The present invention relates to apparatus and methods for generating power and in particular to apparatus for extracting energy from renewable sources, more especially from tidal currents. More broadly, the present invention relates to apparatus and methods for extracting power from moving fluids, in particular moving liquids and more especially moving water.

Apparatus for extracting power from moving water is already known from, for example, WO 99/45268 in which a body is described as having hydroplanes or like structures extending therefrom. The body is arranged in a current of water and the angle of the hydroplanes with respect to the current is adjusted at predetermined times thereby to change the direction of the thrust from the current on the hydroplanes, so that the body oscillates. In one example, the body is mounted on a monopile and oscillates along the essentially vertical axis of the monopile. Means are provided for converting the oscillating motion of the body into useful energy, such as electricity. In another known apparatus, one or more hydroplanes are mounted at one end of an arm. At its other end, the arm is pivotally connected to a base, the base being set, for example, on the seabed in an area of tidal current flow. Again, the angle of the hydroplane with respect to the incident water current is adjusted periodically so that the arm is caused to move to and fro about its pivot point and useful energy is extracted from the motion of the arm. In this apparatus, the hydroplane moves in an arc about the pivot point of the arm.

Although the above-described apparatus are effective, there is scope for improvement. For example, the monopile construction is requires a sliding bearing which is complex and potentially unreliable and the mechanism for energy extraction from the body moving on the monopile is also complex. The apparatus having the hydroplane mounted on the pivoted arm has the advantage of being relatively uncomplicated, but the extractable power and the loading are not uniform through the movement cycle of the arm. Also, it can be difficult to adjust the arm to accommodate operation of the apparatus when the direction of current flow is reversed - i.e. in both flood and ebb tides - as this requires a complex yawing or reversal mechanism for the apparatus.

The present invention seeks to alleviate these disadvantages and to provide an apparatus which is easily adjusted to extract power from current flows in which the flow direction periodically reverses (such as ebb and flow tides) and in which the power and loading throughout the movement cycle is more consistent, with a more favourable ratio between the average extractable power through the cycle and the peak extractable power.

According to a first aspect of the invention there is provided apparatus for extracting power from moving water comprising: a hydroplane; a base for mounting the apparatus in a submerged location on the bed of a body of water; a hydroplane support structure on which the hydroplane is mounted; linkage means by means of which the support structure is connected to the base, said linkage means permitting the hydroplane to move along a substantially linear movement path above said base; means operative to adjust the angular orientation of the hydroplane between: a first angular position in which the action of the current on the hydroplane is effective to generate thrust in one direction thereby operatively to move the hydroplane along said movement path in a first direction; and a second angular position in which the action of the current on the hydroplane is effective to generate thrust in another direction thereby operatively to move the hydroplane along said movement path in a second direction substantially opposite to said first direction, said means operative to adjust the angular orientation of the hydroplane being operative to move the hydroplane from said first angular position to said second angular position at a selected location on said movement path when said hydroplane is moving is said first direction and to move the hydroplane from said second angular position to said first angular position at a selected location on said movement path when said hydroplane is moving in said second direction, thereby causing the hydroplane to execute a controlled oscillation; and means for extracting power from the oscillatory movement of the hydroplane.

In some preferred embodiments, the apparatus comprises a plurality of hydroplanes.

The means operative to adjust the angular orientation the hydroplane will normally, and preferably, move the hydroplane about an axis passing longitudinally through the hydroplane, but this is not essentially so.

The hydroplane supporting structure may, at its simplest, be a connection such as a shaft between the linkage means and a hydroplane. In other embodiments, the hydroplane support structure may be a structure interposed between the linkage means and a hydroplane, on which a hydroplane is mounted.

As noted above, in some preferred embodiments of the invention the apparatus comprises a plurality of hydroplanes. The hydroplanes may be arranged one above the other (e.g. in a stack) or side-by-side, or in both of these arrangements. In this case, a single hydroplane support structure may be used to support a group of hydroplanes, or a hydroplane support structure may be provided at respective end portions of a group of hydroplanes. The hydroplane support structure may contain at least a part of the means for adjusting the angular orientation of the (or each) hydroplane.

Preferably said means operative to adjust the angular orientation of the hydroplane is operative to adjust each hydroplane and most preferably the hydroplanes are adjusted simultaneously by said means. In other words, the hydroplanes move in unison so that they adopt substantially the same angular orientation at the same time. Alternative arrangements may provide that the hydroplanes are independently adjustable with, for example, each hydroplane having its own adjustment means. In this way, each adjustment means is able to respond to the particular local circumstances of its associated hydroplane. Further, different types of adjustment means may be provided for each hydroplane. Alternatively, a common adjustment means may be used to adjust all the hydroplanes, but the specific change in angular orientation may not be the same for each hydroplane.

The apparatus of the invention may preferably comprise a plurality of linkage means. Thus, for example, a linkage means may be disposed at either end of a hydroplane or group of hydroplanes, or a plurality of linkages may be used to support a plurality of hydroplanes when arranged side-by-side

In further preferred embodiments said means operative to adjust the angular orientation of the hydroplane is operable to adjust the hydroplane from a selected one of a range of first angular positions to a selected one of a range of second angular positions. Thus the specific angular orientation of the hydroplane can be adjusted in accordance with, for example, prevailing conditions such as the speed of the incident current. Most preferably the apparatus of the invention further comprises means for rotating the hydroplane with respect to the linkage means from a first orientation in which the hydroplane is operable with a current flow in one direction to a second orientation in which the hydroplane is operable with a current flow substantially opposite to said one direction. Preferably, the means operative to adjust the angular orientation of the hydroplane is further operable to rotate the hydroplane with respect to the linkage means from a first orientation in which the hydroplane is operable with a current flow in one direction to a second orientation in which the hydroplane is operable with a current flow substantially opposite to said one direction. Thus, the, or each, hydroplane is rotated, typically by about 180°, so that power can be extracted from both ebb and flow (outgoing and incoming) tides.

In one arrangement according to the invention, the linkage means comprises a first arm pivotally attached at its first end to the base and a second arm attached to the hydroplane support structure, the second end of the first arm being pivotally attached to a lower end of the second arm. In this arrangement control of the motion of the respective arms is required to ensure that the hydroplane(s) move(s) along a linear movement path. This may be achieved, for example, by suitable additional linkages between the arms.

In an alternative preferred embodiment the linkage means comprises a pair of first arms respectively pivotally attached at their first ends to the base and a pair of second arms at least one of which is attached to the hydroplane support structure, the second ends of the first arms being pivotally attached to lower ends of the second arms. In a much preferred symmetrical arrangement of the linkage means both the upper ends of the second arms are pivotally attached to the hydroplane support structure.

In an alternative asymmetrical arrangement, an upper end of one of said second arms is pivotally attached to the hydroplane support structure and the upper end of the other of said second arms is pivotally attached to the said one second arm.

Most preferably the respective first arms are linked together to synchronise their motion and, preferably further to maintain the arms in the desired configuration with respect to each other. In a convenient variation, the respective first arms are geared together.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made, by way of example only to the following drawings, in which:

Figure 1 is a side view of an apparatus according to the invention when at a lower limit of the hydroplane's motion;

Figure 2 is a side view of an apparatus according to the invention when at an upper limit of the hydroplane's motion;

Figure 3 is a side view of an apparatus according to the invention when at an intermediate point of the hydroplane's motion;

Figure 4 is a side view of an apparatus according to the invention having asymmetrical linkage means; Figure 5 is a perspective view of an apparatus according to the invention including a plurality of hydroplanes;

Figure 6is a side view of the apparatus of Figure 5;

Figure 7 is a view of a part of an apparatus similar to that of Figures 5 and 6, showing an example of a means for adjusting the angular orientation of the hydroplanes;

Figure 8 is a schematic representation of one means for adjusting the angular orientation of the hydroplanes;

Figure 9 is a schematic representation of another means for adjusting the angular orientation of the hydroplanes;

Figure 10 is a schematic representation of a further means for adjusting the angular orientation of the hydroplanes;

Figure 11 is a schematic representation of a further means for adjusting the angular orientation of the hydroplanes;

Figure 12 is a schematic representation of a further means for adjusting the angular orientation of the hydroplanes; and

Figure 13 is a side view of an apparatus according to another embodiment of the invention in which respective first arms are connected by a linkage.

Referring now to the drawings, and in particular to Figures 1 to 3, the apparatus 10 comprises a base 12 which is located in use on the bed of a body of water such as a sea bed or a river bed. The base 12 is simply intended to provide a fixed location for the apparatus and supports the apparatus in its position of use. The base 12 may be located on suitable foundations such as piles, or may have integral foundations, depending, for example, on the seabed conditions. Alternatively, the base 12 may maintain its fixed location simply through the effect of gravity on the apparatus acting through the base. Mounted on the base 12 is a pair of first arms 14a, 14b. The first arms 14a, 14b are pivotally mounted at their first (lower) ends to move either about a common axis, or, as illustrated, about parallel axes. The first arms 14a, 14b are pivotally attached at their second (upper) ends to respective lower ends of second arms 16a, 16b. In the embodiment of Figures 1 to 3, both upper ends of second arms 16a, 16b are attached, directly or indirectly, to hydroplane 18. Together, the first and second arms 14a, 14b, 16a, 16b comprise a linkage means 20 by means of which the hydroplane 18 is retained at a given time in any of a range of positions above the base 12. The said range pf positions define a movement path lying along line B. The extent of the movement path along which the hydroplane 18 moves is defined by the upper and lower limits of movement of the linkage means 20, as illustrated by Figures 2 and 1 respectively. End stops 22, 24 may be provided to determine the limits of movement of the first arms 14a, 14b and hence to determine the limits of movement of the hydroplane 18. Preferably means (not specifically illustrated) are provided whereby the assembly comprising the linkage 20 and the hydroplane 18 can be moved (e.g. by rotation about a substantially vertical axis) so that the hydroplane 18 is in an optimum orientation with respect to the direction of current flow (bearing in mind that in tidal waters the exact direction of current flow varies over time). In addition, means for adjusting the hydroplane so that power can be extracted from both ebb and flow tides are provided and further described below.

The term hydroplane, as used herein and as known in the art, refers to a body which is so shaped that, when there is relative movement between the body and the water surrounding the body (i.e. when the hydroplane is subjected to a current), the body is urged to move in a direction approximately normal to the direction of said relative movement (i.e. approximately normal to the direction of current flow). In this respect, the operation of the hydroplane is analogous to an aerofoil of an aircraft, which generates lift on movement of the aircraft through the air. The direction of movement of the hydroplane 18 with respect to the current is determined by the angular orientation of the hydroplane 18 with respect to the direction of current flow. The direction of the current in use of the apparatus 10 is illustrated schematically in Figure 3 by arrow A. The hydroplane 18 is mounted on the second arms 16aΛ 16b so that it can be rotated about an axis which is nominally horizontal and nominally perpendicular to the direction of current flow in use. The term "nominally" is used here in the sense that some variation from the respective true horizontal and perpendicular orientations is to be expected, depending on local conditions. Expressed differently, the axis about which the hydroplane rotates is normal to the plane of the paper in Figures 1, 2 and 3. A suitable mechanism (not specifically illustrated in Figures 1, 2 and 3) is provided for effecting the rotation of the hydroplane 18 about said axis, the mechanism being controlled by control means which determines the extent and timing of the rotation. Thus the hydroplane 18 is caused to rotate to a selected one of a range of first positions in which the thrust generated by the action of the current on the hydroplane 18 urges the hydroplane 18 generally upwardly, or to a selected one of a range of second positions in which the thrust generated by the action of the current on the hydroplane 18 urges the hydroplane 18 generally downwardly.

The linkage means 20 is constructed so that the motion executed by the hydroplane 18 under the influence of the current A is along a substantially linear movement path which, in Figures 1 to 3, lies along line B. Most preferably movement path, indicated at B' in Fig 2 is nominally vertical, as illustrated in the Figures. To this end, first arms 14a, 14b are most preferably linked, such as by a mechanical linkage so that their motion is synchronised and further to maintain the arms in the desired configuration with respect to each other. Clearly, considering Figure 2, if, say, first arm 14a were allowed to adopt a near vertical position while first arm 14b was allowed to adopt a near horizontal position, the desired linear, preferably vertical movement path would not result. In the illustrated example, the respective first arms 14a, 14b are fixedly attached to meshing gears 26a, 26b (shown schematically). Other linkages which achieve synchronisation of the movement of first arms 14a, 14b are also possible. An example is shown in Figure 13 (described in more detail below). Electronic control of the movement of the arms 14a, 14b to achieve the same end is also an option.

A cycle of movement of the apparatus 10 of the invention will now be described starting, for convenience, with the apparatus in the configuration illustrated in Figure 1. In Figure 1, the hydroplane 18 is shown at the lower limit of its motion. The angular orientation of the hydroplane 18 is then set so that the current flowing over the hydroplane generates a thrust in an upward direction. The hydroplane is thus caused to move upwards along linear movement path B'. At a selected point before the hydroplane 18 reaches the upper limit of its motion (as illustrated in Figure 2) the angular orientation of the hydroplane 18 is changed so that the current flowing over the hydroplane 18 generates a thrust in a downward direction. The hydroplane 18 is thus decelerated and then accelerated in a downward direction so that it moves downwardly along movement path B'. Similarly, as the hydroplane 18 approaches the lower limit of its motion (as illustrated in Figure 1) the angular orientation of the hydroplane 18 is changed so that the current flowing over the hydroplane 18 generates a thrust in an upward direction. The downward movement of the hydroplane 18 is decelerated and the hydroplane 18 is then accelerated in an upward direction along movement path B ', so that the cycle of movement is repeated and so that the hydroplane oscillates or reciprocates along the linear movement path B'. Means (not specifically illustrated in Figures 1, 2 and 3) are provided for extracting power from the movement of the hydroplane 18. Most typically such means are connected to one or more of the arms 14, 16, preferably the first arms 14a, 14b. Typical power extraction means may be an electro- hydraulic arrangement although a form of direct drive and gearbox or a constant pressure linkage adapted to work against a fixed head may also be suitable. A characteristic of such power extraction means may typically be to convert the relatively slow motion of the apparatus 10 into relatively fast motion suitable for driving an electrical generator. Suitable power transmission means are known in the art.

It is much preferred that the angular orientation of the hydroplane 18 is actively controlled during the movement cycle by a suitable control means in order to optimise the power output of the apparatus 10. For example, the control means may receive and process data such as the instantaneous location, speed and acceleration/deceleration of the hydroplane 18 and external data such as the current speed and adjust the angular orientation of the hydroplane 18 accordingly, for example to begin the deceleration of the hydroplane 18 at an appropriate point, to minimise the time at which the hydroplane 18 is at rest at the extremities of its movement path B ', to maximise the acceleration of the hydroplane in the reverse direction and, if appropriate, to determine the length of the movement path B ' in accordance with prevailing circumstances.

In the present application, and more particularly in the illustrated embodiments, the hydroplanes are primarily described as moving along a substantially linear movement path which is nominally vertical. While a nominally vertical movement path is currently preferred, this is not essential. In, for example, Figures 1 to 3 the hydroplane 18 is shown as nominally horizontal. In variations of the invention, the hydroplane or hydroplanes could be positioned nominally vertically (i.e. turned through 90° with respect to the position shown in Figs 1 to 3) with corresponding adaptations to the linkage means. The hydroplane or hydroplanes would then move along a nominally horizontal movement path. Other hydroplane configurations between the horizontal and vertical are also possible, with corresponding movement paths between vertical and horizontal.

Figure 4 shows an alternative embodiment of the apparatus 100 of the invention, having an asymmetric linkage means 120 mounted on a base 112 and including a pair of hydroplanes 180, 181. The linkage means 120 comprises first arms 140, 141 and second arms 160, 161. The first arms 140, 141 are pivotally mounted at their first (lower) ends to move about parallel axes. The first arms 140, 141 are pivotally attached at their second (upper) ends to respective lower ends of second arms 160, 161. Upper end 162 of arm 161 is pivotally connected at 163 to arm 160.

The hydroplanes 180, 181 are mounted on a support structure shown schematically at 183 which in turn is pivotally mounted on an upper end 164 of second arm 160. The linkage means 20 is constructed so that the motion executed by the support structure 183 carrying the hydroplanes 180, 181, under the influence of the current A, is along a linear movement path BB ' which lies along line BB. Most preferably movement path BB ' is nominally vertical, as illustrated. To this end, first arms 140, 141 are most preferably linked, such as by a mechanical linkage so that their motion is synchronised. In the illustrated example, the respective first arms 140, 141 are fixedly attached to meshing gears 126a, 126b (shown schematically). Other linkages which achieve synchronisation of the movement of first arms 140, 141 are also possible, as is electronic control of the movement of the arms 140, 141 to achieve the same end. End stops 122, 124 may be provided to determine the limits of movement of the first arms 140, 141 and hence to determine the limits of movement of the support structure 183 for the hydroplanes 180. 181. Figures 5 and 6 illustrate another apparatus 200 according to the invention, similar to that of Figures 1 to 3 but including a plurality of hydroplanes (in this case three). In Figures 5 and 6 the apparatus 200 of the invention has two linkage means 220 mounted on a base 212 and includes three hydroplanes 280, 281, 282. The hydroplanes 280, 281, 282 are mounted on support structures 283. Each linkage means 220 comprises symmetrical first arms 240, 241 and symmetrical second arms 260, 261. The first arms 240, 241 are pivotally mounted at a lower portion 242, 243 to move about parallel axes. The first arms 240, 241 are pivotally attached at their second (upper) ends to respective lower ends of second arms 260, 261. Upper ends of arms 260, 261 are pivotally connected at points 263,264 respectively to hydroplane support structure 283. A hydroplane support linkage 284 is provided to ensure that the hydroplane support structure 283 is maintained in a desired orientation, most especially substantially vertically as illustrated.

It is also desirable, where the apparatus of the invention includes more than one linkage means 220, to join corresponding first and/or second arms of the linkage means with one or more torsion structures, to ensure that the movement of the linkage means 220 are synchronised. An example of such a torsion structure is tube 245 in Figure 5. Although not specifically shown, the arrangements in Figures 1 to 4 may also include more than one linkage means 20 and the ends of suitable torsion structures are indicated schematically at 19 in Figures 1 to 3 and 119 in Figure 4.

Figures 5 and 6 also show one example of a power transmission arrangement for extracting useable energy from the linear movement of the hydroplanes, this arrangement also being suitable for use with the apparatus of Figures 1 to 4. The power transmission arrangement 250 comprises extensions 252, 254 of first arms 240, 241 which are pivotally connected to first transmission linkage members 255, 256 which are in turn pivotally connected to second transmission linkage member 257. The transmission linkage members 255, 256, 257 serve to ensure that the movement of the first arms 240, 241 is synchronised which assists in ensuring that the movement path of the hydroplane support structure 283 is substantially linear. Hydraulic piston and cylinder arrangements 258 are provided as the primary power transmission means, of which pistons 259 are connected to the second transmission linkage member 257. Thus movement of the second transmission linkage member, caused ultimately by the movement of the hydroplanes, is converted into movement of the pistons 259, i.e. extension and retraction of the pistons 259. The power available from the arrangements 258 can be converted to more useful forms by various means. For example, the power can be used to generate electricity either on the apparatus 200 itself, or remotely (e.g. on shore), with suitable hydraulic piping connecting the arrangements 258 to the generating facility. This arrangement could use conventional hydraulics of water or seawater hydraulics.

In an alternative system, the hydraulic piston and cylinder arrangements can be used to drive a pump in order to provide a fixed head of liquid (typically water) in a tank or reservoir. The water in the reservoir can then be used to generate electricity by conventional hydroelectric generating means.

In another alternative system, a direct drive arrangement can be used, not necessarily requiring the piston and cylinder arrangement 258. In such a system, the power transmission arrangement is driven directly, such as through a gearbox arrangement by the motion of a first arm 14a, 14b, 140, 141, 240, 241 to produce a rotational motion. For example, a hydraulic motor could be coupled to an electrical generator or an electrical generator could be driven directly. In a further alternative system, linear electrical generator technology can be used in which the hydraulic piston and cylinder arrangement is replaced with equipment capable of producing a direct electrical output.

Figure 13 shows an apparatus 400 which is in many respects similar to that of Figure 5 and like parts are given like reference numbers. The apparatus comprises a linkage means 420 comprising first arms 440, 441 and second arms 260, 261. First arms 440, 441 are pivoted at their lower ends for movement about nominally horizontal axes (i.e. normal to the plane of the paper in Fig 13) indicated at 440a and 441a. Upper ends of first arms 440, 441 are pivotally connected to lower ends of second arms 260, 261. Upper ends of second arms 260, 261 are pivotally connected to hydroplane support structure 283. A hydroplane support linkage 284 acts to maintain the hydroplane support in a nominally vertical orientation. First arms 440, 441 are connected by a linkage 442 comprising parts 443, 444 and 445. Parts 443 and 445 are connected to arms 440, 441 respectively and move about axes 440a, 441a in fixed relation to their respective arms 440, 441. Parts 443 and 445 may be formed integrally with respective arms 440, 441. Parts 443 and 445 are pivotally connected to respective ends of a linking member 444. The relative size and position of the parts 443, 445 with respect to each other and to first arms 440, 441 is selected so that hydroplane support structure 283 moves, under the influence of hydroplanes 280-282 along a substantially linear (in the illustrated case, nominally vertical) movement path. In reality, the movement path is not truly linear, in that these is some "snaking" from side to side around the true linear path. However, such a path which is to a reasonable approximation linear (in contrast to the arcuate path of the prior art apparatus) achieves sufficiently the advantages of the present invention {inter alia that the apparatus is easily adjusted to extract power from current flows in which the flow direction periodically reverses (such as ebb and flow tides) and that the power and loading throughout the movement cycle is more consistent, with a more favourable ratio between the average extractable power through the cycle and the peak extractable power) and the term linear as used herein should be interpreted accordingly.

Figure 13 also illustrates another form of a power extraction arrangement. In this arrangement, a pump means 450 is driven from the linkage means 420. In the embodiment specifically illustrated in Fig 13, the pump means 450 is driven from first arm 441 via a link arm 446 which moves in fixed relation to first arm 441. Link arm 446 drives rod 447 of pump means 450 which operates in a conventional manner. Pump means 450 may be used to pump sea water, water or hydraulic fluid via pipe 451 to an elevated reservoir or accumulator, the fluid then being released to generate lower in a known manner such as by means of a turbine driving a generator. In a preferred arrangement, the pumped flows from a plurality of apparatus according to the invention can be combined in a single accumulator or reservoir which has the advantage of smoothing power output to match demand.

Figure 7 shows on an enlarged scale a part of an apparatus 200 similar to that of Figures 5 and 6, and illustrating one example of a means for adjusting the angular orientation of the hydroplanes 280, 281 and 282 (hydroplane 282 is not shown on Figure 7). The arrangement is, of course, suitable also for moving a single hydroplane, two hydroplanes or more than three hydroplanes, with appropriate modifications. Each hydroplane 280, 281, 282 is mounted on a shaft 290 carried in hydroplane support structure 283. Each shaft 290 carries a pinion 291 which meshes with a rack 292. The racks 292 are arranged on a common rod 293. A piston 294 of a hydraulic piston and cylinder actuating arrangement 295 drives the rod 293 to move linearly and the linear motion of the rod 293 is converted by each rack and pinion 291, 292 into rotational movement of the respective shaft 290 and hence of the respective hydroplane 280, 281, 282. The actuating arrangement 295 is controlled by control means (not shown) so that movement of the piston 294 produces the desired change in the angular orientation of the hydroplanes 280, 281, 282. Thus, the actuating arrangement 295 is used to set the angular orientation (about the axis of shafts 290) of the hydroplanes 280, 281, 282 to a selected one of a range of first positions to move the hydroplane support structure in a first direction along its linear movement path and, when desired, to set the angular orientation to a selected one of a range of second positions to move the hydroplane support structure in a second direction along its linear movement path opposite to the first direction.

The hydraulic actuating arrangement 295 can also be used to adjust the hydroplanes 280, 281, 282 in the event of a reversal of the flow direction of the fluid (water) passing over the hydroplanes, such as with the change of tidal current direction as between ebb and flow tides. In this respect, the adjustment requires only a greater extension or retraction, as appropriate, of the piston 294 such that the hydroplanes are rotated by approximately 180° about the axis of shafts 290 from the position shown in the Figures.

Other suitable actuating methods for the hydroplanes 18, 180, 280 are illustrated schematically in Figures 8 to 12.

Figure 8 shows a hydraulically actuated cable driven arrangement. In this arrangement, a cable 300 passes over a shaft 290 of a hydroplane (18, 180- 182, 280-282). The respective ends of the cable 302, 303 are connected to hydraulic actuators 305, 306 which include pistons 308, 309. The hydraulic actuators 305, 306 are controlled by control means (not shown) so that extension of one piston 308 is accompanied by retraction of the other piston 309 and vice versa. Tensioning means 311 such as bottle screw may be provided to adjust the tension in the cable 300. The cable 300 may pass over one or more pulley wheels, sheaves or the like (312), depending on individual design requirements. Thus, extension and retraction of the pistons 308, 309 causes movement of the cable 300 which in turn rotates the hydroplane shaft 290 about its axis. The cable 300 may include means 313 for engaging the shaft 290, such as a chain section which engages with sprockets on the shaft 290. In an alternative arrangement, the cable 300 need not engage the shaft directly: the cable 300 may engage a rotatable component which is rotatably connected to the shaft 290.

Figure 9 shows an arrangement similar to that of Figure 8, except that the cable 320 is driven by a rotatable means 321 such as a shaft or drive wheel which may, for example, be connected to a suitable motor.

Figure 10 shows an arrangement for adjusting the angular orientation of a plurality of hydroplanes which is substantially similar to that shown in Figure 7. In Figure 10 each hydroplane (not shown) is connected to a shaft 290 carrying a pinion 291. Each pinion 291 meshes with a rack 292. The racks 292 are carried on a common rod 293 which is driven by a hydraulic piston and cylinder arrangement 294, 295.

Figure 11 shows a variation of the arrangement of Figure 10 in which the hydraulic actuator 294, 295 is replaced by a drive arrangement comprising a cable 331 which is driven by rotational drive means 330 and which drives a pulley or the like 332 which is rotatably connected to shaft 290. Shaft 290 carries a pinion 291 which engages a rack 292 as previously described.

Figure 12 shows an arrangement for adjusting the angular orientation of a plurality of hydroplanes which, like the arrangement in Figure 11 has a drive arrangement comprising a cable 331 which is driven by rotational drive means 330 and which drives a pulley or the like 333. Pulley 333 is rotatably connected to a shaft 290a on which a hydroplane (not shown) is mounted. Each of the shafts 290, 290a is further connected by a cable 334 so that rotational drive is transferred from driven shaft 290a to the shafts 290 in order to rotate the hydroplanes about the axes of the respective shafts 290, 290a. In further actuating methods (not illustrated), the angular orientation of one or more hydroplanes may be changed by use of direct drive means, such as a motor and associated gearbox.