WO1997038895A1

WO1997038895A1 - Improved keel for aquatic craft

Info

Publication number: WO1997038895A1
Application number: PCT/AU1997/000230
Authority: WO
Inventors: Paul Vincent Ladd
Original assignee: Paul Vincent Ladd
Priority date: 1996-04-12
Filing date: 1997-04-14
Publication date: 1997-10-23
Also published as: GB9828791D0; GB2333067A; AUPN922296A0; GB2333067B; AU2498797A; AU708693B2

Abstract

A keel assembly for an aquatic craft, the keel assembly including a keel (4), mounting means (8) adapted to connect the keel (4) to an underside of the craft for rotation about a generally vertical axis, and bias means (12) disposed resiliently to urge the keel (4) toward a central rest position substantially parallel to a longitudinal axis of the craft. The vertical axis of rotation is spaced forwardly of an effective centre of pressure of the keel (4).

Description

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 ocean¬

going vessels and ships. Whilst some configuration changes may need to be made for

different sizes and types of water craft, the fundamental principles ofthe 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 ofthe 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 ofthe hull's design. Keel

shapes are many, some extending the length ofthe 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 have 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 modern 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 ofthe 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

ofthe 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 ofthe 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 yacht's hull, thereby effectively limiting pure motion to the line ofthe 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 modern 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 ofthe 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 ofthe 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: 1. "Tracking" along the hull;

2. Momentum due to the yacht's mass and velocity;

3. Counterweight ofthe 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 ofthe 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 aquatic craft continue in its present direction of travel unless exterior forces are applied.

Of special note therefore are the separate mass components ofthe 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 ("L.W.K.") 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 ofthe L.W.K.

While past and present purposes of the keel also prevent slippage of the craft due

to the lateral force component ofthe wind in the sails, it is only the wind's forward force

component that translates to the forward motion ofthe yacht's hull, whilst the lateral

force component contributes no useful energy. The forward component ofthe wind will

always be equal to or less than that ofthe total wind force.

However, the effect ofthe lateral force component ofthe 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. Typically, up to 40% of the total weight of a modern 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 ofthe yacht due to the balance effect ofthe sail

forces and counterweight ofthe keel. Thus, the lead weight design or form at the end of

the keel tends to reduce bobbing ofthe yacht due to the counter inertia ofthe 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 ofthe L.W.K. to basically maintain the

yacht in a single leaning stance for most ofthe 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 ofthe 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 ofthe 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 ofthe aquatic craft to

which it is mounted. 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.

Preferably, the mounting means includes a shaft extending upwardly from the keel

through a hole in the underside ofthe craft to a rotatable bearing. Preferably, the keel is detachable from the shaft.

It is desirable that the mounting means includes two or more supporting sub-shafts

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 comprise one or more linear pneumatic, hydraulic or coil springs, or a

combination thereof.

Desirably, a first end ofthe 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 ofthe

keel away from the central rest position causes the crank to turn. This in turn either

compresses or extends the resilient bias means, thereby causing a restoring torque on the

shaft tending to bias the keel toward the central rest position. 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 ofthe 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 ofthe keel. 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 ofthe keel shown in Figure 1, from the front

or rear ofthe yacht;

Figure 3 is a partially cut away plan view ofthe 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 ofthe yacht shown in Figure 28;

Figure 28 shows a simplified side view of an alternative embodiment ofthe

invention, wherein the keel is located rearwardly of the rudder;

Figure 29 is an inverted plan view ofthe embodiment shown in Figure 26; Figure 30 is a drawing of yet another embodiment ofthe 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 ofthe pivot assembly shown in Figure

31 , in respective extended and partially retracted states;

Figure 3 IB is a cross-sectional view ofthe pivot assembly shown in Figure 31A

Figure 32 is a partial longitudinal section of another embodiment of the invention;

Figure 33 is a simplified side view of yet another embodiment ofthe invention;

Figure 34 is a simplified side view of another embodiment ofthe 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 ofthe invention applied to a powered craft;

Figure 36 is a partial longitudinal section of a non-retractable embodiment ofthe

invention for use with a yacht;

Figure 37 is a combined cross sectional view and plan view ofthe embodiment

shown in Figure 36;

Figure 38 is a sectional view of an alternative embodiment ofthe 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 ofthe embodiment shown in Figures 38 and 39; 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 incoφorating 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 ofthe 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 1

for an aquatic craft 2 includes a generally vertical keel 4. The keel 4 is connected to an

underside 6 ofthe 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 (P) ofthe 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 ofthe 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. 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 hydraulic or

coil spring 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 ofthe 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 ofthe 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 ofthe support need not be included, depending upon the particular

application to which the invention is being put.

Flange bearing plate 20 rests upon a raceway 28 formed in base plate 21 for

bearings 30. This configuration enables relatively smooth swivelling ofthe keel 4 about

the axis of rotation 10 in response to hydrodynamic forces on the keel. Other types of

bearings may, of course, be used, depending upon the application. For example, pin or

sleeve bearings may be used to minimise the impact ofthe support and bearing 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 ofthe keel 4 to counterbalance the rolling effect caused by wind in the sails of

the vessel. Figure 21 shows a presently preferred embodiment ofthe invention, which includes a lateral aperture 34 to allow water to shed through the centre ofthe 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

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 ofthe centre

board needs to be adjustable by hand. Figure 25 simply shows yet another alternative

embodiment incoφorating a number of features described in respect ofthe earlier keels.

Turning to Figures 28 and 29, it is believed that the dynamic response of a yacht

incoφorating 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 ofthe

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 ofthe 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 ofthe 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 replaced 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 skiff 44 incorporating the invention is shown.

As with some earlier embodiments, the keel is positioned rearwardly ofthe central area

ofthe boat, whilst the rudder is placed towards the nose ofthe 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 3 IB, larger powered craft may also benefit from the

principle ofthe 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 ofthe 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. Turning to Figure 33, yet another embodiment is shown which incoφorates a pair

of linked rudders at either end ofthe craft. In response to input from a steering wheel

58, the rudders rotate in opposite directions to push the stern and bow ofthe 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.

Turaing to Figure 34, the keel is displaced rearwardly whilst leaving the lead

counterweight mounted separately at the centre ofthe craft. In this way, all ofthe

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 ofthe keel and

keel assembly. For example, means may be provided to adjust the rest position ofthe

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 speeds. 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

CLAIMS:-

1. A keel assembly for an aquatic craft, said keel assembly including:

a keel;

mounting means adapted to connect the keel to an underside ofthe 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 ofthe craft, the axis of rotation being spaced

forwardly of an effective centre of pressure ofthe keel.

2. A keel assembly for an aquatic craft according to claim 1 , wherein the mounting

means includes a shaft extending upwardly from the keel through a hole in the underside

of the craft to a rotatable bearing.

3. A keel assembly for an aquatic craft according to claim 2, wherein the rotatable

bearing is a flange bearing.

4 A keel assembly for an aquatic craft according to claim 3 wherein the keel is

detachable from the flange bearing.

5 A keel assembly for an aquatic craft according to claim 4 wherein the mounting

means include 2 or more supporting sub-shafts substantially parallel with the axis of

rotation, and keyed joints to transfer torque between the keel and the bearing.

6. A keel assembly for an aquatic craft according to claims 1 to 4, wherein the

mounting means includes a keel shaft extending upwardly from the keel through the hull and the rotatable bearing.

7 A keel assembly for an aquatic craft according to claim 5, wherein the mounting

means includes a crank to transfer torque from hydrodynamic forces on the keel to

resilient bias means.

8 A keel assembly for an aquatic craft according to claim 7, wherein resilient bias

means comprise a linear pneumatic, hydraulic or coil spring, or any combination thereof.

9 A keel assembly for an aquatic craft according to claim 8, wherein 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 ofthe keel away from the central rest

position causes the crank to turn, 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.

10. A keel assembly for an aquatic craft according to claim 8 or claim 9, wherein the

resilient means comprise a plurality of pneumatic, hydraulic or coil springs.

11. A keel assembly for an aquatic craft according to any one ofthe preceding claims,

wherein the keel includes a vertically tapered piercing portion extending forward from

the axis of rotation.

12. A keel assembly for an aquatic craft according to claim 11 , wherein the keel

includes a partially circular portion when viewed from the side, the circular portion

extending substantially rearwardly ofthe axis of rotation.

13. A keel assembly for an aquatic craft according to any one ofthe preceding claims,

wherein the keel includes a lateral aperture to allow passage of water during turning

manoeuvres.

14. A keel assembly for an aquatic craft according to any one ofthe preceding claims,

further including adjustment means whereby the keel may selectively be extended or

retracted vertically with respect to the hull.

15. A keel assembly for an aquatic craft according to claim 14, 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 ofthe keel in response to selective actuation ofthe drive means.

16. A keel assembly for an aquatic craft according to claim 15, wherein said drive

means include a manually operable or powered crank.

17. A keel assembly for an aquatic craft according to any one ofthe preceding claims,

wherein the keel is located on the rearward half of the hull.

18. A keel assembly for an aquatic craft according to claim 17, wherein the main

steering rudder is located forwardly ofthe keel.

19. A keel assembly for an aquatic craft according to claim 18, wherein the main

steering rudder is biased towards a central rest position, such that the rudder tends to

align itself with a longitudinal axis ofthe skiff in the absence of positive steering input.

20. A keel assembly for an aquatic craft according to any ofthe preceding claims,

including a plurality of keels and steering rudders.

21. A keel assembly for an aquatic craft according to claim 20, wherein the keels are

substantially identical and are positioned side by side on the hull.

22. A keel assembly for an aquatic craft according to claim 2, wherein the rotatable

bearing includes bearings to prevent lateral movement ofthe keel. W

- 20 -

23. A keel assembly for an aquatic craft according to any one ofthe 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.

AMENDED CLAIMS

[received by the International Bureau on 22 August 1997 (22.08.97); original claims 1-3, 6-12, 19 and 21-23 amended; remaining claims unchanged (4 pages)]

1. A keel assembly for an aquatic craft, said keel assembly including: a keel; mounting means adapted to connect the keel to an underside ofthe craft for enabling a lateral ability of free glide rotation about a generally vertical axis; and bias means disposed resiliently to resist the hull from the keel and ultimately urge the keel toward a central rest position substantially parallel to a longitudinal axis ofthe craft, the axis of rotation being spaced forwardly from any effective centre of pressure on the keel.

2. A keel assembly for an aquatic craft according to claim 1, wherein the mounting means includes a shaft extending upwardly from the keel through a hole, both to the underside ofthe craft and further appeture above of a supporting permanent base plate fixture, and onto a rotatable bearing.

3. A keel assembly for an aquatic craft according to claim 2, wherein the rotatable bearing is a flange bearing directly seated and supported by a substantially horizontal rigid base plate fastened to the hull.

4 A keel assembly for an aquatic craft according to claim 3 wherein the keel is detachable from the flange bearing.

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

6 A keel assembly for an aquatic craft according to claims 1 to 4, wherein the mounting means includes a keel shaft with single or multiple keyed sides such as chamfered surfaces extending upwardly from the keel through the hull and mating thru an internal slide cavity of the rotatable bearing, to form a joint for torque transfer between the keel and the bearing.

AMENDED SHEET {ARTICLE 19) 7 A keel assembly for an aquatic craft according to claims 1 to 6, wherein the mounting means includes the form of a crank upon the flange bearing to transfer torque from hydrodynamic forces on the keel to resilient bias means

8 A keel assembly for an aquatic craft according to claim 7, wherein resilient bias means compnse a linear pneumatic, hydraulic, coil or spiral spring, or any combination thereof whose forces either act in one direction or towards both directions.

9 A keel assembly for an aquatic craft according to claim 8, wherein a first end ofthe 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 ofthe hull or the keel away from the central rest position causes the hull to rotate about a stalling crank position or alternatively the crank to turn that in tum either elastically extends or compresses the spring or springs, thereby causing each spring to exert a torque upon the connecting keel shaft thus tending to bias the keel after all towards the central rest position. 10. A keel assembly for an aquatic craft according to claim 8 or claim 9, wherein the resilient means comprise singular or a plurality of pneumatic, hydraulic, coil or spiral spnngs

11 A keel assembly for an aquatic craft according to any one ofthe preceding claims, wherein the waterbound keels form includes a vertically tapered piercing portion extending forward from the axis of rotation

12 A keel assembly for an aquatic craft according to claim 11 , wherein the keel includes a partially circular spheroidal portion when viewed from the side, the circular or spheroidal poπion extending substantially rearwardly ofthe axis of rotation

13 A keel assembly for an aquatic craft according to any one ofthe preceding claims, wherein the keel includes a lateral aperture to allow passage of water dunng turning manoeuvres 14 A keel assembly for an aquatic craft according to any one ofthe preceding claims, further including adjustment means whereby the keel may selectively be extended or retracted vertically with respect to the hull.

15. A keel assembly for an aquatic craft according to claim 14, 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 ofthe keel in response to selective actuation ofthe drive means.

16. A keel assembly for an aquatic craft according to claim 15, wherein said drive means include a manually operable or powered crank. 17. A keel assembly for an aquatic craft according to any one ofthe preceding claims, wherein the keel is located on the rearward half of the hull.

18. A keel assembly for an aquatic craft according to claim 17, wherein the main steering rudder is located forwardly ofthe keel.

19. A keel assembly of skiff craft classes according to claim 18, wherein the main steering rudder is biased towards a central rest position, such that the rudder tends to align itself with a longitudinal axis ofthe skiff in the absence of positive steering input.

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

21. A keel assembly for an aquatic craft according to claim 20, wherein the keels are substantially similar or identical and are positioned side by side on the hull.

22. A keel assembly for an aquatic craft according to claim 2, wherein other rotatable bearings includes those bearings to prevent lateral movement ofthe keel. 23. A keel assembly for an aquatic craft according to any one ofthe preceding claims, wherein the keel is disposed rearwardly on the hull, a counterweight is disposed centrally on the hull, and in addition both the keel and the counterweight can also be vertically adjustable with respect to the hull.