The present invention relates to a retractable propulsion system for a marine vessel and a method of operating a marine vessel comprising such a retractable propulsion system.

Known propulsion systems for boats or yachts comprise within the hull thereof an engine which drives a propeller shaft protruding from the submerged part of the hull into the water for a propeller to propel the vessel. Propeller shafts of sailboats are often retractable to reduce the hydrodynamic resistance of the vessel during sailing. A disadvantage of known retractable propulsion systems is the complexity of the retracting mechanism and the exposure of its components to the marine environment, whereas an irretractable propulsion system with a fixedly positioned propeller shaft is generally more robust and has a less complex configuration. Previous attempts to develop a robust retractable propulsion system for propelling a marine vessel did not result in an efficient propulsion system.

It is an object of the present invention, amongst other objects, to provide a retractable propulsion system wherein the previously mentioned drawbacks are at least partially alleviated.

Thereto, a retractable propulsion system for a marine vessel is provided, which is configured to be mounted in an opening that is arranged in a lower part of a hull of the vessel, wherein the system comprises:

- a propeller for propelling the marine vessel, the propeller having an axis of rotation, wherein the axis of rotation is, in the installed state in the vessel, fixed in a direction having a component that is substantially parallel with respect to a plane that is spanned by the surge direction and heave direction of the marine vessel, wherein the propeller is movable, relative to the hull, along a first direction, that is substantially perpendicular to the axis of rotation, between at least a first and a second position;

- an upper flow guide surface that is arranged above the propeller for guiding a flow of water passing the propeller, such that, in an installed state in the marine vessel, the upper flow guide surface is in between the propeller and an upper end of the hull; wherein the upper flow guide surface, upon a movement of the propeller in the first direction, is configured to move, relative to the hull, in the first direction.

More specifically, it is preferred if a non-zero distance of displacement of the upper flow guide surface, upon a movement of the propeller from the first to the second position, or vice versa, is less than, or equal to, the distance between the first and second position of the propeller. By fixing the propeller axis to be parallel to, for example, the plane spanned by the surge direction and the heave direction, the retractable propulsion system can have a straightforward and robust configuration, as the propeller, in this example, is only moved along the plane spanned by the surge direction and the heave direction, and substantially along the heave direction. Other planes along which the propeller is moved can be envisaged, such as a plane at an acute angle with the heave direction as seen along the surge direction, which plane is also spanned by a vector with a component parallel to the heave direction and a vector with a component parallel to the surge direction.

Furthermore, by moving the upper flow guide surface along with the propeller, the upper flow guide surface can guide the flow of water passing the propeller more effectively in at least the extended position of the propeller to improve the overall efficiency of the retractable propulsion system, in particular when the propeller can be driven for propelling the vessel in the first position and in the second position of the propeller. As such, the retractable propulsion system can be the main or only propulsion system of the vessel.

By arranging the propeller to propel the vessel in the first position and in the second position, the propulsion system can propel the vessel in varying extended positions including a retracted position, thus enabling the vessel to be manoeuvred in shallow water.

Preferably, in the first position, the propeller extends below the hull and, preferably, the upper flow guide surface extends from, and preferably substantially parallel to, the adjacent outer surface of the hull. More specifically, the upper flow guide surface is preferably flush with the adjacent outer surface of the hull. This way, the water can be smoothly guided to the propeller, thus avoiding water flow turbulence which may reduce the efficiency of the propeller. In other words, the flow along the hull and the guide surface to the propeller can remain substantially laminar for an efficient propulsion.

Preferably, in the second or partly retracted position, the propeller extends less far below the hull when compared to the first position, wherein the propeller is preferably above the lowest part of the hull, in particular between the lowest part of the hull and the waterline, when the marine vessel is floating in water.

According to a further preferred embodiment of the retractable propulsion system, the propeller is further movable along the first direction between the first and/or second position and a third position, in which the propeller is extended upwards such that, when the marine vessel is floating in water, the propeller extends above the waterline, for instance for service purposes. Furthermore, in the third position fouling can be prevented. The third position may thus be preferred when the propulsion system is not in use.

A preferred embodiment of the retractable propulsion system further comprises a frame member, wherein the propeller is rotatably connected to the frame member. Preferably, the frame member comprises the upper flow guide surface. This way, it can be effectively ensured that the upper flow guide surface moves in the first direction upon a movement of the propeller in the first direction.

Preferably, the frame member comprises a pivoting connection point for pivotally connecting to the marine vessel. The pivoting connection point allows, in the installed state in the vessel, the frame member to pivot with respect to the hull, in particular in the pitch direction of the vessel. As such, a movement of the propeller from the first to the second position, or vice versa, is effected by pivoting the frame member around the pivoting connection point.

A further preferred embodiment of the retractable propulsion system comprises an actuator configured for driving the movement of the propeller from the first to the second and/or the third position, or vice versa. The actuator is preferably connected to the frame member at a first end and is configured to be connected to the marine vessel at the second, opposite, end and is configured to pivot the frame member around the pivoting connection point.

A further preferred embodiment of the retractable propulsion system comprises a driving device, preferably a motor, such as a combustion engine and/or an electrical motor, for rotationally driving the propeller.

A further preferred embodiment of the retractable propulsion system comprises a water-tight enclosed compartment, wherein the upper flow guide surface is formed on a lower wall of the water-tight enclosed compartment. Preferably, the frame member comprises the enclosed compartment.

The driving device, upon a movement of the propeller in the first direction, is preferably configured to move in the first direction. Thereto, the driving device is preferably arranged on the frame member. Preferably at least a part of the driving device, in particular the electrical motor, is arranged in the enclosed compartment to be protected from the marine environment. An electrical motor is generally compact. In particular since an electrical motor does not necessarily require intake air or an outlet, the electrical motor can be conveniently installed in the enclosed compartment.

A further preferred embodiment of the retractable propulsion system comprises at least one rudder for steering the marine vessel, wherein the rudder is preferably configured to, upon a movement of the propeller in the first direction, move in the first direction. Thereto, the rudder is preferably connected to the frame member. Alternatively, the rudder may be connected to the hull of the vessel to remain stationary upon movement of the propeller in the first direction or, according to a further embodiment, the propulsion system may comprise a plurality of rudders connected to the frame member or the hull, wherein preferably at least one rudder is configured to move with the propeller in the first direction and at least one rudder is arranged stationary relative to the hull. For an effective steering, the at least one rudder is preferably arranged downstream of the propeller.

A further preferred embodiment of the retractable propulsion system comprises a drive shaft that is connected, at its first end, to the propeller, and is configured to connect, at its opposite second end, to the driving device for rotationally driving the propeller. Preferably, the first end of the drive shaft, upon a movement of the propeller in the first direction, is configured to move in the first direction. The drive shaft is preferably arranged in between the propeller and the driving device.

It is then further preferred if the enclosed compartment comprises a sealed drive shaft opening that is arranged in a wall of the enclosed compartment, in particular in the lower wall of the enclosed compartment, wherein the drive shaft extends through the sealed drive shaft opening from an inside to an outside of the enclosed compartment and wherein the sealed drive shaft opening is configured for creating a water-tight seal between the drive shaft and the respective wall. This way, at least part of the rotatable drive shaft and/or the driving device can be protected from the marine environment.

Preferably, the drive shaft comprises a plurality of drive shaft elements, of which at least two are interconnected using at least one flexible joint that is configured for allowing a misalignment between the respective two drive shaft elements. That is, the flexible joint allows an angled transfer of torque from the driving device to the propeller. To avoid transfer of axial forces between the drive shaft elements, the flexible joint is preferably arranged to allow an axial plunging movement. Such a flexible joint may be a universal joint or the like, preferably a constant- velocity joint, also known as a homokinetic joint, which ensures a uniform rotational velocity of the shaft elements. According to a further preferred embodiment of the retractable propulsion system, the water-tight enclosed compartment comprises a second opening that is arranged with a flexible sealing member that is configured to connect to, and cover, a corresponding opening in the hull of the vessel, such that, in the installed state in the vessel, the water-tight enclosed compartment is in fluid contact with an internal space in the hull of the vessel. The flexible sealing member is configured for allowing a movement, in particular a pitch rotation, of the water-tight enclosed compartment with respect to the hull of the vessel. In this case, in the installed state, the driving device can be fixedly arranged in the hull of the vessel, wherein the drive shaft extends through the second opening.

Due to the frame configuration, sealing configuration and/or drive configuration described above, the components of the drive means and the retracting mechanism can be protected from the marine environment.

A further preferred embodiment of the propulsion system comprises a sole piece arranged below the propeller, and preferably also below the rudder. Alternatively, or additionally, the propulsion system preferably comprises a protection basket that is arranged around the propeller, and preferably also around the rudder. The basket allows water to flow to the propeller while reducing the risk of damage to the propeller by underwater debris. The basket may be cylindrical and be arranged coaxially around the propeller.

According to a further aspect, a marine vessel comprising the retractable propulsion system according to any of the above embodiments is provided, wherein the vessel comprises a hull comprising an opening arranged in a lower part of the hull, in particular a recess that debouches from a bottom of the hull, wherein the retractable propulsion system is arranged in the opening, such that the axis of rotation of the propeller is fixed to be parallel with respect to a plane that is spanned by a first vector that has a component that is parallel to the surge direction and a second vector that has a component that is parallel to the heave direction of the marine vessel. Preferably, the length of the vessel is at least three times its width, preferably at least four times, more preferably at least six times.

According to a preferred embodiment of the marine vessel, the propeller is arranged, as seen in the surge direction, between the bow and the stern, in particular closer to the stern than to the bow.

A further preferred embodiment of the marine vessel comprises a second propulsion system according to any of the above embodiments. Preferably, the propulsion systems are arranged, preferably substantially parallel, on opposite sides of a longitudinal centre of the marine vessel, preferably symmetrically with respect to the longitudinal centre.

Particularly in the partly retracted position in which the propeller extends less far below the hull when compared to the extended position, rotation of the propeller may attract air, which reduces the efficiency of the propeller. To reduce the attraction of air by rotation of the propeller, a further preferred embodiment of the marine vessel comprises an air-tight sealing system that is arranged in between the hull and the retractable propulsion system, such that a section of the opening that is above the retractable propulsion system is not in direct fluid-communication with a section of the opening that is below the retractable propulsion system. The air-tight sealing system thus minimises the attraction of air by rotation of the propeller.

According to yet another a method of operating a marine vessel according to any of the above embodiments is provided, wherein the method comprises the step of lowering the propeller to be below the waterline for sailing, or raising the propeller to be above the waterline.

Preferably, the method comprises selecting an operational mode of the marine vessel, wherein: in a first mode, the propeller is extended below the hull in a first position; and/or in a second mode, the marine vessel is configured for manoeuvring in shallow water by moving the propeller to a second position, whereby the propeller extends less far below the hull when compared to the first position, preferably the propeller is above the lowest part of the hull, in particular between the lowest part of the hull and the waterline, when the marine vessel is floating in water; and/or in a third mode, the propeller is extended upwards to a third position such that the propeller extends above the waterline.

The invention is further elucidated on the basis of the attached drawings, wherein: figures 1 and 2 show a marine vessel comprising a retractable propulsion system; figure 3A shows a first embodiment of the retractable propulsion system in a first mode; figure 3B shows the first embodiment shown in figure 3A in a second mode; figure 3C shows the first embodiment shown in figures 3A-B in a third mode; figure 4A shows a second embodiment of the retractable propulsion system in a first mode; figure 4B shows the second embodiment shown in figure 4A in a second mode; figure 4C shows the second embodiment shown in figures 4A-B in a third mode. Figure 1 represents an isometric view of a marine vessel 100 comprising a hull 2 and a retractable propulsion system 10 arranged in the stern 1 of the vessel 100. Figure 2 represents an isometric view of the stern 1 from a different angle (as indicated in Figure 1). The propulsion system 10 comprises a propeller 11 on a propeller shaft 12.

Figure 3 A represents a cross-sectional side view (as indicated by a dashed frame in Figure 1) of a first embodiment of the retractable propulsion system 10 installed in the marine vessel 100 floating in water. The marine vessel 100, of which the stern 1 is shown, comprises a hull 2 provided with a recess 3 debouching from the hull 2. A distinct hull part 20, formed as a closed compartment, is mounted in the recess 3 and laterally connected to the hull 2 by waterproof hinges arranged to allow the closed compartment 20 to pitch relative to the hull 2, thus forming a hinged part of the stern 1.

The propulsion system 10 comprises a propeller 11 below a lower wall of the closed compartment 20 on which an upper flow guide surface 21 is formed above the propeller 11 for guiding a flow of water passing the propeller 11. The propeller 11 is connected to an end of a propeller shaft 12, which end is supported by the closed compartment 20 via a bearing boss 13 connected to the closed compartment 20 by brackets 14. Positioning the propeller 11 stationary with the closed compartment 20 ensures that a displacement of the propeller 11 equals the displacement of the upper flow guide surface 21 when pitching the closed compartment 20. The propeller shaft 12 extends rotatably through a sealed opening 22 in the lower wall into the closed compartment 20, in which an opposite end of the propeller shaft 12 is coupled to an intermediate shaft 15 via a first flexible joint 16. A flexible rubber bellow 30, interconnecting the closed compartment 20 and the interior 4 of the hull 2, provides a watertight passage 31 therebetween, through which the intermediate shaft 15 extends into the hull interior 4. A combustion engine 40 for providing torque (or any other rotational driving device or the like), installed in the hull 2, is coupled to the intermediate shaft 15 via a second flexible joint 17. The flexible joints 16, 17 enable an angled transfer of torque from the engine 40 to the propeller shaft 12 for rotationally driving the propeller 11 irrespective of the pitch of the closed compartment 20 relative to the hull 2. The propeller 11 converts the torque into thrust, which is transferred to the closed compartment 20 by a thrust bearing 19 at the first flexible joint 16 in the closed compartment 20 and transferred from the closed compartment 20 to the hull 2 by said hinges for propelling the vessel 100. By providing the flexible joints 16, 17 at either end of the intermediate shaft 15 extending through the flexible bellow 30, the respective flexible joints 16, 17 can be provided in the closed compartment 20 and the hull 2 and thereby accessed for service more conveniently from the closed compartment 20 and the hull 2, respectively. The hull 2 and the closed compartment 20 are further interconnected by an actuator 5 at a distance from the hinges. The actuator 5 is arranged to pivot the hinged hull part 20, relative to the hull 2, about the hinges upon operation, lowering or raising the propeller 11 to be below or above the waterline 6, to selectively move the propulsion system 10 into and out of an extended position, a retracted position and a service position. The actuating system is not limited to linear actuators as the cylinder shown. Alternative configurations for moving the propeller between the various positions, such as rotary actuators in the hinges, can be envisaged.

Figure 3A shows the propulsion system 10 in the extended position, in which the propeller 11 extends below the lowest part of the hull 2 or its baseline, and in which the upper flow guide surface 21 of the closed compartment 20 is flush with the hull 2 of the vessel 100 to hydrodynamically optimise the water flow along the hull 2 and the propeller 11 in normal navigating conditions such as in sufficiently deep water.

Figure 3B shows the propulsion system 10 in the retracted position, in which the closed compartment 20 is pitched upwards and the propeller 11 fully extends just above the lowest part of the hull 2 and below the waterline 6, reducing the total draught D of the vessel 100 and thereby the risk of damage to the propulsion system 10 in shallow water. The retractable propulsion system 10 thus enables the vessel 100 to be manoeuvred in shallow water. When navigating forwards in the retracted position, the efficiency of the propeller 11 may be reduced due to water flowing upwards from the propeller 11. By arranging a flow guide 7 downstream of the propeller 11 to direct water towards the backward direction, the propulsion efficiency can be increased. The flow guide 7 may be connected to, or formed integrally with, the hull 2. When navigating in reverse in the retracted position in which the propeller 11 is closer to the water line 6, the flow guide 7 has the additional advantage that less air is attracted by the rotating propeller 11. Additionally, or alternatively, to further reduce air being attracted by the rotating propeller 11, the section 3a of the recess 3 below the closed compartment 20 may be enclosed or fully closed off from the recess section 3b above the closed compartment 20 in an airtight manner by a sealing system (not shown) arranged in between the hull 2 and the closed compartment 20. Additionally, or alternatively, the recess section 3b above the closed compartment 20 may be closed off from the ambient by an airtight cover 9.

Figure 3C shows the propulsion system 10 in the service position, in which the closed compartment 20 is pitched further upwards such that the propeller 11 extends above the waterline 6 for service. To further facilitate access to the retracted propulsion system 10, the cover 9 of the recess 3 can be opened as shown. As can be seen in Figures 3A-C, the propeller shaft 12 is only moved between positions along the plane spanned by the surge direction S and the heave direction H of the vessel 100.

Figures 4A-C represent a second embodiment of the retractable propulsion system 10 as an alternative to the first embodiment in respectively the extended, the retracted and the service position, wherein like elements are indicated by like reference signs. Again, a distinct hull part 20, formed as a closed compartment, is mounted in a recess 3 of the hull 2 of the vessel 100 and connected to the hull 2 by a waterproof hinge 23 and an actuator 5. The propulsion system 10 comprises a propeller 11 below an upper flow guide surface 21 for guiding a flow of water passing the propeller 11. In this embodiment, the opposite end of the propeller shaft 12 in the closed compartment 20 is coupled to an electrical motor 50 for providing torque (or any other rotational driving device or the like), installed in the closed compartment 20, enabling a transfer of torque from the motor 50 to the propeller shaft 12 for rotationally driving the propeller 11 irrespective of the pitch of the closed compartment 20 relative to the hull 2. The motor 50 may be coupled to the shaft 12, e.g., by means of a driving belt and/or, in case of an angled coupling, by means of a crown- or bevel-geared coupling or the like. The propeller thrust is transferred to the closed compartment 20 by a thrust bearing 19 at the coupling between the propeller shaft 12 and the motor 50. Although a combustion engine may be sufficiently compact for installation in the movable closed compartment 20, an electromotor may not require air intake or an outlet and can thus be easily installed in the closed compartment 20. By arranging the electrical motor 50 in the closed compartment 20, direct propulsion is enabled and the closed compartment 20 can be connected to the hull 2 via a simple pivot joint, such as a hinge 23, as opposed to via a connection for an angled transfer of torque as in, e.g., the first embodiment.

A flow guide 7 is arranged downstream of the propeller 11 to direct water towards the backward direction in the retracted position shown in figure 4B, to increase the propulsion efficiency.

The shown vessel 100 further comprises a rudder 8 arranged downstream of the propeller 11 and upstream of the flow guide 7, and connected to the closed compartment 20 such that, upon pitching of the propeller 11, the rudder 8 pitches along.

A top region of the closed compartments 20 is provided with a hatch 24 providing access to the closed compartment 20 for service. A sole piece 25 extends from the closed compartment 20 all the way underneath the propeller 11 and the rudder 8 to protect the propulsion system 10. The sole piece 25 is additionally connected to the closed compartment 20 via the brackets 14. The present invention is not limited to the embodiment shown, but extends also to other embodiments falling within the scope of the appended claims.