TECHNICAL FIELD

[0001] The disclosure relates generally to boat propulsion. In particular aspects, the disclosure relates to a propeller system for a boat and a boat including the same.

[0002] The disclosure can be applied in boats equipped with one or more propeller systems, with or without sail.

BACKGROUND

[0003] Usually, sailboats include motorized propulsion means, which may include a straight shaft, a S-drive or an outboard propeller system. Some of these motorized propulsion means sometimes have a fixed, folding or feathering single propeller. Some feathering propellers also have the possibilities to adjust pitch not only to reduce drag, in feathering mode, but also to optimize performance by adjusting pitch in forward mode.

SUMMARY

[0004] The invention aims to solve the drawbacks of the prior art, by providing a propeller system for a boat, enabling both reducing drag of the propeller system in a dragging mode, and optimization of the performance in a forward mode.

[0005] According to an aspect of the disclosure, a propeller system for a boat, comprises: a shaft assembly, coaxial with a propeller axis of the propeller system; a primary propeller, comprising primary blades carried by the shaft assembly, driven in rotation by the shaft assembly around the propeller axis, each primary blade being rotatable relative to the shaft assembly, around a respective pitch axis perpendicular to the propeller axis and extending along said primary blade, between a first pitch orientation and a second pitch orientation; and a secondary propeller, comprising secondary blades carried by the shaft assembly, driven in rotation by the shaft assembly around the propeller axis, each secondary blade being rotatable relative to the shaft assembly, around a respective folding axis perpendicular to the propeller axis and to said secondary blade, between a deployed orientation and a folded orientation. [0006] Hereby, a technical effect includes that the propelling system is both versatile and optimized for different situations. Hereby, a technical effect includes that a duo propeller system with low drag is obtained.

[0007] For example, in a forward mode, where the propelling system is driven in rotation, e.g. by a motor of the boat, the primary blades may be put in the second pitch orientation, which may be a forward drive pitch orientation, where the pitch is high. The forward drive pitch orientation may be adjusted depending on the desired speed and/or torque. In the forward mode, the secondary blades may be put in deployed orientation, which enables the secondary propeller to impart hydrodynamic torque when rotated.

[0008] For example, the drag of the propeller system may be reduced in a dragging mode, where the boat is propelled by other means than the propelling system, such as sails or another motorized propelling system similar or different than the one discussed above. To this end, the primary blades may be put in a feathering pitch orientation, which may either be the first pitch orientation or a pitch orientation between the first and the second pitch orientation. For also reducing drag in the dragging mode, the secondary blades may be put in folded orientation, or at least at an intermediate orientation between the deployed orientation and the folded orientation.

[0009] For example, in a power generation mode, where the boat is propelled by other means such as a sail or other propelling means and where the propelling system drives an energy generator, the primary blades may be put in a regeneration pitch orientation, which may be the first pitch orientation or a pitch orientation between the first and second pitch orientation. Here again, the regeneration pitch orientation may be adjusted for obtaining the most efficient power generation and/or minimizing drag. In the power generation mode, the secondary blades may be put in folded orientation for reducing drag.

[0010] In certain examples, the propeller system comprises a motor, operatively coupled to the shaft assembly, via a front shaft end of the shaft assembly, for driving the primary propeller and the secondary propeller in rotation around the propeller axis, by driving the shaft assembly, wherein the primary propeller is positioned between the front shaft end and the secondary propeller.

[0011] Hereby, a technical effect includes that the propeller system may enable folding of the secondary blades to the fullest extent.

[0012] In certain examples, the motor is an electric motor configured to drive the shaft assembly in rotation for propelling the boat and to be driven in rotation by the shaft assembly for generating electrical power. [0013] Hereby, a technical effect includes that the same motor may achieve both functions of driving the propeller system e.g. when in a forward mode and of generating energy e.g. when in a power generation mode, where the propeller system is driven in rotation by dragging while the boat is propelled by other means than the propeller system. [0014] In other examples, a driving motor may be provided for driving the shaft assembly in the driving modes, and a separate electrical generator may be provided for being driven by the shaft assembly in the power generation mode.

[0015] In certain examples, the pitch of said primary blades is adjustable, in that each primary blade is configured to be positioned in at least one desired pitch orientation between the first pitch orientation and the second pitch orientation.

[0016] Hereby, a technical effect includes optimizing the pitch of the primary blades depending on an intended use, such as, optimizing the performance for forward propelling, backward propelling, propelling at high speed, at low speed, or using the primary propeller for power generation.

[0017] In certain examples, the first pitch orientation is a feathering orientation.

[0018] Hereby, a technical effect includes that the primary blades may be oriented in the feathering orientation to reduce drag, in particular in a dragging mode where the boat is propelled by other means than the propeller system.

[0019] In certain examples, the propeller system comprises a differential planetary gear by means of which the electrical motor is operatively connected to the primary propeller and to the secondary propeller. In certain examples, the front propeller is connected to a ring of the differential planetary gear and the rear propeller is connected to a carrier of the planetary gear. In certain examples, the differential planetary gear may be a compound or may be a double pinion planetary gear.

[0020] Hereby, a technical effect is that the differential planetary gear adaptively and automatically distributes the torque imparted by the motor between the front propeller and the rear propeller, depending on the current hydrodynamic conditions applied to each of the primary propeller and secondary propeller. In some examples, the differential planetary gear may be useful for automatically and adaptively balancing the ratio of the respective torques produced by the primary propeller and the secondary propeller. In use, the differential planetary gear always strives to achieve the torque ratio it is designed for. Another technical effect includes that the planetary gear may have a high gear ratio, so that, for obtaining a given rotation speed of the primary propeller and of the secondary propeller, the motor may have a high rotation speed. Thus, the motor may produce the required torque for driving the primary and secondary propeller while remaining relatively compact. Another technical effect includes that, when used in power generation mode, the front propeller may drive the motor at a high rotation speed through the differential planetary gear, so that the generator or electric motor is efficiently driven for producing power. Another technical effect includes that the planetary gear may be configured for causing either a counter-rotation of the primary propeller and the secondary propeller, or for causing a rotation of said propeller in the same rotation direction.

[0021] In certain examples, the propeller system comprises a brake for immobilizing or at least braking the secondary propeller relative to the primary propeller, in rotation around the propeller axis.

[0022] Hereby, a technical effect includes obtaining a direct drive of the primary and secondary propellers.

[0023] In certain examples, the propeller system comprises a brake for immobilizing or at least braking the secondary propeller, in rotation around the propeller axis, relative to a stator of the electric motor or to a hull of the boat or to a stern of the boat.

[0024] Hereby, a technical effect includes stopping or slowing the rotation of the secondary propeller, relative to the hull of the boat.

[0025] In certain examples, the primary blades and the secondary blades are mechanically coupled to each other by a mechanical coupling, so that the orientation of the primary blades around the pitch axes and the orientation of the secondary blades around the folding axes are dependent from each other, i.e. are synchronized. Preferably, the mechanical coupling however enables independent rotation of the primary propeller and the secondary propeller around the propeller axis. For example, through mechanical coupling of the primary and secondary blades, it may be provided that, when the secondary blades are in the folded orientation, the primary blades are in the regeneration pitch orientation, when the secondary blades are in the deployed orientation, the primary blades are in the forward drive pitch orientation, and when the secondary blades are in an intermediate orientation between the deployed and the folded orientation, the primary blades are in the feather pitch orientation. [0026] In certain examples, the primary blades are devoid of actuator for being rotated around the pitch axes and secondary blades are devoid of actuator for being rotated around the folding axes, and the primary and secondary blades being actuated solely by means of dynamic effects applied to the blades, such as by centrifugal effect and/or hydrodynamic forces applied to the primary and/or to the secondary propeller. The centrifugal effect and hydrodynamic forces may put the secondary blades in deployed orientation when the shaft assembly is driven in rotation by a motor. When the shaft assembly is dragged without the secondary propeller being driven in rotation around the propeller axis, the dragging pulls the secondary blades to the folded orientation.

[0027] In some examples, when the propeller system is dragged as the boat is driven forward by other means, such as sails, any brake, if equipped, is preferably released. In this configuration, the secondary blades may be pulled to an intermediate orientation between the folded and the deployed orientation by hydrodynamic forces applied to the secondary blades, as the centrifugal effect is weak or null. By mechanical coupling with the secondary blades, the primary blades are put in the feathering pitch orientation. Overall drag of the propeller system is thereby minimized and the feathering pitch orientation enables that the primary blades tends to prevent rotation of the first propeller. This reduces the need of a brake to be provided for immobilizing the shaft assembly in dragging mode.

[0028] In some examples, when the propeller system is driven in rotation by the motor, any brake, if provided, is preferably released. In this configuration, the secondary blades may be put in the deployed orientation by centrifugal effect. By mechanical coupling with the secondary blades, the primary blades are put in the forward drive pitch orientation.

[0029] In some examples where the propeller system comprises a brake for immobilizing or at least braking the secondary propeller relative to the stator, the hull of the boat or the stem of the boat, the brake may be applied for obtaining the power generation mode. When the brake is applied, the secondary propeller is prevented to rotate, or slowed down, so that the centrifugal force applied to the secondary propeller may reach zero or is at least reduced. The secondary blades may be rotated to the folded orientation under the hydrodynamic forces caused by the dragging. The primary blades may be rotated to the regeneration pitch orientation, by mechanical coupling with the secondary blades. Thus, applying the brake automatically enables putting the propeller system in power generation mode. In this case, the primary propeller drives the motor through the differential planetary gear, while the secondary propeller remains immobile and folded.

[0030] In other examples, the orientation of the primary blades and/or of the secondary blades is actuated by one or more pitch actuators and/or folding/deploying actuators.

[0031] In certain examples, the shaft assembly comprises: an outer hub, coaxial with the propeller axis, the primary propeller being carried and driven by the outer hub; and an inner shaft, coaxial with the propeller axis, rotatably received in the outer hub, the secondary propeller being carried and driven by the inner shaft. [0032] Hereby, a technical effect includes that the lateral and longitudinal bulk of the propeller system is optimized and that the rotation speed and direction of the primary and secondary propellers may be different relative to each other.

[0033] In certain examples, the shaft assembly comprises an axial bearing interposed between the primary propeller and the secondary propeller.

[0034] Hereby, a technical effect includes that the primary propeller may rotate at a speed very different compared to the rotation speed of the secondary propeller, e.g. when the secondary propeller is stopped from rotating and only the primary propeller rotates. It may also be provided that the primary and secondary propeller are counter-rotating, i.e. rotating in opposite rotation directions.

[0035] According to another aspect of the disclosure, a boat includes at least one propeller system as defined above.

[0036] In certain examples, the boat is a sailboat.

[0037] In certain examples, the boat is a motor boat devoid of sail.

[0038] Additional features and advantages are disclosed in the following description, claims, and drawings, and in part will be readily apparent therefrom to those skilled in the art or recognized by practicing the disclosure as described herein. There are also disclosed herein control units, computer readable media, and computer program products associated with the above discussed technical effects and corresponding advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] With reference to the appended drawings, below follows a more detailed description of aspects of the disclosure cited as examples.

[0040] FIG. 1 is an exemplary schematic view of a boat including a propeller system according to one example.

[0041] FIG. 2 is a longitudinal section view of a part of the propeller system of FIG. 1.

DETAILED DESCRIPTION

[0042] Aspects set forth below represent the necessary information to enable those skilled in the art to practice the disclosure.

[0043] FIG. 1. shows, in a schematic manner, a hull 2 of a boat 1, equipped with a propeller system 3, which is a duo propeller system. The boat 1 may be a sailboat, or a motor boat devoid of sail. In the present example, the propeller system 3 may comprise: an electric motor 5 with a stator 6 and a rotor 7; a differential planetary gear 20, with a sun gear 21, a planetary carrier 22 and a ring 23; a brake 25; a shaft assembly 30, with an inner shaft 31, an outer hub 32 and an axial bearing 36; a primary propeller 40, with primary blades 41; a secondary propeller 50 with secondary blades 51; a mechanical coupling 60, with a slider 61, a primary mechanism 62 and a secondary mechanism 63. However, some of these components may be omitted, modified or replaced.

[0044] The illustrated example concerns a straight shaft propeller system. However, the propeller system may be implemented for other types of drive systems, such as a stern-drive drive system or an outboard drive system.

[0045] FIG. 2 shows, in a more detailed manner, the shaft assembly 30, with the inner shaft 31, the outer hub 32 and the axial bearing 36; the primary propeller 40, with the primary blades 41; the secondary propeller 50 with the secondary blades 51; the mechanical coupling 60, with the slider 61, the primary mechanism 62 and the secondary mechanism 63.

[0046] As explained below, the propeller system 3 is configured to be operated in a forward mode, in a dragging mode and in a power generation mode. In FIG. 1, the boat 1 floats in water 9.

[0047] In the illustrated example, the propeller system 3 defines propeller axis X30, fixed relative to the stator 6 and/or the hull 2 and to which the shaft assembly 30 and the propellers 40 and 50 are coaxial. The propeller axis X30 may be horizontal or slightly inclined when the boat 1 is in use in water 9, as in FIG. 1.

[0048] In case of a stem drive, the axis X30 may be fixed relative to the stern instead of the hull. In case of an outboard drive, the axis X30 may be fixed relative to the outboard motor casing instead of the hull.

[0049] The boat 1 defines a forward direction XI, oriented from the stern to the bow. [0050] In the illustrated example, the stator 6 of the motor 5 is attached to the hull 2 of the boat 1. The rotor 7 may be coaxial with axis X30, as shown, and operatively connected to the shaft assembly 30, here via the differential planetary gear 20, or may be positioned otherwise, and operatively connected to the shaft assembly via additional angled gears (not shown), which connect said rotor 7 to the differential planetary gear 20.

[0051] In the case of a stern-drive system, the stator 6 may be attached to a stern instead of being attached to the hull.

[0052] In the case of an outboard system, the stator 6 may be attached to an outboard motor casing instead of the hull. [0053] The rotor 7 may be driven in rotation relative to the stator 6 under electromagnetic interaction between the rotor 7 and the stator 6 when the motor 5 is electrically powered. The rotor 7 may also be driven in rotation by the shaft assembly 30, as explained below, so that the rotation of the rotor 7 relative to the stator 6 generates electrical power for the boat 1, by electromagnetic interaction in the motor 5.

[0054] In the illustrated example, the rotation of the rotor 7 is operated about axis X30, but could be operated around a different rotor axis, depending on the configuration of the motor relative to the shaft assembly 30.

[0055] In the present example, the outer hub 32 and the inner shaft 31 of the shaft assembly 30 is coaxial with the propeller axis X30. The inner shaft 31 is received in the outer hub 32, i.e. may rotate relative to the outer hub 32 and to the stator 6, about axis X30. The outer hub 32 may also rotate relative to the inner shaft 31 and to the stator 6 about the axis X30.

[0056] In the example, the primary propeller 40 is arranged frontwards, i.e. in the direction XI relative to the secondary propeller 50. The propeller 40 is a front propeller and the propeller 50 is a rear propeller.

[0057] The primary propeller 40 is attached to the outer hub 32, so that the outer hub 32 carries the primary propeller 40, and may drive the propeller 40 in rotation around the axis X30, when the shaft assembly 30 is driven by the motor 5. The propeller 40 may also drive the outer hub 32 in rotation about the axis X30, in dragging mode and in power generation mode.

[0058] The secondary propeller 50 is attached to the inner shaft 31, so that the inner shaft 31 carries the secondary propeller 50, and may drive the propeller 50 in rotation around the axis X30, when the shaft assembly 30 is driven by the motor 5. The propeller 50 may also drive the inner shaft 31 in rotation about the axis X30, in dragging mode. For carrying the propeller 50, the inner shaft 31 may protrude from the outer hub 32 in a direction opposite to direction XI. The secondary propeller 50 is preferably attached to the protruding part of the inner shaft 31. To this end, the shaft assembly preferably comprises a secondary hub 35, fixedly attached to the shaft 31, at the protruding part thereof. The secondary hub 35 is arranged adjacent to the outer hub 32, along axis X30. The outer hub 32 is positioned in direction XI relative to the secondary hub 35.

[0059] In the illustrated example, the axial bearing 36 is interposed between the propellers 40 and 50, parallel to the axis X30. As shown in FIG 1 and FIG 2, the axial bearing 36 may be positioned around the inner shaft 31, and axially bear against the outer hub 32 and the secondary hub 35.

[0060] The shaft assembly 30 is connected to the rotor 7 via the differential planetary gear 20, so that the motor 5 may drive the shaft assembly 30 in rotation for propelling the boat 1 in the forward mode via the differential planetary gear 20, or may be driven in rotation by the shaft assembly 30 in power generation mode via the differential planetary gear 20, the motor 5 thereby generating electrical power. Preferably, the differential planetary gear 20 is coaxial with axis X30, i.e. the sun gear 21, the planetary carrier 22 and the ring 23 are coaxial with axis X30 and may rotate relative to axis X30. In the present example, the sun gear 21 is fixedly attached to the rotor 7, the planetary carrier 22 is fixedly attached the inner shaft 31 of the shaft assembly 30 and the ring 23 is fixedly attached to the outer hub 32 of the shaft assembly 30. In particular, the carrier 22 is fixedly attached to a front shaft end 37 of the shaft 31, the front shaft end 37 being opposite to the protruding part of the shaft 31 along axis X31. In a manner known per se, the carrier 22 carries one or more planet gears 24 which are rotatable around a respective planet axis relative to the carrier 22, each planet axis being parallel to the axis X30 and fixed relative to the carrier 22, the planet gears 24 being meshed, inwardly, with the sun gear 21, and, outwardly, with the ring 23. To this end, the sun gear 21 has teeth oriented outwardly, and the ring 23 has teeth oriented inwardly.

[0061] The differential planetary gear 20 adaptively and automatically distributes the torque imparted by the rotor 7, between the propeller 40, via the outer hub 32, and to the propeller 50, via the inner shaft 31, depending on the hydrodynamic efforts that may oppose to the rotation of the propellers 40 and 50 around the axis X30. In other words, a first portion of the torque generated at the rotor 7 is transmitted to the propeller 40 and a second portion is transmitted to the propeller 50, and the ratio between the first portion and the second portion may vary depending on the ratio of the efforts respectively opposing the rotation of the propellers 40 and 50. In other words, the differential planetary gear 20 automatically and adaptively balances the ratio of the respective torques produced by the propellers 40 and 50, when the motor 5 powers the rotation of the rotor 7. More precisely, the differential planetary gear 20 adaptively changes the rotation speed of the propellers to fulfill a torque ratio between the propellers 40 and 50, the ratio depending on the design of the planetary gear 20 (in particular, depending on the respective number of teeth for the sun gear 21, planetary gears 24 and ring 23).

[0062] In the event one of the propellers 40 and 50 is completely prevented from rotating around axis X30, then the other of the propellers 40 and 50 may rotate and benefit from all the torque imparted by the motor 5, through the differential planetary gear 20. The differential planetary gear 20 is mechanically reversible, which means that if one of the propellers, such as the propeller 40, is rotated, the torque is distributed to the motor 5 and to the other propeller, for example the propeller 50, via the differential planetary gear 20. If the propeller 50 is prevented from rotating and the propeller 40 is rotated, then all the torque of the propeller 40 is transmitted to the motor 5 via the differential planetary gear 20.

[0063] The brake 25 is configured for, when applied, immobilizing or at least braking the secondary propeller 50, in rotation around the propeller axis X30 relative to the stator 6. For the illustrated case where the stator 6 is fixed relative to the hull 2, the propeller 50 is immobilized or at least slowed in rotation about axis X30 relative to the hull 2. In other cases where the stator 6 is mobile relative to the hull 2 (such as, for outboard drive systems), the propeller 50 is immobilized or at least slowed in rotation about axis X30 relative to the stator 6. In other cases where the axis X30 is mobile relative to the stator 6 and the hull 2 (such as, for stern drive systems), the propeller 50 is immobilized in rotation around axis X30 relative to the stem. When released, the rotation of the propeller 50 around axis X30 relative to the stator 6 is enabled. The brake 25 may comprise a slider, which may slide along axis X30, between a release position shown in FIG. 1, and an applied position where the brake 25 is coupled with the carrier 22, or applies braking pressure to the carrier, thus preventing the carrier 22 to rotate, in that the carrier 22 is fixed relative to the hull 2 via the brake 25.

[0064] In this case, when the brake 25 is applied, the torque generated by the motor 5 is entirely transmitted to the propeller 40 through the differential planetary gear 20, while the propeller 50 does not rotate. When the brake 25 is applied, a rotation of the propeller 40, obtained by hydrodynamic forces applied to the propeller, is entirely transmitted to the motor 5 via the differential planetary gear 20 while the propeller 50 does not rotate.

[0065] Alternatively or additionally, a brake may immobilize or at least brake the propellers 40 and 50 in rotation around the axis X30, so that the propellers 40 and 50 always rotate at the same rotation speed around the axis X30 relative to the hull 2, when the brake is applied. In this case, the brake synchronizes the rotation of the propellers 40 and 50.

[0066] Since the carrier 22 is attached to the shaft 31 at the front shaft end 37 thereof and since the ring 23 is connected to the outer hub 32, the motor 5 is coupled to the shaft assembly 30 via the front shaft 37 end and the outer hub 32 for driving the propellers 40 and 50 in rotation around the axis X30. Since the propeller 40 is a front propeller and the propeller 50 is a rear propeller, the primary propeller 40 is positioned between the front shaft end 37 and the secondary propeller 50, and between the differential planetary gear 20 and the secondary propeller 50.

[0067] Each primary blade 41 is carried by the shaft assembly 30, in particular by the outer hub 32, so as to rotate together with the outer hub 32 around axis X30. Each blade 41 is oriented radially relative to the axis X30. Preferably, the primary blades 41 are equally distributed around the axis X30. A number of primary blades 41 such as two, three, four or more primary blades 41 may be provided. Each primary blade 41 has a helical profile, i.e. is cambered, so that the blades 41 generate a propelling force directed parallel to the axis X30 when the propeller 40, and thus the blades 41, are rotated by the motor 5 through the shaft assembly 30, in particular through the outer hub 32. Thanks to their helical profile, each blade 41 may also be driven in rotation by the water 9, i.e. by hydrodynamic forces applied to the blades 41, when the propeller 40 is dragged in the water 9 along the axis X30. In this case, the propeller 40 may actuate the rotation of the rotor 7 through the differential planetary gear 20, provided the brake 25 is applied and prevents, or reduces, the rotation of the propeller 50. [0068] Each primary blade 41 is individually rotatable relative to the shaft assembly 30, in particular to the outer hub 32, around a respective pitch axis R41, between a first pitch orientation and a second pitch orientation. In other words, the primary propeller 40 is of variable pitch. Each pitch axis R41 is perpendicular to the propeller axis X30, preferably radial to the propeller axis X30, and extends along the concerned blade 41, as shown in FIG 1 and FIG 2.

[0069] For example, the first pitch orientation is a regeneration pitch orientation, which may be used in power generation mode or in a backward mode. In the regeneration pitch orientation, the primary blades 41 are oriented with a front side thereof directed opposite to direction XI. In the power generation mode, the regeneration pitch orientation is most appropriate for the propeller 40 for being rotated by hydrodynamic forces when dragged as the boat 1 moves in direction XI. In the backward mode, the regeneration pitch orientation is most appropriate for propelling the boat backwards, i.e. opposite to direction XI.

[0070] For example, the second pitch orientation, shown in FIG.l, is a forward drive pitch orientation, that may be used in forward mode, where the blades 41 are oriented with their front side directed in direction XI, most appropriate for propelling the boat in the direction XI.

[0071] Preferably, the pitch orientation of the primary blades 41 is adjustable, in that each primary blade 41 is configured to be positioned in at least one desired pitch orientation between the first pitch orientation and the second pitch orientation. In the present example, the blades 41 continuously rotate from the first to the second pitch orientation, so that any desired pitch orientation between the first and second pitch orientations may be adopted by the blades 41. In particular, the blades 41 may reach a feather pitch orientation, shown in FIG 2, between the first and second pitch orientations. In the feather pitch orientation, each blade 41 is oriented so that their sides are substantially parallel to the axis X30, i.e. oriented sideways, so that the drag of the blades 41 parallel to axis X30 is reduced. When the blades 41 are in feather pitch orientation, their drag is maximal in an orthoradial direction, i.e. in a rotational direction around axis X30, thereby tending to prevent the propeller 40 from rotating around axis X30 due to rotational hydrodynamic forces.

[0072] Each secondary blade 51 is carried by the shaft assembly 30, in particular by the inner shaft 31, via the secondary hub 35, so as to rotate together with the inner shaft 31 around axis X30. Preferably, the secondary blades 51 are equally distributed around the axis X30. A number of secondary blades 51 such as two, three, four or more secondary blades 51 may be provided. Each secondary blade 51 has a helical profile, i.e. is cambered, so that the blades 51 generate a propelling force directed parallel to the axis X30 when the propeller 50, and thus the blades 51, are rotated by the motor 5 through the shaft assembly 30, in particular through the inner shaft 31. Thanks to their helical profile, each blade 51 may also be driven in rotation by the water 9, i.e. by hydrodynamic forces applied to the blades 51, when the propeller 50 is dragged in the water 9 along the axis X30.

[0073] Each secondary blade 51 is individually rotatable relative to the shaft assembly 30, in particular to the inner shaft 31, around a respective folding axis R51, between a deployed orientation, shown in FIG 1 at references 51, and a folded orientation, shown in FIG 1 at references 51’. In other words, the secondary propeller 50 is foldable. Each folding axis R51 is perpendicular to the propeller axis X30 and to the concerned blade 51, and crosses through a proximal end of the blade 51 by which the blade 51 is attached to the shaft assembly 30, in particular to the secondary hub 35. In the deployed orientation, the blades 51 may be oriented radially to the axis X30, whereas in the folded orientation, the blades 51 may be oriented parallel to the axis X30. In folded orientation, the blades 51 are preferably directed opposite to the direction XI, i.e. a respective free end of the blades 51 is directed opposite to the direction XI.

[0074] For example, the deployed orientation, shown in FIG.l at references 51, may be used in forward mode or in backward mode, where the blades 51 are oriented radially relative to the axis X30 with their front side directed in direction XI. [0075] For example, the folded orientation, shown in FIG. l at references 51’, may be used in dragging mode, where the blades 51 are less subject to drag along axis X30.

[0076] Preferably, the folding orientation of the secondary blades 51 is adjustable, in that each secondary blade 51 is configured to be positioned in at least one desired folding orientation between the deployed orientation and the folded orientation. In the present example, the blades 51 continuously rotate from the deployed to the folded orientation, so that any desired folding orientation between the deployed and folded orientations may be adopted by the blades 51. In particular, the blades 51 may reach an intermediate orientation, shown in FIG. 2, between the deployed and the folded orientations. The intermediate orientation may be considered as a half-folded or partially-folded orientation, where the drag of the blades 51 is reduced compared to the deployed orientation, as the blades 51 are almost parallel to the axis X30, or at least are oriented between a radial and an axial orientations.

The intermediate orientation may be used in the power generation mode. In other examples, the folded orientation used in power generation mode, instead of the intermediate orientation. [0077] Preferably, the blades 41 are mechanically coupled for being at the same pitch orientation, i.e. are synchronized for their pitch orientation. Preferably, the blades 51 are mechanically coupled for being at the same folding orientation, i.e. are synchronized for their folding orientation.

[0078] Preferably, the primary blades 41 and the secondary blades 51 are mechanically coupled to each other via the mechanical coupling 60, so that the pitch orientation of the primary blades 41 around the pitch axes R41 and the folding orientation of the secondary blades 51 around the folding axes R51 are dependent from each other, i.e. are synchronized. Preferably, by means of the mechanical coupling 60, the primary blades 41 are in the forward drive pitch orientation when the secondary blades 51 are in the deployed orientation, the primary blades 41 are in the feathering pitch orientation when the secondary blades 51 are in the intermediate orientation, and the primary blades 41 are in the regeneration pitch orientation when the secondary blades 51 are in the folded orientation.

[0079] The slider 61 is configured for sliding parallel to axis X30. To this end, the slider is for example formed by a sleeve, mounted around the inner shaft 31 and inside the outer hub 32, and, if implemented, inside the secondary hub 35. Preferably, said sleeve comprises a primary sleeve part 65, in the secondary hub 35, and a secondary sleeve part 66 around the protruding part of the inner shaft 31. The parts 65 and 66 slide together along the axis X30, but are enabled to rotate relative to each other around axis X30. When not sliding, the part 66 rotates with the inner shaft 31 and the propeller 50 around the axis X30. When not sliding, the part 65 rotates with the hub 32 and the propeller 40 around the axis X30. Preferably, the slider 61 comprises an axial bearing 67 connecting the sleeve part 65 to the sleeve part 66. [0080] The primary mechanism 62 synchronizes the rotation of the blades 41 around their respective pitch axes R41 with the sliding of the slider 61 along axis X30, in particular with the sliding of the sleeve part 65. To that end, the primary mechanism 62 may comprise crankshafts actuated by the slider 61, as visible in FIG 2. and/or a rack and pinion system, coupling the blades 41 to the slider 61.

[0081] The secondary mechanism 63 synchronizes the rotation of the blades 51 around their respective folding axes R51 with the sliding of the slider 61 along axis X30, in particular with the sleeve part 66. To that end, the secondary mechanism 63 may comprise crankshafts and/or conical gears, coupling the blades 51 with the slider 61.

[0082] Thus, the orientation of the blades 41 around their pitch axes R41 is synchronized with the orientation of the blades 51 around their folding axes R51, via the mechanisms 62 and 63 and via the slider.

[0083] In use, the blades 51 may be actuated in rotation about their axes R51, without the need of a folding actuator, by centrifugal effect, when the propeller 50 rotates, and by hydrodynamic forces, when the propeller 50 is dragged. By centrifugal effect, the blades 51 tend to reach the deployed orientation. By dragging in the direction XI, the hydrodynamic forces tend to fold the blades 51 back to the folded orientation, since the blades 51 are oriented opposite to the direction XI when in the folded orientation.

[0084] In use, the blades 41 may be actuated in rotation about their axes R41, without the need of a pitching actuator, by actuation of the blades 51 about their axes R51, via the mechanical coupling 60. Thus, no pitch or folding actor is necessary.

[0085] In the forward drive mode, where the motor 5 actuates the shaft assembly 30 in rotation about the axis X30 and the brake 25 is released, the blades 51 are put and maintained to deployed orientation by centrifugal effect, and thus put and maintain the blades 41 in forward drive pitch orientation via the coupling 60.

[0086] In dragging mode, where the motor 5 does not actuate the shaft assembly 30, the brake 25 is released and the boat 1 is propelled by other means, such as sails or other propellers, the blades 51 are put to an intermediate orientation by the hydrodynamic forces caused by the dragging in direction XI. Thus, the blades 51 put the blades 41 in feathering pitch orientation via the coupling 60. The blades 41 being oriented in feathering pitch orientation, they tend to prevent rotation of the propeller 40. The motor 5 is not actuated by the propellers 40 and 50 since they do not rotate, or is actuated very slowly since the rotation of the propeller 40 and 50 is slow. Thus, no brake, or only a weaker brake, is required for immobilizing the rotor 7 in dragging mode.

[0087] In power generation mode, where the motor 5 does not actuate the shaft assembly 30, the brake 25 is applied and the boat 1 is propelled by other means, such as sails or other propellers, the blades 51 are put to a folded orientation, due to the absence of centrifugal effect since they are prevented from rotating about the axis X30 by the brake, and due to hydrodynamic forces caused by dragging. Thus, the blades 51 put the blades 41 in regeneration pitch orientation via the coupling 60, so that the propeller 40 is rotated by the hydrodynamic forces caused by the dragging. The rotation of the propeller 40 drives the motor through the differential planetary gear 20, thereby producing electrical power.

[0088] The terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting of the disclosure. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms "comprises," "comprising," "includes," and/or "including" when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0089] It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element without departing from the scope of the present disclosure.

[0090] Relative terms such as "below" or "above" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe a relationship of one element to another element as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present. [0091] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0092] It is to be understood that the present disclosure is not limited to the aspects described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the present disclosure and appended claims. In the drawings and specification, there have been disclosed aspects for purposes of illustration only and not for purposes of limitation, the scope of the inventive concepts being set forth in the following claims.