AU708693B2 - Improved keel for aquatic craft - Google Patents

Description

WO 97/38895 PCT/AU97/00230 -1- TITLE: IMPROVED KEEL FOR AQUATIC CRAFT FIELD OF THE INVENTION The present invention relates to keel assemblies for use with aquatic craft to reduce stabiliser drag during turning manoeuvres.

Whilst the invention has been developed primarily for use with sailing vessels ranging from small dinghies to large yachts, it will be appreciated that the invention is not limited to this particular field of use. In particular it is presently envisaged that the invention will provide advantages for powered craft of all sizes, including larger oceangoing vessels and ships. Whilst some configuration changes may need to be made for different sizes and types of water craft, the fundamental principles of the invention apply uniformly.

BACKGROUND OF THE INVENTION The popularity of sailing world wide has increased in recent years, as evidenced by media and consumer interest in such events as the America's Cup, the Australian Sydney to Hobart Yacht Classic, and the numerous types of skiff racing shown on television sports programs. The intense nature of the competitions themselves, along with the race for commercial sponsorship, has lead to an ongoing search for ways to maximise the performance of sail craft.

For about 100 years, the keel has been an integral part of the hull's design. Keel shapes are many, some extending the length of the hull, gently sloping and tapering to the tail, whilst vertical "centre board" keels with their added lead weights or winged fittings on their ends are becoming more widespread. Ocean-going and racing yachts WO 97/38895 PCT/AU97/00230 -2have had drawn-out hulls in the shape of a central tapering keel, the keel's inner cavity forming a mould for their pouring of the original lead.

In any event, most modem yachts have a large, rigid keel, generally mounted about half way along the craft's length. Often, the keel is heavily weighted with lead at its lower end, thereby to reduce leaning of the craft during tacking, as well as generally stabilising the craft against excessive rolling or pitching such as in rough seas. However, whilst keels clearly serve a number of useful purposes, the fact that the keel is always configured in the same direction as the hull can give rise to certain disadvantages.

At relatively high speeds, turning a yacht's rudder to instigate a turn greater than, say, a minor course adjustment can easily cause the rudder to stall rather than creating the necessary turning force. This means that the rudder is instead dragged through the water, rather than turning the craft.

For prior art craft, a turn at such a relatively high speed requires the sheer displacement or shifting of a large mass of fast moving "hard" water, adjacent either side of the keel. This is relatively difficult, especially due to the more or less exponential correlation between the speed of water passing the keel and the pressures involved.

More recently, it is believed that the reduction or removal of the keel would allow the hull more easily to turn from any desired speed.

Until now drag force has been considered a restricting factor on a craft's turning performance. In this context, drag arises due to increasing water pressure over a keel's surface area as a result of increasing water flow or increasing speeds. This in turn produces strong holding forces at the keel, effectively holding or locking a keel into its present direction. The keel in one sense acts like a large rudder, jammed in line with the WO 97/38895 PCT/AU97/00230 -3yacht's hull, thereby effectively limiting pure motion to the line of the hull's present course. This causes a normal hull's performance to be inefficient, only performing best in a straight line along the keel's orientation within the water. Accordingly, a yacht's potential tracking and performance effectively is biased in a single direction.

We equate the term vertical stabiliser drag to the hard water effect on turning ability, as shown by the very real problems associated with escalating pressures at speed, and notably for relatively large vertical surface areas, as are present with modem keels, ship drafts and even rudders. In specific reference to sailing, for turning to be improved, it is necessary therefore to lessen this clamping effect of water upon the keel to be able to turn against it. In the past, this has been achieved by lowering a craft's velocity or speed prior to turning, thereby lowering the ratio of water pressures on the keel's surface area or draft to within a workable resistance in water. The rudder is then able to steer the hull by displacing "softer" water on either side of the keel, making the turn appear as if it was quite a normal function but this is only possible at relatively low speeds. An attempt to turn at relatively high speeds tends to result in stalling of the keel and rudder, leading to a significant reduction in speed through the water.

If drag can be reduced or by-passed during turns, then a yacht or any other vessel could more easily turn at higher speeds, without such a significant loss of speed during turning, hence the higher speeds going into the turn could substantially be preserved.

To assist with understanding the invention, the basics of sailing, in particular yachting, will now briefly be explained.

The dynamics of a yacht moving through water are a function of, amongst other things: WO 97/38895 PCT/AU97/00230 -4- 1. "Tracking" along the hull; 2. Momentum due to the yacht's mass and velocity; 3. Counterweight of the keel; and 4. Keel planing.

To elaborate on each of these points: 1. Hull Tracking Moving objects in water or air will tend to move along a line of least resistance, which will tend to be dictated by the shape of the object. If it is longer than it is wide, as is typically the case with a yacht, the narrowest cross section will provide the least resistance to water flow. Accordingly, a yacht will tend to travel along a line coincident with its centre line. To increase efficiency, ships and yachts are built in a very streamlined fashion to ensure that they move easily through the water in a directionally stable manner. A turning manoeuvre would therefore involve redirecting this movement.

2. Momentum in a Particular Direction.

Momentum acts to keep an object moving in a particular direction at its developed speed unless an exterior force causes the momentum to change such as wind or water. In this way momentum can be conceptualised as something which resists attempts to increase or decrease the speed of an object, and change of the direction in which it is moving. Presently there is a relationship between the direction of a moving object and the speed in which it is travelling.

Momentum evens out pulses and variations in applied energy situations as evidenced in craft motion within larger wave troughs. Momentum thus tends to make an WO 97/38895 PCT/AU97/00230 aquatic craft continue in its present direction of travel unless exterior forces are applied.

Of special note therefore are the separate mass components of the hull and the planing counterweighted keel body, whose separate masses proportionally generate their own momentum.

When the hull of a yacht is turned, it is evidenced the nature of any motion and tracking naturally tends to follow the hull's direction, regardless of a keel's mass displacement or to the urge of a held or directed course from the keel.

3. Counterbalance.

In modern yachts, the lead weighted keel is not integral with the theoretical act of sailing.

The hull provides a further role as a floating platform for sails, masts, people and equipment including floating and lift for the weight of the hull and the keel. The hull also acts as a point of rotation about which sails, mast and wind are counterbalanced by the lower mass of the L.W.K.

While past and present purposes of the keel also prevent slippage of the craft due to the lateral force component of the wind in the sails, it is only the wind's forward force component that translates to the forward motion of the yacht's hull, whilst the lateral force component contributes no useful energy. The forward component of the wind will always be equal to or less than that of the total wind force.

However, the effect of the lateral force component of the wind must still be managed. If this sideways force component is not counterbalanced, a sizeable lateral force could easily lean a yacht to the point of capsizing. The keel originated to enable yachts and the like to remain upright even in strong lateral winds.

WO 97/38895 PCT/AU97/00230 -6- Typically, up to 40% of the total weight of a modem yacht is disposed within the keel to counterbalance the potential effects of lateral wind components.

4. Keel Planing If a keel was merely a rod with a lead mass at its end, there would be nothing to stop the yacht from rolling with each gust of wind into the sails, or even due to minor wave motion.

However, the usually large surface area of a keel through a planing principle helps to maintain a reasonably stable lean of the yacht due to the balance effect of the sail forces and counterweight of the keel. Thus, the lead weight design or form at the end of the keel tends to reduce bobbing of the yacht due to the counter inertia of the keel mass.

While the L.W.K.'s inertia "brakes" rapid sway caused by wind or waves, the planing effect from the keel's substantial flat surface through water, from the action of normal water pressures, clamps the present position of the L.W.K. to basically maintain the yacht in a single leaning stance for most of the time.

A keel's cross section is typically similar to that of a symmetrical aerofoil, orientated so that, at least when the yacht is heading straight downwind, there are equal water pressures acting on either side of the keel. In this configuration, reaction pressures acting upon the keel from almost any range of speeds, especially faster speeds, tend to lock the keel generally in line with the present direction of water flow.

To that end, it is an object of the present invention to provide a new keel which offers improved steering dynamics in operation when compared to the prior art, and which, at least in a preferred form, increases the manoeuvrability of the aquatic craft to which it is mounted.

WO 97/38895 PCT/AU97/00230 RECEIVED 0 5 JUN 1998 DISCLOSURE OF THE INVENTION Accordingly, the invention provides a keel assembly for an aquatic craft, said assembly including: a keel; mounting means adapted to connect the keel to an underside of the craft for rotation about a generally vertical axis; and bias means disposed resiliently to urge the keel toward a central rest position substantially parallel to a longitudinal axis of the craft, the axis of rotation being spaced forwardly of an effective centre of pressure of the keel.

Preferrably, the mounting means includes a shaft extending upwardly from the keel through a hole in the underside of the craft to a rotatable bearing. Preferrably, the keel is detachable from the shaft.

It is desirable that the mounting means includes two or more supporting subshafts substantially parallel with the axis of rotation.

In a preferred form, the mounting means includes a crank to transfer torque from hydrodynamic forces on the keel to resilient bias means. Preferably, the resilient bias means compromise one or more linear pneumatic, hydraulic or coil springs, or a combination thereof.

Desirably, a first end of the spring is connected with the crank and a second end distal the first end is attached to a support within the hull, such that movement or displacement of the keel away from the central rest position in relation to the hull turning, causes the crank to rotate or, equally, appear to rotate. This in turn either compresses or extends the resilient bias means, thereby causing a restoring torque on 1 the shaft, and at the least tending to bias the keel toward the central rest position.

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WO 97/38895 PCT/AU97/00230 -8- At least in a preferred embodiment, the keel includes a vertically tapered piercing portion extending forward from the axis of rotation. Similarly, it is preferable that the keel includes a partially circular portion when viewed from the side, the circular portion extending substantially rearwardly of the axis of rotation.

In a preferred embodiment, the keel includes a lateral aperture to allow passage of water during turning manoeuvres. Desirably, the keel assembly further includes adjustment means whereby the keel may selectively be extended or retracted vertically with respect to the hull.

In a preferred embodiment, the keel is located on the rearward half of the hull, whilst the main steering rudder is located forwardly of the keel.

Preferably, the mounting means allows the keel to be removed for maintenance, or interchanged with a keel of different shape, depth or weight, depending upon weather or course conditions.

In use, reaction pressures due to the interaction of water flow over the keel and the momentum of the boat itself cause the keel to divert from its central rest position during turning manoeuvres. This diversion tends to allow the keel to continue aligning itself with the water flow, thereby reducing drag. It is believed that this will enable improved turning characteristics, including increased speed through turns by delaying onset to "stalling" of the keel during turns.

With this configuration, it is believed that natural reaction pressures on the keel during turning manoeuvres, or other natural surface adjustments or compensations by the hull to, say, wave trough motion, will now result in less exterior forces towards the hull's motion, without substantially impeding the stabilising function of the keel.

WO 97/38895 PCT/AU97/00230 -9- BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a partially cut away elevation of a keel assembly according to the invention; Figure 2 is a partially cut away view of the keel shown in Figure 1, from the front or rear of the yacht; Figure 3 is a partially cut away plan view of the keel shown in Figures 1 and 2: Figures 4 through 13 show sub-components of the embodiment shown in figures 1 to 3; Figure 14 shows the path of a prior art yacht under turning conditions; Figures 15 and 16 show various aspects of the turning path of a yacht embodying the present invention; Figures 17 and 18 show the self righting effect of a prior art weighted keel on a hull; Figure 19 shows a schematic plan view of wind striking a prior art yacht; Figures 20 through 25 show various embodiments of keels for use in keel assemblies according to the invention; Figure 26 is a simplified schematic side view of a prior art yacht; Figure 27 is an inverted plan view of the yacht shown in Figure 28; Figure 28 shows a simplified side view of an alternative embodiment of the invention, wherein the keel is located rearwardly of the rudder; Figure 29 is an inverted plan view of the embodiment shown in Figure 26; WO 97/38895 PCT/AU97/00230 Figure 30 is a drawing of yet another embodiment of the invention, adapted for use with a smaller sailing craft; Figure 31 is a schematic view of a retractable keel according to the invention, on a larger powered vessel; Figure 31 A is a detailed sectional drawing of the pivot assembly shown in-Figure 31, in respective extended and partially retracted states; Figure 31 B is a cross-sectional view of the pivot assembly shown in Figure 31 A Figure 32 is a partial longitudinal section of another embodiment of the invention; Figure 33 is a simplified side view of yet another embodiment of the invention; Figure 34 is a simplified side view of another embodiment of the invention, wherein the central counterweight is separated from a rearwardly displaced keel; Figure 35 is a combined schematic side view and inverted plan view of yet another embodiment of the invention applied to a powered craft; Figure 36 is a partial longitudinal section of a non-retractable embodiment of the invention for use with a yacht; Figure 37 is a combined cross sectional view and plan view of the embodiment shown in Figure 36; Figure 38 is a sectional view of an alternative embodiment of the invention which includes a winch box adaptation; Figure 39 is a side view of a winch box for use with the embodiment shown in Figure 38; Figure 40 is a plan view of the embodiment shown in Figures 38 and 39; WO 97/38895 PCT/AU97/00230 -11- Figure 41 is a longitudinal section of a retractable embodiment of the invention for use with a yacht.

Figure 42 is a simplified perspective view of a skiff incorporating a keel assembly according to the present invention; Figure 43 is a plan view of a rudder assembly for the skiff shown in Figure 42; and Figure 44 is a longitudinal section of the rudder assembly of Figure 43.

PREFERRED EMBODIMENT OF THE INVENTION Referring to the drawings, and in particular to Figures 1 to 13, a keel assembly I for an aquatic craft 2 includes a generally vertical keel 4. The keel 4 is connected to an underside 6 of the craft 2 via mounting means shown generally as 8 for rotation about a generally vertical axis 10, shown by a centre line. Bias means shown generally as 12 are disposed within hull 14 resiliently to urge the keel towards a central rest position substantially parallel to a longitudinal axis 15 of the craft. The axis of rotation 10 is spaced forwardly of an effective centre of pressure of the keel.

The mounting means includes a pair of shafts 16 and 18 which bolt the keel to a flange bearing 20 which in turn rest on base plate 21. Although safety strength factors should be made adequate so that a single shaft could operationally support the keel, it is recommended that if one of the shafts did break, say, in heavy conditions, the remaining shaft would allow the craft to function normally and to return home without actually losing the keel. In other embodiments such as those shown in Figures 20-23, 25 and 39, a single keel shaft 23 is used.

WO 97/38895 PCT/AU97/00230 RECEIVED 0 5 JUN 1998 The keel assembly also includes a crank 22 which is a T-shaped vertical element rising diagonally above the flange bearing 20, being adapted to transfer torque generated by hydrodynamic forces on keel 4 to resilient bias means 12.

The resilient bias means in the preferred embodiment are telescopic pneumatic, hydraulic or coil springs devices, which extend between a support 24 mounted within hull 14 and respective peripheral mounting points on the crank 22. Support 24 consists of a framework mounted to the floor of the hull 14 to act as both a mounting point for the resilient bias means and a stabilising pivot point for the keel. It should be borne in mind that the illustrated configuration of the support is simply a preferred embodiment, and may be embodied in a number of different forms. For example, as illustrated in figure 39, the top piece 26 of the support need not be included, depending upon the particular application to which the invention is being put.

Lower Spigoted, Flange bearing plate 20 rests upon a raceway 28 formed in base plate with matching, mating, lower cylindrical sleeve 21 for vertical load bearings 30. This configuration enables a centrally mounted, relatively smooth, swivelling of the keel 4 about fixed axis of rotation 10 in response to hydrodynamic forces on the keel. Other types of vertical load bearings may, of course, be used, depending upon the application.

For example, pin or tapered bearings may be used to minimise the impact of the support and bearing plate areas on deck or floor space.

Turning to Figures 20 to 25, there are shown a number of different keels intended for different applications according to the invention. Figure 20, for example, is for use with an ocean-going yacht or like where a lead counterweight 32 is positioned at the bottom of the keel 4 to counterbalance the rolling effect caused by wind in the sails of the vessel. Figure 21 shows a presently preferred embodiment of the invention, which AMENDED SHEET

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WO 97/38895 PCT/AU97/00230 -13includes a lateral aperture 34 to allow water to shed through the centre of the keel 4, as described previously as well as around its outer edges. It is believed that this will further reduce the drag effect of "hard water" during fast turns.

Figure 22 shows a keel embodying concepts from both Figures 20 and 21, whilst s Figure 23 is merely another embodiment of a keel according to the invention. Figure 24 shows a centre board keel useable with a skiff or dinghy, where the height of the centre board needs to be adjustable by hand. Figure 25 simply shows yet another alternative embodiment incorporating a number of features described in respect of the earlier keels.

Turning to Figures 28 and 29, it is believed that the dynamic response of a yacht incorporating the invention can be improved by moving the main steering rudder 36 to the front of hull 14, and moving the keel 4 rearwardly. During turning, the front of the boat is now driven strongly along a new course, whilst the rear keel is forced and maintained away from its central resting position by the effect of differential water pressure across its surface. This enables the yacht to turn more efficiently due to increased lateral deflection forces acting on the keel, thus reducing the drag effects of a static keel. By contrast, in the prior art situation shown in Figures 26 and 27 a relatively large build up of pressure occurs on the keel, which in turn slows the yacht as it turns.

Turning to Figure 30, an embodiment of the invention for use with a skiff is shown. A pin 38 is used to hold the keel at a selected vertical depth. The keel may be adjusted by removing the pin, altering the vertical depth of the keel using handle 40 and then replacing the pin through one of holes 42. In this embodiment, only two springs are shown, although even one could be adequate in some circumstances. In the latter case a bi-directional spring would be required. Alternatively, the pin retaining means may be WO 97/38895 PCT/AU97/00230 -14replaced with any other suitable retaining means, such as, for example, a manually operable clamp. In one form, this could be a frictional cam arranged to wedge the keel in a desired vertical position.

Turning to Figure 42, a small racing skiff44 incorporating the invention is shown.

As with some earlier embodiments, the keel is positioned rearwardly of the central area of the boat, whilst the rudder is placed towards the nose of the craft. As well as embodying the invention as discussed earlier, the skiff illustrated also has a spring loaded front rudder. As best shown in Figures 43 and 44, a pair of opposing torsional spiral springs 46 and 47 associated with the front rudder cause it to return to a central position whenever the control ropes are loosened sufficiently. It is believed that this will give the craft better stability at speed, as well as allowing the coxswain to concentrate on tactics and navigation rather than the mechanics of steering the craft.

Turning now to Figures 31 to 31B, larger powered craft may also benefit from the principle of the invention. In the embodiment shown, a retractable keel 38 is shown.

When not in use, the retractable keel 38 sits within a recess 50 and is protected by a cover 52. Drive means in the form of a circular crank 54 and hydraulic ram 55 driving an alignment sleeve 53 about a keel shaft pivot point 56 enables selective levered extension and retraction of the keel when the cover 52 is retracted. In use, the alignment sleeve 53 encapsulates intermeshing plates 55, thereby to accurately and stably hold the keel shaft, and thereby the keel, in the correct position. It is envisaged that high speed craft such as that shown in Figure 35 could benefit from the use of keels in accordance with the invention, as this would allow a far shallower draft.

WO 97/38895 PCT/AU97/00230 Turning to Figure 33, yet another embodiment is shown which incorporates a pair of linked rudders at either end of the craft. In response to input from a steering wheel 58, the rudders rotate in opposite directions to push the stern and bow of the craft about a central pivot point, upon which is mounted a keel assembly according to the invention.

It is believed that this will significantly increase the turning abilities of the craft; Turning to Figure 34, the keel is displaced rearwardly whilst leaving the lead counterweight mounted separately at the centre of the craft. In this way, all of the advantages of a centrally mounted counterweight will be achieved, whilst the advantages of mounting the keel more rearwardly may also be utilised.

Turning to the embodiment shown in Figures 38 to 40, there is illustrated a depth adjustable keel assembly according to the invention. The keel may vertically be adjusted by operation of winch box 60. Turning the winch handle causes pinion gear 64 to drive rack gear 66 with respect to the hull. An additional feature is the ability to "quick release" the keel to its full depth by disengaging pawl 68 from ratchet 70. The keel assembly also includes crank locking pins which are designed to lock the keel in a forward position. This arrangement allows the bias means to conveniently be removed, adjusted or replaced, whilst maintaining the keel in its central rest position.

Other arrangements may also be used to alter the characteristics of the keel and keel assembly. For example, means may be provided to adjust the rest position of the keel, which may provide improved tracking characteristics in certain winds.

The invention provides a novel assembly for use with yachts and other aquatic craft, which, is believed, will allow improved turning dynamics, especially at higher WO 97/38895 PCT/AU97/00230 16speeds. For this reason, the present invention represents a significant commercial improvement over prior art keels.

Although the invention has been described by reference to a large number of examples, it will be appreciated by those skilled in the art that the invention may be embodied in other forms.

Claims (19)

1. A keel assembly for an aquatic craft, said keel assembly including: a keel; mounting means adapted to connect the keel to an underside of the craft for rotation about a generally vertical axis; and bias means disposed resiliently to urge the keel toward a central rest position substantially parallel to a longitudinal axis of the craft, the axis of rotation being spaced forwardly from any effective centre of pressure on the keel. with the said keel, being spaced rearwardly of a forward hull rudder assembly, and the Keel having one or more supporting shafts for mounting means, with an upper bearing and its crank body whereby the latter transfers hydrodynamic forces on the Keel, via resilient bias means to supports within the hull.

2. A keel assembly for an aquatic craft according to claim 1, wherein the mounting or support means includes a shaft extending upwardly from the keel through a hole in the underside of the craft to a rotatable bearing for affixing atop thereof.

3. A keel assembly for an aquatic craft according to claim 2, wherein the rotatable bearing is a flanged, spigot bearing.

4. A keel assembly for an aquatic craft according to claim 3 wherein the keel is detachable from the flanged, spigot bearing. AMENDED SHEET IPEA/AU WO 97/38895 PCT/AU97/00230 RECEIVED 0

5 JUN 1998 -18- A keel assembly for an aquatic craft according to claim 4 wherein the mounting or support means include 2 or more supporting sub-shafts fastened to the flanged, spigot bearing and substantially parallel to the axis of rotation, with keyed joints to transfer torque between the keel and the bearing.

6. A keel or rudder assembly for an aquatic craft according to claim 1, wherein resilient bias means comprise a single or plurality of linear pneumatic, hydraulic or coil springs, or any combination thereof and thru which their forces may act in one or both directions.

7. A keel assembly for an aquatic craft according to claim 6, wherein a first end of the spring is connected with the crank and a second end distal the first end is attached indirectly or directly onto a support within the hull, such that displacement of the keel away from the central rest position causes the crank to offset, which in turn either elastically compresses or extends the spring, thereby causing the spring to exert a torque on the shaft tending to bias the keel toward the central rest position.

8. A keel and rudder assembly for an aquatic craft according to any one of the preceding claims, wherein the keel or rudder includes a vertically tapered piercing portion extending forward from the axis of rotation.

9. A keel and rudder assembly for an aquatic craft according to claim 8, wherein the keel or rudder includes a partially circular portion when viewed from the side, the circular portion extending substantially rearwardly of the axis of rotation. A keel and rudder assembly for an aquatic craft according to any one of the preceding claims, wherein the keel or rudder includes a lateral aperture to allow passage of water during turning manoeuvres.

AMENDED SHEET IPEAIAU WO 97f3=95 pCIAM97no20o RECEIVED .12 JUN 199a

11. A keel assembly for an aquatic; craft according to any one of the preceding claims, further including adjustment nicans where the keel shaft having keyed surfaces to transfer torque between the keel and bearing may also selectively be extended or retracted vertically with respect to the hull.

12. A keel assembly for an aquat"c craft according to claim 11. wherein the adjustment means comprises a rack gear associated with the keel and a pinion gear associated with drive means mounted with the hull, the rack and pinion gears being configured to cause retraction and extension of the keel in response to selective actuation of the drive means.

13. A keel assembly for an aquatio craft according to claim 12, wherein said drive means include a manually operable oe~powered crank

14. A Keel assembly for an aquatirr craft according to Claim I1I wherein further adjustment means comprise a hull rectss with a retractable cover for Keel stowage of a laterally pivoted, retractable Keel actuated thru a hull crank and hydraulic ram with hoist-sleeve and with the latter held sthitionary encapsulating the aligned keel shaft pivot joint in the Keels exended worlcing ppsition, allows for the full and constant translation of bias and external forces. A keel assembly for an aquatio craft according to claim 1, wherein the passed form of a rear hull tiller rudder his rej~ositioned to the forward hull, having new controls thru a pair of hand held steerong ropes set on the ends of diaposed arms of a forward rudder and furthermore where said rudder, tensioned from coil or other spring&, is biased towars a central rest position, such that the rudder tends to align itself with a longitudinal axis of the skiff or yacht In the absence of positive steering input.

AUMDED SHEET IPEAIAU WO 97/38895 PCT/AU97/00230 RECEIVED 0 3 JUL 1998

16. A keel assembly for an aquatic craft according to any one of the preceding claims, including a plurality of keels and steering rudders.

17. A keel assembly for an aquatic craft according to claim 16, wherein the keels are substantially identical and are positioned side by side on the hull.

18. A keel assembly for an aquatic craft according to claim 2, wherein the rotatable bearings lower spigot includes other horizontal load bearings to prevent lateral loading by a pitched keel thereby assisting rotation.

19. A keel assembly for an aquatic craft according to any one of the preceding claims, wherein the keel is disposed rearwardly on the hull, a counterweight is disposed centrally on the hull, and both the keel and the counterweight are vertically adjustable with respect to the hull. SHEET IPEA/AU