GB2344333A

GB2344333A - Marine propulsion unit

Info

Publication number: GB2344333A
Application number: GB9826565A
Authority: GB
Inventors: Robert Duncan Glen; Michael Burnham
Original assignee: HALMATIC Ltd; Barrus EP Ltd; EP Barrus Ltd
Current assignee: HALMATIC Ltd; Barrus EP Ltd; EP Barrus Ltd
Priority date: 1998-12-04
Filing date: 1998-12-04
Publication date: 2000-06-07
Also published as: GB9826565D0

Abstract

A marine propulsion unit has a propeller 3 mounted on a shaft 4. The propulsion unit also includes a blade 13 mounted downstream of the propeller 3. Rotating the blade about a substantially vertical axis deflects the thrust generated by the propeller 3 so as to steer the boat to which the propulsion unit is applied. The propulsion unit can also incorporate a second blade 16,16' provided downstream of the propeller 3 and mounted for rotation about a horizontal axis, so as to adjust the trim of the boat.

Description

A Marine Propulsion Unit and A Boat Having A Marine Propulsion Unit The present invention relates to a marine propulsion unit. In particular, it relates to a marine propulsion unit that is provided with means for deflecting the thrust produced by the propulsion unit. The present invention also relates to a boat provided with such a marine propulsion unit.

One form of marine propulsion unit is the conventional outboard motor. The principal components of an outboard motor are a diesel or petrol engine, a propeller mounted on a propeller shaft, and a drive train or drive shaft for transmitting the output torque of the engine to the propeller shaft. In use, an outboard motor is suspended from the back of a boat so that the propeller is below the water level and the engine is above the water level.

An outboard motor is usually attached to a boat in such a way that it can pivot about a substantially vertical axis. (Throughout the specification and claims, by"vertical axis" is meant an axis that is vertical when the boat is in its normal orientation, for example at rest in a calm sea. Similarly, by"horizontal axis"is meant an axis which is horizontal when the boat is in its normal orientation, for example at rest in a calm sea.) The boat is steered by rotating the outboard motor about the axis, so that the shaft of the propeller, and hence the thrust of the propeller, is no longer directed exactly along the longitudinal axis of the boat.

Outboard motors can be heavy-for example, with a mass of 50-200kg-so that they have large moments of inertia. It is therefore relatively difficult to rotate an outboard motor in order to steer a boat. This means that there is a significant response time before a boat will change course. This is the case even where the outboard motor is rotated by means of a power steering, or power-assisted steering, arrangement.

It is usual for a boat to be provided with means for trimming the boat, to take account of the distribution of the load in a boat. In a boat fitted with a conventional outboard motor, the boat is generally trimmed by altering the angle of the propeller shaft relative to a horizontal axis, so as to vary the angle between the propeller thrust and the horizontal. This is done by rotating the entire outboard motor about a horizontal axis.

Since it is necessary to rotate the entire outboard motor so as to trim the boat, the same problems occur in trimming a boat as occur in steering.

Another form of marine propulsion unit is the jet outboard motor. A jet outboard motor is similar in appearance to a conventional outboard motor, except that the propeller of a conventional outboard motor is replaced by a jet. The boat is driven by the thrust from the jet, rather than by thrust generated by rotation of a propeller.

A jet outboard motor is generally suspended from the back of a boat in the same manner as a conventional outboard motor. The boat is steered in a similar manner to a boat fitted with a conventional outboard motor, by rotating the jet outboard motor about a substantially vertical axis. The trim of a boat fitted with a jet outboard motor is adjusted in a similar manner to a boat fitted with a conventional outboard motor, by rotating the jet outboard motor about a horizontal axis. The disadvantages listed above with reference to an outboard motor therefore also apply to a jet outboard motor.

Small boats are commonly powered by diesel outboard engines typically having power outputs of up to around 50hp. If a greater power output is required, a conventional stem drive propulsion unit can provide a power output of around 150hp or greater, and a jet outboard motor will provide a horsepower of greater than 300hp. It can thus be seen that there is a gap in the range of power outputs provided by today's commercially available propulsion units, in the range 50 to 150hp. While this gap can sometimes be filled by using two outboard motors-for example, using two 50hp outboard motors to obtain a total power output of 1 00hp-this is not an ideal solution. For one reason, some boats physically cannot be fitted with two outboard motors. Another disadvantage is that, if a boat is provided with two outboard motors, their full combined power output may be required only for short periods because maximum power is often only required at specific times such as, for example, when a boat is accelerating from rest to its cruising speed. The provision of two outboard motors is thus an expensive way of achieving greater initial acceleration. Furthermore, since outboard motors are usually available only up to around 50hp, the maximum power output that can be obtained from two outboard motors is around 100hp. This still leaves a gap between 100hp and 150hp.

An outboard motor is inflexible in its positioning, since it must be located at the stem of a boat. If it is desired to replace an existing outboard motor, it is advisable to replace it with an outboard motor of approximately the same weight, as otherwise the balance and performance of the boat can be adversely affected. This severely limits the choice of a possible replacement outboard motor, and makes it very difficult to replace an outboard motor with a significantly more powerful outboard motor, since a more powerful outboard motor will almost certainly be heavier than the existing outboard motor. A jet outboard motor is again suspended from the stem of a boat, and thus suffers from this disadvantage of the conventional outboard motor.

A first aspect of the present invention provides a marine propulsion unit comprising : a propeller shaft adapted to be coupled to a drive source; a propeller mounted on the propeller shaft; and a first blade provided downstream of the propeller and mounted for rotation about a first axis for directing the thrust of the propeller.

If the first axis is substantially vertical, the boat is steered simply by rotating the first blade about its axis. The blade is much lighter than an outboard motor, and has a much lower moment of inertia. It is therefore much easier to control the rotation of the blade, and hence the direction of thrust. The response time of the blade is also reduced compared to the response time of an outboard motor.

The marine propulsion unit may comprise a second blade provided downstream of the propeller and mounted for rotation about a second axis for directing the thrust of the propeller, the second axis being substantially perpendicular to the first axis. If the first axis is substantially vertical, the axis of rotation of the second blade will then be substantially horizontal and the second blade can be used to trim the boat. It is again much easier to trim the boat, since it is necessary to rotate only the second blade rather than an entire outboard motor.

The marine propulsion unit may further comprise a cowl provided under the propeller, the cowl extending at least partially around the circumference of the propeller.

Providing a cowl will concentrate the thrust provided by the propeller so that the entire thrust is directed substantially towards the first blade (and the second blade if provided).

This will increase the efficiency of steering and/or trimming. The cowl will also provide physical protection for the propeller and the blades.

A second aspect of the present invention provides a boat comprising a marine propulsion unit as described above.

Further features of the present invention are defined in the dependent claims.

The present invention will now be described by way of illustrative examples with reference to the accompanying Figures in which: Figure 1 is a schematic side view of a boat equipped with a propulsion unit according to an embodiment of the invention; Figure 2 is a view of the stem of the boat of Figure 1; Figure 3 is a perspective view of the cowl of the propulsion unit of Figure 1; Figure 4 (a) is a partial perspective view of a propulsion unit according to another embodiment of the invention; Figure 4 (b) is a view of the stem of a boat equipped with the propulsion unit of Figure 4(a); Figure 5 (a) is a partial perspective view of a propulsion unit according to another embodiment of the invention; Figure 5 (b) is a view of the stem of a boat equipped with the propulsion unit of Figure 5(a); Figure 6 (a) is a partial side view of a boat equipped with a propulsion unit according to another embodiment of the invention; Figure 6 (b) is a partial side view of a boat equipped with a propulsion unit according to another embodiment of the invention; Figure 7 is a partial side view of a boat equipped with a propulsion unit according to another embodiment of the present invention; Figure 8 (a) is a schematic side view of a boat equipped with a propulsion unit according to another embodiment of the invention; Figure 8 (b) is a partial enlarged view of the propulsion unit of Figure 8 (a); Figure 9 is a partial side view of a boat having a propulsion unit according to another embodiment of the invention; Figure 10 is a partial perspective view of a propulsion unit according to another embodiment of the invention; Figure 11 is a partial perspective view of a propulsion unit according to another embodiment of the invention ; Figure 12 is a view of the stem of a boat having two propulsion units according to the present invention; Figure 13 is a schematic cross-sectional view of a boat equipped with a marine propulsion unit according to an embodiment of the invention; Figure 14 is a schematic view of the stem of the boat of Figure 13 ; Figure 15 is a view of the stem of a boat equipped with a marine propulsion unit according to a modified embodiment of the invention; Figure 16 is a schematic cross-sectional view of a boat equipped with a marine propulsion unit according to a further embodiment of the present invention; and Figure 17 is a view of the boat of Figure 16.

Figure 1 is a schematic sectional view of a boat fitted with a propulsion unit according to the present invention, and Figure 2 is a view of the stem of the boat of Figure 1.

The boat 1 has a bottom which is provided with a recess 2. As is indicated by the broken line in Figure 1, the height of this recess is greatest at the stem of the boat, and decreases away from the stem. The height of the recess becomes zero around one-third to one-half of the length of the boat from the bow of the boat.

At the stem of the boat, the height and width of the recess 2 are sufficient to accommodate the propeller 3 of the boat. The propeller 3 is mounted on a propeller shaft 4, which is mounted for rotation in bearings 5,6. The forward bearing 5 is provided where the propeller shaft passes through the bottom of the boat, and is a watertight bearing. The rear bearing 6 is mounted in a support 7, the lower end of which is secured to the cowl and the upper end of which is secured to the boat at or near its stern. The support 7 may be a conventional"P-bracket".

The propeller shaft 4 is connected to the output shaft 8 of a power unit 9. The power unit 9 is mounted inside the boat 1.

The bottom of the recess is partially closed by a cowl 10. The cowl 10 acts so as to enclose the propeller in a"tunnel". Water enters the"tunnel"upstream of the propeller, as indicated by the arrows.

The cowl 10 has two principal functions. Firstly, it provides physical protection for the propeller if, for example, the boat is grounded. Secondly, the cowl promotes the flow of water past the propeller, and thus increases the thrust provided by the propeller.

In order to remove the propulsion unit from a boat, all that is necessary is to release the cowl 10 from the boat, to separate the upper end of the support 7 from the boat, and to separate the propeller shaft 4 from the output shaft 8 of the power unit 9. (Although the joint between the propeller shaft 4 and the output shaft 8 is shown inside the boat-i. e., to the left of the bearing 5 in Figure 1-the joint could alternatively be placed within the recess so as to facilitate disconnection and removal of the propeller shaft.) A further avantage of a propulsion unit according to the present invention is that it provides increased protection for divers. The cowl 10 serves to prevent divers from inadvertently coming into contact with the propeller and thereby suffering injury. To provide greater protection for divers, propeller protection bars as disclosed in UK Patent No. 2 152 459 can be incorporated into the propulsion unit.

Figure 3 is a perspective view of the cowl 10. The cowl is provided with abutments 11, 11. When the propulsion unit is fitted to a boat, the abutments 11, 11 of the propulsion unit fit against corresponding abutments provided on the bottom of the boat, so as to position the propulsion unit relative to the boat. The propulsion unit can be secured to the boat by means of suitable fastening units (not shown) such as, for example, bolts or studs that protrude from the hull of the boat and pass through complementary holes 19 in the abutments 11, 11 of the cowl 10. The cowl is then secured in place with nuts.

The cowl 10 can also be provided with means (not shown) for securing the lower end of the support 7 to the cowl.

The cowl 10 shown in Figure 3 is provided with"vector vanes"12. These are provided to spread the thrust of the propeller. In Figure 3, four vanes 12 are shown but, in practice, any number of vanes can be provided. If only a single vane 12 is provided, it should be placed on the centre line of the cowl 10. If more than one vane is provided, they should be arranged symmetrically about the longitudinal centre line of the cowl 10.

It is possible for the vanes 12 to be omitted.

An alternative embodiment of the invention is shown in Figures 4 (a) and 4 (b). Figure 4 (a) is a partial perspective view of a boat having a propulsion unit according to this embodiment, and Figure 4 (b) is a view of the stem of the boat of Figure 4 (a). This embodiment differs from the embodiment of Figures 1 to 3 primarily in that a rudder plate 13 is provided on the boat 1. The rudder plate 13 is mounted downstream of the propeller, within the recess 2. The position of the propeller 3, and the axis 4'of the propeller shaft 4, are shown in broken lines in Figure 3 (a).

The rudder plate 13 is mounted on a rudder shaft 14 for rotation about the axis X-X shown in Figure 4 (a). The upper end of the rudder shaft 14 is mounted in a watertight bearing 15 provided in the bottom of the boat, and the lower end of the rudder shaft is mounted in a bearing 15'provided in the cowl 10. The axis X-X is substantially vertical when the boat is in its normal orientation, for example in a calm sea.

In use, the boat is steered by rotating the rudder 13 about the axis X-X. When the rudder is rotated such that the blade, as seen in plan view, is no longer parallel to the axis of the propeller shaft 4, and hence parallel to the thrust provided by the propeller, a sideways force will be developed on the rudder 13. This will cause the boat to turn.

Apart from the provision of the rudder shaft 13, the embodiment of Figures 4 (a) and 4 (b) is generally similar to the embodiment of Figures 1 to 3. Although the support 7 is upstream of the propeller 3 in Figure 4 (a) (whereas it is downstream of the propeller in Figure 1) it would be possible for the support 7 in Figure 4 (a) to be downstream of the propeller.

A modified embodiment of the invention is illustrated in Figures 5 (a) and 5 (b). This embodiment is generally similar to the embodiment of Figures 4 (a) and 4 (b), except that trimming planes 16,16'are provided in addition to the rudder plate 13. A further difference is that the support 7 of Figure 4 (a) is replaced by a support 7 that is positioned downstream of the propeller 3.

The trimming planes 16,16'are mounted on a shaft 17 which is supported in a bearing (not shown) in the support 7. The shaft 17 can be rotated about the axis Y-Y. This axis is substantially horizontal and perpendicular to the axis X-X. The trimming planes 16 16'are fixedly mounted on the shaft 17. If the shaft 17 is rotated about the axis Y-Y, the trimming planes 16,16'will rotate so that they are no longer parallel to the axis 4'of the propeller shaft 4 (not shown in Figure 5 (a)). As a result, an upwards or downwards force will be exerted on the trimming planes, depending on whether they have been rotated upwards or downwards from their position as shown in Figure 5 (a). This force will act to trim the boat in the water.

Figure 5 (a) illustrates one possible way in which the shaft 17 can be rotated. The shaft 17 is provided with lugs 18,18'. If the upper end of the lugs is moved to the left as shown in Figure 5 (a), for example using a push-pull cable or other suitable mechanical actuator, the rear edges of the trimming planes will be elevated. Conversely, if the upper ends of the lugs 18,18' are moved to the right, then the rear edges of the trimming planes will be lowered.

In place of purely mechanical actuation, it would be possible to use other means of actuation such as, for example, electrical or hydraulic actuation.

In the embodiments described above, the boat is steered using the rudder plate 13. The rudder 13 will, however, not be very efficient at low speeds. To overcome this problem, the embodiments described above can be modified by the provision of an auxiliary steering means provided below the cowl 10. If no cowl 10 is provided, the auxiliary steering means should be provided so that they are protrude out from the recess 2.

Figure 6 (a) shows an embodiment in which auxiliary steering means are provided. In this embodiment, the auxiliary steering means simply consist of an extension 13'of the rudder 13 that it extends below the cowl 10.

In an alternative embodiment, illustrated in Figure 6 (b), the rudder shaft 14 extends through the cowl 10. An auxiliary rudder 13"is attached to the portion of the rudder shaft 14 that extends below the cowl 10.

Figure 7 shows a modification of the embodiment of Figure 1. In this embodiment a nozzle 20 is provided in the bottom of the boat and opens at a point within the recess.

Air can be conducted through this nozzle into the recess 2. The nozzle 20 is provided with a non-return valve (not shown), to prevent water from passing upwards through the nozzle 20.

Once the air has been introduced into the recess 2 through the nozzle 20, it will follow the path indicated by the arrows 21 in Figure 7. Air will thus be introduced to the upper part of the area swept by the blades of the propeller 3. This air will induce cavitation at the propeller, and this will reduce the drag exerted by the water on the propeller. In consequence, the propeller will be able to rotate faster for a given torque, and thus provide a greater power output.

Introducing air into the recess of Figure 7 effectively creates a"false water line". The propeller acts as if the upper part of the area swept by the propeller was above the water line, rather than being submerged, so that the drag on the propeller is reduced.

In one method of operation, air is introduced through the nozzle 20 for a limited period when the boat is started. This will provide an increased power output on starting, so that the boat can quickly accelerate. Once a particular speed has been reached, the air supply can be shut off. It is likely that it will be sufficient to supply air for around 15 seconds after starting, but this will depend on the flow rate of air into the recess, the power of the engine, the desired cruising speed, and other performance characteristics of the boat.

The air can be supplied from any suitable source such as, for example, a compressor or a compressed air tank. A control means is provided to control the supply of air. Where the air is supplied from a compressed air tank, the control means can simply be a valve to shut off the supply of compressed air from the tank. If the compressed air is supplied by a compressor, the air supply could be controlled by providing a valve between the compressor and the nozzle 20. As an alternative, the supply of compressed air could be controlled by switching on, and shutting off, the compressor as required.

It is possible for the supply of air to be controlled automatically. This could be done, for example, by controlling the supply of compressed air on the basis of the speed of revolution of the propeller shaft 4 as measured by a suitable sensor. The control means would start supplying air when it detected that the propeller shaft had started to revolve, and would shut off the supply of air when the speed of revolution of the propeller shaft reached a pre-set value. As an alternative, the control means could control the supply of compressed air on the basis of other variables such as, for example, the speed of the boat.

In principle, the supply of air could be controlled manually rather than automatically. In a further modification, the automatic control methods described above could be provided with a manual override.

A modified embodiment of the invention is illustrated in Figures 8 (a) and 8 (b). This embodiment differs from the embodiment of Figure 7 in that the support 7 for the propeller 3 is replaced by a support 7'which is positioned upstream of the propeller.

The support 7'is secured to the bottom of the boat by any suitable means. The lower end of the support 7'can be secured to the cowl 10.

In this embodiment, the nozzle 20 is not provided in the bottom of the boat. Instead, the support 7'is provided with an internal air passage 22 as shown in Figure 8 (b). The rear bearing 6 is disposed within the support 7'. In use, the propeller shaft (not shown) is supported by the rear bearing 6, and the propeller 3 is carried on the propeller shaft 4.

The position of the propeller 3 when it is installed is indicated in broken lines in Figure 8 (b).

Air is introduced into the inlet A of the internal air passage. Air leaving the internal air passage at the exit B is directed towards the upper part of the volume swept by the blades of the propeller 3, so as to induce cavitation. Although the intemal air passage is shown as having only one exit in Figure 8 (b), it is possible to provide more than one exit.

In a further embodiment (not illustrated), an internal air passage is provided in a conventional P-bracket. The P-bracket is positioned up-stream of the propeller.

In the embodiment of Figures 8 (a) and 8 (b), the inlet A to the internal air passage 22 is in the upper face of the support. It is not, however, necessary to place the inlet in this location, and it could be provided elsewhere. In this case it would be necessary to transport air to the inlet; this could be done, for example, by providing a nozzle similar to nozzle 20 in the bottom of the boat, and connecting the nozzle 20 to the inlet A to the internal air passage of the support 7'using, for example, a pipe.

A further embodiment of the invention is illustrated in Figure 9. In this embodiment, a pipe 23 is provided towards the bottom of the recess. Compressed air is introduced into the end C of the pipe 23, and leaves at the end D. The air leaving the end D of the pipe 23 is directed towards the propeller 3, so as to promote cavitation. The cowl 10 is shown in broken lines in Figure 9, for clarity.

A further embodiment of the invention is shown in Figure 10. In this embodiment, the pipe 23 is again provided towards the base of the recess. The end D of the pipe 23 is not open in this embodiment, however, but is connected to an annular tube 24. The annular tube is provided with a plurality of outlet holes 25 distributed around the circumference of the annulus. Air introduced into the tube 23 at its end C passes into the annular tube 24, and is directed towards the propeller by the outlets in the annular tube as indicated by the arrows. In this embodiment, the air is directed primarily towards the outer portion of the area swept by the propeller blades-that is, towards the tips of the propeller blades.

Alternatively, the annular tube 24 could be provided with outlets only at its upper most portion. In this case, air would be directed only towards the radially outer portion of the upper part of the area swept by the blades of the propeller 3.

In the embodiments described above, cavitation is induced by directing compressed air towards the propeller. This air can be provided either by a compressor on board the boat, or by a compressed air reservoir. The use of a compressed air reservoir is particularly convenient in the case of boats which carry a supply of compressed air for other purposes such as, for example, operating a self-righting mechanism.

Although the above embodiments use air, the invention is not limited to air. Any gas that will induce cavitation at the propeller can be used.

As one alternative to a supply of compressed air, it is possible to use the exhaust from, for example, a diesel engine as the gas for promoting cavitation.

In a further modification of the invention (not illustrated), the means for delivering gas to the propeller (for example, the nozzle 20, the pipe 23, the annular tube 24, or the internal air passage 22 of the support 7') are arranged such that rotation of the propeller tends to draw gas towards the propeller. The gas is fed into the recess, for example in any of the ways described above, and it is then drawn towards the propeller as a result of the suction caused by the rotation of the propeller.

In a further embodiment, air could be provided from an inlet positioned above the waterline of the boat, for example in the stem or in a side of the boat. The air would be fed from the inlet to a nozzle such as the nozzle 20 of Figure 7, positioned in the base of the boat so as to feed air into the recess 2. If this arrangement is used, providing the nozzle in the side of the boat has the advantage that, once the boat starts to move, air will be forced through the inlet into the delivery system.

The invention has been described above with reference to promoting cavitation in a stem drive propulsion system. However, the invention is not limited to a stem drive propulsion system. It can be applied to other propulsion systems such as, for example, an outboard motor, or the"keel drive"propulsion system described in European patent application No. 97303818. 5.

Although the introduction of air to promote caviatation is of particular use when a boat is accelerating from rest, it is not limited to this. It can equally be used when a boat has been travelling at reduced speed with the engine idling.

It is possible for a propulsion unit of the present invention to be provided with a gas introducing means to promote cavitation of the propeller in combination with a rudder plate and/or a trimming plane. As an example, Figure 11 shows a further embodiment of the invention, which combines the rudder 13 and trimming planes 16,16'of Figure 5 (a) with the pipe 23 and annular tube 24 for introducing air of Figure 10.

Although only one embodiment having both a gas introducing means and a steering/ trimming plate is illustrated in this application, it is possible for a propulsion unit of the present invention to incorporate any of the gas introducing means described hereinabove and any of the arrangements of rudder plates and/or trimming planes described hereinabove.

It is possible for a boat to be provided with more than one propulsion unit of the present invention. As an example, Figure 12 illustrates an embodiment of the invention in which two propulsion units are provided. The hull of the boat shown in Figure 12 is provided with two recesses 2, which are arranged side by side in the hull of the boat. A propulsion unit according to the invention is fitted in each recess.

In the embodiment shown in Figure 12, each propulsion unit is of the type illustrated in Figures 5 (a) and 5 (b). However, the boat of Figure 12 could be provided with any two propulsion units of the invention. It is, in principle, possible for the boat of Figure 12 to be fitted with two different propulsion units according to the invention.

In a further embodiment of the invention (not illustrated), any of the propulsion units described hereinabove can be provided with an after-planing shoe. This can be formed integrally with the cowl 10, or it can be made as a separate item and secured to the cowl by any suitable means (for example, such as welding or using bolts).

It is possible for the cowl 10 to be omitted from the embodiments described above, with all components of the propulsion unit being secured to the hull of the boat.

Figures 13 and 14 show an embodiment in which the cowl 10 is not present. This generally corresponds to the embodiment of Figure 7, except for the omission of the cowl. The upper end of th substantially vertical when the boat is in its normal orientation, for example in a calm sea.

It will be noted that the embodiment of Figures 16 and 17 generally corresponds to the embodiment of Figures 1 and 2, except that the rudder plate 13 is provided and the cowl 10 is omitted. Since the cowl 10 is omitted, the lower ends of the support 7 and the rudder shaft 15 are unsupported.

Claims

CLAIMS : 1. A marine propulsion unit comprising : a propeller shaft adapted to be coupled to a drive source; a propeller mounted on the propeller shaft; and a first blade provided downstream of the propeller and mounted for rotation about a first axis for directing the thrust of the propeller.
2. A marine propulsion unit as claimed in claim 1 and further comprising a second blade provided downstream of the propeller and mounted for rotation about a second axis for directing the thrust of the propeller, the second axis being substantially perpendicular to the first axis.
3. A marine propulsion unit as claimed in claim 1 or 2 wherein the first axis is substantially vertical so that rotation of the first blade about the first axis deflects the thrust of the propeller so as to steer the boat.
4. A marine propulsion unit as claimed in claim 3 and further comprising a third blade disposed below the first blade and mounted for rotation about the first axis and a fourth blade disposed side-by side with the second blade and mounted for rotation about the second axis.
5. A marine propulsion unit as claimed in claim 4 wherein the first and third blades are disposed symmetrically about the second axis, and wherein the second and fourth blades are disposed symmetrically about the first axis.
6. A marine propulsion unit as claimed in any preceding claim and further comprising a cowl provided under the propeller and extending at least partially around the circumference of the propeller.
7. A marine propulsion unit as claimed in claim 5 wherein the cowl extends completely around the circumference of the propeller.
8. A marine propulsion unit as claimed in claim 6 or 7 wherein at least one vane is provided on the upper surface of the cowl, the vane being positioned downstream of the propeller.
9. A marine propulsion unit as claimed in claim 6,7, or 8 wherein the cowl substantially encloses the or each blade.
10. A marine propulsion unit as claimed in claim 6,7 or 8 and further comprising auxiliary steering means provided below the bottom of the cowl.
11. A marine propulsion unit as claimed in claim 10 wherein the first blade extends below the cowl, the portion of the first blade extending below the cowl comprising the auxiliary steering means.
12. A marine propulsion unit substantially as described herein with reference to Figures 4 (a) and 4 (b), or Figures 5 (a) and 5 (b), or Figure 6 (a), or Figure 6 (b), or Figure 7, or Figures 16 and 17 of the accompanying drawings.
13. A boat provided with a marine propulsion unit as specified in any of claims 1 to 12.
14. A boat as claimed in claim 13 and further comprising a drive source coupled to the propeller shaft.
15. A boat as claimed in claim 13 or 14 wherein the marine propulsion unit is a marine propulsion unit as specified in claim 6, the cowl and the bottom of the boat together completely enclosing the circumference of the propeller.
16. A boat as claimed in any preceding claim wherein a recess is provided in the hull of the boat, the propulsion unit being mounted on the boat such that the propeller and the or each blade are disposed within the recess.