TECHNICAL FIELD OF THE INVENTION

The present invention relates to a propulsion and steering unit for a waterborne vessel. The unit is of a kind comprising a housing and a propeller fixed to a front end of the housing for generating a pulling thrust. The housing can be turned to direct the pulling thrust of the propeller into a desired direction.

BACKGROUND OF THE INVENTION

Among the propulsion and steering units of the above kind there are known thrusters and pods. In general, thrusters have a mechanical drive where a diesel engine or an electric motor inside the waterborne vessel is connected to the propeller by gearing. On the other hand, pods have an electric drive where an electric motor inside the housing is connected directly to the propeller without gears, the electricity being produced by an onboard engine.

In a pulling type thruster or pod, the propeller is fixed to the front end of the housing to provide a pulling thrust, and the propeller receives a clear inflow of water for improved propulsion efficiency. Further, because the housing can be turned together with the propeller, the propeller can apply its pulling thrust in a desired

direction to make the vessel manoeuvrable. A thruster or pod which can be turned by 360° is also known as an azimuth thruster or pod.

EP 1 847 455 Al discloses an azimuth thruster

comprising a propeller fixed to a front end of a bulb- shaped housing. The housing accommodates a propeller shaft and a gearing. The housing further comprises a shank that extends upwardly from an upper central region of the housing and accommodates a drive shaft which is connected with drive means onboard the vessel, and a fin that extends downwardly from a lower aft region of the housing. The fin is claimed to reduce a steering torque needed for turning the thruster during rotation of the propeller.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the propulsion and steering unit disclosed in EP 1 847 455 Al .

According to a first aspect of the present invention, there is provided a propulsion and steering unit for a waterborne vessel, comprising a housing that accommodates at least a propeller shaft, a propeller that is connected to the propeller shaft at a front end of the housing for generating a pulling thrust, a shank that is fixed to and extends upwardly from an upper region of the housing and that is to be mounted to steering means of the vessel which are adapted to turn the propulsion and steering unit into a desired direction, and a fin that is fixed to and extends downwardly from a lower region of the housing. The

propulsion and steering unit is characterized in that the fin has a leading edge facing away from a longitudinal axis of the propeller shaft to meet a slipstream which is produced during rotation of the propeller below the

housing .

In the conventional propulsion and steering unit of EP 1 847 455 Al, the shank and the fin have leading edges that face into the same direction as the longitudinal axis of the propeller shaft. As a result, the slipstreams of the propeller form opposite attack angles with the shank and the fin so that the force component of the slipstream acting on the fin counteracts the force component of the slipstream acting on the shank. In contrast to that, the present invention contemplates a propulsion and steering unit in which the leading edge of the fin meets the slipstream produced below the housing so that no attack angle is formed or the attack angle is reduced. As a result, the propulsion efficiency of the propulsion and steering unit according to present invention can be

increased .

In order to avoid an excessive increase in the

steering torque needed for turning the propulsion and steering unit during rotation of the propeller, the

propulsion and steering unit preferably fulfils at least one of the following four conditions:

(1) The shank also has a leading edge facing away from the longitudinal axis of the propeller shaft.

(2) The angle between the leading edge of the fin and the longitudinal axis of the propeller shaft is made less than an angle which reduces the force component of the slipstream to a minimum.

(3) The size of the fin is increased.

(4) The fin has an asymmetric configuration with an arched profile wherein a trailing edge of the fin follows the longitudinal axis of the propeller shaft.

If condition (1) is fulfilled, it is not only the force component acting on the fin, but also the force component acting on the shank which is reduced. As a result, the steering torque needed for turning the

propulsion and steering unit during rotation of the

propeller can be reduced. At the same time, the propulsion efficiency of the propulsion and steering unit can be further increased.

If condition (2) is fulfilled, a balance is achieved between the conflicting aims of increasing the propulsion efficiency of the propulsion and steering unit and reducing the steering torque needed for turning the propulsion and steering unit during rotation of the propeller.

If condition (3) is fulfilled, the lift acting on the fin can be increased, which counteracts the lift acting on the shank. As a result, the steering torque needed for turning the propulsion and steering unit during rotation of the propeller can be reduced. Furthermore, an improved rudder effect is achieved. The fin has a projected area that is preferably not less than one-half, more preferably not less than two-thirds of a projected area of the shank.

If condition (4) is fulfilled, the attack angle of the slipstream of the propeller is increased in the vicinity of the trailing edge of the fin. As a result, the lift acting on the trailing end of the fin is increased, and the steering torque needed for turning the propulsion and steering unit during rotation of the propeller can be reduced .

Further, the respective leading edge of the fin or shank may form, with respect to the longitudinal axis of the propeller shaft, either a constant angle or an angle which decreases with a distance from the housing. If the angle decreases with a distance from the housing, the angle is usually set such that the propulsion efficiency is maximized at a steering angle of zero degrees. If the angle is constant, it is likely that the propulsion efficiency is slightly lower at the steering angle of zero degrees, but it may be higher under certain conditions like manoeuvring at high or medium speeds. Further, a constant angle of the leading edge facilitates manufacture of the fin or shank. The angle of the leading edge of the fin is preferably between 3° and 15°, more preferably between 3° and 10°, and most preferably between 4° and 10° at a transition between the fin and the housing.

According to a second aspect of the present invention, there is provided a waterborne vessel having the afore ^¬ mentioned propulsion and steering unit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic side view showing a propulsion and steering unit according to a first embodiment of the present invention.

FIG. 2 is a schematic top view of the propulsion and steering unit according to the first embodiment.

FIG. 3 is a schematic bottom view of the propulsion and steering unit according to the first embodiment.

FIG. 4 is a schematic top view of a propulsion and steering unit according to a second embodiment of the present invention.

FIG. 5 is a schematic bottom view of the propulsion and steering unit according to the second embodiment.

FIG. 6 is a schematic top view of a propulsion and steering unit according to a third embodiment of the present invention.

FIG. 7 is a schematic bottom view of the propulsion and steering unit according to the third embodiment.

FIG. 8 is a schematic top view of a propulsion and steering unit according to a fourth embodiment of the present invention.

FIG. 9 is a schematic bottom view of the propulsion and steering unit according to the fourth embodiment.

FIG. 10 is a schematic top view of a propulsion and steering unit according to a fifth embodiment of the present invention.

DETAILED DISCLOSURE OF THE INVENTION

The invention is now explained in greater detail with reference to FIGS. 1 to 10 which illustrate preferred embodiments of the present invention.

First Embodiment

FIGS. 1 to 3 show a propulsion and steering unit according to a first embodiment of the present invention. The propulsion and steering unit is to be mounted to a stern of a waterborne vessel. The vessel may be equipped with one or more of the propulsion and steering units. The vessel may be further equipped with a fixed centre line propeller and a rudder. In this case, it is preferable to use two of the propulsion and steering units, one on each side of the centre line propeller and the rudder.

As best shown in the schematic side view of FIG. 1, the propulsion and steering unit according to the first embodiment comprises a housing 2 and a propeller 4 fixed to a front end of the housing 2. The propeller 4 generates a pulling thrust when rotating in anticlockwise direction when looking in the direction from the front end of the housing 2 to an aft end thereof. The propulsion and

steering unit further comprises a shank 6 that is fixed to and extends upwardly from an upper region of the housing 2. The shank 6 is to be mounted to steering means of the vessel which are adapted to turn the propulsion and

steering unit by 360°. The housing 2 accommodates a

propeller shaft and a gearing. The shank 6 accommodates a drive shaft which is to be connected with drive means onboard the vessel. The driving force of the drive means is transmitted via the drive shaft, the gearing and the propeller shaft to the propeller 4. It follows that the propulsion and steering unit of the first embodiment is embodied as an azimuth-type pulling thruster.

The propulsion and steering unit further comprises a fin 8 that is fixed to and extends downwardly from a lower region of the housing 2. As shown in the schematic side view of FIG. 1, the fin 8 has a projected area that is about three-quarters of the projected area of the shank 6 so that a rudder-like performance is achieved.

The edges 6e, 8e of the shank 6 and the fin 8 which are closest to the propeller 4 are called leading edges 6e, 8e. The edges 6t, 8t close to the aft end of the housing 2 are called trailing edges. As shown in the schematic side view of FIG. 1, the leading edge 6e of the shank 6 extends in a vertical direction, and the trailing edge 6t of the shank 6 is slanted towards the aft end of the housing 2. The trailing edge 8t of the fin 8 is aligned with the trailing edge 6t of the shank 6 and has approximately the same slanted angle as the trailing edge 6t of the shank 6. The leading edge 8e of the fin 8 is slanted with a steeper angle in the same direction as the trailing edge 8t of the fin 8. The centre of area of the fin 8 is located further towards the aft end of the housing 2 than the centre of area of the shank 6.

As shown in the schematic top and bottom views of FIGS. 2 and 3, both the shank 6 and the fin 8 have a streamlined shape whose cross-sections are axially

symmetric with respect to a longitudinal axis of the shank 6 or fin 8. These shapes minimize flow resistance. Assuming that the steering angle is zero degrees, the leading edge 6e of the shank 6 is orientated to the portside of the vessel so that it meets a slipstream 10 produced by the rotation of the propeller 4 above the housing 2. The longitudinal axis of the shank 6 forms a constant angle a of about 7° with the longitudinal axis of the propeller shaft. As opposed to that, the leading edge 8e of the fin 8 is orientated to the starboard of the vessel so that it meets a slipstream 12 produced by the rotation of the propeller 4 below the housing 2. The longitudinal axis of the fin 8 forms a constant angle β of about 6° with the longitudinal axis of the propeller shaft.

In the first embodiment, the shapes of the shank 6 and the fin 8 and the orientation of the leading edges 6e, 8e are set such that there is achieved a good balance between the propulsion efficiency of the propulsion and steering unit and the steering torque needed for turning the

propulsion and steering unit during rotation of the

propeller 4. Second Embodiment

FIGS. 4 and 5 show schematic top and bottom views of a propulsion and steering unit according to a second

embodiment of the present invention. The propulsion and steering unit according to the second embodiment is similar to the propulsion and steering unit according to the first embodiment except for the shapes of the shank 6a and the fin 8a.

Similar to the first embodiment, the shank 6a and the fin 8a have streamlined shapes whose cross-sections are axially symmetric with respect to the longitudinal axis of the shank 6a or fin 8a. The second embodiment differs from the first embodiment in that the leading edges 6e, 8e of the shank 6a and the fin 8a do not form constant angles with respect to the longitudinal axis of the propeller shaft, but angles , β which decrease with a distance from the housing 2. That is, the leading edge 6e of the shank 6a forms an angle a of about 7° with the longitudinal axis of the propeller shaft at the transition between the shank 6a and the housing 2, and the angle a gradually decreases to 0° towards the upper end of the shank 6a. As opposed to that, the leading edge 8e of the fin 8a forms an angle β of about 6° with the longitudinal axis of the propeller shaft at the transition between the fin 8a and the housing 2, and the angle β gradually decreases to 0° towards the lower end of the fin 8a.

In the second embodiment, the angles , β of the leading edges 6e, 8e are set such that the propulsion efficiency is maximized at a steering angle of zero

degrees. Therefore, the second embodiment is particularly useful for decreasing fuel consumption when the propulsion and steering unit is used for propulsion in open water.

Third Embodiment FIGS. 6 and 7 show schematic top and bottom views of a propulsion and steering unit according to a third

embodiment of the present invention. The propulsion and steering unit according to the third embodiment is similar to the propulsion and steering unit according to the first embodiment except for the shapes of the shank 6b and the fin 8b.

Similar to the first embodiment, the shank 6b and the fin 8b have leading edges 6e, 8e which form constant angles , β of 7° and 6° with respect to the longitudinal axis of the propeller shaft. The third embodiment differs from the first embodiment in that the shank 6b and the fin 8b have an asymmetric configuration with an arched profile which makes the trailing edges 6t, 8t of the shank 6b and the fin 8b follow the longitudinal axis of the propeller shaft. Each of the trailing edges 6t, 8t forms an angle with respect to the longitudinal axis of the propeller shaft which is smaller than the angle of the corresponding leading edge 6e, 8e. In particular, each of the trailing edges 6t, 8t forms an angle of 0° with respect to the longitudinal axis of the propeller shaft. Accordingly, the trailing edges 6t, 8t of the shank 6b and the fin 8b arch towards the slipstreams 10, 12 of the propeller 4.

In the third embodiment, the attack angles of the slipstreams 10, 12 of the propeller 4 are increased at the trailing edges 6t, 8t of the shank 6b and the fin 8b as compared with the first embodiment. However, the trailing edge 8t of the fin 8b is slanted further towards the aft end of the housing 2 than the trailing edge 6t of the shank 6b. Therefore, the force component of the slipstream 12 acting on the trailing edge 8t of the fin 8b produces a larger torque than the force component of the slipstream 10 acting on the trailing edge 6t of the shank 6b. As a result, the steering torque needed for turning the propulsion and steering unit during rotation of the

propeller 4 can be further reduced.

Fourth Embodiment

FIGS. 8 and 9 show schematic top and bottom views of a propulsion and steering unit according to a fourth

embodiment of the present invention. Similar to the third embodiment, the shank 6c and the fin 8c have an asymmetric configuration with an arched profile which makes the trailing edges 6t, 8t of the shank 6c and the fin 8c follow the longitudinal axis of the propeller shaft. In

particular, each of the trailing edges 6t, 8t forms an angle of 0° with respect to the longitudinal axis of the propeller shaft. The fourth embodiment differs from the third embodiment in that the leading edges 6e, 8e of the shank 6c and the fin 8c do not form a constant angle with respect to the longitudinal axis of the propeller shaft, but an angle , β which decreases with a distance from the housing 2. That is, the leading edge 6e of the shank 6c forms an angle a of about 7° with the longitudinal axis of the propeller shaft at the transition between the shank 6c and the housing 2, and the angle a gradually decreases to 0° towards the upper end of the shank 6. As opposed to that, the leading edge 8e of the fin 8c forms an angle β of about 6° with the longitudinal axis of the propeller shaft at the transition between the fin 8c and the housing 2, and the angle β gradually decreases to 0° towards the lower end of the fin 8c.

In the fourth embodiment, the angles , β of the leading edges 6e, 8e are set such that the propulsion efficiency is maximized at a steering angle of zero

degrees, and the angles of the trailing edges 6t, 8t are set such that the steering torque needed for turning the propulsion and steering unit during rotation of the

propeller 4 can be reduced. Fifth Embodiment

According to a fifth embodiment of the present

invention, it is only the fin 8, but not the shank 6d which has a leading edge 6e, 8e facing away from the longitudinal axis of the propeller shaft such that the leading edge 8e of the fin 8 meets the slipstream 12 produced by the rotation of the propeller 4 below the housing 2. In this case, the fin 8 may have a shape as shown in any one of FIGS. 3, 5, 7 and 9, and the shank 6d may have a

conventional shape as shown in FIG. 10. That is, the shank 6d has a streamlined shape whose cross-sections are axially symmetric with respect to a longitudinal axis of the shank 6d, and the longitudinal axis of the shank 6d is parallel to the longitudinal axis of the propeller shaft.

As compared with the first to fourth embodiments, the propulsion and steering unit according to the fifth

embodiment has a lower propulsion efficiency and needs a larger steering torque for turning the propulsion and steering unit during rotation of the propeller 4. However, as compared with a conventional propulsion and steering unit having the shank 6d shown in Fig. 10 but no fin, the steering torque needed for turning the propulsion and steering unit is reduced and the propulsion efficiency is increased. It follows that the performance of a

conventional propulsion and steering unit having a shank but no fin can be easily improved by retrofitting a fin 8, 8a, 8b or 8c as shown in any one of FIGS. 3, 5, 7 and 9.

Further Embodiments

The present invention is not limited to the above- mentioned embodiments. For example, it is not necessary that the propulsion and steering unit according to the above-mentioned embodiments are azimuth-type thrusters. It is acceptable if the propulsion and steering unit allows turning in a range of less than 360°. Furthermore, the propulsion and steering unit may have a pod configuration in which the housing 2 accommodates the propeller shaft and an electric motor, and the shank 6 accommodates an

electrical connection between the electric motor and a power source onboard the vessel.

In the first to fourth embodiments the angles , β of the leading edges 6e, 8e are 7° and 6°, respectively. It goes without saying that the angles , β of the leading edges 6e, 8e can be varied according to the outer shape of the propulsion and steering unit, the slipstreams 10, 12 produced by the propeller 4, and the requirements on the propulsion efficiency and the steering torque. In

particular, it is possible to further reduce the angle β of leading edge 8e of the fin 8 with respect to the angle a of the leading edge 6e of the shank 6 so as to increase the attack angle of the slipstream 12 with the fin 8 and thus the lift acting on the fin 8. In this case, the steering torque needed for turning the propulsion and steering unit during rotation of the propeller 4 is further reduced.

However, in view of the propulsion efficiency of the propulsion and steering unit, the angle β of the leading edge 8e of the fin 8 is preferably at least 3° and more preferably at least 4°.

In the first to fourth embodiments, the angles of the trailing edges 6t, 8t are either 0° or the same as the angles , β of the corresponding leading edges 6e, 8e.

Similar to the angles , β of the leading edges 6e, 8e, the angles of the trailing edges 6t, 8t can be varied according to the outer shape of the propulsion and steering unit, the slipstreams 10, 12 produced by the propeller 4, and the requirements on the propulsion efficiency and the steering torque. In particular, it is possible to make the trailing edge 8t of the fin 8 follow the longitudinal axis of the propeller shaft to a greater extent than the trailing edge 6t of the shank 6 so as to increase the attack angle of the slipstream 12 in the vicinity of the trailing edge 8t of the fin 8. In this case, the steering torque needed for turning the propulsion and steering unit during rotation of the propeller 4 is further reduced.

In the above-mentioned embodiments, the projected area of the fin 8 is about three-quarters of the projected area of the shank 6. Similar to the angles of the leading edge 8e and the trailing edges 8t of the fin 8, the size of the fin 8 can be varied according to the outer shape of the propulsion and steering unit, the slipstream 12 produced by the propeller 4 below the housing 2, and the requirements on the propulsion efficiency and the steering torque. In particular, it is possible to further increase the

projected area of the fin 8. In this case, an improved rudder effect is achieved. In order to produce sufficient torque on the fin 8 for counteracting the torque on the shank 6, the projected area of the fin 8 is preferably not less than one-half, more preferably not less than two- thirds of the projected area of the shank 6.

Further, it is not necessary that the propeller 4 rotates anticlockwise. In particular, if two of the

propulsion and steering units are mounted to the vessel, it is preferable to make one of the propellers 4 rotate anticlockwise and make the other propeller 4 rotate

clockwise. In the case of the propeller 4 rotating

clockwise, the shapes of the shank 6 and the fin 8 are mirrored lengthwise so that the leading edges 6e, 8e of the shank 6 and the fin 8 meet the flipped slipstreams 10, 12.