MARINE PROPULSION AND CONSTRUCTIONAL DETAILS THEREOF
A hub (101) for a marine propeller, the hub being formed from first (1012) and second (1011) cooperating sections which, when forming the hub, are aligned along the rotational axis of the hub, wherein the two cooperating sections define a plurality of recesses (1061) into each of which a fin assembly can be inserted, thereby forming a propeller, and wherein of the first and second sections, only the first cooperating section is arranged to be connected, in use, to a drive shaft.
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The present invention relates to marine propulsion systems and to methods of providing controlled moments about the yaw, pitch and roll axes of a craft independently whilst also providing controlled thrust forces in the axial and transverse senses. The invention also covers a propeller, a fin or blade for a marine propulsion systems and methods of providing dihedral control and folding, including mechanisms for controlling the pitch and dihedral (as defined below) of the fins on the propellers.
Whilst the inventions have particular application to propellers for boats and ships, they will have application to other propellers and lift-generating rotors.
PURPOSE OF THE INVENTIONThe various aspects of the invention have one or more of the following principal objectives:
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- 1. Hull trim control
- The drag or resistance of high-speed hulls, and planing hulls in particular is very dependent on the hull trim angle. The optimum trim angle is particularly dependent on craft surge speed and displacement.
- In the case of stepless ‘V’ hulls in particular, but generally for high-speed hulls, the centre of lift moves aft as the speed increases. Whilst the use of some form of dynamic ballasting such as water ballasting, may be used to vary the centre of gravity position to correct for relatively slow variations in the centre of gravity position, much faster response is required for effective craft trimming whilst accelerating or running in a seaway.
- The first objective of the present invention is to provide effective forces and moments to enable effective static and dynamic craft trimming by means of thrust vectoring with minimal change to propulsive efficiency.
- 2. Turning
- Turning a craft efficiently requires that the roll, trim and yaw angles are established and maintained such as to optimise variables including hull drag or resistance, side-slip, and ride comfort.
- A second objective of the present invention is to provide effective means of thrust vectoring to enable the roll, trim and yaw angles and rates of a craft to be independently set up and maintained such that these angles and angular rates may be optimised by a corresponding control system without recourse to rudders, trim tabs or like drag-creating control surfaces.
- Optimisation of these variables results in minimal power loss in turns.
- 3. Improved docking & Low-speed control
- Boats in general, but in particular lightweight planing craft show very poor directional stability at low speeds and helm-reaction is slow. Maneuvering frequently requires operating the helm, clutching and throttling one or more engines and operating a bow thruster. Furthermore, all drives other than controllable pitch propellers and water jets have a minimum boat speed which is fixed by engine speed. For fast craft this minimum speed may be 8 knots or even higher. Improved gearboxes have recently become available TwinDisc U.S. Pat. Nos. 4,186,829, 6,443,286, 6,666,312, Caterpillar U.S. Pat. No. 4,125,039 and Volvo Penta U.S. Pat. No. 5,992,599, ZF Patent No DE10035480. These rely on modulating the drive clutch and largely remove the minimum speed limitation.
- Docking and low speed maneuvering in confined spaces requires considerable tuition and practice and remains a time of high stress for pilots.
- A third objective of the present invention is to enable low speed thrust vectoring such that the velocities and accelerations in the fore/aft, lateral and yaw directions may be independently controlled such as to enable dynamic positioning and simplified docking and low-speed maneuvers without declutching or gear changing or the use of complex controls.
- 4. Motion Control
- A fourth objective of the present invention is to provide fast and efficient thrust vectoring to enable effective 6-dimensional stabilisation for fast craft without the requirement for stabilising foils or other drag-producing control surfaces
- 5. Speed Range for high propulsive efficiency
- A fifth objective is the provision of a propulsion system with very high propulsive efficiency over a broad operating spectrum.
- 6. Fin Folding
- A sixth objective is the provision of means for folding the blades of such propulsion devices for the purposes of beaching or operating in very shallow water.
- 7. To enable individual changing of fins
- When propellers are damaged due to striking logs or other debris, invariably only one or two blades are damaged. The ability to readily change only damaged components can lead to a reduction in cost of replacement
- 8. To allow fins to be changed in the water
- 9. To provide a streamlined root section of minimum cross section to minimise churning losses at low craft speeds when the propeller is largely immersed
- 10. To provide a shape which allows large dihedral angles and folding
- 11. To allow for simple manufacturing and metrology in volume
- 12. To minimise weight and optimise stress distribution
- 13. To minimise control moments and allow for fail safe operation
- 14. To provide a clean form which does not tend to trap logs and other flotsam in order to reduce damage in case of log strikes.
- 1. Hull trim control
System control variables are:
For dual propulsor installations:
With 5 variables to control, the speed of the two propulsors may be kept the same Alternatively, for single or dual propulsor installations:
Collective and Cyclical pitch
Collective and cyclical trim
Engine speed
For dual propulsor installations this provides too many variables so that cyclical controls could be used to optimise for non-axial incoming flow, etc.
It is only possible to control a marine vessel about 5 axes (Fore/aft, side/side, Roll, Pitch and Yaw). The sixth axis (Vertical) is not required as the craft is a surface craft, thus only 5 control variables are required. Thus being able to control the pitch, dihedral (the term ‘trim’ used in this specification is a marine term for trimming a drive to control the pitch attitude of a boat hull) of two propulsors and to control a common engine speed is a sufficient number of variables. In the case of a single propeller, control of collective and cyclical pitch and dihedral and engine speed is required to achieve the required 5 control variables.
In the above cases the word “trim” is taken to mean the means of creating forces to trim the craft, e.g. variable dihedral, variable rake, variable sw ash, or variable tilt. Whilst aspects of this invention have application to fixed pitch propellers, it may be more generally applied to controllable pitch propellers which may be provided with collective pitch control or both collective and cyclical pitch control. Cyclical control may be a simple sinusoidal variation or some more complex motion involving higher-order harmonics.
Whilst variable dihedral is the preferred mechanism, similar, if less desirous effects may be achieved by the use of controllable rake. The conventional methods using variable tilt, or variable swash, are not suitable due to the large variation in cyclical angle of incidence which results from the use of such mechanisms. This is discussed further in the specific description of
Aspects of the invention further relate to methods of folding the blades of such propulsion devices for the purposes of beaching or operating in very shallow water. A craft to which devices including these new inventions are fitted will preferentially be provided with a propulsion management system.
Rake: Rotation of a fin about an axis located in the root of the fin and which is in or parallel to the plane of rotation of the propeller hub
Dihedral: Rotation of a fin about an axis located in the root of the fin, irrespective of the orientation of the fin relative to the hub
It will be appreciated that the term dihedral angle is not generally used in the definition of propellers. It will be defined by reference to
The invention enables the propulsion device to control all the motions of the craft, namely: forward and reverse displacement, axial velocity and acceleration or braking; lateral displacement, velocity and acceleration; heave position, heave velocity and acceleration; roll angle, velocity and acceleration, pitch angle, velocity and acceleration, yaw (steering) angle, velocity and acceleration. Such devices as rudders, flaps, foils or stabilisers are not required for these functions, although they may be used in conjunction with the propulsive device of the current invention.
The propulsors of this invention may be used in conjunction with a control system to optimise the attitude of the craft to maximise its speed, acceleration, the general handling and steering of the craft, and its motions in roll, yaw or pitch. An independent aspect of this invention allows for the controlled use of water ballast to optimise the steady-state trim (pitch angle) of the craft. Further aspects of this invention relate to docking a craft and to its operation in the case of the failure of a prime mover or a propulsor.
The propulsors of this invention may also be used in conjunction with a control system for the purposes of dynamic positioning, simplified docking and maneuvering at low speeds, and for the navigation of the craft.
Aspects of the invention are applicable to craft comprising a single or multiple prime movers. The propulsion system may consist of a single propeller, a single or multiple pairs of contra-rotating propellers in which each pair of propellers rotates about a common axis, or dual or multiple propellers rotating about parallel or individually or collectively orientable axes.
Surface-piercing propellers are known to produce a great deal of vibration due to the cyclical torque and thrust variation which can be as much as 100% of the mean value of either. Undertaking a Fourier transform of the torque or thrust reaction forces indicates that the most energy is in the first and second fin orders, especially for 5 or 6 fins
3 to 6 fins means that forcing frequencies will be present for substantial periods of time over the total frequency spectrum between typically 10 hz and 500 Hz
Rear mount deflect—keeping the fin roots within the shadow of the flow guides means that the deflection of the rear mount must be restricted. Conventional mounts require a deflection of up to 6 mm in order to achieve the required low frequency isolation, but a mounting having such a characteristic would deflect excessively when subjected to the very much higher forces generated by a propulsion system designed to produce thrust vectoring.
Heretofore, the reaction forces have largely been absorbed by the transom of the craft. Vibratory forces reacted by the transom will generally result in considerable vibration being transmitted throughout the hull structure resulting in high levels of noise and vibration.
Marine propulsion systems to control yaw, axial thrust and side thrust have been proposed by BUECHLER Dirk (EP1008514—Ship propulsion) and others. Such devices have used controllable pitch and engine speed for these purposes. However, the net side thrust achievable by this means is modest under some conditions and reversing the side thrust direction once motion has been established is difficult.
According to an aspect of the invention, there is provided a propeller comprising: a hub; and a plurality of fins extending radially outwards from the hub, wherein each fin is moveably connected to hub such that, during rotation of the propeller, either the dihedral, the rake or both the dihedral and the rake of the fin can be altered and controlled by movement of the fin about one or more axes of rotation.
The propeller preferably further comprises a closed loop control system to vary either the dihedral angle, the rake angle or both the dihedral and the rake angle of the fins during use.
The fin may be pivotable about a radial axis so as to control the pitch of the fin. The fin may be pivotable about an axis located in the plane of the rotational axis of the propeller. The fin may be pivotable about an axis generally parallel to the root chord of the fin.
The fins are preferably discretely moveable about at least one axis between at least two positions. The fins may be continuously moveable about at least one axis between two positions.
The propeller may further comprise dihedral angle control means for moving the fins between the first and the second position. The dihedral angle control means is preferably arranged to vary the dihedral angle of the fins during use.
The propeller may comprise pitch control means for controlling the pitch of the fins. The pitch of the fins may be continuously variable. The pitch control means may be arranged to vary the pitch of the fins during operation.
The propeller may be provided with means for varying the dihedral angle of one or more fins during one revolution of the propeller, such that the dihedral angle of any fin at top dead centre can be different to the dihedral angle of the fin at bottom dead centre, and similarly for the cross-axis.
The propeller may be provided with means to vary the pitch of one or more fins during one revolution of the propeller, such that the pitch of any fin at top dead centre can be different to the dihedral angle of the fin at bottom dead centre, and similarly for the cross-axis.
The closed loop control system may also vary the pitch of the fins during use.
The propeller is preferably a surface-piercing propeller.
The range of dihedral angle is independent of the pitch angle.
The propeller may further comprise a master spline within the dihedral section for, in use, aligning the fin in the desired orientation with respect to a hub.
The centre of pressure of the fin is preferably located at a distance of between 50% and 60% of the working surface span from the root section.
The centre of pressure of the fin is preferably located at a distance of between 60% and 70% of the span from the dihedral axis.
The present invention also provides a boat having a hull and at least one propeller as described above, wherein the hull is provided with a fairing in front of the or each propeller and wherein fins are moveable such that they may be wholly positioned within the hull or the fairing profile when viewed from the front.
The hub of the propeller may be contained wholly within the hull profile when viewed from the front.
The invention also provides a vehicle such as a water craft, ship, aircraft or helicopter having at least two propellers as described above, wherein at least one propeller is arranged to the starboard side of the longitudinal axis of said vehicle and at least one propeller is arranged to the port side of the longitudinal axis of said vehicle.
Any propeller arranged to the starboard side of the longitudinal axis of said vehicle preferably rotates in the opposite sense to any propeller arranged to the port side of the longitudinal axis of said vehicle.
Any propeller arranged to the starboard side of the longitudinal axis of said vehicle preferably rotates in a clockwise sense when viewed from the rear and any propeller arranged to the port side of the longitudinal axis of said vehicle rotates in a counter-clockwise sense when viewed from the rear.
The present invention also provides a craft comprising a hull and at least one propeller as described above, wherein the control system is arranged to optimise at least one of the following: the trim angle and motion of the craft by varying the dihedral angle of the fins of the propeller(s); the roll motion of the craft by varying the dihedral angle of the fins of the propellers relative to each other; the roll motion of the craft by varying the dihedral angle of the fins of the propellers and their rotational speed relative to each other; the roll motion of the craft by varying the dihedral angle of the fins of the propellers and their pitch angle relative to each other; and the yaw motion of the craft by varying the pitch of the fins of the propellers relative to each other.
The propeller may comprise pitch control means in which the pitch means comprises two bearings disposed along the pitch axis of any fin in which either or both the first and the second bearings are angular contact bearings disposed such as to increase the effective centres of said bearings.
The propeller may comprises pitch control means in which the pitch means comprises two bearings disposed along the pitch axis of any fin in which either or both the first and the second bearings are spherical bearings disposed such as to increase the effective centres of said bearings.
The pitch control means may rotate any fin about a spherical cup by which said fin is pivotally attached to the hub for dihedral angle control.
The dihedral angle control means may comprise a lever arranged to exert a moment about the dihedral angle axis and which comprises a spherical pivot at its inner end.
The dihedral angle control means may further comprise a link pivotally attached to the inner end of the trim lever which is configured such that a first pivot of said link is aligned on the pitch axis when the corresponding fin is arranged at its maximum design dihedral angle and a second pivot is aligned on the pitch axis when said fin is arranged at its minimum design dihedral angle.
The shuttle is preferably configured to move in a substantially helical trajectory about the rotational axis of the propeller with such trajectory passing through, or at a proscribed distance from, the pitch axis of the corresponding fin.
The trajectory of the fixing point of the link onto the shuttle preferably bisects the angle of the link at the maximum forward pitch angle and the maximum reve rse pitch angle, but it may be at some other desirable angle.
The pivot at the inner end of the dihedral angle lever is preferably free to slide in a radial direction in the shuttle. The shuttle may be constrained to move along a helical trajectory.
The pitch angle is preferably arranged to be independent of the dihedral angle throughout its controlled range and is preferably arranged to be independent of the dihedral angle throughout its desirable working range.
The pitch control moment range is prefer ably minimised.
Preferably, the pitch moment is arranged to be such that its direction is not changed during normal operation of the propeller.
One or more actuators are preferably provided to control each of the pitch and dihedral movement. These actuators may be arranged such that the actuators work together to control the dihedral movement and such that the actuators work differentially and/or independently to control the pitch movement.
The dihedral actuation may have a discontinuity between the normal operating range and the folding range such that the pitch angle can be set to some optimum value prior to the folding operation.
The dihedral actuation may comprise a sector gear operating on the fin holder.
According to an aspect of the invention, there is provided a fin for use on a propeller, the fin comprising: a lift generating section having a leading edge and a trailing edge, and a pair of surfaces extending between the leading and the trailing edges thereby defining a tip section and a root section; and a dihedral section connected to the root section of the lift generating section and having an axis about which, in use, the lift generating system can be rotated to vary the dihedral of the fin, the axis being generally parallel to the root section of the lift generating section.
The axis of rotation is preferably within a 15 degrees deviation from the root section, more preferably the axis of rotation deviates by no more than 3 degrees from the root section. The axis of rotation may be parallel to the root section.
The dihedral section may comprise a front boss adjacent the leading edge and a rear boss adjacent the trailing edge. Each boss is preferably provided with a through hole which is splined for engagement with a corresponding spline on a shaft in use. The axis of rotation preferably passes through the centres of the through holes in each boss.
The front boss may advantageously blend smoothly into the leading edge such that no re-entrant is formed between the boss and the leading edge.
The fin may have a pitch axis and a stack axis and the pitch axis is preferably closer than the stack axis to the leading edge. The stack axis preferably does not pass through the axis of rotation. The stack axis may be angled relative to the pitch axis. The centre of pressure of a fin may be located forward of the pitch axis.
The fin may further comprise a stop element mounted on either or both of the front and rear bosses for limiting rotation of the fin about the axis of rotation in at least one direction.
The fin may be provided with a surface between the front and rear bosses at the root of the fin shaped such that, when the fin is mounted on a yoke, the surface at the root of the fin acts, during rotation, to remove marine growth, sediment or other unwanted material from the yoke.
According to an aspect of the invention, there is provided a modular propeller system having: a plurality of substantially identical fin assemblies; and a plurality of hubs, each hub being arranged to receive a different number of the substantially identical fin assemblies; wherein a propeller of the desired number of fins can be created by the selection of the appropriate hub and the required number of the substantially identical fin assemblies.
Each fin assembly may include a fin having a lift generating surface and a leading and a trailing edge.
Each fin assembly preferably comprises a turret on which the fin is mounted, the turret being arranged to be connected, in use, to an engagement means in one of the hubs.
Each fin assembly may be connected directly to the hub.
Each hub may be provided with a plurality of engagement means, each engagement means being associated with a respective fin assembly. Each engagement means on each hub of the plurality of hubs is preferably identical.
Each hub may be formed from two cooperating sections and the two sections are, when constructed, preferably aligned along the rotational axis of the propeller.
The hub sections may be provided with cooperating recesses which, when the sections are aligned, define the desired number of receiving locations for the plurality of fin assemblies.
The invention also provides a hub for a marine propeller, the hub being formed from two cooperating sections which, when forming the hub, are aligned along the rotational axis of the hub.
The two cooperating sections preferably define a plurality of recesses into each of which a fin assembly can be inserted, thereby forming a propeller.
According to an aspect of the present invention there is provided a mount for supporting part of a marine drive system to a marine hull, the mount comprising:
a rigid outer housing defining a radially outer circumferential surface on its radially inner surface;
a resilient mounting disposed radially with in the outer housing and defining a radially inner circumferential surface, the marine drive system passing through and supported by the resilient mounting in use;
wherein a circumferential radial gap is provided either between the resilient mounting and the rigid outer housing or within the resilient mounting.
The rigid outer housing may be substantially annular.
The rigid outer housing may be circular.
The rigid outer housing may have an eccentric form.
The resilient mounting may be substantially annular.
The resilient mounting may be circular.
The resilient mounting may have an eccentric form.
Preferably, upon operation of the marine drive system the marine drive system radially deflects an arcuate portion of the resilient mounting into a corresponding arcuate portion of the gap.
Preferably, wherein the gap is dimensioned such that the rate of force produced by the marine drive system to the radial deflection of the marine drive system is substantially linear.
Preferably, the gap is dimensioned such that the highest solid-body resonant frequency of the marine drive system is less than 1/√{square root over (2)} times the lowest forcing frequency generated by the marine drive system.
Preferably, upon operation of the marine drive system the marine drive system deflects an arcuate portion of the resilient mounting into a corresponding arcuate portion of the gap and compresses the arcuate portion of the resilient mounting against the rigid outer housing.
Preferably, the gap is dimensioned and the resilient mounting is formed such that the rate of force produced by the marine drive system to the radial deflection of the marine drive system increases progressively.
Preferably, the resilient mounting is formed from a micro-cellular polyurethane material.
Preferably, the circumferential radial gap is an air gap and is provided over an axial length of the resilient mounting. The remaining axial length of the resilient mounting is compressed, in use, such that it forms a seal between the outer circumferential surface and the inner circumferential surface.
A marine craft may be provided having a hull and a transom positioned at the rear of the craft, the craft comprising a marine drive system attached to the transom by means of the mount described above.
According to the present invention there is provided a mount for supporting part of a marine drive system on a marine hull, the mount comprising:
an annular resilient member having radially spaced inner and outer circumferential surfaces, wherein an annular groove forming an air gap is provided between the inner and outer circumferential surfaces.
According to the present invention there is provided a marine drive system comprising:
an engine connected to a gearbox by means of a drive shaft;
a propeller connected to the gearbox by means of a propeller shaft;
wherein the gearbox and the propeller are formed as a single rigid body for mounting in the hull of a marine craft as a single unit; and
wherein the engine, gearbox and propeller are positioned such that they are generally aligned with the longitudinal axis of the craft.
The gearbox may be disposed remotely from the engine and a rigid frame is connected between the gearbox and the craft.
The drive shaft may be formed from carbon fibre.
The single rigid body may comprise the engine, gearbox and propeller for mounting in the hull of a marine craft as a single unit.
The engine and the gearbox may be positioned adjacent to each other.
Preferably, the engine, gearbox and propeller are aligned along the longitudinal axis of the craft.
The marine drive system may further comprise a second propeller connected to the gearbox by means of a second propeller shaft, the engine and the gearbox being aligned along the longitudinal axis of the craft and the propellers being aligned with the longitudinal axis of the craft.
The marine drive system may further comprise a second single rigid body; and
wherein each single rigid body is generally aligned with the longitudinal axis of the craft.
Preferably, the ratio of the propeller diameter to the propeller shaft diameter is in the range of 4.5 to 5.5
Preferably, the ratio of the propeller shaft diameter to the propeller shaft length is in the range of 0.65 to 0.9.
Preferably, the gearbox has an input shaft driven by the engine and the ratio of the propeller shaft diameter to the input shaft diameter is in the range of 2.2 to 3.5.
Preferably, the ratio of the input shaft diameter to the input shaft length is in the range of 0.5 to 0.62.
Preferably, the single unit is provided with front mounting points adjacent to the engine and rear mounting points adjacent to the gearbox, for attaching to the hull of the marine craft,
the rear mounting points being positioned at 8% to 12% of the overall length of the marine drive system from the centre of the propeller; and
the front mounting points being positioned at 40% to 60% of the overall length of the marine drive system from the rear mounts.
A marine craft may be provided having a hull and a transom, the marine craft comprising a marine drive system according to any one of the preceding claims;
wherein the marine drive system is flexibly mounted to the hull of the marine craft.
According to an aspect of the present invention there is provided a marine drive system comprising:
an engine connected to a gearbox by means of a drive shaft;
a propeller connected to the gearbox by means of a propeller shaft;
wherein the engine, gearbox and propeller are formed as a single rigid body for mounting in the hull of a marine craft as a single unit; and
wherein the engine, gearbox and propeller are positioned such that they are generally aligned with the longitudinal axis of the craft.
Preferably, the rotational axes of the drive shaft, the propeller shaft and the propeller are generally aligned with the longitudinal axis of the craft.
According to the present invention there is provided a propulsion assembly comprising:
a hub;
a plurality of fins moveably connected to the hub such that the pitch angle of each fin can be controlled;
a pitch shuttle moveable, by means of a pitch actuator, along an axis coaxial with the axis of the hub;
a plurality of pitch linkages each connecting the pitch shuttle with one of the fins arranged such that upon movement of the pitch shuttle, the pitch angle of each fin is varied.
Preferably the propulsion assembly further comprises means for controlling the dihedral angle of each fin.
Preferably the means for controlling the dihedral angle of each fin comprises:
a dihedral shuttle moveable, by means of a dihedral actuator, along an axis coaxial with the axis of the hub;
a plurality of dihedral linkages each connecting the dihedral shuttle with one of the fins arranged such that upon movement of the dihedral shuttle, the dihedral angle of each fin is varied.
According to the present invention there is provided a propulsion assembly comprising:
a hub;
a plurality of fins moveably connected to the hub such the dihedral angle of each fin can be controlled;
a dihedral shuttle moveable, by means of a dihedral actuator, along an axis coaxial with the axis of the hub;
a plurality of dihedral linkages each connecting the dihedral shuttle with one of the fins arranged such that upon movement of the dihedral shuttle, the dihedral angle of each fin is varied.
Preferably the propulsion assembly further comprises means for controlling the pitch angle of each fin.
Preferably the means for controlling the pitch angle of each fin comprises:
a pitch shuttle moveable, by means of a pitch actuator, along an axis coaxial with the axis of the hub;
a plurality of pitch linkages each connecting the pitch shuttle with one of the fins arranged such that upon movement of the pitch shuttle, the pitch angle of each fin is varied.
Preferably each of the dihedral linkages has a first end which is spherically pivoted to the dihedral shuttle; and
a plurality of swing links are provided at the point of connection of each dihedral linkage to the dihedral shuttle;
each swing link having a first end spherically pivoted to the dihedral shuttle and a second end spherically pivoted to a point on the hub.
Preferably each swing link is arranged such that upon movement of the dihedral shuttle along the axis coaxial with the axis of the hub, the dihedral shuttle is caused to rotate around the axis coaxial with the axis of the hub in a helical manner.
Preferably the pitch angle and the dihedral angle can be independently varied.
A plurality of dihedral levers may be provided, each connecting one of the fins with a second end of one of the dihedral linkages, each dihedral lever having an inner end which is spherically pivoted to the second end of one of the dihedral linkages.
Preferably the dihedral levers and the dihedral linkages are configured such that the first end of each dihedral linkage is coaxial with the pitch axis when the fin is at a minimum design dihedral angle, during operation of the propulsion assembly.
Preferably the dihedral levers and the dihedral linkages are configured such that the second end of each dihedral linkage is coaxial with the pitch axis when the fin is at a maximum design dihedral angle, during operation of the propulsion assembly.
The propulsion assembly may further comprise two bearings disposed along the pitch axis of each fin and either or both of the bearings are angular contact bearings disposed such as to increase the effective centres of said bearings or either or both of the bearings are spherical bearings disposed such as to increase the effective centres of said bearings.
Embodiments of the various inventions will now be described with reference to the accompanying drawings, in which:
A propulsor 1 is arranged to rotate about axis CC and comprises a hub 101 and six fins 3 each mounted to a turret 20 arranged for independent pivotal motion about a pitch axis AA and a dihedral axis BB. Axis AA is generally normal to the propeller axis CC. Axis BB is generally arranged at 90 degrees to axis AA.
Fin 3 comprises a stop 316 which reacts against a planar area of yoke 21.
FIG. 2A propulsor fin 3 comprises a lift generating section 30 and a root section 31. The lift generating section comprises a normally wetted surface 301, a leading edge 302, a trailing edge 303 and a normally un-wetted surface 304.
The normally un-wetted surface is vented to atmospheric pressure during normal forward thrust conditions. During reverse-thrust conditions surface 304 may become fully wetted and surface 301 may become fully un-wetted. Under intermediate conditions either surface may be partially wetted.
The root section (also referred to as “dihedral section” as it is the section that allows the dihedral to be altered) blends smoothly into the normally wetted and normally un-wetted section along two lines 305. The root section is un-wetted during normal operation but becomes partially or fully wetted at low speeds.
The root section has a generally smooth profile which may be a surface of revolution about an axis BB and comprises a leading edge boss 314, a trailing edge boss 313 and a bridge section 310. The leading and trailing edge bosses comprise a common spline profile 315 preferentially having a master spline 3151 (see
The trailing edge boss 313 comprises an inner face 3131 and an outer face 3133 (see
The leading edge boss 314 comprises an inner face 3141 (see
A root section of fin 3 is angled at a small angle θR to axis BB. A tip section of fin 3 is angled at a larger angle θT to axis BB. The angles θR and θT are preferentially disposed on the same side of axis BB, but may be disposed on different sides of said axis. The twist of the fin is defined by the expression
Twist=θT−θR
Wherein the twist is normally below 20 degrees and preferentially in the range 12 degrees to 16 degrees.
Propeller blades and other such lifting surfaces have traditionally been made by stacking a number of discrete sections together along a common axis and then fairing the complete surface. Even though the use of numerically controlled machines has largely replaced this practice the concept of the use of a “stacking axis” is still in common use.
FIG. 3The stacking axis 320 is shown having an offset 3202 from axis BB and inclined at an angle 3203 relative to the pitch axis AA. Such an offset may be used to reduce the centripetal moment exerted by fin 3 about axis BB when the fin is in its normal range of dihedral angle inclined anticlockwise about axis BB by 5 degrees to 35 degrees. The trailing edge thickness 324 reduces from 3242 at the root to 3241 (see
The outer profile 3135 of the trailing edge boss is arranged to achieve a minimum wall thickness around the splined centre consistent with maintaining adequate fatigue life and a smooth hydrodynamic shape.
FIG. 4The stacking axis 320 is shown arranged at a distance 3201 be hind the pitch axis AA such that the fin centre of pressure 325 is at a distance 3251 forward of the pitch axis AA. It will be appreciated that the centre of pressure position will vary during operation, but that the control moment about the pitch axis should be kept low in order to limit the size of the pitch control mechanism. By keeping the centre of pressure in front of the pitch axis as depicted any failure of the pitch mechanism will normally ensure that the fin rotates to its maximum forward pitch position.
The centre of pressure 325 is shown at a distance 3252 from the dihedral axis BB. The dihedral control moment is governed by this distance and it will be appreciated by those skilled in the Art that this distance is very small in comparison with of her methods of thrust vectoring.
The fin has a root chord 3222 and a tip chord 3221. The root chord is shown as being identical to the length of the root section for manufacturing simplicity and compactness, but it will be realised that the root section could extend forward and/or rearwards relative to the root chord.
A key feature of the present invention is the ability to create a family of fins from a single forging. The fin is shown with a span 321, but the same forging may conveniently be cut down to tip profiles indicated by 3061 and 3062 having spans 3211 and 3212 respectively.
The trailing edge root section 313 has an outer plane face 3133 and an inner plane face 3131 both arranged to be normal to the dihedral axis BB, and is arranged with a width 3134.
The leading edge root section 314 (
The root bridge section 310 is arranged with a profile of varying radius 3102 swept about axis BB. The swept bridge profile is blended into the inner faces 3131 and 3141 with blending surfaces 3103, 3104.
The inner surfaces 3132, 3142 of root bosses 313, 314 are arranged with surfaces finished for dynamic face seals. The outer surfaces 3133, 3143 are finished for static seals.
FIG. 5The top view shows the relationship between θT and θR.
FIG. 6Whereas the preferred embodiment has a trailing edge normal to the dihedral axis BB to simplify manufacture and metrology, the trailing edge may be skewed positively or negatively through angle ψ as desired.
A removable fin according to this invention may beneficially have dimensions within the ranges shown in the table below:
A propulsor 1 is shown with a single fin 3 mounted by a retaining pin assembly 35 to a turret 20. Turret 20 is fixed to a hub 101 arranged for a plurality of fins. Turret 20 is arranged such that movement of a pitch pivot pin 211 results in rotation of the turret assembly and its attached fin about axis AA, and movement of a dihedral lever 241 results in rotation of the fin about axis BB.
FIG. 9A dihedral lever 241 is fitted to the external splines and may be locked in place by one or more grub screws 245 reacting against a thinned down section of the lever. Thrust washers 227 either side of the dihedral lever react side thrust from lever 241 against internal flanks in the bore of yoke 21. Journal bearings 226 react forces from the fin and dihedral lever into yoke 21. Seals 223 maintain a seal between the internal bore of yoke 21 and the external surfaces of fin carrier 221 and also seal against the internal faces (not shown) of fin 3 (not shown). A four-point, bearing and seal cartridge 25 retains the turret assembly 21 in the propeller hub 101 and is sufficiently dimensioned to take the radial, axial and tilting loads imposed by fin 3 and dihedral lever without brinelling under the continuous load fluctuation exerted by fin 3 (
A dihedral link, 131 comprising a high-capacity plain spherical bearing 132 is retained to the inner end of dihedral lever 241 by screw 133.
FIG. 11A dihedral link, 131 comprising a high-capacity plain spherical bearing 132 is retained to the inner end of dihedral lever 241 by screw 133.
The root section of fin 3 is yoke-shaped. The fin is assembled by pushing the inner faces of the leading and trailing edge bosses over the projecting sealing lips of seals 223. Once in position pin 3511 is inserted. Nose cone 353 and end cap 3512 complete with O-rings 354 are then pushed in either end of the fin bore. Once in position screw 355 is inserted and tightened and plug 3514 with its seal 3515 is inserted. This arrangement provides an exceptionally robust pivot and allows the simple exchange of individual fins when required.
FIG. 12A bearing cassette 25 fitted to an abutment at the lower end of yoke 21 is arranged with contact angles such that reaction vectors 2593, 2594 intersect the pitch axis AA at 2591, 2592 respectively. The distance between these two intersection points is 259. The distance 2595 between the dihedral pivot axis BB and point 2591 and the distance 259 should each be as great as possible to resist the turning moment about the pivot axis BB exerted by the lift and centripetal moment generated by fin 3 and the control moment exerted by the dihedral lever 241.
FIG. 13Seal 254 preferentially comprising an outer lip 2541 and inner lip 2542 and an external static sealing arrangement 2543 is preferentially trapped between the outer race of bearing 261 and an abutment 1011 formed in the propeller hub 101. Sealing lips 2541, 2542 rotate about the seal counterface 2544.
Bearing 261 is preferentially maintained against an abutment 1012 in the hub by a pressure ring 262.
FIG. 14As the dihedral shuttle is moved along axis CC from a zero dihedral angle point P1, the shuttle will be rotated about axis CC by the influence of the swing links 14. When swing link 14 is rotated to point P2 the propulsor 1 will achieve its maximum dihedral angle. As shuttle 453 is further displaced under the action of the actuator to the point where fin 3 (not shown) is fully folded, swing link 14 will further rotate to point P3.
FIG. 16The dihedral lever side force has to be reacted by the pitch moment applied. This is undesirable as it will increase or decrease the pitch moment. If it increases the pitch moment it will increase the size of the pitch actuator and the energy required to control pitch. If it detracts from the pitch moment it could cause reversal of the pitch moment causing potential control problems and the potential for the pitch angle to swing to full reverse in case of a failure in the pitch control function. This could result in a catastrophic failure.
It will be evident that by optimising the position in which the swing link 14 is attached to the propeller hub 101, the direction and magnitude of the dihedral lever side force can be optimised, particularly over the range of pitch angles where high thrust forces are generated.
FIG. 20It will be appreciated that the fins, turrets and mechanisms including bearing arrangements, seals, yokes, links, etc. required for this sort of propeller require to be heavily tooled and produced in high volumes to ensure economic production costs together with high quality standards and low weight. Accordingly, the object of the current invention is to devise a propeller using a plurality of common components.
A wide variety of marine propellers are normally required to cover any particular band of power, boat speed and engine speed and this results in a major manufacturing and logistical problem. As a result larger propellers, in particular, are frequently manufactured to order with a consequent delivery delay.
Prior art propellers have usually consisted of a fixed number of blades integrally manufactured on a hub of cylindrical or swept section
Prior art controllable pitch propellers for marine applications have generally been constructed as shown in Duncan (U.S. Pat. No. 6,332,818)
A propeller hub comprises a hub which may accept a number of blade turrets. The preferred number is between three and six, although in practice the number may vary from two upwards. This construction allows a six bladed propeller with common components to absorb approximately twice the power of a three bladed propeller with four and five bladed propellers being able to absorb intermediate power.
This construction also allows a single fin type to be used for a propeller size capable of transmitting between 50% and 100% of its design power with little variation in propulsive efficiency. As described in Patent Application EP 06120796.5 of equal date, propeller blades may be cut down from some maximum length such that even finer adjustment of the propeller to the power to be transmitted and the performance required may be achieved.
In this construction the only components which chan ge for the power band above are the hub and the pitch and dihedral shuttles. All other components and assemblies are common.
The hub may be manufactured as a casting or moulding from modular tooling.
This approach allows a propeller which is closely matched to the hull performance and engine characteristics to be made up from a small number of standard components which can be stocked thus cutting down lead times from weeks or months to hours.
For instance, this construction requires the following part numbers and tools to cover a power band multiplier of 16 (say from 100 hp to 1600 hp):
The cooperating casings 1011, 1012 define a hollow interior into which the control mechanism for controlling the movement of the fins can be placed.
FIG. 27In particular surface piercing propellers arranged for controllable pitch have been designed with an essentially cylindrical hub of large diameter. Such a hub is claimed in Eriksson EP1280694 (US2003157849). Duncan U.S. Pat. No. 6,332,818 disclosed a large diameter hub with a swept profiled surface.
The hubs of propellers of this Art tend to be wholly or substantially immersed when the hull to which they are fitted is at rest or is travelling at displacement speeds. Under theses conditions it has been found that the propeller hub creates substantial churning losses such that the power required to overcome said churning losses result in a significant reduction in propulsive efficiency.
The churning losses are proportional to the following factors:
-
- (RΩ)2=The square of the speed of any part of any surface in contact with the water, and,
- A=The surface area of any surface in contact with the water at a radius of R from the rotational axis, and,
- Cdf=the friction drag coefficient of any surface A in contact with the water, and,
- Cd=the drag coefficient due to the form of any surface A in contact with the water.
The total churning power (Pc) is given by the expression below:
Pc=Σ(R×Ω×(R×Ω)2×A×(Cdf+Cd))=Σ(R3×Ω3×A×(Cdf+Cd))
From which it is evident that to minimise churning and the consequent power loss during low-speed operation the following requirements should be met:
-
- The rotational speed Ω should be kept as low as possible, and,
- The area A of any surface in contact with the water should be kept as low as possible and the radius R at which any such surface area is arranged from the axis of rotation should be minimised, and,
- The form drag coefficient Cd and the friction drag coefficient Cdf should be kept as low as possible.
It will be noted that at both the tip and the root sections of fins 3 the leading and trailing edges describe different circles as they rotate about axis CC. This means that the flow through the propeller at any particular point does not follow a design section.
This has a number major disadvantages:
-
- Due to the twist along the span of a fin as shown in
FIG. 10 , the curvature of the fin section seen by the flow will be less curved or more curved depending on whether the rake angle is more or less than the design figure. - An increase in curvature results in an increased lift coefficient as shown in
FIG. 82 . This in turn means that maintaining the correct torque requirement will require that the pitch is reduced which may result in the leading edge operating at negative incidence which would tend to swap the cavity to the other side of the fin. This would result in an unstable flow pattern and could cause hunting of the propulsion controls or the engine controller or both. - A decrease in curvature would result in a reduction in the lift coefficient which would require an increase in the leading edge incidence. However, this might not be sufficient to prevent the flow breaking away near the trailing edge, this could result in local cavitation.
- Due to the twist along the span of a fin as shown in
Additionally, it will be clear from looking at the drawing that the flow around the root and tip sections would be perturbed. This would result in a significant loss of performance.
FIG. 37It will be evident from this drawing that unlike in the case of rake the both the leading and the trailing edges of the fins sweep closely around circle 151 at the tip and circle 152 at the root. This results in the flow at any radius following a design section such that there are no significant additional losses.
FIG. 41As for the propeller of
It will be evident from this drawing that both the leading and the trailing edges of the fins sweep closely around circle 151 at the tip and circle 152 at the root. This results in the flow at any radius following a design section such that there are no significant additional losses.
FIG. 45It will be evident from this drawing that both the leading and the trailing edges of the fins do not sweep closely around circle 151 at the tip and circle 152 at the root, other than when the fins are aligned with axis BB. Not only do the fins become angled to the flow under other conditions, but they sweep an elliptical trajectory. This results in the flow at any radius failing to follow a design section such that there will be significant additional losses.
FIG. 47The combined effects shown in
It will be evident from this drawing that both the leading and the trailing edges of the fins sweep closely around circle 151 at the tip and circle 152 at the root. This results in the flow at any radius following a design section such that there are no significant additional losses.
FIG. 50It will be evident from this drawing that the both the leading and the trailing edges of the fins do not sweep closely around circle 151 at the tip and circle 151 at the root, other than when the fins are aligned with axis BB. Not only do the fins become angled to the flow under other conditions, but they sweep an elliptical trajectory. This results in the flow at any radius failing to follow a design section such that there will be significant additional losses.
FIG. 52The combined effects shown in
In this embodiment the engine load is constrained to values determined by the engine speed set points or the actual engine speeds.
FIG. 55It will be evident that additional sets of front and rear mounts could be positioned to accommodate additional engine/propulsor units.
FIG. 58The mount will preferentially be designed for a static deflection in the range of 3.5 mm to 5.5 mm shown at point X and to have a quasi linear load deflection characteristic around this point. As the load increases beyond point Y, the rate increases progressively.
In the opposite sense, the load decreases quasi linearly initially and then starts to decrease rapidly as the deflection decreases further. This situation corresponds to unloading the mount due to lifting forces generated by the propulsor or by negative acceleration in rough sea conditions.
In an optional design the rate curve is designed to be mirrored about point X to provide close control of the propulsor position whilst providing the required noise and vibration isolation.
FIG. 60A resilient moulding 831, preferentially manufactured from a micro-cellular polyurethane material is inserted into an outer housing 832 and pushed onto the aft gear casing 512 to which a retainer 837 has been loosely placed. Prior to pushing the resilient moulding 831 onto the gearcasing 512 its interior profile is substantially smaller than the corresponding profile of the gearcase. The act of pushing the moulding in to position creates a substantial pre-compression of the mount which ensures that sufficient sealing pressure is maintained between the resilient moulding 831 and both the outer housing 832 and the gearcasing 512 under all conditions. A plastic ring 5121 (see
The gearcasing 512 together with its associated propeller shaft 501, rear propeller shaft bearing 502 and shaft seal 110 and with retainer 837, outer housing 832 and resilient moulding 831 is inserted from inboard the craft (right-hand side of the drawing) and is retained in position by retainer 837 which is fixed by a number of bolts 836.
The outer housing 832 is preferentially ring-shaped and manufactured from a water resistant reinforced plastic material. A eccentric form is indicated in the drawing but the requirement for this will depend on the specific shape of the mating components.
The transom plate 611 is preferentially manufactured from a plastic material.
The resilient moulding 831 is preferentially annular in form.
FIG. 61The outer housing 832 preferentially has a forward internal lip 8321 and a rear internal lip 832 which serve to prevent the resilient mount from sliding due to axial forces, especially once it has been pre-compressed. It also comprises an external O-ring and groove 834 or other means of ensuring a static seal between it and the transom plate 611
The aft gearcase preferentially comprises a swept profile 5122 which prevents the resilient mount from sliding along it in the pre-compressed state.
In the static loaded condition there should preferentially be a small radial gap between the nose 8311 of the resilient mount 831 and the internal profile of retainer 837 as shown. This gap may vary around the periphery of the nose piece, but should preferentially be adjusted to 1.0 mm and 3.0 mm in the static laden condition. This is generally sufficient to ensure an adequate range for the desired linear load/deflection curve about point X of
It will be evident to those skilled in the Art that the load/deflection curve of mount 832 will be approximately equal to the sum of the curves due to the main body and the nose of the mount in isolation and that by adjusting the radial thicknesses and axial lengths of the two parts, by adjusting the preload in the main body of the mount, and by variation of the compound from which the mount is manufactured as well as its density, the mount may be tuned to the desired characteristics. It will also be evident that as the material is in shear in the axial direction, its rate will be substantially lower than in the radial sense where the material is in compression.
FIG. 62This alternative arrangement can simplify assembly of the aft gear casing into the mount.
FIG. 63The displacement curve in this figure shows a desirable deflection cu rye for the first order modal frequency of the complete unit about some neutral axis. The front and rear mounts are preferentially arranged such that they are subject to little or no deflection at this frequency.
In practice, the precise deflection curve may vary due to tolerance variations, changes in fluid levels in the engine and gearbox, the number of fins fitted to the propulsor, etc. As a consequence it will not be possible to ensure that the mounting positions are ideally placed under all conditions and it is generally sufficient that they are placed within +/−10% of the total length L, although the acceptable range varies depending on the proximity of forcing frequencies to the first order modal frequency in bending of the complete propulsion unit
Although the resonance is shown in the vertical longitudinal plane for convenience, it is unlikely to be precisely in this plane due to unsymmetry.
Whilst the propulsion unit is shown comprising an internal combustion engine it will be clear to those skilled in the Art that any other form of prime mover such as an electric motor, gas or steam turbine, etc. may be fitted.
FIG. 64The displacement curve in this figure shows a desirable deflection curve for the first order modal frequency of the complete unit about some neutral axis. The front and rear mounts are preferentially arranged such that they are subject to little or no deflection at this frequency.
In practice, the precise deflection curve may vary due to tolerance variations, changes in fluid levels in the engine and gearbox, the number of fins fitted to the propulsor, etc. As a consequence it will not be possible to ensure that the mounting positions are ideally placed under all conditions and it is generally sufficient that they are placed within +/−10% of the total length L, although the acceptable range varies depending on the proximity of forcing frequencies to the first order modal frequency in bending of the complete propulsion unit
Although the resonance is shown in the Horizontal longitudinal plane for convenience, it is unlikely to be precisely in this plane due to unsymmetry.
It is likely that the modal frequency in this plane is at a lower frequency than that in the essentially vertical plane due to differences in stiffness. Accordingly this mode of vibration may be the more critical.
Whilst the propulsion unit is shown comprising an internal combustion engine it will be clear to those skilled in the Art that any other form of prime mover such as an electric motor, gas or steam turbine, etc. may be fitted.
FIG. 65The engine is remote mounted. A drive shaft 861 connects the engine flywheel to the gearbox input shaft. The drive shaft is preferentially manufactured from carbon fibre to achieve the required torsional stiffness to avoid vibration. A torsionally flexible coupling (not shown) may be fitted either at the engine end or the gearbox end of the drive shaft.
The same vibration and mounting criteria apply for the separately mounted unit as shown.
Whilst the propulsion unit is shown comprising an internal combustion engine it will be clear to those skilled in the Art that any other form of prime mover such as an electric motor, gas or steam turbine, etc. may be fitted.
FIG. 66The hull 6 preferentially comprises a combined platform and propeller shroud which comprises a vent (not shown) with a rear opening and two internal openings arranged in the arcuate surfaces forming protective shrouds around the propellers 1.
FIG. 74The root blend profile 305 of each fin 3 sweeps a circle 152 which should remain within the rear profile 631 of the flow guide 63 of
Fins 3 are spherically jointed into blade carrier assemblies 2 of a hub 4 which is integrally formed with a hollow propeller shaft 9. A non-rotating actuator assembly 30 comprises six pistons 32, the position of which is determined by position sensors 31 connected by cables 311 to an electronic controller (not shown). A non-rotating guide shaft 50 has a centralising guide bearing 121 in a hub cover plate 12.
Inboard shuttle 5 and outboard shuttle 6 are spherically pivoted to segmental slider bearings 51 which are free to slide along shaft 50. Shuttles 5 and 6 each comprise non-rotating inner ring elements 502, 602 and rotating outer ring elements 501, 601, respectively.
The inner and outer ring elements each comprise angular contact bearing tracks 503 and 603 which enable the rotating outer ring elements to rotate around the inner ring elements. These bearings are arranged to take both thrust and journal loads.
A first set of three actuator pistons 32a are spherically attached to the non-rotating inner element 502 of the inboard shuttle 5 and a second set of three actuator pistons 32b are spherically attached to the non-rotating inner element 602 of the outboard shuttle 6. The actuator pistons are connected to the non-rotating inner elements 502, 602 by means of rods 33a and 33b, each comprising a spherical end 331 which are connected to spherical sockets in slippers 52 which are slideably attached to the non-rotating inner elements 502, 602.
An inwardly extending lever 14 (
As shown in
There is an axial datum position along the shaft 50 where the inner shuttle 5 and the outer shuttle 6 are positioned such that the pitch of each fin is ninety degrees and the rake of each fin is zero degrees.
By displacing the first and second set of actuator pistons 32a and 32b to the same extent along the shaft 50 in the same axial direction, the fins 3 will be pivoted fore or aft to change the rake angle be cause the fin is caused to pivot in the blade carrier assembly 2, which is located at a fixed radial distance from the shaft 50.
By displacing the inboard shuttle 5 and the outboard shuttle 6 in opposite directions away from the datum position and by the same amount, the pitch of each fin will be changed collectively.
In one example, where the outboard shuttle 6 and the inboard shuttle 5 are displaced along the shaft 50 away from the axial datum position and displaced relative to each other, the dihedral of each fin will be changed collectively.
By extending the pistons attached to the outboard shuttle 6 and the pistons attached to the inboard shuttle 5 to a varying degree, shuttles 5 and 6 can be pivoted relative to the shaft 50 providing independent amounts of cyclical motion as shown in
Claims
1. A hub for a marine propeller, the hub being formed from first and second cooperating sections which, when forming the hub, are aligned along the rotational axis of the hub;
- wherein the two cooperating sections define a plurality of recesses into each of which a fin assembly can be inserted, thereby forming a propeller; and
- wherein of the first and second sections, only the first cooperating section is arranged to be connected, in use, to a drive shaft.
2. A hub according to claim 1, wherein the first and second cooperating sections define a hollow interior to the hub into which control mechanisms for controlling movement of the fins can be placed.
3. A hub according to claim 1, wherein a plurality of axially extending, circumferentially arrayed holes are provided in the first cooperating section for, in use, connection to a drive shaft.
4. A hub array according to claim 3, wherein the number of holes is twice the number of recesses.
5. A hub according to claim 1, further comprising:
- a plurality of substantially identical fin assemblies, one in each of the recesses.
6. A hub according to claim 5, further comprising one or more control mechanisms within the hub and connected to each of the fins for controlling the pitch, and/or rake and/or dihedral of each fin.
7. A modular propeller system having:
- a plurality of substantially identical fin assemblies; and
- a plurality of hubs according to any one of the preceding claims, each hub being arranged to receive a different number of the substantially identical fin assemblies;
- wherein a propeller of the desired number of fins can be created by the selection of the appropriate hub and the required number of the substantially identical fin assemblies.
8. A system according to claim 7, wherein each fin assembly comprises a turret on which the fin is mounted, the turret being arranged to be connected, in use, to an engagement means in one of the hubs.
9. A system according to claim 8, wherein, in use, each fin assembly is connected directly to the hub.
10. A system according to claim 7, wherein each hub is provided with a plurality of engagement means, each engagement means being associated with a respective fin assembly.
11. A system according to claim 10, wherein each engagement means on each hub of the plurality of hubs is identical.
Type: Application
Filed: Sep 14, 2007
Publication Date: Jun 17, 2010
Applicant: YELLOWFIN LIMITED (Southampton, Hampshire)
Inventor: Ian James Duncan (Hampshire)
Application Number: 12/441,522
International Classification: B63H 3/00 (20060101); B63H 1/20 (20060101);
