The present invention relates to a ring propeller and a thruster comprising a ring propeller which is driven by a permanent magnet motor.

This type of thruster with a ring propeller driven by means of a permanent magnet motor is employed on different types of vessels. Known permanent magnet-driven ring propellers, however, have been designed without much skew.

In developing the present ring propeller and thruster which are driven by a permanent magnet motor, an object has been to provide a permanent magnet-driven thruster with greater efficiency than known thrusters. It has been a further object to provide a permanent magnet-driven thruster with a ring propeller which offers better control of when cavitation sets in and the extent of the cavitation.

These objects are achieved with the present ring propeller as defined in claim 1 , a thruster as defined in claim 7 and an application of the ring propeller as defined in claim 8. Further embodiments of the ring propeller are defined in claims 2-6.

A ring propeller is provided for a thruster. The ring propeller comprises a ring, a centre element and at least one propeller blade extending between the centre element and the ring and attached to the centre element and the ring. The propeller blade has a leading edge profile and a trailing edge profile and is provided with forward skew. The leading edge profile of the at least one propeller blade, moreover, has an S-shape in a cross section perpendicular to the ring propeller's axis of rotation. The at least one propeller blade's trailing edge profile may also have an S-shape in a cross section perpendicular to the ring propeller's axis of rotation. The combination of the S-shape on the at least one propeller blade's leading edge profile and/or trailing edge profile and the forward skew of the at least one propeller blade provide better cavitation conditions, i.e. reduced cavitation.

The leading edge has a leading edge profile viewed in a section perpendicular to the ring propeller's axis of rotation. Correspondingly, the propeller blades' trailing edge will then naturally be the edge on the opposite side of the propeller blade with a trailing edge profile viewed in a section perpendicular to the ring propeller's axis of rotation. The fact that the ring propeller's blades have a forward skew means that the propeller tip, which is attached to the ring, is skewed forwards towards the propeller's normal direction of rotation, with the result that the outermost part of the blade meets zones with altered speed earlier. In connection with the forward skew of the propeller blades, a skew angle can be defined. The skew angle is the greatest possible angle, viewed in a cross section perpendicular to the ring propeller's axis of rotation, measured between the straight line drawn from the point where the propeller blade's centre chord line/skew line meets the ring propeller's encompassing ring and the axis of rotation and a line tangential to a point on the propeller blade's centre chord line and the axis of rotation. The propeller blade's centre chord line and the skew angle of a propeller blade on the present ring propeller are shown in the attached figures.

In a portion at the point of attachment to the ring, the S-shaped leading edge profile may have a concave shape. In practice this will mean that the tangent to the leading edge profile at the point of attachment to the ring propeller's encompassing ring and the tangent to the encompassing ring at the same attachment point form an angle which is greater than 0° and less than 90°.

In an embodiment of the invention the S-shaped trailing edge profile may also have a concave shape in a portion at the point of attachment to the ring. In the same way as above, this means that the tangent to the trailing edge profile at the point of attachment to the ring propeller's encompassing ring and the tangent to the encompassing ring at the same attachment point form an angle which is greater than 0° and less than 90°.

With regard to the strength of the blades, they have preferably been given a thickened shape (a fillet) in the transition to the propeller ring. By employing a concave shape on the outermost part of the blades, space is provided for a slimmer fillet and thereby better hydrodynamic conditions on the outermost part of the propeller.

In an embodiment of the present invention the ring propeller's ring is preferably provided with permanent magnets, where the permanent magnets form a part of a permanent magnet motor when the ring propeller is mounted in the thruster.

A thruster is also provided comprising a ring propeller and a permanent magnet motor. The thruster comprises a ring propeller as described above and a thruster housing which encloses the ring propeller's ring and comprises the permanent magnet motor's stator windings. A permanent magnet motor is thereby provided for driving the ring propeller. The thruster' s ring propeller is otherwise preferably designed as described above and may advantageously be used on a vessel.

A non-limiting embodiment of the present invention will now be described with reference to the figures, in which

Figure 1 is a cross section of a thruster with a ring propeller according to the present invention perpendicular to the ring propeller's axis of rotation A. Figure 2 illustrates the same figure as above, but where the angles between the ring and the leading edge profile and the trailing edge profile respectively are indicated. Figure 1 illustrates a thruster 10 according to the present invention. The thruster 10 comprises a thruster housing 13 and a ring propeller 12 which may be rotatably mounted in the thruster housing 13 about the axis of rotation A. The ring propeller 12 comprises a ring 15 and a centre element 16. Between the ring 15 and the centre element 16 a number of propeller blades 18 are preferably mounted, attached to the centre element 16 and the ring 15. The ring propeller 12 is therefore a monoblock where the propeller blades 18 have fixed pitch. The thruster 10 is arranged for attachment to a vessel (not shown in the figures). For this purpose the thruster 10 may be provided with an attachment element 17, thereby enabling the thruster 10, for example, to be screwed, bolted or welded to the vessel.

The ring propeller 12 further comprises permanent magnets (not shown in the figures) which are preferably mounted in the ring 15. In the thruster housing 13 stator windings (not shown in the figure) are similarly provided, with the result that the ring propeller is driven by a permanent magnet motor. Electric power for the permanent magnet motor may, for example, be supplied via the attachment element 17.

The propeller blades 18 have a leading edge profile 19 and a trailing edge profile 20 in a section perpendicular to the ring propeller's 12 axis of rotation A as indicated in figure 1. The leading edge profile 19 and the trailing edge profile 20 are defined in relation to the ring propeller's 12 direction of rotation R as illustrated in figure 1.

The propeller blades 18 have an imaginary centre chord line 24 extending from the centre element 16 to a point 27 where the centre line intersects the ring 15. The centre chord line 24 is the imaginary line located at the same distance from the leading edge profile 19 as from the trailing edge profile 20 on the propeller blade 18.

As indicated in the figures, the propeller is designed with forward skew, i.e. the propeller blades 18 are skewed forwards in the direction of the propeller's normal direction of rotation R, with the result that the outermost part of the blade meets zones with changed speed earlier. The degree of forward skew may be indicated by means of the skew angle V. The skew angle V is the greatest angle formed between a first line 25 through the axis of rotation A and a point 27 where the centre line 24 crosses the ring's 15 internal diameter and a second line 26 through the axis of rotation A and a point 28 on the centre line 24. Depending on the propeller blade's degree of forward skew, the point 28 on the centre line may vary. In figure 1 the point 28 on the centre line 24 which will give the greatest angle, i.e. the skew angle V, is right in at the centre element 16. In other designs the point 28 may be located somewhere on the centre line between the centre element 16 and the ring 15. By providing the ring propeller 12 with forward skew in this manner, the ring propeller 12 will acquire better cavitation properties since the tip of the propeller blades 18 takes a smaller part of the total thrust.

As illustrated in the figures the propeller blades' 18 leading edge profile 19 is designed with a slight S-shape. This means that in the transition to the ring 15, it will be possible to design the propeller blades 18 with a slim section, giving a good hydrodynamic effect while at the same time providing sufficient strength. The trailing edge profiles may also be designed with a slight S-shape as indicated in the figures.

In the transition between the propeller blades' leading edge profile 19 and the ring 15, the propeller blades 18 preferably have a concave shape. This is illustrated in greater detail in figure 2 where the tangent 30 to the propeller blade's leading edge profile 19 in the attachment point 37 and the tangent 31 to the ring 15 in the attachment point 37 to the ring are indicated. Due to the fact that the propeller blades 18 have a concave shape, the angle 35 opening on to the ring is less than 90° and greater than 0°.

In a similar manner, in the transition between the propeller blades' trailing edge 19 and the ring 15, the propeller blades 18 preferably have a concave shape. This is also illustrated in figure 2 where the tangent 32 to the propeller blade's trailing edge profile 20 in the attachment point 38 and the tangent 33 to the ring's 15 attachment point 38 to the ring are indicated. Due to the fact that the propeller blades 18 have a concave shape, the angle 36 opening on to the ring is also less than 90° and greater than 0°.