A folding propeller with an adjustable rake angle, having a hub and at least three propeller blades which are pivotally mounted in the hub about pivot axes extending radially outwards from a central area in the hub, said propeller blades being pivotal about their respective pivot axis between unfolded, radially protruding operating positions, and a passive position in which the propeller blades extend axially rearwards in extension of the hub with respective rear edges of the propeller blades facing each other, said propeller blades being adapted to pass each other in said passive position so that the propeller blades are movable to both sides from the passive position to an operating position, whereby said propeller blades are pivotal about their pivot axes for restricted angular movement to both sides of said passive position.

The operating positions are e.g. assumed when the propeller during use is rotated to propel a ship through the water, and the passive position is e.g. assumed when the shaft does not rotate, but the ship is moving forward- ly through the water. It is noted that folding propellers of the above art inherently have an adjustable rake angle the propeller blades assuming differ- ent rake angles when propelling a ship forwards and rearwards.

A folding propeller of the above art is known from WO-A-95/17331, which discloses two different embodiments, namely a first embodiment allowing the propeller blades to rotate 360° around their pivot axis and a second embodiment in which the angular movement of the propeller blades around their pivot axis is restricted. Being folding propellers, the propeller blades of both embodiments will in use, when the propeller is mounted on a ship which is forwarded through the water without the propeller being rotated, assume their passive position the propeller blades extending axially rearwards. In operation the propeller blades of the first embodiment, when the propeller is rotated to propel the ship, unfold and assume an unfolded position with a rake angle, in which the propeller blades are balanced under a combined action of centrifugal forces and hydrodynamic forces. Hereby the propeller may adapt optimally to a given operating situation, whether the ship is sailing ahead or astern. The propeller blades of the second embodiment will, though their rotation is limited, unfold in the exactly same manner as the in the first embodiment. The respective rear edges of the propeller blades facing each other when the propeller blades are in the passive position entails that the propeller blades generally extend radially outwards in a star-configuration which provides for a very low drag when the ship is moving through the water e.g. propelled by sails.

The propeller blades of the first embodiment of WO-A-95/17331 may as mentioned rotate 360° around their axis and accordingly the propeller blades will always have the same edge, front edge, leading while the rear edge will always be trailing.

However, for the second embodiment of WO-A-95/17331 the propeller blades are not free to rotate to a position in which the propeller blades extend axially forwards, and thus it is possible to operate the propeller to propel the ship ahead with the propeller blades in a reverse position in which the front edge is trailing and the rear edge is leading. The rear edge of the propeller blades is in an operating position extending approximately along a radius intersecting the axis of rotation of the propeller and accordingly at least the major part of the propeller blade is extending asymmetrically to one or the other side in a circumferential direction, i.e. relative to a radius. This asymmetry provides for a change of the effective pitch of the propeller blade when the rake angle is changed. Thus it is possible to operate the propeller of the second embodiment of WO-A-95/17331 with two different effective pitches by inverting the propeller blades to have either the front edge or the rear edge in lead. The larger the rake angle the larger the effective pitch difference. This possibility of changing the effective pitch or "change gear" so-to- say, is mentioned in WO-A-95/17331 to be a considerable advantage e.g. making it possible to use a "normal gear" when propelling a ship such as a sail boat by motor alone and to use a "higher gear" when using the motor to aid propulsion by sails, thus running the motor at a lower rotational speed resulting in lower noise and fuel consumption.

From practical use the rake angle of the propeller blades of a propeller according to the second embodiment of WO-A-95/17331 is known to be + or - 14°-15°.

However, a change of (effective) pitch entails a corresponding change of load on the motor and the drive train connecting the motor and the propeller, which for some ships, motors, and/or drive trains is found to be disadvantageous.

It is an object of the present invention to overcome this disad- vantage while maintaining the benefit of the low drag of the known propeller.

This object is obtained according to the invention in that a folding propeller of the kind mentioned in the opening paragraph is provided with at least one abutment defining in forward propulsion the rake angle of the propeller blades.

By introducing an abutment defining the rake angle of the propeller blades in forward propulsion the propeller blades are prevented from adjusting their rake angle during operation when the ship is sailing forward or ahead being propelled by the propeller and thus variations of the load on the drive, including motor, propeller, and the drive train connecting the motor and the propeller, due to variations of the rake angle are avoided. Still the propeller blades may fold to the passive position in star-configuration providing a minimal drag when the motor is stopped and sails are driving or propelling the ship.

In an embodiment the abutment is provided by an exchangeable el- ement. Hereby is obtained the possibility of adjusting the defined rake angle according to needs or, if the abutment is provided by an element of a wearable material, said element may be renewed. The defined rake angle may e.g. be adjusted to adjust the effective pitch if conditions are changed such as the displacement of the ship e.g. in case of loading the ship with luggage for a long trip.

In an embodiment the exchangeable element is a combined element comprising parts of different hardness. Thus the combined element may comprise a hard part providing rigidity for the abutment to ensure the definition of the rake angle, and a softer part providing for shock absorption when the propeller blades unfold and hit the abutment.

In another embodiment the exchangeable element is a unitary element.

In an embodiment the exchangeable element comprises material having a hardness of at least 60 Shore A, preferable at least 80 Shore A, and more preferably at least 90 Shore A. Especially if the exchangeable element is a unitary element the hardness should be high enough to ensure the definition of the rake angle.

In a practical embodiment an abutment is provided for each propeller blade.

The propeller blades may be inverted as described above referring to

WO-A-95/17331 whereby either the front edge or the rear edge of the propeller blades are leading in the rotational movement of the propeller.

The defined rake angle is may be zero i.e. so-called no-rake. At no- rake the effective pitch and the load on the drive, including motor, propeller, and the drive train connecting the motor and the propeller, will substantially not change if the propeller blades are inverted. The defined rake may however be in the range - 3° to 3°, preferably - 2° to 2°. By using a small rake larger than zero the possibility of a small change of effective pitch by inverting the propeller blades is obtained. Still the small rake angle is defined i.e. constant during forward propulsion of the ship by the propeller rotating.

In an embodiment each propeller blade comprises a propeller root through which the pivot axis extends, the root has an abutment surface on either side of the pivot axis for abutment against the abutment, the abutment surfaces each having a distance from centre line of the propeller blade inter- secting the pivot axis and a tip of the propeller blade. In a further embodiment the distances of the abutment surfaces from the centre line are mutually different. Hereby is obtained that different numerical values for the rake angle may be obtained in the normal position and the inverted position of the propeller blades, whereby the propeller may e.g. have a no-rake configura- tion in normal forward propulsion and a small rake angle (positive or negative) in forward propulsion with inverted propeller blades, or vice versa.

In an embodiment the distance of at least one of the abutment surfaces is adjustable. Hereby it is possible to adjust the rakes and thereby the effective pitches individually for the normal operating position and the invert- ed operating position.

In a practical embodiment the adjustable abutment surface is provided by an adjustable element, such as a screw element.

In an embodiment the pivot axes of the propeller blades are disposed in a plane which is at right angles to the axis of rotation of the propel- ler. In a further embodiment the pivot axes of the propeller blades intersect the axis of rotation of the propeller.

In a further embodiment the propeller blades are retained in the hub for mutual synchronised movement about their pivot axes.

In still a further embodiment in the area around their pivot axes the propeller blades are provided with toothed portions which cooperate with a common rotatable, axially mounted conical gear wheel.

In the following the invention will be explained in further detail by way of an example of an embodiment having reference to the drawings, in which

Fig. 1 is a perspective view of an embodiment of a folding propeller according to the invention in a normal operating position for forward propulsion;

Fig. 2 is a perspective view of the propeller of Fig. 1 in a passive po- sition;

Fig. 3 is a rear view of the propeller of Fig. 1;

Fig. 4 is a rear view of the propeller of Fig. 1 in an inverted operating position;

Fig. 5 is a rear view of the propeller in the passive position;

Fig. 6 is a side view of the propeller of Fig. 1 in the normal operating position;

Fig. 7 shows a section along the line VII-VII in Fig. 3;

Fig. 8 is a second side view of the propeller of Fig. 1 wherein the propeller is rotated 30° relative to the position shown in Fig. 6 in the direction of forward propulsion;

Fig. 9 is a perspective view of a propeller blade; and

Fig. 10 is a top view of the propeller blade seen in direction of arrow X in Fig. 9.

The propeller 1 shown in Figs. 1-8 comprises a hub 10 for connection with a drive shaft of a drive train for transmitting rotational power from a motor as it will be well known to the person skilled in the art. The propeller further comprises three propeller blades 11 each of which has a root 12 with a through hole 13 and a toothed portion 14 constituting a section of a conical gear wheel, and a blade portion 15. The latter comprises a front edge 16 and a rear edge 17. The propeller has an axis of rotation 18 around which the propeller rotates during operation.

The hub 10 comprises a central pin 20 extending axially in the hub 10 and three circumferential wall sections 21 separated by axially extending grooves 22 extending into the hub 10 from a rear end 23 thereof. At a root or forward end thereof the central pin 20 has a circular cylindrical section receiving rotatably a central conical gear wheel 24. Rear of the circular cylindrical section the central pin 20 has a generally triangular cross section to provide for receiving ends of pivots 25 for mounting the propeller blades 11.

In the assembled condition shown in the figures the central conical gear wheel 24 is positioned on the root of the central pin 20, the roots 12 of the propeller blades 11 are inserted in between the circumferential wall sections 21 and the central pin 20, and the pivots 25 are inserted through radially extending holes 26 in the circumferential wall sections, through the holes 13 in the roots 12, which are provided axially relative to the toothed portions 14, and into holes 28 provided in respective sides of the generally triangular portion of the central pin 20. The pivots 25 are secured by locking screws 29. The pivots 25 each provides a pivot axis 25a for the respective propeller blade 11 to rotate or pivot about.

As described so far the present embodiment of the folding propeller according to the present invention is similar to the embodiment disclosed in Figs. 6-10 of WO-A-95/17331. Thus the propeller blades 11 of the folding propeller 1 of the present embodiment are rotatable around the pivots 25 between a normal operating position shown in Figs. 1, 3 and 6-8 in which the propeller blades 11 are unfolded to protrude radially, a folded passive posi- tion shown in Figs. 2 and 5, and an inverted operating position shown in Fig. 4 in which the propeller blades 11 are also unfolded to protrude radially, but have been rotated to the opposite side from the passive position compared to the normal operating position.

Engagement of the toothed portions 14 of the propeller blades 11 with the central conical gear wheel 24 provides for synchronization of the rotation of the propeller blades 11 during folding and unfolding.

In the passive position the propeller blades assume a star- configuration as seen in Fig. 5 which provides for a very little draft when a ship provided with the folding propeller is propelled by sails and the folding propeller is not rotating. According to the present invention abutments 30 are provided at forward closed ends of the grooves 22. The root 12 of the respective propeller blades 11 comprises abutment surfaces 31 and 32 on respective sides of a centre line 33 of the propeller blade 11 intersecting the pivot axis 25a and a tip of the blade. In the normal and the inverted operating position, respectively, the abutment surface 31 or the abutment surface 32 abuts on the adjacent abutment 30 when the propeller is driven to propel a ship, on which the propeller is mounted, forward. The abutment restricts the rotation of the propeller blade 11 and thereby the abutment 30 defines a rake angle of the propeller blade.

As it is known in the art the rake angle (or simply "rake") is defined as an angle of inclination of a propeller blade relative to a plane perpendicular to the axis of rotation 18 of the propeller. A rewards inclination (the blade portion is rear of the root of the propeller blade) is defined as a positive rake, a forward inclination is defined as a negative rake, and the propeller blade extending radially from the axis of rotation is defined as zero or no rake.

In the embodiment shown in the figures the abutment 30 is provided by an exchangeable element comprising two parts, namely a relatively hard part 34 and a relatively soft part 35. The hard part 34 is in the present em- bodiment made as a cylindrical pin of stainless steel and the soft part 35 is made of a synthetic rubber material with a Shore A hardness in the range of 60 to 90. The soft part 35 extends a little further in the rearwards axial direction, see Fig. 2, than the hard part 34 to act as a shock absorber when the propeller blades unfold to the operating position shown in Figs. 1, 3, 4 and 6- 8.

As mentioned above Fig. 3 shows the propeller 1 in a normal operating position and Fig. 4 shows the propeller 1 in an inverted operating position. As also described in WO-A-95/17331 and assuming the propeller 1 is mounted on a ship to propel the same, the propeller 1 is brought in the two different operating positions as follow: When the propeller 1 is forwarded through the water without being rotated the propeller blades 11 will move into the passive position shown in Figs. 2 and 5 by action of the water i.e. by hydrodynamic forces. When the propeller 1 is now rotated in a rotational forward direction 36 for propelling forward the ship a combined action of hydro- dynamic forces, inertial forces, and centrifugal forces (or centripetal forces) will move the propeller blades to the normal operating position shown e.g. in Fig. 3. In the normal operating position, the front edges 16 of the propeller blades 11 are leading and the rear edges 17 are trailing. When subsequently the rotation of the propeller 11 is stopped the ship will continue to move through the water due to inertia and hydrodynamic forces will move the propeller blades 11 back into the passive position. If now the propeller 1 is rotated in a rotational direction opposite the rotational forward direction 36 the propeller blades 11 will by a combined action of hydrodynamic forces, inertial forces, and centrifugal forces be moved to the inverted operating position and the front edges 16 will still be leading, but in an opposite rotational direction compared to the situation just mentioned. Due to the opposite direction of the rotational movement the propeller will now propel the ship astern or backwards. If now the rotation of the propeller 1 is stopped the hydrodynamic forces will tend to push the propeller blades 11 forwards opposite the direc- tion towards the passive position. Is then the propeller 1 again rotated in the rotational forward direction 36 the propeller blades 11 will maintain their inverted operating position and the front edges 16 will be trailing and the rear edges 17 will be leading while the propeller 1 will propel the ship forward.

In a situation of no rake in both the normal operating position and the inverted operating position the same effective pitch or at lease substantially the same effective pitch will be present in the normal operating position and the inverted operating position. In case of a small rake in either of the normal operating position and the inverted operating position the effective pitch will change when the propeller blades are inverted from the normal op- erating position to the inverted operating position and vice versa.

A small defined rake, either positive or negative, may be obtained by adjusting the abutment 30 which may be done by exchanging the element comprising the hard part 34 and the soft part 35 by a thicker or thinner element, i.e. an element having a larger or smaller extent in the axial direction of the propeller 1 in the mounted position.

Another possibility of adjusting the defined rake is to adjust the distances of the abutment surfaces 31 and 32 from the centre line 33. Such adjustment may be provided in the construction of the root 12 of the propeller blade 11, by constructing said root asymmetrical, whereby the defined rake will be different in the normal operating position and the inverted operating position.

Figs. 9 and 10 shows a variant of the propeller blade 11' which differs from the propeller blade 11 by comprising a threaded through bore 40 which accommodates two screws 41 and 42 the ends of which provides ad- justable abutment surfaces 31a and 32a respectively. The screws 41 and 42 may be screwed completely into the bore 40 whereby the surfaces 31 and 32 adjacent the bore 40 provides the abutment surfaces, or one or both of the screws 41 and 42 may extend slightly out of the bore to provide slightly raises abutment surfaces 31a and/or 32a compared to the abutment surfaces 31 and 32.

In a variant only one of the two screws 41 and 42 is mounted, because it is possible by only one screw 41, 42 to adjust a difference between the rakes of the normal operating position and the inverted operating position, whereas an overall level of the rakes may be adjusted by exchanging the abutment element comprising the hard and the soft part 34, 35.

It is also possible to provide a single screw extending through most of the bore 40 to be shifted by screwing to a position in which this single screw extends from one or the other of the two abutment surfaces 31 and 32.

Instead of the element comprising the hard part 34 and the soft part

35 it is possible to use an integrated unitary element having a sufficient hardness to define the rake. Such hardness should be more than 60 Shore A, but is preferably at least 80 Shore A, e.g. 90 Shore A.