The present disclosure pertains to the field of propulsion systems for vessels. The present disclosure relates to a propulsion unit for a vessel and vessel comprising the propulsion unit.

BACKGROUND

The field of propulsion systems for vessels is concerned with converting energy output from a vessel’s prime mover into forward motion. Depending on the type of vessel and service provided by the vessel, fuel costs may represent as much as 50-60% of total ship operating costs. A screw propeller is the mainly used propulsion device on vessels today. Maximum achievable open-water efficiencies by modern screw propellers are about 70%. For commercial shipping, it is desirable to improve the efficiency of the propulsion system, to avoid wasting the energy provided by the prime mover.

SUMMARY

Accordingly, there is a need for a propulsion system, which mitigates, alleviates or addresses the shortcomings existing and provides a more efficient propulsion of a vessel.

Disclosed is a propulsion unit for propelling a vessel. The propulsion unit comprises a main body configured to be arranged at a keel of the vessel and comprising a pivot point, a fin being movably arranged in relation to the main body, and an actuator assembly for generating a heave motion of the fin in relation to the main body and/or the keel. The actuator assembly comprises at least one actuator. The fin is connected to the pivot point such that the fin is arranged to pivot around the pivot point when the at least one actuator generates the heave motion of the fin, thereby generating a pitch motion of the fin.

It is an advantage of the present disclosure that the fin produces less hydrodynamic drag than common propulsion systems, such as propellers driven by shafting moving in the water. The actuator of the propulsion unit provides a driving mechanism for generating the motion of the fin which is simple and efficient and reduces energy losses and maintenance requirements of the propulsion unit. The motion of the fin may further be controllable by means of the actuator, such that the hydrodynamic characteristics of the fin may be adapted to improve efficiency of the fin. Furthermore, the main body of the propulsion system provides directional stability to a vessel comprising the propulsion unit. Thereby, the efficiency of the propulsion unit is increased.

Disclosed is a vessel comprising the propulsion unit for propelling the vessel, as disclosed herein. The main body is arranged to the keel of the vessel. The fin is configured to perform a pitch motion and/or a heave motion in relation to the keel of the vessel.

It is an advantage of the present disclosure that the propulsion system of the vessel increases efficiency and reduces maintenance requirements compared to common propulsion systems, such as propellers driven by shafting moving in the water. Thus, a fuel consumption of the vessel and downtime due to maintenance of the vessel may be reduced, which reduces operating costs and emissions of the vessel. Furthermore, the main body of the propulsion system provides directional stability to a vessel comprising the propulsion unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present disclosure will become readily apparent to those skilled in the art by the following detailed description of exemplary embodiments thereof with reference to the attached drawings, in which:

Fig. 1 A illustrates an exemplary perspective view of a propulsion unit comprising a single actuator according to this disclosure,

Fig. 1 B illustrates an exemplary side view of the propulsion unit comprising the single actuator according to this disclosure,

Fig. 2A-2D illustrate an exemplary overall design and motion pattern of a propulsion unit comprising a first actuator and a second actuator according to this disclosure,

Fig. 3 illustrates an exemplary connection for connecting the fin to the first and second actuators according to this disclosure, Fig. 4 illustrates an exemplary main body of the propulsion unit according to this disclosure,

Fig. 5 is an exemplary graph illustrating a motion pattern of a fin of the propulsion system when the first and second actuators are operated with a phase shift according to this disclosure, Fig. 6 illustrates an exemplary propulsion unit for being rotatably arranged to a vessel according to this disclosure

Fig. 7 illustrates a perspective outside view of an exemplary vessel comprising an exemplary propulsion unit according to this disclosure,

Fig. 8 illustrates a perspective inside view of an exemplary vessel comprising an exemplary propulsion unit according to this disclosure,

Fig. 9 illustrates an exemplary propulsion unit comprising a first and a second actuator being arranged to the fin via fixed pivot points according to this disclosure,

Fig. 10 illustrates an exemplary propulsion unit comprising a first and a second actuator being arranged to the fin via fixed pivot points according to this disclosure, and

Fig. 11 is an exemplary graph illustrating a motion pattern of the fin of the propulsion system using different angle of attack profiles.

DETAILED DESCRIPTION

Various exemplary embodiments and details are described hereinafter, with reference to the figures when relevant. It should be noted that the figures may or may not be drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the disclosure or as a limitation on the scope of the disclosure.

In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated, or if not so explicitly described.

The figures are schematic and simplified for clarity, and they merely show details which aid understanding the disclosure, while other details have been left out. Throughout, the same reference numerals are used for identical or corresponding parts.

A propulsion unit for propelling a vessel is disclosed. The propulsion unit comprises a main body configured to be arranged at a keel of the vessel and comprising a pivot point (such as a first pivot point), a fin being movably arranged in relation to the main body, and an actuator assembly for generating a heave motion of the fin in relation to the main body. The heave motion may herein refer to a reciprocating motion of the fin, such as a linear vertical upwards/downwards motion of the fin in relation to the main body, when the propulsion unit is arranged on the vessel. The actuator assembly comprises at least one actuator. The fin is connected to the pivot point such that the fin is arranged to pivot around the pivot point when the at least one actuator generates the heave motion of the fin, thereby generating a pitch motion of the fin. The pitch motion of the fin herein refers to a rotation of the fin about its transverse axis, such as an axis extending from a first tip of the fin to a second tip of the fin, such as port side tip of the fin to a starboard side tip of the fin). The heave motion and/or pitch motion of the fin generates a thrust force for propelling the vessel.

The at least one actuator may be a linear actuator. The actuator may be a hydraulic, electric or mechanic actuator. The actuator may in one or more example propulsion units be a ram style actuator. The propulsion unit may comprise at least one actuating rod. The actuator assembly may be connected to the fin via the at least one actuating rod. The actuator assembly may be configured to operate with an oscillating pattern, thereby generating an oscillating heave and pitch motion of the fin.

In one or more example embodiments of the propulsion unit, the pivot point may be fixedly arranged to the main body. The propulsion unit may comprise a lever arm. The lever arm may be attached to the pivot point, such that the lever arm may pivot around the pivot point. The fin may be attached to the pivot point via the lever arm. Since the fin may be attached to the lever arm being pivotably attached to the pivot point, the fin may pivot around the pivot point.

In one or more example embodiments of the propulsion unit, the fin may be pivotably arranged to the lever arm. The propulsion unit may further comprise a pitch rod configured to change the pitch of the fin when the actuator generates the heave motion of the fin. A first end of the pitch rod may be pivotably arranged to the fin at a distance from the pivot point at which the fin is attached to the lever arm. The propulsion unit may comprise a crank, such as a bell crank, having a first and a second arm and being pivotably arranged to the main body of the propulsion unit at a distance from an end of the first and second arm, respectively. A second end of the pitch rod may be pivotably arranged on a first end of the crank and the actuator may be pivotably arranged to the second end of the crank. Upon the actuator performing a reciprocating motion, the crank may rotate around the pivot point thereby changing the position of the second arm of the crank which changes the position of the second end of the pitch rod. This causes the first end of the pitch rod to change the position in relation to the pivot point joining the fin to the lever arm which leads to the fin changing the pitch angle relative to the lever arm. The crank may further comprise a first contacting surface and a second contacting surface facing each other and being configured to contact an upper side and a lower side of the lever arm, respectively, to generate the heave motion of the fin. Upon extension of the actuator, the crank may pivot in relation to the main body, so that the first contacting surface of the crank contacts the upper side of the lever arm and pushes the lever arm downwards. Upon retraction of the actuator, the crank may pivot in an opposite direction relative to the main body, so that the second contacting surface of the crank contacts the lower side of the lever arm and pushes the lever arm upwards.

In some example embodiments, the pitching motion of the fin may also be generated by the actuator being a rotating actuator. In some example embodiments of the propulsion unit, the actuator assembly may comprise a first actuator and a second actuator. The first actuator and the second actuator may be connected to the fin via a first pivot point and optionally a second pivot point, respectively. The pivot point, such as the first pivot point and/or the second pivot point, may thus be movably arranged in relation to the main body. The positions of the pivot points and/or the actuators connected to them may be moved forward or aft on the fin body, so that they are placed in order to optimize their induced torque on the fin.

The first actuator and the second actuator may be independently operable in relation to each other, so that a phase difference between the heave motion and the pitch motion of the fin is variable. The first actuator and the second actuator may thus be operated in phase or out of phase. In order to create a pitch and heave motion which is out of phase, the actuators may be operated with a phase difference. Varying the phase difference allows an adjustment of the maximum angle of pitch of the fin. The two actuators may in one or more embodiments be operated by independently varying the amplitude of their travel or motion. The first actuator and the second actuator may thus vary the amplitude and/or the phase of their travel or motion independently of each other. The angle of pitch herein is an angle of the fin to a horizontal plane. When the first actuator and the second actuator are operated in phase, the fin only performs a heave motion since the relative position of the first and second pivot points does not change. In other words, when the first and the second actuators are operated in phase, they perform the same movement, the pitch of the fin is thus not changed, and the fin performs the upward/downward motion without changing the angle of the fin to the horizontal plane. When the first actuator and the second actuator are operated out of phase, the fin performs a heave motion and a pitch motion since the relative position of the first and second pivot points changes. In other words, when the first and the second actuators are operated out of phase, they perform the different movements, the pitch of the fin is thus changed, and the fin performs the upward/downward motion while changing the angle of the fin to the horizontal plane. Since the first actuator and the second actuator may be independently operable, a motion pattern of the fin may be adapted and optimized to increase the lift of the fin, and thus the thrust force generated by the fin. By significantly changing and/or reversing the phase or amplitude of the first actuator and/or the second actuator the fin may produce a reverse thrust for reversing the vessel. By controlling the phase difference and/or the amplitude of the of the first actuator and the second actuator individually, a precise control of the motion of the fin may be achieved. Thus, the one or more example embodiments disclosed herein provide a simple system which allows a precise control of the motion of the fin.

In one or more example embodiments of the propulsion unit, the fin may be slidably arranged inside of the main body. The fin may comprise a first fin section, a second fin section and a connecting element for connecting the first fin section and the second fin section. The connecting element may comprise a first connecting rod and optionally a second connecting rod. The first fin section and the second fin section may be connected via the first connecting rod and the second connecting rod. The first connecting rod and the second connecting rod may be arranged in parallel and at a respective first distance and second distance from a leading edge of the first and second fin sections. The first and second connecting rods may have a respective first and second end section configured to be arranged inside the first and the second fin section, respectively. A center section of the first connecting rod and the second connecting rod may be arranged inside the main body. The first fin section and the second fin section may be arranged on opposite sides of the main body, such that the main body separates the first fin section and the second fin section. The first fin section may thus be arranged on a first side, such as on a starboard side, of the main body and the second fin section may be arranged on a second side, such as on a port side, of the main body.

The first connecting rod may be connected to the first actuator and the second connecting rod may be connected to the second actuator. The first connecting rod and the second connecting rod may be connected to the respective first actuator and second actuator via a first actuating rod and a second actuating rod, respectively. The first connecting rod may be connected to the first actuating rod via a first freely rotating pinned connection. The second connecting rod may be connected to the second actuating rod via a second freely rotating pinned connection. The first freely rotating pinned connection may thus constitute the first pivot point and optionally the second freely rotating pinned connection may constitute the second pivot point around which the fin is arranged to pivot.

The at least one actuator may comprise one or more pinned connection at its ends for connecting the actuator to the fin, the actuating rod and/or the hull of the vessel. The at least one actuating rod may also comprise one or more pinned connections at its ends for connecting the actuator rod to the actuator and/or to the fin. The pinned connection allows the actuator to rotate freely at the connecting points and thus allows the actuator to adjust its alignment to the lever arm and/or the vessel during the pivotal movement of the fin around the pivot point. The at least one actuator comprising the pinned connection can thus not carry any bending moment.

The at least one actuator and/or the at least one actuating rod may in one or more example embodiments be restrained, e.g. by one or more bearings, such as sliding bearings, to only move in a longitudinal direction, such as in the direction of the linear actuation of the at least one actuator, so that it cannot rotate and can thus support a bending moment in the actuator. The at least one actuator and/or the at least one actuating rod, such as at least one of the first and the second actuator, may be restrained such that it can only be displaced in a vertical direction. The vertical direction may correspond to a vertical direction of the vessel when the propulsion unit is arranged on the vessel. Only being displaced in a vertical direction may herein be seen as the displacement of the actuator not having a component in either longitudinal or lateral direction of the vessel. The at least one of the first actuator and the second actuator may be configured to be displaced in a vertical direction. The at least one of the first actuator and the second actuator may be restrained in a lateral and/or longitudinal direction. The at least one actuator may be restrained such that it cannot pivot in relation to the vessel, such as in relation to the hull of the vessel. The at least one actuator, such as at least one of the first actuator and the second actuator may restrained in a direction perpendicular to a direction of extension of the at least one actuator, such as in a direction perpendicular to a direction of extension of at least one of the first actuator and the second actuator. The one or more bearings may be arranged in the main body of the propulsion unit and/or in the hull of the vessel. The one or more bearings may be arranged around a circumference of the at least one actuator and/or the at least one actuating rod. The one or more bearings may be configured to support forces acting on the at least one actuator and/or the at least one actuating rod in a direction perpendicular to the direction of the linear actuation of the at least one actuator, such as in a vertical direction when the propulsion unit is mounted to the vessel. Restricting the movement of the at least one actuator and/or the at least one actuating rod to a longitudinal direction facilitates a sealing of the at least one actuator and/or the at least one actuating rod to the vessel. Since the at least one actuator and/or the at least one actuating rod cannot rotate in relation to each other, and/or in relation to the hull of the vessel, simpler and less costly seals may be used than compared to solutions where the at least one actuator and/or the at least one actuating rod can rotate in relation to vessel. Restraining the movement of the at least one actuator and/or actuating rod improves the controllability of the movement of the fin and allows the motion of the fin to be controlled precisely. Restraining the movement of the at least one actuator and/or actuating rod prevents the fin from oscillating fore and aft during the heave and/or the pitch motion of the fin in relation to the hull of the vessel.

In one or more example propulsion units, the propulsion unit comprises a plurality of actuators and/or actuating rods, such as for example a first actuator and/or actuating rod, a second actuator and/or actuating rod and a third actuator and/or actuating rod. One of the plurality of actuators and/or actuating rods may be restrained, such that the actuator and/or actuating rod is prevented from rotating relative to the hull of the vessel. The other actuators and/or actuating rods of the plurality of actuators and/or actuating rods may be configured to pivot in relation to the hull of the vessel while performing the heave and/or the pitch motion of the fin. The fin may be configured to pivot in relation to each one of the plurality of actuators and/or actuating rods, such that the fin can change its angle of attack in relation to each of the plurality of actuators and/or actuating rods. The fin may for example be connected to the plurality of actuators and/or actuating rods via a respective pivot point.

When the fin performs a pitch motion, the distance between the first pivot point and the second pivot point, seen from a horizontal plane, such as a plane perpendicular to the longitudinal extension of the actuating rods, varies. In order to allow for a change in the distance between the first pivot point and the second pivot point, seen from the horizontal plane, at least one of the pivot points may be connected to the fin via a sliding joint. The sliding joint may e.g. be an elongated slot in which the pivot point may be arranged to slidably move.

At least one of the first and second connecting rods, such as the first connecting rod, may comprise a first and a second elongated slot arranged at opposite ends of the connecting rod, respectively. The elongated slots may have a longitudinal extension in a direction perpendicular to a longitudinal axis of the at least one of the first and second connecting rods. The elongated slots may be arranged inside the first and second fin section, respectively. The first elongated slot and the second elongated slot may be connected to a pin or a rod fixedly arranged inside the first fin section and the second fin section, respectively, such that the pin or rod is slidably arranged within the elongated slot of the at least one of the first and second connecting rods. The first and the second fin sections may also comprise an elongated slot for receiving the first connecting rod and/or the second connecting rod, thereby allowing the first connecting rod and/or the second connecting rod to slide inside the elongated slot relative to the first and the second fin section. The first connecting rod and/or the second connecting rod may thus be slidably arranged inside the first and the second fin sections in a direction perpendicular to the longitudinal axis of the at least one of the first and second connecting rods, such that the distance of the first connecting rod and/or the second connecting rod from the leading edge can be varied. By varying the distance of the first connecting rod and/or the second connecting rod from the leading edge, the distance between the first pivot point and the second pivot point, seen from the horizontal plane, may change with the pitch motion of the fin. Thereby, the first actuator and the second actuator may perform a purely vertical motion for generating both the pitch and the heave motion of the fin. The location of the first connecting rod and the second connecting rod, and thus the location of the connection points to the first and second actuators, may be up to implementation and may be moved along the chord of the fin body, so that the connection points to the first and second actuators, are placed in order to optimized the torque induced onto the fin.

The first actuating rod and/or the second actuating rod (or at least center sections thereof) may be arranged inside the main body. Thus, the main body may comprise a first through- going slot and/or a second through-going slot for receiving the first connecting rod and/or the second connecting rod respectively. The first through-going slot and/or the second through-going slot allows the first connecting rod and the second connecting rod to protrude through the main body and to be slidably arranged within the first through-going slot and the second through-going slot. The first connecting rod and the second connecting rod may connect to the first actuating rod and the second connecting rod inside the main body. The first through-going slot and the second through-going slot have a longitudinal extension in a vertical direction of the main body, to allow the first connecting rod and the second connecting rod to perform a vertical movement to generate the pitch and the heave motion of the fin. The first through-going slot and the second through-going slot may comprise an opening at a top end of the first through- going slot and the second through-going slot, respectively. The openings at the top end of first through-going slot and the second through-going slot, allow the first and second actuators and/or first actuating rod and the second actuating rods to protrude into main body to connect to the connecting rods. The openings at the top end of first through-going slot and the second through-going slot may extend upwards into the main body and/or hull of the vessel and may be sealed above the waterline of the vessel.

The propulsion unit may comprise a rudder. In one or more example embodiments, the main body may be configured to be fixedly arranged to the keel of the vessel, and the rudder may be pivotably arranged to the skeg. When the main body is fixedly arranged to the keel it may constitute a skeg of the vessel. Skeg herein means a sternward extension of the keel of the vessel. The skeg may provide directional stability to the vessel. The main body may have a rudder mounted on its center line. The rudder may be attached to the main body via a rudder stock arranged at a trailing edge of the main body. The rudder stock is a vertical shaft through which the turning force of the steering gear may be transmitted to the rudder. The rudder stock may thus provide the pivot point for the rudder, around which the rudder may be pivotably arranged to the main body.

In one or more example embodiments, the main body may comprise a rudder stock for rotatably arranging the main body to the keel of the vessel. The main body may thus be configured to rotate and act as the rudder. The rudder stock may be hollow to accommodate the one or more actuators and/or the at least one actuating rod. The one or more actuators and/or the at least one actuating rod may be arranged inside the rudder stock. The rudder stock may comprise a bearing, such as a rotating bearing, for rotatably arranging the rudder stock to the keel of the vessel. The rudder stock may constitute a bearing race of the bearing. In order to prevent water from entering the vessel through the hollow rudder stock, the rudder stock may be sealed. The rudder stock constituting one of the bearing races may be extended above the waterline of the vessel, such that the rudder stock may be sealed above the waterline. Sealing the rudder stock above the waterline allows simpler and less expensive seals to be used compared to when the rudder stock is sealed below the waterline of the vessel.

The main body may comprise a hollow tube protruding from the main body on a vessel- or keel-facing side of the main body. The first actuating rod and/or the second actuating rod may be arranged within the hollow tube. The hollow tube may be configured to protrude into the vessel, such as into the inside of a hull of the vessel, when the main body is arranged to the keel of the vessel. When the main body is fixedly arranged to the keel of the vessel, the hollow tube may be a steel tube welded to the hull of the vessel. When the main body is rotatably arranged to the keel of the vessel, the rudder stock may be hollow and may constitute the hollow tube. The hollow tube may comprise a first end arranged to protrude into the hull of the vessel and a second end arranged in and/or at the main body of the vessel. The first end of the hollow tube is thus located further away from the main body and/or the fin than the second end of the hollow tube. Thus, the first end of the hollow tube may herein be referred to as a distal end of the hollow tube. The main body may be wholly or partially open to the water surrounding the vessel. In order to allow a movement of the actuators, actuator rods and/or connecting rods inside the main body, the main body comprises openings, such as slots, for receiving the actuators, actuator rods and/or connecting rods. Water may thus protrude through the openings in the main body. In order to prevent water to enter into the hull of the vessel, the hollow tube may comprise a seal arranged at the first end, such as the distal end, of the hollow tube, for sealing the actuators and/or the actuator rods to the hollow tube. The hollow tube may comprise a flanged connection for receiving the seal. Sealing between the actuators and the hull shell, may be particularly complicated and difficult if the seal is arranged below a waterline of the vessel. By providing the hollow tubes on the main body, that protrude into the hull of the vessel, and arranging the seal at the first end, such as the distal end, of the hollow tube inside the hull of the vessel, the seal may be moved upwards, such that the connection between the actuators and the hull of the vessel and/or between the actuating rods and the hull of the vessel can be arranged above the waterline of the vessel.

Thereby the connection between the actuators and the hull of the vessel and/or between the actuating rods and the hull of the vessel may be sealed using less complicated and less expensive seals. The first actuator and/or the second actuator may be sealed internally to prevent water from entering the vessel through the actuator.

In one or more example propulsion units, the first actuator and the second actuator and/or the first actuating rod and the second actuating rod may be arranged in the same hollow tube. In one or more example embodiments of the propulsion unit, such as e.g. when the main body is fixedly arranged on the keel of the vessel, the main body may comprise a first and a second hollow tube. The first actuator and the second actuator and/or the first actuating rod and the second actuating rod may be arranged in a respective hollow tube. The first actuator and/or the first actuating rod may be arranged within the first hollow tube and the second actuator and/or the second actuating rod may be arranged within the second hollow tube.

In one or more example propulsion units, the propulsion unit, such as the actuator assembly, comprises a third actuator, such that the propulsion unit comprises at least three actuators. The at least three actuators may be arranged between the hull of the vessel and the fin. The at least three actuators are configured to provide the heave and pitch motion of the fin in relation to the hull of the vessel. At least one of the three actuators may be configured to only perform a movement in a vertical direction of the main body. The other actuators may be configured to perform a movement in a vertical and in a longitudinal direction of the main body. The at least one actuator being configured to only perform a movement in the vertical direction may be restrained in its movement by a support. The support may prevent the at least one actuator and/or the at least one actuating rod being configured to only perform the vertical movement to move in any other direction than the vertical direction.

In one or more example propulsion units, at least two of the first actuator, the second actuator and the third actuator may be correlatively operable, such as operable simultaneously and relative to each other, to variably adjust the pitch angle and/or the heave of the fin. For example, in order to change the pitch of the fin, two of the three actuators may be operated simultaneously and relative to each other, such as being retracted or extended relative to each other, to change the vertical distance between the respective pivot points to which the two actuators are connected. One of the actuators may not be displaced, such that the fin can pivot around the pivot point connecting the actuator not being displaced to the fin. In order to perform a heave motion of the fin, all three of the actuators may be operated simultaneously

In order to control the heave and the pitch motion of the fin, the first actuator, the second actuator and the third actuator may be individually controlled. The first actuator, the second actuator and the third actuator may be connected to the main body of the propulsion unit (or to the hull of the vessel) and/or to the fin via respective pivot points. The pivot points of the fin may be arranged at respective distances from the leading edge of the fin, such that the first, the second and the third actuators are acting on the fin at respective distances from the leading edge of the fin. For example, the first actuator may be arranged closer to a leading edge of the fin than the second actuator and the third actuator. The second actuator may be arranged closer to the leading edge of the fin than the third actuator but farther from the leading edge than the first actuator. The third actuator may be arranged farther from the leading edge than both the first actuator and the second actuator.

The third actuator may be of the same type as the first and second actuators or of a different type than the first and the second actuators. The third actuator may in one or more example propulsion units be a hydraulic ram-type actuator. The third actuator may be an actuating rod. The third actuator may be connected to the fin at a distance from the first and the second actuators in a longitudinal direction, such as in a fore/aft direction, of the vessel. The first actuator, the second actuator and/or the third actuator may be connected to the fin and/or to the hull of the vessel via one or more fixed pivot points. Fixed pivot point herein means that the pivot point is fixedly, such as not slidably, arranged, to the fin, to the hull of the vessel and/or to the main body of the propulsion unit. The pivot points may in one or more example propulsion units be pinned joints that are fixed to the fin and/or to the main body of the propulsion unit and/or to the hull of the vessel. By fixedly arranging the first and the second pivot points in the fin, the stability and/or controllability of the fin may be increased, such that the pitch of the fin may be controlled with an increased precision. Furthermore, by not having a sliding joint, such as a slidably arranged pivot point, in the fin, the friction induced into the system may be reduced thereby increasing the efficiency of the propulsion unit. Reducing the movable parts of the propulsion unit being arranged under water, further reduces the risk of corrosion and potential seizing of the moving parts and a potential malfunction of the system. Thereby, the performance of the propulsion unit may be improved.

The displacements, such as the respective displacement, of the actuators, such as the first actuator, the second actuator and/or the third actuator, may for example be controlled such that one of the actuators only performs a vertical displacement with no motion component in a fore and/or aft direction, such as in the longitudinal direction, of the vessel. By restraining the movement of one of the three actuators, the motion of the fin can be controlled in a more precise manner. For example, the fin can be prevented from oscillating in a fore/aft direction of the vessel while performing the heave and/or pitch motion, which improves the efficiency of the propulsion unit since an oscillation of the fin in a fore/aft direction may change the pitch angle of the fin.

The displacement of the actuators may be controlled to adjust the heave and pitch based on conditions of the water surrounding the fin, such as based on an incoming water velocity, such as a velocity of the water meeting the leading edge of the fin. Thereby, the motion of the fin may be adapted to increase the performance of the propulsion unit based on the current conditions of the water surrounding the fin. The actuators may for example be controlled based on a plurality of angle of attack profiles depending on the conditions of the water. For example, a first example angle of attack profile may be optimized for open water. A second example angle of attack profile may be optimized for operation in a wake behind the vessel. The propulsion unit according to the current disclosure, such as the propulsion unit comprising two or more actuators, can continuously adapt the heave and/or the pitch of the fin and can thus adjust the motion of the fin to more or less an infinite number of angle of attack profiles. The plurality of angle of attack profiles may for example be determined using Computational Fluid Dynamics (CFD) codes with optimization routines in order to optimize a movement pattern and/or fin geometry based on a given water condition, such as a given inflow of water to the fin.

In one or more example propulsion units, the propulsion unit may comprise a control system configured to optimize and control the movement pattern, such as the displacement, of the actuators for different angle of attack profiles based on a detected angle of attack of the fin. The control system may, in one or more example propulsion units, control the movement pattern in real time, for example by using machine learning. The control system may receive information about the conditions of the fin, such as a water flow around the fin, information about the pitch and/or the heave of the fin, such as a pitch angle, a heave and/or a velocity of the motion of the fin, such as a velocity of the pitch and/or the heave. The control system may use the received information to optimize the pitch and/or the heave motion of the fin to improve the efficiency of the propulsion unit.

In one or more example propulsion units, at least one of the actuators may be offset transversely from one or more other actuators, in order to absorb twisting moments on the fin, that may for example be induced by waves. Transversely herein means along the transverse axis of the fin, such as along an axis extending from a starboard side of the vessel to a port side of the vessel. Thereby, the performance of the fin may be further increased. The at least one of the actuators may, in one or more example propulsion units, be offset from the one or more other actuators by a distance in the range of 50- 500mm, such as in a range of 100-200mm.

In one or more example propulsion units, such as when the propulsion unit comprises a third actuator, the fin may comprise a third connecting rod. The third actuator may be pivotably connected to the third connecting rod. The third connecting rod may be connected to the third actuator via a third actuating rod. The third actuating rod may be arranged inside the main body of the propulsion unit. The first fin section and the second fin section may be connected via the third connecting rod. The first connecting rod, the second connecting rod and the third connecting rod may be arranged in parallel and at a respective first distance, second distance and third distance from the leading edge of the first and second fin sections.

In one or more example propulsion units, the main body may comprise a third through- going slot for allowing the third connecting rod to protrude through the main body and to be slidably arranged within the third through-going slot.

The fin may have an elliptical planform. In some embodiments, such as in one or more example propulsion units, the fin may have a high aspect ratio elliptical planform.

The aspect ratio of the fin is the ratio of the its span to its mean chord. The span of the fin is the distance from one fin tip to the other fin tip. The chord is an imaginary straight line joining the leading edge and the trailing edge of the fin. The aspect ratio is equal to the square of the wingspan divided by the wing area. Thus, a long, narrow fin has a high aspect ratio, whereas a short, wide fin has a low aspect ratio. A lift-to-drag ratio of the fin increases with the aspect ratio, hence having a high aspect ratio fin improves the performance and efficiency of the fin, which may improve fuel economy of the vessel. The lift-to-drag ratio, which may also be referred to as L/D ratio, is the amount of lift generated by the fin, divided by the hydrodynamic drag it creates by moving through a viscous fluid, such as water. The chord shape and/or the planform shape of the fin may be altered, e.g. based on the shape of the hull of the vessel, to reduce drag of the fin.

The fin may comprise winglets. The winglets are endplates arranged on the tip of the fin, which may extend in a direction perpendicular, or at least substantially perpendicular, to a main plane of the fin. The winglets may improve the efficiency of the fin by reducing the hydrodynamic drag of the fin. The drag of the fin may be reduced by partial recovery of the tip vortex energy. The winglets smoothen the water flow across the fin near the tip which may increase the lift generated at the tip of the fin, which reduces a lift-induced drag caused by vortices around the tip of the fin. Reducing the lift-induced drag improves a lift-to-drag ratio of the fin. This increases efficiency of the fin. The winglets may also increase efficiency of the fin by reducing vortex interference with a laminar flow of water near the tips of the fin, by moving the confluence of low-pressure and high- pressure areas of water away from the surface of the fin. Vortices around the tip of the fin may create turbulence, originating at the leading edge of the fin tip and propagating backwards and inboard the fin. This turbulence may delaminate the flow of water over a small triangular section of the outboard fin, which may destroy the lift created by the fin in that area. The winglets move the area where the vortex forms away from the fin surface, since the center of the resulting vortex is now at the tip of the winglet. Since the fin performs an upwards/downwards motion, which may also be referred to an up/down stroke, during operation of the propulsion unit, winglets may be arranged on both an upper and a lower side of the fin. Since the lift is reversable on the fin between the up and down stroke, arranging winglets on both the upper and the lower side of the fin increases the efficiency of the fin on both the up and the down stroke.

The stroke of the fin, such as the displacement of the heave motion, may be dependent on the size of the vessel and/or the draught of the vessel. The larger the vessel and/or the deeper the draught of the vessel, the larger the stroke may be. By increasing the stroke of the fin, the efficiency of the fin and thereby the propulsion unit may be increased. In one or more example propulsion units, the stroke may be in a range of 3 to 20 meters, such as in a range of 5 to 15 meters.

Further, a vessel comprising the propulsion unit for propelling the vessel according to this disclosure is disclosed. The vessel comprises a keel. The main body of the propulsion unit may be arranged to the keel of the vessel. The fin is configured to perform a pitch and heave motion in relation to the keel of the vessel. In one or more exemplary embodiments, the vessel may comprise a plurality of propulsion units, such as when the vessel comprises several engines.

In one or more example embodiments, the main body may be fixedly arranged to the keel of the vessel. Thereby the main body is strongly attached to the hull and the hydrodynamic drag of the main body is reduced.

In one or more example embodiments, the main body may be rotatably arranged to the keel of the vessel. The main body and rudder parts may thereby be integrated into one unit and the whole unit made to turn like a rudder. This may improve the maneuverability of the vessel.

Fig. 1A and 1B illustrate a propulsion unit 1 for propelling the vessel 100 according to one or more first example embodiments herein. In the one or more example embodiments shown herein the propulsion unit 1 comprises a single actuator for generating the heave and pitch motion. The actuator acts on a lever arm attached to a pivot point at a first end and to the fin in the other end. The pitch motion of the fin is generated by the lever arm rotating around the pivot point when the actuator exerts a heaving motion on the lever arm.

Fig 1 A shows a side view of the one or more first exemplary embodiments of the propulsion unit 1. The propulsion unit 1 comprises a main body 2 configured to be arranged at a keel 101 of the vessel 100 and comprising a pivot point 5a, a fin 3 being movably arranged in relation to the main body 2, and an actuator assembly 4 for generating a heave motion of the fin 3 in relation to the main body 2, the actuator assembly 4 comprising an actuator 4a. The fin 3 is connected to the pivot point 5a via a lever arm 7 such that the fin 3 is arranged to pivot around the pivot point 5a when the actuator 4a generates the heave motion of the fin 3, thereby generating a pitch motion of the fin 3. In the one or more first example embodiments herein, a first end of the lever arm 7 is connected to the pivot point 5a and a second end of the lever arm 7 is connected to the fin 3. The pivot point 5a is fixedly arranged to the main body 2. The pivot point 5a may be a pinned rotation point. The pivot point 5a may be arranged inside the main body 2, such that lever arm 7 may be connected to the pivot point on the inside of the main body 2. The main body 2 may thus comprise a slot 2a open towards an aft end of the main body 2 for allowing the lever arm 7 to protrude through the main body 2 and to perform the heave and pitch motion in relation to the main body 2. Since the slot 2a is open the slot 2a is configured to be in contact with water surrounding the vessel 100 during operation.

The actuator assembly 4 comprises a single actuator 4a, for generating the heave and pitch motion of the fin 3. The propulsion unit 1 and/or the actuator assembly 4 may comprise an actuating rod 9. The actuator 4a may be connected to the lever arm 3 via the actuating rod 9. The actuating rod 9 may be connected to the lever arm 7 at a distance from the pivot point 5a. The actuator 4a thus exerts a force on the lever arm 7 via the actuating rod 9 at a distance from the pivot point 5a, which force causes a torque on the lever arm 7 around the pivot point 5a.

The actuator 4a, which may also be referred to as a first actuator 4a, may be a linear actuator performing a linear motion. The actuator 4a may be arranged to perform a vertical (up/down) motion. Since the lever arm 7 is connected to the pivot point 5a, the linear vertical motion of the actuator 4a causes the lever arm 7 to rotate around the pivot point 5a. The rotation of the lever arm 7 around the pivot point 5a causes the fin 3, which is attached to an opposite end of the lever arm 7 than the pivot point 5a, to rotate around the pivot point 5a and thus to perform a heave and pitch motion in relation to the hull of the vessel 100. Since the pitch of the fin 3 is generated by the pivoting motion of the lever arm 7 during the heaving motion of the fin 3, the pitch of the fin 3 is directly dependent on the heave of the fin 3. The pitch and the heave of the fin 3 are thus always in phase with each other. The actuating rod 9 may be connected to the lever arm 7 via a second pivot point 5b thus allowing the actuating rod 9 and the lever arm 7 to rotate in relation to each other. This allows an angle between the linear actuator 4a and the lever arm 7 to vary during the heave and pitch motion of the lever arm 7 and the fin 3. When the actuating rod 9 is connected to the lever arm 7 via a second pivot point 5b, the pivot point 5a may be referred to as a first pivot point 5a.

The actuator 4a may be configured to be operated with an oscillating pattern, thereby generating an oscillating heave and pitch motion of the fin 3. The oscillating heave and pitch motion may comprise an upward stroke and a downward stroke. The upward stroke refers to a motion where the actuator 4a pulls the fin 3 towards the vessel 10. The downward stroke refers to a motion where the actuator 4a pushes the fin 3 away from the vessel 10. In Fig. 1 A the lever arm 7 and the fin 3 are shown in an upper position, such as at an end position of the upward stroke.

When the fin 3 performs the pitch motion the distance between the first pivot point 5a and the second pivot point 5b, seen from a horizontal plane, such as a plane perpendicular to the longitudinal extension of the actuating rod 9, varies. In order to allow for a change in the distance between the first pivot point 5a and the second pivot point 5b, seen from the horizontal plane, the second pivot point 5b may be connected to the lever arm 7 via a sliding joint 7a. The sliding joint 7a may e.g. be an elongated slot in which the second pivot point 5b may be arranged to slidably move. The second pivot point 5b may thus be movably arranged in relation to the fin 3. The fin 3 comprise winglets 15. The winglets 15 are endplates arranged on the tip of the fin 3, and extending in a direction perpendicular, or at least substantially perpendicular, to a main plane of the fin 3. The winglets 15 improve the efficiency of the fin 3 by reducing the hydrodynamic drag of the fin 3. Winglets 15 may be provided on a top and/or a bottom side of the fin 3. In the one or more embodiments shown herein, winglets 15 are provided on both the top and the bottom side of the fin 3. Thereby, the efficiency of the fin 3 is improved on both the upward stroke and the downward stroke of the fin 3.

The propulsion unit 1 may further comprise a rudder 6. The main body 2 may be rotatably arranged to the vessel 100, such that the main body 2 may act as a rudder 6. The fin 3 is thus arranged to rotate together with the rudder 6. The main body 2 may be arranged to the vessel via a rudder stock. The rudder stock may be hollow to accommodate the actuator 4a and/or the actuating rod 9. The actuator 4a and/or the actuating rod 9 may thus connect to the lever arm 7 through the hollow rudder stock. The rudder stock may be rotatably arranged to the vessel 100 via a bearing, such as a rotating bearing. The rudder stock may constitute a bearing race of the bearing. In order to prevent water from entering the vessel through the hollow rudder stock, the rudder stock may have to be sealed. The rudder stock constituting one of the bearing races may be extended above the waterline of the vessel 100, such that the rudder stock may be sealed above the waterline. Sealing the rudder stock above the waterline allows simpler and less expensive seals to be used compared to when the rudder stock is sealed below the waterline. The propulsion unit 1 may however also be fixedly arranged to the vessel 100. When the propulsion unit 1 is fixed to the vessel 100, the vessel 100 may comprise a traditional rudder 6 arranged behind the fin 3.

Fig 1B shows a perspective view of the propulsion unit 1 for propelling the vessel 100, according to one or more example embodiments shown in Fig. 1A. As can be seen the main body 2 is arranged on the keel 101 of the vessel 100. The lever arm 7 is pivotably connected to the main body 2 via the pivot point 5a (not shown in Fig. 1B) arranged inside the main body 2. A second end of the lever arm 7 is connected to the fin 3. The main body 2 comprises the slot 2a open towards an aft end of the main body 2 for allowing the lever arm 7 to perform a vertical motion in relation to the main body 2. The lever arm 7 is connected to the actuator 4a (not shown in Fig. 1 B) via the actuating rod 9. The actuating rod 9 may perform a linear vertical motion, such as an up/down motion, which motion is transferred to the fin 3 via the lever arm 7. In Fig. 1B the lever arm 7 and the fin 3 are shown in an upper position, such as in an end position of an upward stroke. The fin 3 has an elliptical planform, such as a high aspect ratio elliptical planform.

The aspect ratio of the fin is the ratio of the its span s to its mean chord c. Thus, a fin having a high aspect ratio planform means that the fin 3 is long and narrow. The lift-to- drag ratio of the fin increases with the aspect ratio, hence high aspect ratio fin 3 disclosed herein improves the performance and efficiency of the fin 3, which may improve fuel economy of the vessel. The lift-to-drag ratio, which may also be referred to as L/D ratio, is the amount of lift generated by the fin, divided by the hydrodynamic drag it creates by moving through a viscous fluid, such as water. The shape of the chord c and/or the planform shape of the fin 3 may be up to implementation and may be altered, e.g. based on the shape of the hull of the vessel 100, to reduce the hydrodynamic drag of the fin 3.

As can be seen in Fig. 1 B, according to the one or more first embodiments of the propulsion unit 1 shown herein, winglets 15 are provided on both the top and the bottom side of the fin 3 to improve the efficiency of the fin 3 on both the upward stroke and the downward stroke.

The propulsion unit 1 according to the one or more first example embodiments herein has a simple layout comprising only one actuator for performing the heave and the pitch motion and thus has a high mechanical efficiency and low maintenance requirements.

Fig. 2A to 2D illustrate the propulsion unit 1 and a motion pattern of the propulsion unit 1 according to one or more second example embodiments herein. The actuator assembly 4 comprises the actuator 4a, which may also be referred to as a first actuator 4a, and a second actuator 4b. The pivot point 5a (not shown in Fig 2A to 2D) is connected to the second actuator 4b, such that the pivot point 5a is movably arranged in relation to the main body 2. The first actuator 4a and the second actuator 4b may be independently operable in relation to each other, so that a phase difference between the heave motion and the pitch motion of the fin 3 is variable. The first actuator 4a and the second actuator 4b may be linear actuators. The pitch motion and the heave motion of the fin 3 may thus be controlled independently of each other. The fin 3 is movably arranged to the main body 2. The fin 3 comprises a first fin section 3a and a second fin section 3b, a first connecting rod 8a (not shown in Fig. 2A to 2D) and a second connecting rod 8b (not shown in Fig. 2A to 2D). The first fin section 3a and the second fin section 3b are connected via the first connecting rod 8a and a second connecting rod 8b. The first connecting rod 8a and a second connecting rod 8b may form a structural element of the fin 3. The main body 2 comprises a first through-going slot 13a and a second through-going slot 13b for receiving the first connecting rod 8a and the second connecting rod 8b of the fin 3. The first through-going slot 13a and a second through-going slot 13b allow the first connecting rod 8a and the second connecting rod 8b to protrude through the main body 2 in a lateral direction. The first connecting rod 8a and the second connecting rod 8b are slidably arranged within the first through-going slot 13a and the second through-going slot 13b, respectively. The first through-going slot 13a and the second through-going slot 13b have a longitudinal extension in a lateral direction of the main body 2, thereby allowing the first connecting rod 8a and the second connecting rod 8b of the fin 3 to perform a vertical movement in relation to the main body 2, thereby allowing the fin 3 to perform a heave and pitch motion in relation to the main body 2. The first actuator 4a and the second actuator 4b may be connected to the first connecting rod 8a and the second connecting rod 8b, respectively. In order to accommodate the actuators 4a and 4b, the first through- going slot 13a and the second through-going slot 13b may be open at a top end, such as an end of the first through-going slot 13a and the second through-going slot 13b facing the vessel 100, when the propulsion unit 1 is arranged on the vessel 100. The actuators 4a and 4b may thus extend through the main body 2 in a vertical direction of the main body 2 and may connect to the first connecting rod 8a and the second connecting rod 8b protruding through the main body 2 in a lateral direction of the main body 2.

The propulsion unit 1 disclosed herein, may further comprise a rudder 6. The main body 2 may be configured to be fixedly arranged to the vessel 100, such as to the keel 101 of the vessel 100, and the rudder 6 may be pivotably arranged to the skeg 2. The rudder 6 may be mounted to a center line of the main body 2. The rudder 6 may be attached to the main body 2 via a rudder stock arranged at a trailing edge of the main body 2.

Fig. 2A shows the fin 3, such as the fin sections 3a and 3b, of the propulsion unit 1 in a top position during an exemplary motion pattern for moving the fin 3. In this exemplary motion pattern, the first actuator 4a and the second actuator 4b are operated with a phase difference. The top position of the fin 3 corresponds to the top of the travel of the fin 3, such as when the fin 3 reaches and end position during an upwards stroke. In this position the first and the second actuators are both in an upper position and the fin sections 3a and 3b are horizontally arranged, such as having a pitch angle of 0°.

Fig. 2B shows the fin 3 of the propulsion unit 1 during a downward stroke of the fin 3. Both the first actuator 4a and the second actuator 4b are moving downward, thereby applying a downward heave motion to the fin 3. However, the second actuator 4b has started the downward stroke before the first actuator 4a started the downward stroke. This causes the leading edge 10 of the fin 3, such as the fin sections 3a and 3b, to move downward before the trailing edge 16 moves downward, which causes the fin 3 to have a pitch angle to a horizontal having a first sign. The pitch angle may have a positive or a negative sign depending on the definition of the coordinate system. In the example shown in Fig. 2B and in Fig. 5 herein, this pitch angle, such as the pitch angle having the first sign, corresponds to a positive pitch angle.

Fig. 2C shows the fin 3 of the propulsion unit in a bottom position during the exemplary motion pattern for moving the fin 3. The bottom position of the fin 3 corresponds to the bottom of the travel of the fin 3, such as when the fin 3 reaches and end position during the downwards stroke. In this position the first and the second actuators are both in a lower position and the fin sections 3a and 3b are horizontally arranged, such as having a pitch angle of 0°.

Fig. 2D shows the fin 3 of the propulsion unit 1 during an upward stroke of the fin the exemplary motion pattern for moving the fin 3. Both the first actuator 4a and the second actuator 4b are moving upward, thereby applying an upward heave motion to the fin 3. Since the second actuator 4b started its downward stroke before the first actuator 4a started the downward stroke it, will also start the upward stroke before the first actuator 4a. This causes the leading edge 10 of the fin 3, such as the fin sections 3a and 3b, to move upward before the trailing edge 16 moves upward, which causes which causes the fin 3 to have a pitch angle to a horizontal having a second sign. In the example shown in Fig. 2D and in Fig. 5 herein, this pitch angle corresponds to a negative pitch angle.

Fig. 3 illustrates a connection for connecting the fin 3 to the first actuator 4a and the second actuator 4b according to the one or more second example embodiments herein. The first connecting rod 8a and the second connecting rod 8b of the fin 3 are arranged in parallel and at a respective first distance and second distance from the leading edge 10 of the first fin section 3a and the second fin section 3b of the fin 3.

The first connecting rod 8a is connected to the first actuator 4a and the second connecting rod 8b is connected to the second actuator 4b. The first connecting rod 8a and the second connecting rod 8b may be connected to the respective first actuator 4a and second actuator 4b via a first actuating rod 9a and a second actuating rod 9b, respectively. The first actuator 4a and the second actuator 4b may be connected to the first connecting rod 8a and the second connecting rod 8b via a first actuating rod 9a and a second actuating rod 9b, respectively, e.g. via a freely rotating pin connection. The first actuating rod 9a and a second actuating rod 9b may e.g. comprise a through-going hole at their respective lower end, through which the first connecting rod 8a and the second connecting rod 8b may be inserted respectively. A translational movement from the first actuator 4a and the second actuator 4b may thus be transferred to the first connecting rod 8a and the second connecting rod 8b via the first actuating rod 9a and the second actuating rod 9b, respectively.

The rotating pin connection however allows the allows the first connecting rod 8a and the second connecting rod 8b to rotate freely within the first actuating rod 9a and the second actuating rod 9b, respectively. The rotating pin connection between the actuating rods and the connecting rods thereby constitute a respective pivot point. In the example shown in Fig. 3, the connection between the second actuating rod 9b and the second connecting rod 8b constitutes the first pivot point 5a and the connection between the first actuating rod 9a and the first connecting rod 8a constitutes the second pivot point 5b. The fin 3 may thus rotate in relation to the actuating rods 9a, 9b, in order to change the pitch of the fin 3.

In order to compensate for a change in distance between the first and the second connecting rods 8a, 8b, seen from a horizontal plane, when the fin 3 performs a pitch motion, one of the first connecting rod 8a and the second connecting rod 8b may be movably arranged within the first fin section 3a and the second fin section 3b. In one or more example embodiments, the second connecting rod 8b is movably arranged within the fin sections and may comprise a first and a second elongated guide section 17 arranged at opposite ends of the connecting rod 8b. The first and second elongated guide sections 17 may have a longitudinal extension in a direction perpendicular to a longitudinal axis of the at least one of the first and second connecting rods. The first and second elongated guide sections 17 may be configured to be arranged inside the first fin section 3a and the second fin section 3b, respectively. The first and second elongated guide sections 17 may be configured to guide a first pin 18 fixedly arranged inside the first fin section 3a and a second pin 18 fixedly arranged inside the second fin section 3b, respectively. The first and second elongated guide sections 17 may be elongated slots, elongated bearings or tracks being configured to guide the first pin 18 and the second pin 18, respectively. The pin 18 may thus be slidably arranged within the elongated slot 17 of the second connecting rod 8b. In one or more example embodiments, the first and second pins 18 may be instead be arranged on opposite ends of the one of the first connecting rod 8a and the second connecting rod 8b and the elongated guide section may be arranged on the inside of the first fin section 3a and the second fin section 3b. The second connecting rod 8b may for example comprise the first and the second pin 18 arranged at opposite ends of the connecting rod 8b. The first and second elongated guide sections 17 arranged inside the first fin section 3a and second fin section 3b may have a longitudinal extension in a direction perpendicular to the span s of the first fin section 3a and the second fin section 3b. The first pin 18 and the second pin 18 may be arranged to slidably engage the first and second elongated guide sections 17 fixedly arranged inside the first fin section 3a and the second fin section 3b, respectively. The first fin section 3a and the second fin section 3b may also comprise an elongated slot 19 for receiving the second connecting rod 8b, thereby allowing the second connecting rod 8b to slide back and forth inside the elongated slot 19 in a direction perpendicular to the longitudinal axis of the second connecting rod 8b, such that the distance between the second connecting rod 8b and the leading edge 10 can be varied. This permits the actuating rod 9b and/or the actuator 4b to perform a purely vertical motion, during a pitch motion of the fin 3.

Fig. 4 shows the main body 2 comprising the first through-going slot 13a and the second through-going slot 13b for receiving the first connecting rod 8a and the second connecting rod 8b of the fin 3. The first connecting rod 8a and the second connecting rod 8b protrude through the main body 2 in a lateral direction. The first connecting rod 8a and the second connecting rod 8b are slidably arranged within the first through-going slot 13a and the second through-going slot 13b, respectively. The first connecting rod 8a and the second connecting rod 8b may also protrude through the through-going holes at the lower end of the first actuating rod 9a and the second actuating rod 9b, respectively The first through- going slot 13a and the second through-going slot 13b have a longitudinal extension in a lateral direction of the main body 2, thereby allowing the first connecting rod 8a and the second connecting rod 8b to perform a vertical up/down movement within the through- going slots 13a and 13b respectively. The first through-going slot 13a and the second through-going slot 13b are open at a top end, such as at an end facing the vessel 100, when the propulsion unit 1 is arranged on the vessel 100, for allowing the first actuating rod 9a and the second actuating rod 9b to enter the through-going slots 13a, 13b from a vertical direction. The open ends of the first through-going slot 13a and the second through-going slot 13b may be joined to holes in the hull of the vessel above them to allow the actuating rods to travel vertically into the hull of the vessel 100 were the actuator assembly may be arranged.

Fig. 5 shows a graph illustrating an exemplary motion pattern of the fin 3 when the first actuator 4a and the second actuator 4b are operated with a phase shift. The curve h2 shows the extension of the first actuating rod 9a from a position mid-stroke, such as a position in the middle of the top of travel and the bottom of travel of the first actuating rod. The curve hi shows to the extension of the second actuating rod 9b from a position mid stroke. The curve theta shows the corresponding pitch angle of the fin in degrees to a horizontal. In order to create pitch and heave motion, which is out of phase, the actuators are operated with a phase difference. Varying this phase difference allows an adjustment of the maximum angle of pitch of the fin 3 during the heave motion. As can be seen in the graph, the second actuating rod 9b performs its motion slightly ahead of the second actuating rod 9a. As can be seen, the pitch angle of the fin is highest when the first and the second actuating rods 9a and 9b are mid-stroke and moving in the same direction, such as at t=0 during a downwards stroke and at t=3 during an upwards stroke. When the first actuating rod and the second actuating rod are in the end positions, such as at the top of travel or bottom of travel, such as at t_1 ,5 and t=4,5.

Fig. 6 discloses the propulsion unit 1 according to one or more third exemplary embodiments, in which the main body 2 is configured to be rotatably arranged to the vessel 100. The propulsion unit 1 disclosed herein is similar to the propulsion unit 1 as disclosed in relation to Fig. 2a to 2D, Fig. 3 and Fig. 4. The main body 2 comprises a rudder stock 11 for rotatably arranging the main body 2 to the keel 101 of the vessel 100. The rudder stock is arranged on a top side of the main body 2, such as on a side facing a bottom of the vessel 100 when the main body 2 is mounted to the vessel 100. The main body 2 may thus be configured to rotate around the rudder stock 11 and act as the rudder 6. The rudder stock 11 may be hollow to accommodate the one or more actuators 4a, 4b and/or the first actuating rod 9a and/or the second actuating rod 9b. The one or more actuators 4a, 4b and/or the first actuating rod 9a and/or the second actuating rod 9b may be arranged inside the rudder stock 11 , such that they can move vertically inside the rudder stock 11. The rudder stock 11 may comprise a bearing, such as a rotating bearing, for rotatably arranging the rudder stock to the vessel 100, such as to the keel 101 of the vessel 100. In some embodiments, the rudder stock 11 may constitute a race of the bearing, such as an inner race of the bearing. Fig. 7 discloses a vessel 100 comprising a propulsion unit 1 for propelling the vessel 100 according to the one or more second exemplary embodiments disclosed herein. The vessel 100 comprises a keel 101. The main body 2 of the propulsion unit 1 may be arranged to the bottom of the vessel 100, such as to the keel 101 of the vessel 100. The fin 3a, 3b is configured to perform a pitch and heave motion in relation to the bottom of the vessel 100, such as to the keel 101 of the vessel 100. When the fin 3, 3a, 3b performs the heave and pitch motion a thrust force is generated for propelling the vessel 100. In the exemplary embodiment shown in Fig. 7, the propulsion unit 1 comprises two actuators for generating the heave and pitch motion of the fin 3, 3a, 3b. The motion pattern of the fin 3 may thus be precisely controlled to match the hull and the load of the fin 3 in a boundary layer of the hull. The main body 2 herein, is fixedly arranged to the vessel 100, such as to the keel 101 of the vessel 100, and may thus constitute a skeg of the vessel 100. The propulsion unit further comprises a rudder attached to the trailing edge of the main body 2. However, the vessel 100 may in one or more embodiment comprise a propulsion unit according to the one or more first and third exemplary embodiments disclosed herein.

Fig. 8 illustrates a perspective view of an inside of an exemplary vessel 100 comprising an exemplary propulsion unit 1 according to this disclosure. The main body 2 is fixedly mounted to the vessel and the first actuator 4a and the second actuator 4b are arranged on the inside of the vessel 100. The propulsion unit 1 shown in Fig. 8 comprises a sealing arrangement according to one or more example embodiments herein, for preventing water from entering into the vessel. In order for the first actuator 4a and the second actuator 4b to generate the heave and pitch motion of the fin 3, 3a, the hull and the main body comprises openings for receiving the first and the second actuators 4a, 4b, and/or the first actuating rod and the second actuating rods 9a, 9b. The openings are open to the water surrounding the vessel 100. In order to prevent water from entering the vessel through the openings, the main body may comprise one or more hollow tubes 12; 12a, 12b protruding from the main body and into the vessel 100. The first actuating rod 9a and/or the second actuating rod 9b and or the first actuator 4a and/or the second actuator 4b may be arranged within the one or more hollow tubes 12; 12a, 12b. The one or more hollow tubes 12; 12a, 12b may be steel tube(s) welded to the hull of the vessel 100. The one or more hollow tube(s) 12; 12a, 12b may comprise a first end, such as a distal end, arranged to protrude into the hull of the vessel 100 and a proximal end arranged in the main body 2 of the vessel 100. The first end, such as the distal end, of the one or more hollow tube(s) 12; 12a, 12b may be configured to extend above a waterline of the vessel 100. In order to prevent water to enter into the hull of the vessel 100 through the openings, the one or more hollow tube(s) 12; 12a, 12b may comprise a seal 20 arranged at the distal end of the one or more hollow tube(s) 12; 12a, 12b, respectively. The one or more hollow tube(s) 12; 12a, 12b may comprise a flanged connection for receiving the seal 20. By providing the one or more hollow tube(s) 12; 12a, 12b on the main body 2, that protrude into the hull of the vessel 100, and arranging the seal 20 at a distal end of the one or more hollow tube(s) 12; 12a, 12b above the waterline of the vessel 100, less complicated and less expensive seals may be used.

Fig. 9 illustrates an example propulsion unit 1 according to the current disclosure. The example propulsion unit 1 comprises two actuators 4, such as the first actuator 4a and the second actuator 4b. One of the two actuators 4, such as the second actuator 4b, is restrained in its movement, such that it can only be displaced, such as being extended or retracted, in a vertical direction, such as along a vertical axis, of the vessel. The example propulsion unit 1 comprises one or more support surfaces 21 for restraining the movement of the second actuator 4b. The one or more support surfaces may be roller supports and/or sliding bearings. The one or more support surfaces 21 may be fixedly arranged to the hull of the vessel or to the main body of the propulsion unit 1. The one or more support surfaces 21 may be configured to prevent the second actuator 4b from pivoting in relation to the hull of the vessel. By restraining the movement of the second actuator 4b, the pivot point 5b connecting the second actuator 4b to the fin 3 can only be displaced in the vertical direction and can thus only perform a heave motion. The fin 3 can however still pivot around the pivot point 5b to allow a change of the pitch of the fin 3. By restraining the movement of one of the actuators, such as the second actuator 4b, the movement of the motion of the fin 3 can be precisely controlled. For example, the fin 3 can be prevented from oscillating in a fore/aft direction of the vessel while performing the heave and/or pitch motion. The first actuator 4a may be unrestrained, such that it can pivot in relation to the hull of the vessel when the first actuator is being extended and/or retracted. In one or more example propulsion units 1, the two actuators 4a; 4b may be arranged at an angle to each other, such that a direction of extension of the two actuators 4a; 4b are not parallel. For example, the first actuator 4a may be arranged at an angle to the vertical axis of the vessel. The first pivot point 5a and the second pivot point 5b may be fixedly arranged, such as not slidably arranged, in the fin 3. In other words, the fin 3 can pivot around the first pivot point 5a and the second pivot point 5b. The first pivot point 5a and/or the second pivot point 5b may however not be slidably arranged in relation to the fin 3. The first pivot point 5a and the second pivot point 5b may in one or more example propulsion units be pinned joints. By fixedly arranging the first and the second pivot points in the fin 3, the stability and/or controllability of the fin 3 may be increased, such that the pitch of the fin 3 may be controlled with an increased precision.

Furthermore, by fixedly arranging the first and the second pivot points 5a, 5b to the fin 3, the friction induced in the propulsion system may be reduced, since there is no sliding movement in the one or more connections between the actuators 4a, 4b and the fin 3. Reducing the movable parts arranged underwater of the propulsion unit 1, further reduces the risk of corrosion and potential seizing of the moving parts and a potential malfunction of the system. Thereby, the performance of the propulsion unit may be improved.

Fig. 10 illustrates an example propulsion unit 1 according to the current disclosure. The example propulsion unit 1 illustrated in Fig. 10 comprises three actuators 4, such as the first actuator 4a, the second actuator 4b and a third actuator 4c. The first actuator 4a, the second actuator 4b and the third actuator 4c may be connected to the fin via a respective pivot point, such as the first pivot point 5a, the second pivot point 5b and a third pivot point 5c. In the example propulsion unit 1 disclosed in Fig. 10, one of the three actuators 4, such as the second actuator 4b, is configured to be displaced, such as being extended or retracted, in the vertical direction, such as along the vertical axis, of the vessel. The one of the three actuators 4, such as the second actuator 4b, may be restricted in the lateral and/or the longitudinal direction, such that the one of the three actuators cannot be displaced, such as being extended or retracted, in the lateral and/or the longitudinal direction, such as along the lateral and/or the longitudinal axis, of the vessel. Being configured to be displaced only in the vertical direction herein means that the second actuator does not move in a fore and/or aft direction of the vessel when the actuator is extended and/or retracted. The restricted displacement of the second actuator 4b in the vertical direction only may be achieved by controlling an extension and/or retraction of each of the actuators 4a, 4b, 4c. By providing the propulsion unit 1 with a third actuator, the motion of the fin can be precisely controlled without using support surfaces for preventing the movement of the fin in a fore and/or aft direction. By not using a support surface for controlling the motion of the fin, the friction between the support surface and the at least one actuator may be reduced, thereby reducing the losses in the propulsion unit. Furthermore, by not having a support surface arranged under water reduces the number of moving elements of the propulsion unit being arranged underwater. Reducing the movable parts of the propulsion unit being arranged underwater, further reduces the risk of corrosion and potential seizing of the moving parts and a potential malfunction of the system. Thereby, the performance of the propulsion unit may be improved. Due to the restraining of the movement of the second actuator 4b, the pivot point 5b connecting the second actuator 4b to the fin 3 can only be displaced in the vertical direction and can thus only perform a heave motion. The fin 3 is configured to pivot around the pivot point 5b to allow a change of the pitch of the fin 3. By restraining the movement of one of the three actuators, such as by restricting the movement of the second actuator 4b, the motion of the fin 3 can be precisely controlled. For example, the fin 3 can be prevented from oscillating in a fore/aft direction of the vessel while performing the heave and/or pitch motion. The first actuator 4a and/or the third actuator 4c may be unrestrained, such that they can pivot in relation to the hull of the vessel when the first actuator 4a and/or the third actuator 4c is being extended and/or retracted. In one or more example propulsion units 1, the three actuators 4a, 4b, 4c may be arranged at an angle to each other, such that a direction of extension of the three actuators 4a, 4b, 4c are not parallel to each other. The actuators 4a, 4b, 4c, such as the ram-type actuators, may be arranged to the hull of the vessel so that the actuators are arranged so that forces acting on the actuators 4a, 4b, 4c do not fight, such as work against, each other. One or more of the actuators 4a, 4b, 4c may be arranged such that the one or more of the actuators 4a, 4b, 4c are able to absorb forces in a longitudinal direction of the vessel and in a vertical direction of the vessel. The forces acting in a longitudinal direction may e.g. be longitudinal thrust forces. For example, the first actuator 4a may be arranged at a first angle to the vertical axis of the vessel, the second actuator 4b may be arranged at a second angle, such as parallel, to the vertical axis of the vessel and the third actuator may be arranged at a third angle to the vertical axis of the vessel. The first pivot point 5a, the second pivot point 5b and the third pivot point 5c may be fixedly arranged, such as not slidably arranged, to the fin 3. In other words, the fin 3 can pivot around the first pivot point 5a, the second pivot point 5b and the third pivot point 5c. By arranging two of the actuators, such as the first actuator 4a and the third actuator 4c at an angle different than zero to the vertical axis, the first actuator 4a and the third actuator 4c can compensate for a change in distance between the first pivot point 5a, the second pivot point 5b and the third pivot point, as seen from a horizontal plane, such as a plane perpendicular to vertical axis of the vessel, when the pitch angle of the fin 3 varies. The first pivot point 5a, the second pivot point 5b and the third pivot point 5c may in one or more example propulsion units be pinned joints. By fixedly arranging the first pivot point 5a, the second pivot point 5b and the third pivot point 5c in the fin 3, the stability and/or controllability of the fin 3 may be increased, such that the pitch of the fin 3 may be controlled with an increased precision. In order to provide the pitch and/or heave motion of the fin 3, two or more of the actuators 5a, 5b, 5c may be operated in a correlated manner, such that the displacement of the actuators 5a, 5b, 5c are controlled in a correlated manner. For example, in order to change the pitch of the fin 3, the first actuator 5a may be extended while the third actuator 5c is retracted. This causes the fin 3 to pivot around the second pivot point 5b, such that the leading edge 10 of the fin 3 is lowered and the trailing edge 16 of the fin 3 is raised. In order to raise the leading edge 10 and lower the trailing edge 16 of the fin 3, the first actuator 5a may be retracted while the third actuator 5c is extended. In order to change the heave of the fin 3, all of the actuators may be operated in a correlated manner. By retracting the first actuator 4a, the second actuator 4b and the third actuator 4c simultaneously, the fin 3 may be lifted, such that the distance between the fin 3 and the hull of the vessel decreases. By extending the first actuator 4a, the second actuator 4b and the third actuator 4c simultaneously the fin 3 may be lowered, such that the distance between the fin 3 and the hull of the vessel increases. By independently controlling the displacement, such as a rate of extension and/or a rate of retraction, of the first actuator 4a, the second actuator 4b and the third actuator 4c, a combined pitch and heave motion of the fin 3 may be generated. This allows the heave and/or the pitch of the fin 3 of the propulsion unit 1 to be continuously adjusted to an infinite number of angle of attack profiles. Thereby performance of the propulsion unit 1 may be increased, as the angle of attack of the fin 3 towards the incoming water is a crucial factor for performance of the propulsion unit 1. The heave and/or pitch of the fin 3 may e.g. be controlled to increase the efficiency of the fin 3 based on conditions of the water surrounding the fin, such as based on an incoming water velocity, such as a velocity of the water meeting the leading edge 10 of the fin 3. The displacements of the actuators 5a, 5b, 5c may for example be controlled such that all forces acting on the actuators 5a, 5b, 5c can be absorbed as either tension or compression in one or more of the actuators 5a, 5b, 5c. The displacements of the actuators 5a, 5b, 5c may for example be controlled such that one of the actuators, such as the second actuator 5b only performs a vertical displacement with no motion component in a fore and/or aft direction, such as in the longitudinal direction, of the vessel.

Fig. 11 shows a diagram illustrating two different angle of attack profiles for controlling the motion, such as the heave and/or the pitch, of the fin 3 based on the conditions of the water surrounding the fin 3, such as an inflow of water or a velocity of the incoming water. The diagram shows the pitch angle of the fin 3 during one reciprocating motion, such as during one stroke of the fin. The dotted line of Fig. 11 illustrates an angle of attack profile optimized for a fin operating in open water, such as when the propulsion unit is simulated without a vessel. The solid line illustrates an angle of attack profile optimized for operation in a wake behind the vessel. Instead of the water flow moving along the longitudinal axis of the ship, the hull of the vessel may cause a vector of the water flow to angle upward along the rise of a stern of the vessel. This generates a flow field having a vertical component and an aftwards component. The boundary layer of the hull may also cause the flow of water to slow down, such that the flow magnitude of the water is less than a velocity of water flowing freely without being affected by the hull of the vessel. The angle of attack profile illustrated by the dotted line takes the influence of the vessel on the water flowing to the fin 3 into account when determining the optimal angle of attack profile for the fin. By adapting, such as optimizing, the angle of attack profile to the conditions of the water surrounding the vessel the efficiency of the propulsion unit may be significantly improved.

It shall be noted that the features mentioned in the embodiments described in Figs. 1-11 are not restricted to these specific embodiments. Any features relating to the fin, the one or more actuators, and/or the seals of the fin comprised therein and mentioned in relation to the one or more first example embodiments of Figs. 1 a-1 b, such as dimensions of the fin and the type of actuators or sealing solutions, are thus also applicable to the one or more second example embodiments described in relation to figs. 2-5, and/or to the example embodiments described in relation to Fig. 9-11 , and vice versa.

It shall further be noted that a vertical axis, when referred to herein, relates to an imaginary line running vertically through the ship and through its center of gravity, a transverse axis or lateral axis is an imaginary line running horizontally across the ship and through the center of gravity and a longitudinal axis is an imaginary line running horizontally through the length of the ship through its center of gravity and parallel to a waterline. Similarly, when referred to herein, a vertical plane relates to an imaginary plane running vertically through the width of the ship, a transverse plane or lateral plane is an imaginary plane running horizontally across the ship and a longitudinal plane is an imaginary plane running vertically through the length of the ship.

Embodiments of products (propulsion unit and vessel) according to the disclosure are set out in the following items:

Item 1. A propulsion unit (1 ) for propelling a vessel, the propulsion unit (1 ) comprising: a main body (2) configured to be arranged at a keel of the vessel and comprising a pivot point (5a), a fin (3) being movably arranged in relation to the main body (2), and an actuator assembly (4) for generating a heave motion of the fin (3) in relation to the main body (2), the actuator assembly (4) comprising at least one actuator (4a, 4b), wherein the fin (3) is connected to the pivot point (5a) such that the fin (3) is arranged to pivot around the first pivot point (5a) when the at least one actuator (4a, 4b) generates the heave motion of the fin (3), thereby generating a pitch motion of the fin (3).

Item 2. The propulsion unit (1 ) according to Item 1 , wherein the at least one actuator (4a, 4b) is a linear actuator.

Item 3. The propulsion unit (1 ) according to any one of the previous Items, wherein the propulsion unit (1) comprises at least one actuating rod (9, 9a, 9b), the actuator assembly (4) being connected to the fin (3) via the at least one actuating rod (9, 9a, 9b).

Item 4. The propulsion unit (1 ) according to any one of the previous Items, wherein the actuator assembly (4) is configured to be operated with an oscillating pattern, thereby generating an oscillating heave and pitch motion of the fin.

Item 5. The propulsion unit (1 ) according to any one of the previous Items, wherein the pivot point (5a) is fixedly arranged to the main body (2).

Item 6. The propulsion unit (1) according to Item 5, wherein the propulsion unit (1) comprises a lever arm (7), the fin (3) being attached to the pivot point (5a) via the lever arm (7).

Item 7. The propulsion unit (1 ) according to any one of the Items 1 to 4, wherein the actuator assembly (4) comprises a first actuator (4a) and a second actuator (4b), wherein the pivot point (5a) is connected to the second actuator (4b) such that the pivot point (5a) is movably arranged in relation to the main body (2). Item 8. The propulsion unit (1 ) according to Item 7, wherein the first actuator (4a) and the second actuator (4b) are independently operable in relation to each other, so that a phase difference between the heave motion and the pitch motion of the fin (3) is variable.

Item 9. The propulsion unit (1 ) according to Item 7 or 8, wherein the fin (3) comprises a first fin section (3a), a second fin section (3b), a first connecting rod (8a), and a second connecting rod (8b), wherein the first fin section (3a) and the second fin section (3b) are connected via the first connecting rod (8a) and the second connecting rod (8b), wherein the first connecting rod (8a) and the second connecting rod (8b) are arranged in parallel and at a respective first distance and second distance from a leading edge (10) of the first and second fin sections (3a, 3b).

Item 10. The propulsion unit (1) according to Item 9, wherein the main body (2) comprises a first through-going slot (13a) and a second through-going slot (13b) for allowing the first connecting rod (8a) and the second connecting rods (8b) to protrude through the main body (3) and to be slidably arranged within the first through- going slot (13a) and the second through-going slot (13b).

Item 11. The propulsion unit (1 ) according to Item 9 or 10, wherein the first connecting rod (8a) is connected to the first actuator (4a) and the second connecting rod (8b) is connected to the second actuator (4b).

Item 12. The propulsion unit (1 ) according to Item 11 , wherein the first connecting rod (8a) and the second connecting rod (8b) are connected to the respective first actuator (4a) and second actuator (4b) via a first actuating rod (9a) and a second actuating rod (9b), respectively.

Item 13. The propulsion unit (1) according to Item 12, wherein the first actuating rod (9a) and the second actuating rod (9b) are arranged inside the main body (2).

Item 14. The propulsion unit (1) according to any one of the Items 7 to 13, wherein the actuator assembly (4) comprises a third actuator (4c).

Item 15. The propulsion unit (1) according to Item 14, wherein at least two of the first actuator (4a), the second actuator (4b) and the third actuator (4c) are correlatively operable, to variably adjust the pitch angle and/or the heave of the fin (3).

Item 16. The propulsion unit (1) according to Item 14 or 15 when dependent on any one of Items 9 to 12, wherein the fin (3) comprises a third connecting rod, wherein the first fin section (3a) and the second fin section (3b) are connected via the third connecting rod, wherein the first connecting rod (8a), the second connecting rod (8b) and the third connecting rod are arranged in parallel and at a respective first distance, second distance and third distance from the leading edge (10) of the first and second fin sections (3a, 3b). Item 17. The propulsion unit (1) according to Item 16, wherein the main body (2) comprises a third through-going slot (13c) for allowing the first connecting rod (8c) to protrude through the main body (3) and to be slidably arranged within the three through-going slot (13c).

Item 18. The propulsion unit (1) according to Item 16 or 17, wherein the third connecting rod (8c) is connected to the third actuator (4c).

Item 19. The propulsion unit (1) according to Item 18, wherein the third connecting rod (8c) is connected to the third actuator (4c) via a third actuating rod (9c).

Item 20. The propulsion unit (1) according to Item 19, wherein the third actuating rod (9c) is arranged inside the main body (2). Item 21. The propulsion unit (1 ) according to any one of the Items 7 to 20, wherein at least one of the first actuator (4a) and the second actuator (4b) is configured to be displaced in a vertical direction.

Item 22. The propulsion unit (1 ) according to Item 21 , wherein the at least one of the first actuator (4a) and the second actuator (4b) is restrained in a direction perpendicular to a direction of extension of the at least one of the first actuator

(4a) and the second actuator (4b).

Item 23. The propulsion unit (1 ) according to Item 21 or 22, wherein the at least one of the first actuator (4a) and the second actuator (4b) is restrained in a lateral and/or longitudinal direction. Item 24. The propulsion unit (1) according to any one of the previous Items, wherein the propulsion unit comprises a rudder (6).

Item 25. The propulsion unit (1) according to Item 24, wherein the main body (2) is configured to be fixedly arranged to the keel of the vessel, and wherein the rudder (6) is pivotably arranged to the main body (2).

Item 26. The propulsion unit (1) according to Item 24, wherein the main body (2) comprises a rudder stock (11) for rotatably arranging the main body (2) to the keel of the vessel, wherein the one or more actuators (4a, 4b) and/or the at least one actuating rods (9, 9a, 9b) are arranged inside the rudder stock (11), and wherein the main body (2) is configured to act as the rudder (6).

Item 27. The propulsion unit (1) according to any one of the previous Items, wherein the main body (2) comprises a hollow tube (12) protruding from the main body (2) on a keel-facing side of the main body (2), wherein the first actuating rod (9a) and/or the second actuating rod (9b) is/are arranged within the hollow tube (12), wherein the hollow tube (12) is configured to protrude into the vessel when the main body (2) is arranged to the keel of the vessel.

Item 28. The propulsion unit (1 ) according to Item 27, wherein the hollow tube (12) comprises a seal (14) arranged at a distal end of the hollow tube (12) for sealing the first actuating rod (9a) and/or the second actuating rod (9b) to the hollow tube (12).

Item 29. The propulsion unit (1 ) according to Item 27 or 28, wherein the rudder stock (11 ) is hollow and constitutes the hollow tube (12).

Item 30. The propulsion unit (1) according to any one of the previous Items, wherein the fin (3) has an elliptical planform.

Item 31. The propulsion unit (1 ) according to any one of the previous Items, wherein the fin (3) comprises winglets (15).

Item 32. A vessel (100) comprising a propulsion unit (1 ) for propelling the vessel according to any one of Items 1 to 31, wherein the main body (2) is arranged to the keel (101) of the vessel (100) and wherein the fin is configured to perform a pitch and heave motion in relation to the keel (101) of the vessel (100). Item 33. The vessel (100) according to Item 32, wherein the main body (2) is fixedly arranged to the keel (101).

Item 34. The vessel (100) according to Item 32, wherein the main body (2) is rotatably arranged to the keel (101).

The use of the terms “first”, “second”, “third” and “fourth”, “primary”, “secondary”, “tertiary” etc. does not imply any particular order, but are included to identify individual elements. Moreover, the use of the terms “first”, “second”, “third” and “fourth”, “primary”,

“secondary”, “tertiary” etc. does not denote any order or importance, but rather the terms “first”, “second”, “third” and “fourth”, “primary”, “secondary”, “tertiary” etc. are used to distinguish one element from another. Note that the words “first”, “second”, “third” and “fourth”, “primary”, “secondary”, “tertiary” etc. are used here and elsewhere for labelling purposes only and are not intended to denote any specific spatial or temporal ordering. Furthermore, the labelling of a first element does not imply the presence of a second element and vice versa.

It is to be noted that the word "comprising" does not necessarily exclude the presence of other elements or steps than those listed.

It is to be noted that the words "a" or "an" preceding an element do not exclude the presence of a plurality of such elements.

Although features have been shown and described, it will be understood that they are not intended to limit the claimed disclosure, and it will be made obvious to those skilled in the art that various changes and modifications may be made without departing from the scope of the claimed disclosure. The specification and drawings are accordingly to be regarded in an illustrative rather than restrictive sense. The claimed disclosure is intended to cover all alternatives, modifications, and equivalents.

LIST OF REFERENCES

1 propulsion unit

2 main body

3 fin

3a first fin section

3b second fin section

4 actuator assembly

4a actuator, first actuator

4b second actuator

5a pivot point, first pivot point

5b second pivot point

6 rudder

7 lever

8a first connecting rod

8b second connecting rod

9 actuating rod

9a first actuating rod

9b second actuating rod

10 leading edge 11 rudder stock 12 hollow tube 13a first through-going slot 13b second through-going slot

14 seal

15 winglet

16 trailing edge

17 elongated slot

18 pin

19 elongated slot

20 seal 21 support surface 100 vessel 101 keel