast weight to assist in maintaining roll stability. With the configurations shown in figures 3 and 4, achieving roll stability purely by means of the hydrofoil system is likely to lead to a craft overall width that is excessively large for a given rig height. By contrast, it is anticipated that the use of tee-foil units as described above will allow roll stability to be maintained by hydrodynamic forces alone over a wider range of operating conditions whilst also allowing an acceptable craft width once the tee-foil units have been moved to their stowed positions.

It is envisaged that a mechanism could be provided such that if an overload of cable (b) in figure 10 occurs, for example due an active tee-foil unit striking a floating object, the tee-foil unit could automatically detach from the craft to minimise damage. The tee foil unit might then be retained on a length of rope to aid recovery^

The advantages of the present invention are summarised as:

1) Roll stability of a hydrofoil sailing craft is achievable over at least a part of the range of operating conditions by utilising only hydrodynamic forces, this offering potentially better high speed efficiency than is possible by utilising a combination of hydrodynamic and bouyancy forces.

2) Roll stability of a hydrofoil craft is achievable with hydrofoils that are fully immersed, at least in calm water Fully immersed hydrofoils are generally more efficient than hydrofoils that penetrate the water surface.

3) Roll stability of a hydrofoil sailing craft is achievable with an arrangement that allows a craft overall width with the hydrofoils in a stowed position that is less than for various alternative configurations of similar rig height.

4) The arrangement of hydrofoil and supporting strut is such that the hydrofoil is supported close to the centre of hydrodynamic pressure, this being structurally advantageous to alternative arrangements in which a hydrofoil is cantilevered from one end.

5) The arrangement of hydrofoil and supporting strut is typically attached to a craft at three mounting points. This makes it relatively straightforward to provide for the hydrofoil and supporting to strut to break away from the craft with minimal damage in the event of collision. It could also simplify retrofitting to an existing craft.

6) The arrangement of hydrofoil and supporting strut allows for a possible means to control height/roll angle of a craft by utilising the imbalance of the hydrodynamic forces on the two 'arms' of the hydrofoil as that hydrofoil approaches, then breaks though, the water surface.

Claims

1. A unit comprising a hydrofoil supported by a nominally perpendicular strut, this unit being pivoted about an axis ahead of the strut such that the hydrodynamic force on the unit is primarily that on the hydrofoil.

2. A unit according to claim 1 which is mounted from a sailing watercraft such that the line of action of the hydrodynamic force on the unit coincides with the lines of action of the aerodynamic force acting on the craft, the gravitational force acting on the craft and any buoyancy force acting on the craft sufficiently closely that the craft is stable in roll.

3. A combination of units each according to claim 1 which are mounted from a sailing watercraft such that the lines of action of the hydrodynamic forces on the units all coincide with the lines of action of the aerodynamic force acting on the craft, the gravitational force acting on the craft and any buoyancy forces acting on the craft sufficiently closely that the craft is stable in roll.

4. A unit according to claim 1 that is arranged such that as the unit rises relative to the water surface, causing part of the hydrofoil to approach and then possibly break through the water surface, the resulting imbalance of hydrodynamic forces on the parts of the hydrofoil above and below the pivot axis generates torque about the pivot axis that in turn generates hydrodynamic force on the strut that counteracts the change in height of the unit.

5. A unit or combination of units according to claims 1 and 4 such that the hydrodynamic force(s) generated on the strut(s) of the unit(s) control(s) the overall height of a water craft above the mean level of the water surface.

6. A unit or combination of units according to claims 1 and 4, this unit(s) including a supplementary immersed surface(s) nominally parallel to the surfaces of the strut(s) this supplementary surface(s) providing additional hydrodynamic force to supplement that generated by the strut so augmenting the roll/height control effect considered under claim 4.

7. A unit according to claim 1 that includes an arrangement to provide damping, for example viscous damping, of rotational motion about the pivot axis of the unit so as to prevent excessive oscillatory motion of the unit about this axis.

8. A unit according to claim 1 that includes an arrangement to provide a torque, for example by means of a spring, about the pivot axis of the unit so as to align the unit with the water flow at low speed.

9. A unit according to claim 1 which includes an arrangement to provide manually or automatically controlled torque about the pivot axis of the unit such as to generate hydrodynamic force on the strut for the purpose of controlling the roll angle and/or height above the water surface of a water craft from which the unit is mounted.

10. A unit according to claim 1 that is mounted from a water craft with an additional pivot axis that is nominally parallel to the span of the hydrofoil such that rotation about this pivot axis can be used to adjust the angle of attack of the hydrofoil and hence the lift force generated by the hydrofoil.

11. A unit according to claim 1 that is mounted from a water craft with an additional pivot axis that is alighned so that rotation about this pivot axis can be used to adjust the elevation angle, relative to a horizontal plane, of the line of action of the hydrodynamic force on the unit.

12. A unit according to claim 1 that is mounted from a water craft with an additional pivot axis that is aligned so that rotation about this pivot axis can be used to move the unit into a stowed position where it is clear of the water.

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