Field of the Invention The present invention relates generally, but not exclusively, to Flettner rotors and, in particular, to drive mechanisms for Flettner rotors.

Background to the Invention Flettner rotors, also known as Magnus rotors, can be used on waterborne vessels for propulsion. Such rotors make use of the Magnus effect for vessel propulsion.

Flettner rotors are generally placed upright, in use, on the deck of waterborne vessels. Typically, Flettner rotors comprise an outer rotor in the form of a cylindrical tube disposed about a stator, the rotor being coupled to the stator via a rotatable coupling, normally comprising an upper bearing towards the top end of the stator and a lower bearing lower down the stator, often at about a midpoint thereof. The stator is typically connected to a base, which is in turn connected to the vessel deck. The rotor may be longer than and project above the stator, such that the upper bearing may be at about the mid-point of the rotor height.

In the case of a Flettner rotor fitted to a waterborne vessel, the rotor is caused to rotate about its vertical axis and, as the surrounding airflow moves over the spinning rotor, the relative motion between the spinning body of the rotor and the air gives rise to pressure differences in the air. The side of the rotor which is rotating into the airflow retards the airflow locally as a result of the drag caused by the surface of the rotor, whereas the side of the rotor which is rotating away from the airflow speeds up the airflow locally. A high pressure region then develops on the side of the rotor which is rotating into the airflow and a low pressure region develops on the side of the rotor which is rotating away from the airflow. As such, a force in the direction of the low pressure region of the rotor is generated and the force is transferred to the vessel, this force may assist in the propulsion of the vessel. Multiple Flettner rotors can be used in conjunction on a single vessel.

A modern day application of the Flettner rotor is on large vessels such as cargo ships. Flettner rotors are used during transit in conjunction with a vessel's primary propulsion system so as to reduce the burden on the primary propulsion system. This can lead to significant fuel savings, in particular, for long distance journeys under suitable wind conditions. Flettner rotors can present a more efficient means of ship propulsion as compared to the main drive system and therefore the environmental impact of vessels fitted with Flettner rotors can be significantly reduced as compared to vessels which are not so equipped.

Flettner rotors are most commonly driven by means of a drive mechanism housed within the stator. The drive mechanism typically transmits a rotation force to the rotor via the rotatable coupling between the stator and the rotor, and usually via the upper bearing thereof.

By virtue of the positioning along the stator of the conventional rotational upper and lower bearings, such systems can be prone to cross coupling effects between the bearings, whereby any radial or axial movement experienced at one bearing will be experienced to a greater or lesser extent by the other. For example, if the stator were subjected to a deflection at its upper end as a result of a radial load on the upper bearing, then the load would also be 'felt' at the lower bearing, having been transmitted through the deflecting stator. This can lead to an increase in complexity of the rotor dynamic behaviour and deleterious resonance.

Furthermore, if the rotor is connected rigidly to the stator, both axially and radially, via the conventional rotational upper and lower bearings, such systems can be prone to secondary coupling effects, whereby axial forces acting on the upper bearing, for example resulting from residual stresses due to temperature changes, or due to vertical components of stator/rotor deflection, are transmitted to the lower bearing and vice versa. In conventional Flettner rotors, maintenance and replacement of components parts can be difficult from a number of perspectives. Where the drive mechanism is housed within the stator, physical access to the drive mechanism can be difficult, especially for smaller sized rotors. Moreover, the lower bearing usually has to be sleeved over the stator into position, with the rotor then being mounted over that. Accordingly, in order to access the lower bearing for maintenance or replacement, it is necessary to take the rotor off, lift the lower bearing off the stator and place the old bearing, or a replacement bearing, back over the stator and place the rotor back over the stator. This can be a very costly and time consuming process. A further consideration is the availability of bearings of suitable size and speed rating for the application.

Accordingly, there is a need to provide an improved drive mechanism and bearing arrangement for a Flettner rotor; in particular one in which radial and axial movements and loads at the upper and lower bearings are de-coupled from one another, and one in which ease of maintenance is improved.

Summary of the Invention According to a first aspect of the invention, there is provided a Flettner rotor, the Flettner rotor comprising: a stator; a rotor body disposed about and rotatably coupled to the stator; and a drive mechanism comprising at least one drive wheel or roller configured to engage the outer surface of the rotor body for rotation of the rotor body.

An advantage of such a Flettner rotor is easy access to the drive mechanism for repair and maintenance as the drive mechanism is located outside of the stator and rotor. The at least one drive wheel or roller may be coupled to an associated motor for rotation thereof.

The at least one drive wheel or roller may comprise first and second drive wheels or rollers. The first and second drive wheels or rollers may engage the rotor body at diametrically opposite sides of the rotor body. The drive wheel or roller may comprise a wheel rim and a tyre mounted thereon.

The drive wheel or roller may comprise a rubber material.

The Flettner rotor may further comprise at least one guide wheel or roller configured to engage the outer surface of the rotor body.

The Flettner rotor may comprise a plurality of guide wheels and/or rollers configured to engage the outer surface of the rotor body. The plurality of guide wheels and/or rollers may be disposed circumferentially around the rotor body.

Each of the first and second drive wheels or rollers may be disposed between two of the guide wheels and/or rollers.

The at least one guide wheel or roller may comprise a wheel rim and a tyre.

The at least one guide wheel or roller may engage the outer surface of the rotor body in a play-free manner.

Each at least one guide wheel or roller may comprise an associated biasing arm, for biasing said at least one guide wheel or roller against the outer surface of the rotor body. The drive mechanism may be configured to engage the outer surface of the rotor body at a base of the rotor body. The drive mechanism may comprise at least one biasing arm and the drive mechanism may be biased against the outer surface of the rotor body by the biasing arm. In use, the drive mechanism may cause the rotor body to rotate relative to the stator via a frictional force generated between the drive mechanism and the outer surface of the rotor body.

According to a second aspect of the invention, there is provided a drive mechanism for a Flettner rotor comprising at least one drive wheel or roller configured to engage the outer surface of the rotor body for rotation of the rotor body.

The at least one drive wheel or roller may be coupled to an associated motor for rotation thereof.

The at least one drive wheel or roller may comprise first and second drive wheels or rollers. The first and second drive wheels or rollers may be configured to engage a rotor body of a Flettner rotor at diametrically opposite sides of the rotor body.

The drive wheel or roller may comprise a wheel rim and a tyre mounted thereon. The drive wheel or roller may comprise a rubber material.

The drive mechanism may be configured to engage the outer surface of the rotor body at a base of the rotor body. The drive mechanism may comprise at least one biasing arm and the drive mechanism may be biased against the outer surface of the rotor body by the biasing arm. The drive mechanism may be configured to cause the rotor body to rotate relative to a stator of the Flettner rotor via a frictional force generated between the drive mechanism and the outer surface of the rotor body. A Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 depicts an exemplary Flettner rotor;

Figure 2 depicts the base of the Flettner rotor of Figure 1 with an upper covering removed to reveal a drive and guide wheel mechanism according to one embodiment of the invention, housed within a pedestal;

Figure 3a shows an exemplary drive wheel assembly;

Figure 3b is an exploded view of the components of Figure 3a; Figure 4a is a perspective view of an exemplary guide wheel assembly; and

Figure 4b is a perspective view from beneath of the exemplary guide wheel assembly of Figure 4a. Detailed Description

Figure 1 depicts an exemplary Flettner rotor 100 in accordance with the present invention, comprising a rotor 102 (shown transparent so that the elements located within are visible), a stator 104, a pedestal 106, an upper bearing 108 and a lower bearing 110. As in known in the art, the rotor may be capped by a top disc 1 12 for improved aerodynamic properties. The top disc 112 may typically have a diameter of twice that of the rotor 102. The rotor 102 may further include additional discs, or other features for aerodynamic enhancement. The rotor 102 is disposed about the stator 104 and is rotatably coupled to the stator 104 via an upper bearing 108 and a lower bearing 1 10. A base end of the stator 104 sits on and is fixed to the pedestal 106 and a base end of the rotor 102 sits within the pedestal 106. The lower bearing 1 10 constricts radial movement of the rotor at its base end. The lower bearing 110 comprises a drive mechanism 200 (as best seen in Figure 2), which causes the rotor 102 to rotate about the stator 104 and which constrains radial movement of the rotor 102 relative to the stator 104, as explained in greater detail below. Figure 2 shows the pedestal 106 of Figure 1 with the upper covering removed to reveal the drive mechanism 200 that is housed within the pedestal 106. Also shown are a cross-section of the lower portions of the rotor 102 and the stator 104 extending into the pedestal 106. The stator 104 is fixed relative to the pedestal 106, whereas the rotor 102 is free to rotate relative to the pedestal 106.

The drive mechanism 200 shown in Figure 2 comprises two diametrically opposed drive wheels 202 that are resiliently biased against the outer surface of the rotor 102. In other embodiments, the drive mechanism 200 may comprise only a single drive wheel 202 resiliently biased against the outer surface of the rotor 102, or the drive mechanism 200 may comprise three or more drive wheels 202 resiliently biased against the outer surface of the rotor 102. As shown in Figure 2, the lower bearing 110 further comprises eight guide wheels 204 disposed around the peripheral circumference of the rotor 102 between the two drive wheels 202. The guide wheels 204 together with the drive wheels 202 can hence be considered to comprise the lower bearing 110 as they constrict radial movement of the rotor 102 at its base end, to ensure that the rotor rotates concentrically with regard to the stator whilst not restraining any axial movement or deformation of the rotor.

In other embodiments, there may be a greater or lesser number of guide wheels 204. For example, in an embodiment where there is only a single drive wheel 202, there may be nine guide wheels 204 disposed around the peripheral circumference of the rotor 102 either side of the drive wheel 202. In an embodiment where there are three or more drive wheels 202, there may be fewer than eight guide wheels 204 disposed around the peripheral circumference of the rotor 102 between the drive wheels 202. There may also be more than eight guide wheels 204 disposed around the peripheral circumference of the rotor 102 between the drive wheels 202 to provide increased rotational stability. The drive wheels 202 comprise a wheel rim 206, a tyre 208 disposed around the wheel rim 206, and a stub axle 306 with a wheel mounting portion 308. The guide wheels 204 each comprise a wheel rim 206, a tyre 208 disposed around the wheel rim 206 and a bearing hub 412. The tyres 208 may be of the conventional pneumatic type used by road vehicles meaning that replacement parts would be readily available. The tyres 208 may comprise a conventional rubber compound material. The wheel rims 206 and the associated bearing hubs 412 may also be of conventional, readily-available type.

The drive wheels 202 are each connected to an associated motor 301 (as best seen in Figures 3a and 3b) by a direct-drive coupling. Friction generated between the drive wheels 202 and the outer surface of the rotor 102 when the drive wheels 202 are rotated causes the rotor 102 to rotate about the stator 104 and about its axis. The guide wheels 204 are in a play-free relationship with the rotor 102 and rotate passively as the rotor rotates, by virtue of the friction generated between the outer surface of the rotor 102 and the guide wheels 204.

It will be understood that in place of a wheel rim 206 and a tyre 208, the guide wheels 204 and drive wheels 202 may be replaced with any suitable form of wheel or roller.

In other embodiments, ranks of drive and/or guide wheels may be provided, whereby each drive or guide wheel comprises two or more wheels mounted vertically above each other on a common axis. Figure 3a shows an exemplary drive wheel assembly 300 comprising a drive wheel 202 directly mounted onto a motor 301. The motor 301 is attached to a drive mounting plate 302 which is pivotably mounted to the pedestal 106 (not shown in Figures 3a and 3b) via a hinged connection with a pedestal plate 310. To control wheel/tyre slip against the outer surface of the rotor 102 due to drive torque, the drive wheels 202 may be biased against the outer surface of the rotor 102 with a predetermined force via the pivoted mounting of the drive mounting plate 302 to the pedestal 106. A hydraulic actuator 304 (shown in Figure 2) may be connected between the pedestal 106 and the drive mounting plate 302 in such a way that the drive wheels 202 can be held with said predetermined force against the outer surface of the rotor 102. The predetermined force may be constant or may be variable and determined, for example, due to the conditions under which the rotor is operating. The hydraulic actuator 304 may also be used to disengage the drive wheels 202 from the outer surface of the rotor 102.

Figure 3b is an exploded view of the exemplary drive wheel assembly 300 of Figure 3a, which shows how the wheel rim 206 of the drive wheel 202 is mounted to the motor 301 via a stub axle 306 with a wheel mounting portion 308 and how the mounting plate 302 is mounted to the motor 301. The drive mounting plate 302 is arranged to allow vertical and radial positioning of the motor 301 , for example through the use of shims or packing of appropriate thickness at the hinged connection between the mounting plate 302 and the pedestal plate 310, such that a range of drive wheel radii and widths may be used as drive wheels 202 with the motor 301.

Where there are two drive wheel assemblies 300, the drive wheel assemblies 300 may be positioned to be diametrically opposed so as to make the lower bearing arrangement orthogonal. Where there are more than one drive and guide wheel assemblies, the assemblies may be disposed symmetrically around the circumference of the rotor in order to make the lower bearing behaviour similar in all directions. Advantageously the drive wheel assembly 300 is easy to access for maintenance as it is disposed outside of the rotor within pedestal 106.

Furthermore, because there is no drive mechanism housed within the stator 104, the upper bearing 108 is easier to access for maintenance as there is less congestion inside the stator 104.

Figure 4a and Figure 4b each show an exemplary guide wheel assembly 400 comprising a guide wheel 204, a swing arm 403, a torsion bar 404, a control arm 406, a control strut 408 and two bush mounting plates 410.

The guide wheel assembly 400 controls the radial position of the guide wheel 204 within the pedestal 106 and relative to the outer surface of the rotor 102. The guide wheel assembly 400 also allows the guide wheel 204 to be retracted, such that it is swung clear of the outer surface of the rotor 102.

The guide wheel 204 shown in Figures 4a and 4b comprises a wheel rim 206, a tyre 208 and a bearing hub 412. The bearing hub 412 is fixed to a swing arm 402 which is fixedly coupled to a torsion bar 404. The torsion bar 404 is mounted within bush mounting plates 410 at two positions on the torsion bar 404. The bush mounting plates 410 may each be fixed to the pedestal 106. The bush mounting plates 410 contain bushes allowing rotation of the torsion bar 404 therewithin; about its axis and relative to the pedestal 106. As such, the swing arm 402 and guide wheel 204 are free to rotate about the axis of the torsion bar 404 as the torsion bar 404 rotates.

The pedestal 106 may comprise several fixation points for the bush mounting plates 410 such that the bush mounting plates can be positioned in various distinct radial positions relative to the rotor 102. This enables the guide wheel assembly 400 to accommodate a range of different guide wheel 204 diameters. The alignment of the swing arm 402 can be set to be parallel to the tangent of the outer surface of the rotor 102 at the point of guide wheel contact for a range of guide wheel radii. The torsion bar 404 is removably coupled to a control arm 406 at its other end and the relative angle of the control arm 406 may be adjusted. The control arm 406 is coupled to a control strut 408, as can be seen in Figures 4a and 4b. The control strut 408 is in turn attached to the pedestal 106. As such, any movement of the guide wheel 204 is transmitted to the swing arm 402 and in turn to the control arm 406 and the control strut 408 via the torsion bar 404. The control strut 408 may be set so that it is perpendicular to the control arm 406.

The control strut 408 may be detachable from the control arm 406 so as to allow the guide wheel 204 to be fully retracted without requiring the control arm 406 to be detached. This allows for maintenance and replacement of the guide wheel 204.

The guide wheel 204 may be a vehicle wheel comprising a wheel rim 206, a tyre 208 and bearing hub 412. To ensure compatibility, the tyre 208, wheel rim 206, and bearing hub 412 could be sourced from a specific, preferably widely- available, manufacturer or vehicle.

The axis of the torsion bar 404 may be parallel to the axis of the wheel 204 and also parallel to the axis of the rotor 101. This would achieve zero toe and camber angle on the contact between the guide wheel and the outer surface of the rotor.

The control strut 408 of each guide wheel assembly 400 determines how each assembly 400 reacts when a load is applied to the assembly 400 via the movement of outer surface of the rotor 102 towards the guide wheel 204 and when a load is removed from the assembly 400 via movement of the rotor 102 away from the guide wheel assembly 400.

A fixed length of the control strut 408 may be set by using a turnbuckle as the control strut 408.

The control strut 408 may also be configured such that it can lengthen such that when a load is removed from the guide wheel assembly 400 via relative movement of the rotor 102 away from the guide wheel assembly 400, the control strut 408 lengthens and the guide wheel 204 remains in contact with the outer surface of the rotor 102 as the lengthening of the control strut 408 results in rotation of the rest of the components of the guide wheel assembly 400 about the torsion bar 404.

The control strut 408 may accordingly comprise a rod within a cylinder and a spring (not shown), which may be a soft spring, disposed within the cylinder moving the rod through the cylinder such that the control strut 408 may lengthen. Where a minimum length of the control strut 408 is set by a turn buckle, once the control strut 408 has reached its minimum length the guide wheel assembly 400 is effectively a rigid system with respect to any further load applied to the guide wheel assembly 400 as the rotor 102 is urged to move towards the guide wheel assembly 400. The only further deflection in the system would be the deformation of the guide wheel 204 (i.e. deflection of the tyre 208). The spring (if present) would extend the control strut 408, ultimately pushing the guide wheel 204 back towards the outer surface of the rotor 102 as the outer surface of rotor 102 moves away from the guide wheel assembly 400 keeping the tyre 208 of the guide wheel 204 in contact with the outer surface of the rotor 102.

In other embodiments the control strut 408 could comprise one or more hydraulic cylinders (not shown). In such embodiments, the position and displacement of the guide wheels 204 could be controlled via suitable hydraulic control. For example, the length of the hydraulic cylinder may be set by a control system (not shown). The control system could be actuated to retract all of the guide wheels 204 simultaneously or according to a predetermined pattern from the surface of the rotor, for example to allow for maintenance access. Likewise, the protocol could be reversed to re-engage the guide wheels to the rotor surface. Moreover, the control system would allow for active control of the position of the guide wheels 204 so as to be able to respond actively to the position of the rotor and the associated radial loads as it rotates. The position of the rotor 102 may be determined by sensors connected to the control system. The rotor 102 may comprise a base ring (not shown) attached to the base of the rotor 102. For example, the base ring may be an annular, fully welded steel fabrication located at the base of the rotor 102 such that it forms the lowest part of the assembled rotor 102. The base ring may comprise an outer cylindrical plate to which drive torque is applied through friction generated between the outer cylindrical plate and the drive wheels 202 of the drive mechanism 200.

As outlined above, the rotor 102 and the stator 104 of the present Flettner rotor 100 are rotatably coupled via the upper bearing 108. The rotor 102 and the stator 104 are also coupled via the lower bearing 110. However, the rotor 102 is only radially constrained and not axially restrained by the lower bearing 110. As such, the rotor 102 and stator 104 are axially decoupled from one another at the lower bearing 110. Advantageously, this means that any axial change in the dimensions of either or both of the rotor 102 and stator 104, for example, as a result of a change in temperature, will not lead to an increase in stress on any of the constituent parts of the Flettner rotor 100.