Patent Publication Number: US-2022234708-A1

Title: Marine drive unit and marine vessel

Description:
TECHNICAL FIELD 
     The present invention relates to a marine drive unit and a marine vessel with a hybrid driveline comprising such a drive unit. 
     BACKGROUND 
     Known marine vessels comprising a propulsion unit in the form of a pod drive are usually provided with an internal combustion engine (ICE) arranged within the hull of the vessel. Torque is then transmitted from the ICE to the drive via a transmission comprising shafts and gearing in order to drive a set of propellers on a steerable drive unit mounted to the hull. 
     When operating a vessel of this type at low speed it is sometimes desirable to be able to drive the vessel at reduced noise levels and/or without exhaust emissions. Operating conditions when this is an advantage is for instance when manoeuvring within a marina, while trolling or during docking. A possible solution to the above problems can be to provide an individual electric motor. However, such motors are more suited for smaller vessels with an outboard motor and are usually too small for operating vessels comprising one or more inboard engines with pod drives. A further solution to the problem is to provide a hybrid driveline with the inboard engine and electric motor arranged in series. Such a solution is known from US 2011/195618. A problem with this solution is that it takes up more space within the hull, reducing accommodation space for the occupants. Further, the control system for the engine and electric motor must be combined and becomes more complex. Such a control system will at best be difficult to adapt to an existing inboard driveline comprising one or more engines. Also, combining such a hybrid driveline with a pod drive will require additional space for the transmission and steering arrangement extending through the hull to the steerable pod beneath the hull. 
     The invention provides an improved marine drive unit aiming to solve the above-mentioned problems. 
     SUMMARY 
     An object of the invention is to provide a marine drive unit for a vessel, which drive unit solves the above-mentioned problems. 
     The object is achieved by a hybrid marine drive unit and a marine vessel with a hybrid driveline comprising such a drive unit according to the appended claims. 
     In the subsequent text, the term “drive unit” means an assembly comprising an outdrive having two sub-units. An upper sub-unit comprises a drive housing containing at least one source of drive torque and a transmission comprising a vertical driveshaft partly enclosed by the drive housing. The drive housing is preferably, but not necessarily, completely submerged. A lower sub-unit forms a propulsor or propelling unit and contains an extension of the vertical driveshaft and a transmission comprising a gearbox providing power to a propeller shaft/-s for driving at least one propeller. The transmission in the lower sub-unit supplies power from the transmission in the upper sub-unit to the propellers. The component parts of the transmission in the lower sub-unit are enclosed in a gearbox housing. At least one drive unit is mounted to the transom of a marine vessel and forms part of a hybrid driveline comprising a first source of drive torque within the drive unit and an inboard, second source of drive torque. The terms “inboard” or “on-board” are used to indicate that a component is located within the hull of the vessel, i.e. not within the drive unit or its housing. 
     According to a first aspect of the invention, the invention relates to a hybrid marine drive unit arranged to be mounted to a transom on a marine vessel. The drive unit comprises a drive housing that is configured to be rigidly mounted at or on the transom, and is preferably, but not necessarily, submerged during operation. That the drive housing is rigidly mounted to the transom means that it is stationary in relation to the transom during operation, i.e. in contrast to a conventional stern drive where the housing is arranged to pivot (together with a propelling unit) about a vertical axis at the transom for at least steering. The drive unit further comprises a propelling unit rotatable about a vertical axis and mounted to a lower surface of the drive housing and a transmission with at least a vertical drive shaft located in the drive housing. The drive unit is an azimuthing pod drive removably attached to the transom. The propelling unit is arranged to be rotatable relative to the lower surface of the drive housing by a steering arrangement in order to steer the vessel. The vertical drive shaft is arranged to transmit drive torque from multiple sources of drive torque to the propelling unit for propelling the vessel. The vertical drive shaft is operably connected to at least one first source of drive torque arranged within the drive housing. In addition, the vertical drive shaft is also operably connected to a horizontal output shaft extending into the drive housing through the transom, wherein the horizontal output shaft is connectable to a second source of drive torque. 
     The first source of drive torque is preferably an electric motor with an independently excited rotor, wherein the rotor is arranged to be freewheeling when its excitation current is deactivated to demagnetize the rotor. A non-exhaustive list of suitable electric motors comprises polyphase synchronous motors, switched reluctance motors or synchronous reluctance motors. 
     The vertical drive shaft is operably connected to at least one first source of drive torque in the form of an electric motor arranged within the drive housing. According to one example, an electric motor with a vertical output shaft can be operably connected to the upper end or portion of the vertical drive shaft. According to this example, the electric motor comprises a vertical output shaft drivingly connected to the vertical drive shaft extending directly into the propelling unit. For this electric motor, switching between a connected torque transmitting state and a disconnected freewheeling state relative to the vertical drive shaft is achieved by demagnetizing its rotor. This allows the vertical drive shaft to rotate without resistance from the electric motor, for instance, when propelling the vessel using the second source of drive torque only. 
     One or more additional sources of drive torque can be operably connected to the vertical drive shaft by a suitable gear unit. The gear unit can comprise a number of gears, such as bevel gears, wherein each gear is associated with a horizontal driving input shaft from a first source of drive torque or a driven output shaft from the second source of drive torque. Preferably, a single common gear unit is used for this purpose. The gears are preferably switchable between a connected, torque transmitting state and a disconnected, freewheeling state relative to their respective shaft. For additional first sources of drive torque comprising electric motors switching can be achieved by demagnetizing the rotor of the respective motor. According to a further example the at least one first source of drive torque comprises an electric motor with a vertical output shaft, as described above, and at least one electrical motor with a horizontal output shaft which can be operably connected to the gear unit. According to a further example the at least one first source of drive torque comprises at least one electrical motor with a horizontal output shaft which can be operably connected to the gear unit. 
     The drive unit is part of a hybrid driveline, wherein a first source of drive torque is an electric motor and a second source of drive torque can be an internal combustion engine. Consequently, the vertical drive shaft is operably connected to a second source of drive torque in the form of an internal combustion engine. The horizontal output shaft from the second source of drive torque is operably connected to the vertical drive shaft via the common gear unit. A separate clutch is provided for disconnecting the second source of drive torque from the gear unit during electrical operation of the drive unit. This clutch can be a friction clutch located adjacent the second source of drive torque within the hull of the vessel. Preferably, the second source of drive torque is operably connected to the vertical drive shaft via the gear unit comprising multiple bevel gears in driving connection. The at least one electric motor is operably connected directly to the vertical drive shaft and/or indirectly via the gear unit, as described above. 
     The horizontal output shafts from the internal combustion engine and/or at least one electric motor are operably connected to the vertical drive shaft via the common gear unit. The common gear unit can comprise a bevel gear mounted on each of the horizontal output shafts from the one or more electric motors and the internal combustion engine. Each driving bevel gear is operably connected with a pair of driven opposed bevel gears operatively connectable to the vertical drive shaft. When driven, the bevel gear on either one of the driving horizontal shafts will drive both the opposed bevel gears. The bevel gears on the vertical drive shaft are provided with controllable actuators allowing each gear to be placed in driving connection with the vertical drive shaft in turn. For the second source of drive torque one bevel gear is connected for forward propulsion and the opposite bevel gear is connected for reverse propulsion. Alternatively, both bevel gears can rotate freely relative to the vertical drive shaft. 
     Switching the bevel gears between a connected torque transmitting state and a disconnected freewheeling state relative to the vertical drive shaft is achieved by actuation or deactuation of a suitable controllable actuator in the form of a mechanical actuator or a fluid (hydraulically or pneumatically) operated clutch. An example of a suitable clutch is a wet or dry multi-plate clutch, also termed lamella clutch. Hence, torque transmission from each drive source is controllable between its connected and disconnected states by a corresponding actuator mounted adjacent the respective gear, preferably within the gear unit. 
     As described above, the marine drive unit comprises a propelling unit, such as a propeller, impeller or pod drive mounted to the lower surface of the drive housing. The propelling unit is arranged to be rotatable relative to the lower surface of the drive housing by a steering system in order to steer the vessel. The steering arrangement is located in the drive housing and comprises a steering system with a control unit and a steering drive unit for rotating the propelling unit about its vertical axis. The steering drive unit can comprise an electric motor. The propelling unit can comprise counter rotating forward facing propellers in the form of an azimuthing pod. 
     The drive housing can comprise a control unit and power electronics controller (PEC) for the at least one electric motor and for the steering arrangement. The outer enclosure for the drive housing provides a thermal mass to absorb the heat generated by the electric motor or the power electronics. In operation, the drive housing is immersed in water and the water provides effective convection cooling. The electric motor is connected to the PEC, which supplies current to the at least one electric motor from an energy storage, such as a high voltage battery pack via a propulsion voltage system comprising high voltage DC buses and a high voltage junction box. The high voltage junction box can also be used for joining and distributing high voltage buses to a number of different electrical components on-board the vessel. The battery pack can comprise a separate power electronics controller (PEC) and an electronic controller for calibrating and charging the battery pack. Power electronics controllers of this type are known in the art and will not be described in further detail here. 
     According to a further example, the drive housing can comprise a closed coolant and lubrication circuit for the transmission, including the gear unit and propeller unit, and the at least one electric motor. The drive housing can comprise a reservoir for a liquid lubricant and coolant. The closed coolant and lubrication circuit comprises a pump, a supply conduit connected to conduits for the electric motors and the transmission, and a return conduit connected to the reservoir. The pump is preferably, but not necessarily, located in the reservoir. The provision of a closed coolant and lubrication circuit allows the drive unit to be cooled without the use of water from the surrounding body of water. This is a particular advantage if the vessel is operated in saline or polluted waters. A further advantage is that the same system can be used for lubrication, wherein separate pumps and circuits for cooling and lubrication can be dispensed with, which provides a reduction of both cost and space requirement. 
     According to a second aspect of the invention, the invention relates to a marine vessel with a hybrid driveline comprising multiple sources of drive torque to propel the vessel, wherein the vessel is provided with at least one marine drive unit as described above. The at least one drive unit comprises at least one electric motor arranged within a drive housing and that the drive unit is operatively connected to an internal combustion engine arranged within the hull of the vessel. Exhaust from the internal combustion engine can be discharged through a suitable port through the hull or below the waterline through the propelling unit. 
     The drive unit according to the invention provides a way to mount a pod drive with a hybrid driveline without requiring significant modifications of a marine vessel intended for stern drive applications. In most cases the outer drive unit can be advantageously provided with a drive housing having the same or approximately the same shape and size as conventional stern drive housings. Further, the interface for mounting a pod drive and its steering gear connections to the transom can be maintained. For marine vessel intended for pod drive applications the invention eliminates the need for a sizable opening through the lower surface of the hull which is required for most types of pod drives, such as an IPS © pod drive manufactured by Volvo Penta. Further, by mounting the electric motors in the outer drive housing, it is possible to provide a hybrid drive unit without taking up space for electric motors or the pod drive itself within the hull. The provision of one or more on-board battery packs can be achieved without taking up accommodation space. The electric motor/-s and the inboard engine can drive the propellers together, independently or in variable combinations in response to different torque and power demands whereby the efficiency of the drive unit is improved. By allowing independent operation of at least a single motor the arrangement provides a redundancy for the drive unit and ensures that the vessel can be operated even if the engine or one or more electric motors are inoperable. 
     Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples. In the drawings: 
         FIG. 1  shows a lower perspective view of a schematically illustrated vessel comprising a pair of drive units; 
         FIG. 2  shows a schematic side view of a driveline according to a first example; 
         FIG. 3  shows a schematic side view of a driveline according to a second example; 
         FIG. 4  shows a schematic side view of a driveline according to a third example; 
         FIG. 5  shows a schematic transmission for the driveline in  FIG. 2 ; 
         FIG. 6  shows a schematic transmission for the driveline in  FIG. 3 ; and 
         FIG. 7  shows a schematic transmission for the driveline in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION 
       FIG. 1  shows a lower perspective view of a schematically illustrated marine vessel  100  comprising two marine drive units  103 ,  103 ′ according to the invention. In this example, the marine drive units  103 ,  103 ′ are identical and only one will be described in further detail below. The marine drive units  103 ,  103 ′ are mounted to a transom  102  on the vessel  100 . Each marine drive unit  103 ,  103 ′ comprises an upper and a lower unit, wherein the upper unit is a drive housing  104 ,  104 ′ rigidly mounted on the transom  102 . The lower unit is a propelling unit  105 ,  105 ′ rotatable about a vertical axis and mounted to a lower surface  106 ,  106 ′ of each drive housing  104 ,  104 ′. The schematically indicated marine drive units  103 ,  103 ′ in  FIG. 1  are preferably located below the waterline of the vessel hull  101 . The example shown in  FIG. 1  shows propelling units in the form of steerable pods which comprise twin forward facing, pulling propellers  107 ,  107 ′. As will be described below, alternative propelling units can be employed within the scope of the invention. 
     The marine drive units in  FIG. 1  are controllable by a control means (not shown) such as a throttle lever located at an operating position on-board the vessel. The throttle lever can be connected to an electronic control unit (ECU) via suitable wiring, which ECU is connected to a source of energy, such as a battery pack or a fuel cell via additional wiring. Such an energy source is located within the hull of the vessel and can comprise a power electronic controller (PEC) and an electronic controller for calibrating and charging a battery pack. The throttle lever be used for controlling the first source of drive torque, such as at least one electric motor within the drive housing, and the second source of drive torque, such as an engine located within the hull of the vessel. The first and second sources of drive torque form a hybrid driveline and the sources can be operated individually or together. Electronic controllers of this type are known in the art and will not be shown or described in further detail here. 
       FIG. 2  shows a cross-sectional side view of a drive unit  203  according to a first example shown in  FIG. 1 .  FIG. 2  shows the drive unit  203  mounted to a transom  202  of a marine vessel (see  FIG. 1 ). The drive unit  203  comprises an upper drive housing  204 , and a lower propelling unit  205 , where the propelling unit  205  is rotatably mounted to a lower surface  206  of the drive housing  204  in order to steer the vessel. The drive housing  204  encloses a transmission comprising a vertical drive shaft  210  arranged transmit drive torque from at least one source of drive torque to a pair of forward-facing counter rotating propellers  207  on the propelling unit  205 . The transmission further comprises a gear unit  213  operably connectable to an upper end of the vertical drive shaft  210 . In  FIG. 2 , a first source of drive torque is an electric motor  211  with a vertical output shaft  212  that is operably connected to the vertical drive shaft  210  directly through the gear unit  213 . The electric motor  211  can be disconnected from the through shaft comprising the vertical drive shaft  210  and the vertical output shaft  212  by demagnetizing the rotor. A horizontal output shaft  220  is connected to a second source of drive torque in the form of an inboard ICE  221  located within the hull of the vessel (see  FIG. 2 ). 
     The gear unit  213  comprises a set of bevel gears  214 ,  215 ,  216  which are in constant driving contact with each other. Each bevel gear is associated with a respective driving or driven shaft  212 ,  210 ,  220  and is switchable between a connected state and a disconnected state for transferring torque to the vertical drive shaft  210 . The bevel gear  216  is fixed to the horizontal output shaft  220  and is switchable between a driven state and a freewheeling state by a main clutch  224  adjacent the ICE  221 . Each bevel gear  214 ,  215  on the vertical drive shaft  210  is controllable between its connected and disconnected states by a corresponding actuatable clutch  214 ′,  215 ′ mounted adjacent the respective bevel gear (see  FIG. 5 ). Switching can be achieved by actuation or deactuation of a suitable controllable clutch or mechanical actuator. In the subsequent text switching is performed using wet or dry multi-plate clutches, or lamella clutches, hereafter referred to as “clutches”. Lamella clutches of this type can be pneumatically or hydraulically actuated using a suitable source of fluid pressure. The design or control of such clutches is known in the art and will not be described in further detail here. 
     In  FIGS. 2 and 6 , the vertical output shaft  212  of the electric motor  211  passes through the gear unit  213 . The gear unit  213  comprises an upper first bevel gear  214  arranged on the vertical output shaft  212  and a lower second bevel gear  215  arranged on the vertical drive shaft  210 . The first and second bevel gears  214 ,  215  are in driving connection with the intermediate third bevel gear  216  arranged on the horizontal output shaft  220 . The horizontal output shaft  220  is connected to a second source of drive torque in the form of an inboard ICE  221  located within the hull of the vessel (see  FIG. 1 ). The horizontal output shaft  220  passes through a seal  222  in the transom  202  and is fixed in a vibration absorbing bushing  223  supported by the ICE output shaft. The clutch  224  is provided between the horizontal output shaft  220  and the ICE crankshaft to control the rotation of the horizontal output shaft  220 . The first and second bevel gears  214 ,  215  are freely rotatable about the vertical output shaft  212  and the vertical drive shaft  210 , respectively, in their disconnected state. Similarly, the third bevel gear  216  is freely rotatable with the horizontal output shaft  220  when the clutch  224  adjacent the ICE 221  in its disconnected state. The bevel gears  214 ,  215  are selectably connected to the vertical drive shaft  210  in order to transmit torque from the ICE  221  to the vertical drive shaft  210  and the propellers. In this way, the vertical drive shaft  210  can be operably connected to the horizontal output shaft  220  which extends out of the drive housing  204  through the transom  202 . 
     In operation, the driveline can be operated in electric mode using the electric motor  211  rotating the vertical output shaft  212  and the vertical drive shaft  210  directly as shown in  FIGS. 2 and 5  to drive the vessel in a forward direction.  FIG. 5  shows a schematic view of the transmission for the driveline in  FIG. 2 . In the electric mode, the rotor of the electric motor  211  is magnetized and the bevel gears  214 ,  215  are disconnected from the vertical drive shaft  210 . Propelling the vessel in a reverse direction is achieved by switching the direction of rotation of the electric motor  211 . 
     Alternatively, the driveline can be operated in ICE mode, wherein the rotor (not shown) of the electric motor  211  is demagnetized making the vertical output shaft  212  freely rotatable relative to the motor. In the gear unit  213 , the first bevel gear  214  is maintained disconnected while the second bevel gear  215  is connected to the vertical drive shaft  210  by actuation of the clutch  215 ′. At the same time, the third bevel gear  216  is driven by the horizontal output shaft  220  by actuation of the main clutch  224 . The ICE  221  can then be operated to transmit torque to the horizontal shaft  220  and the vertical drive shaft  210  via the third bevel gear  216  and the second bevel gear  215 , in order to propel the vessel in a forward direction. In order to propel the vessel in a reverse direction the main clutch  224  is deactuated. The second bevel gear  215  is then disconnected by deactuation of the clutch  215 ′, while the first bevel gear  214  is connected to the vertical output shaft  212  by actuation of the clutch  214 ′. Subsequently, the third bevel gear  216  continues to be driven by the horizontal output shaft  220  by actuation of the main clutch  224 . The ICE  221  can then be operated to transmit torque to the horizontal shaft  220  and the vertical drive shaft  210  via the third bevel gear  216  and the first bevel gear  214 . 
     According to a further example, the driveline can be operated in a hybrid mode using the electric motor  211  and the ICE  221  together. In the hybrid mode, the gear unit  213  is operated in the same way as in the ICE mode described above, wherein the rotor of the electric motor  211  is magnetized so that the motor can be operated to drive the vertical output shaft  212  to assist the ICE  221 . The direction of rotation of the electric motor  211  is selected to correspond with the direction of rotation of the currently connected first or second bevel gear  214 ,  215 . 
     The propelling unit  205  contains a gearbox  208  operably connected to a lower end of the vertical drive shaft  210 , which can be rotated as shown by the arrow A 1  to drive the counter rotating propellers  207 . Gearboxes for driving counter-rotating shafts of this type are well known and will not be described in further detail. 
     The drive housing  204  further comprises a control unit and power electronics controller (PEC)  230  for the electric motor  211 . The combined control unit and power electronics controller (PEC)  230  is also used for controlling a steering arrangement  240  described below. The outer enclosure for the drive housing  204  provides a thermal mass to absorb the heat generated by the electric motor  211  and the PEC  230 . In operation, the drive housing  204  is immersed in water and the water provides effective convection cooling. The electric motor  211  is connected to the PEC  230 , which supplies current to the electric motor  211  from an inboard energy storage (not shown). Control means such as a throttle and a steering means (not shown) are provided at an operator station on-board the vessel. 
     The propelling unit  205  is arranged to be rotatable relative to the lower surface  206  of the drive housing by a steering arrangement  240  in order to steer the vessel. The steering arrangement  240  is located in the drive housing comprises a steering system with a control unit and a steering drive unit for rotating the propelling unit about its vertical axis. The steering drive unit can comprise an electric motor. The steering drive unit drives a steering transmission comprising a pinon gear that drives a gear fixed to the propelling unit  205  about the central axis X of the vertical drive shaft  210  as indicated by the arrow A 2 . 
     The drive housing  204  in  FIG. 2  further comprises a coolant and lubricant circuit  250 .  FIG. 2  schematically indicates a closed coolant and lubrication circuit for the gear unit  213 , the vertical drive shaft  210 , the steering arrangement  240  and the electric motor  211 . The closed coolant and lubrication circuit comprises a pump, a reservoir, a supply conduit connected to conduits for cooling the electric motor  211  and a conduit supplying the coolant/lubricant to the gear unit and steering arrangement. The provision of a closed coolant and lubrication circuit allows internal components to be cooled without using water from the surrounding body of water. As described above, the outer enclosure of the drive housing  204  can provide additional cooling by using it as a thermal mass to absorb the heat generated by the electric motor  211  and the PEC  230 . The arrangement also allows the same system to be used for both cooling and lubrication. 
       FIG. 3  shows a schematic side view of a driveline according to a second example.  FIG. 3  shows the drive unit  303  mounted to a transom  302  of a marine vessel (see  FIG. 1 ). The drive unit  303  comprises an upper drive housing  304 , and a lower propelling unit  305 , where the propelling unit  305  is rotatably mounted to a lower surface  306  of the drive housing  304  in order to steer the vessel. The drive housing  304  encloses a transmission comprising a vertical drive shaft  310  arranged to transmit drive torque from at least one source of drive torque to a pair of forward-facing counter rotating propellers  307  on the propelling unit  305 . The transmission further comprises a gear unit  313  operably connectable to an upper end of the vertical drive shaft  310 , which passes directly through the gear unit  313 . In  FIGS. 3 and 6 , a first source of drive torque is an electric motor  311  with a horizontal output shaft  312  that is operably connectable to the vertical drive shaft  310  directly through the gear unit  313 .  FIG. 6  shows a schematic view of the transmission for the driveline in  FIG. 3 . The electric motor  311  can be disconnected from the horizontal output shaft  312  and the through shaft comprising the vertical drive shaft  310  and a vertical support shaft  318  by demagnetizing the rotor. A horizontal output shaft  320  is connected to a second source of drive torque in the form of an inboard internal combustion engine (ICE)  321  located within the hull of the vessel (see  FIG. 3 or 1 ). 
     The gear unit  313  comprises a set of bevel gears  315 ,  316 ,  317 ,  319  which are in constant driving contact with each other. Each bevel gear is associated with a respective driving or driven shaft  310 ,  320 ,  312 ,  318  and is switchable between a connected state and a disconnected state for transferring torque to the vertical drive shaft  210 . The bevel gear  316  fixed to the horizontal output shaft  320  from the ICE  321  is switchable between a driven state and a freewheeling state by a main clutch  324  adjacent the ICE  321 . The bevel gear  317  fixed to the horizontal output shaft  312  from the electric motor  311  is switchable between a driven state and a freewheeling state by magnetizing and demagnetizing the rotor of the electric motor  311 . Each bevel gear  319 ,  315  on the vertical drive shaft  310  is controllable between its connected and disconnected states by a corresponding actuatable clutch  319 ′,  315 ′ mounted adjacent the respective bevel gear. Switching the bevel gears  319 ,  315  can be achieved by actuation or deactuation of a suitable controllable clutch or mechanical actuator. In the subsequent text switching is performed using wet multi-plate clutches, or lamella clutches, hereafter referred to as “clutches”. Hence, each bevel gear  319 ,  315  on the vertical drive shaft  310  is controllable between its connected and disconnected states by a corresponding actuatable clutch  319 ′,  315 ′ mounted adjacent the respective bevel gear. 
     With reference to  FIG. 6 , the vertical drive shaft  310  passes directly upwards through the gear unit  313  and exits as the upper supporting shaft  318 . The gear unit  313  comprises an upper first bevel gear  319  and a lower second bevel gear  315  arranged on the vertical drive shaft  310 . The first and second bevel gears  319 ,  315  are in driving connection with an intermediate third bevel gear  316  fixed to a first horizontal output shaft  320  connected to a main clutch  324  via a main clutch  324 . The first and second bevel gears  319 ,  315  are further in driving connection with an intermediate fourth bevel gear  317  arranged on a second horizontal output shaft  312 . The fourth bevel gear  317  is arranged opposite the third bevel gear  316  coaxially with the first horizontal output shaft  320 . The second horizontal output shaft  312  is connected to a first source of drive torque in the form of an electric motor  311 . The first horizontal output shaft  320  is connected to a second source of drive torque in the form of an inboard ICE  321  located within the hull of the vessel (see  FIG. 1 ). The first horizontal output shaft  320  passes through a seal  322  in the transom  302  and is fixed in a vibration absorbing bushing  323  supported by the ICE output shaft. A main clutch  324  is provided between the first horizontal output shaft  320  and the ICE crankshaft to control the rotation of the first horizontal output shaft  320 . The first and second bevel gears  319 ,  315  are freely rotatable about the supporting shaft  318  and the vertical drive shaft  310 , respectively, in their disconnected state. Similarly, the third bevel gear  316  is freely rotatable with the first horizontal output shaft  320  in its disconnected state. The fourth bevel gear  317  is freely rotatable about the second horizontal output shaft  312  with the main clutch  324  in its disconnected state. The upper and lower bevel gears  319 ,  315  are selectably connected to the vertical drive shaft  310  in order to transmit torque from the electric motor  311  and/or the ICE  321  to the vertical drive shaft  310  and the propellers. In this way, the vertical drive shaft  310  can be operably connected to the first horizontal output shaft  320 , which extends out of the drive housing  304  through the transom  302 , and to the second horizontal output shaft  312 . 
     In operation, the driveline can be operated in electric mode using the electric motor  311  for rotating the horizontal second output shaft  312  and the vertical drive shaft  310  to drive the vessel in a forward direction. In this mode, the third bevel gear  316  is allowed to rotate freely by disconnection of the main clutch  324 . In the gear unit  313 , the first bevel gear  319  is maintained disconnected while the second bevel gear  315  is connected to the vertical drive shaft  310  by actuation of the lower clutch  315 ′. At the same time, the rotor of the electric motor  311  is magnetized allowing it to be operated to transmit torque to the second horizontal output shaft  312  and the vertical drive shaft  310  via the fourth bevel gear  317  and the second bevel gear  315 , in order to propel the vessel in a forward direction. Propelling the vessel in reverse direction is achieved by switching the direction of rotation of the electric motor  311 . 
     Alternatively, the driveline can be operated in ICE mode, wherein the rotor (not shown) of the electric motor  311  is demagnetized making the second horizontal output shaft  312  freely rotatable. In the gear unit  313 , the first bevel gear  319  is maintained disconnected while the second bevel gear  315  is connected to the vertical drive shaft  310  by actuation of the clutch  315 ′. At the same time, the third bevel gear  316  and the first horizontal output shaft  320  are operatively connected to the ICE  321  by actuation of the main clutch  324 . The ICE  321  can then be operated to transmit torque to the horizontal shaft  320  and the vertical drive shaft  310  via the third bevel gear  316  and the second bevel gear  315 , in order to propel the vessel in a forward direction. In order to propel the vessel in a reverse direction the main clutch  324  is deactuated. The second bevel gear  315  is then disconnected by deactuation of the clutch  315 ′, while the first bevel gear  319  is connected to the vertical support shaft  318  by actuation of the clutch  319 ′. Subsequently, the third bevel gear  316  continues to be driven the horizontal output shaft  320  by actuation of the main clutch  324 . The ICE  321  can then be operated to transmit torque to the horizontal shaft  320  and the vertical drive shaft  310  via the third bevel gear  316  and the first bevel gear  319 . 
     According to a further example, the driveline can be operated in a hybrid mode using the electric motor  311  and the ICE  321  together. In the hybrid mode, the gear unit  313  is operated in the same way as in the ICE mode described above, wherein the rotor of the electric motor  311  is magnetized so that the motor can be operated to drive the vertical output shaft  312  to assist the ICE  321 . The direction of rotation of the electric motor  311  is selected to correspond with the direction of rotation of the currently connected first or second bevel gears  314 ,  315  selected for forward or reverse operation of the vessel using the ICE  321 . 
     The propelling unit  305  contains a gearbox  308  operably connected to a lower end of the vertical drive shaft  310 , which can be rotated as shown by the arrow A 1  to drive the counter rotating propellers  307 . Gearboxes for driving counter-rotating shafts of this type are well known and will not be described in further detail. 
     The drive housing  304  further comprises a control unit and power electronics controller (PEC)  330  for the electric motor  311 . The combined control unit and power electronics controller (PEC)  330  is also used for controlling a steering arrangement  340  described below. The outer enclosure for the drive housing  304  provides a thermal mass to absorb the heat generated by the electric motor  311  and the PEC  330 . In operation, the drive housing  304  is immersed in water and the water provides effective convection cooling. The electric motor  311  is connected to the PEC  330 , which supplies current to the electric motor  311  from an inboard energy storage (not shown). Control means such as a throttle and a steering means (not shown) are provided at an operator station on-board the vessel. 
     The propelling unit  305  is arranged to be rotatable relative to the lower surface  306  of the drive housing by a steering arrangement  340  in order to steer the vessel. The steering arrangement  340  is located in the drive housing comprises a steering system with a control unit and a steering drive unit for rotating the propelling unit about its vertical axis. The steering drive unit can comprise an electric motor. The steering drive unit drives a steering transmission comprising a pinon gear that drives a gear fixed to the propelling unit  305  about the central axis X of the vertical drive shaft  310  as indicated by the arrow A 2 . 
     The drive housing  304  in  FIG. 3  further comprises a coolant and lubricant circuit of the same type as described with reference to  FIG. 2  above. 
       FIG. 4  shows a schematic side view of a driveline according to a third example.  FIG. 4  shows the drive unit  403  mounted to a transom  402  of a marine vessel (see  FIG. 1 ). The drive unit  403  comprises an upper drive housing  404 , and a lower propelling unit  405 , where the propelling unit  405  is rotatably mounted to a lower surface  406  of the drive housing  404  in order to steer the vessel. The drive housing  404  encloses a transmission comprising a vertical drive shaft  410  arranged to transmit drive torque from at least one source of drive torque to a pair of forward-facing counter rotating propellers  407  on the propelling unit  405 . The transmission further comprises a gear unit  413  operably connected to an upper end of the vertical drive shaft  410 . In  FIGS. 4 and 7 , one first source of drive torque is an electric motor  411  with a vertical output shaft  412  that is operably connectable to the vertical drive shaft  410  directly through the gear unit  413 . A further first source of drive torque is a second electric motor  417  with a horizontal output shaft  418  that is operably connectable to the vertical drive shaft  410  via the gear unit  413 . 
       FIG. 7  shows a schematic view of the transmission for the driveline in  FIG. 4 . The electric motors  411 ,  417  can be disconnected from their respective shaft  412 ,  418  and the through shaft comprising the vertical drive shaft  410  by demagnetizing their respective rotors. A horizontal output shaft  320  is connected to a second source of drive torque in the form of an inboard ICE  321  located within the hull of the vessel (see  FIGS. 4 and 1 ). 
     The gear unit  413  comprises a set of bevel gears  414 ,  415 ,  416 ,  419  which are in constant driving contact with each other. Each bevel gear is associated with a respective driving or driven shaft  412 ,  410 ,  420 ,  418  and is switchable between a connected state and a disconnected state for transferring torque to the vertical drive shaft  410 . 
     The bevel gear  416  fixed to the horizontal output shaft  420  from the ICE  421  is switchable between a driven state and a freewheeling state by a main clutch  424  adjacent the ICE  421 . The bevel gear  419  fixed to the horizontal output shaft  418  from the electric motor  417  is switchable between a driven state and a freewheeling state by magnetizing and demagnetizing the rotor of the electric motor  417 . Each bevel gear  414 ,  415  on the vertical drive shaft  410  is controllable between its connected and disconnected states by a corresponding actuatable clutch  414 ′,  415 ′ mounted adjacent the respective bevel gear. Switching the bevel gears  414 ,  415  can be achieved by actuation or deactuation of a suitable controllable clutch or mechanical actuator. In the subsequent text switching is performed using wet multi-plate clutches, or lamella clutches, hereafter referred to as “clutches”. Hence, each bevel gear  414 ,  415  is controllable between its connected and disconnected states by a corresponding actuatable clutch  414 ′,  415 ′ mounted adjacent the respective bevel gear. 
     With reference to  FIG. 7 , the vertical output shaft  412  of the electric motor  411  passes directly through the gear unit  413 . The gear unit  413  comprises an upper first bevel gear  414  arranged on the vertical output shaft  412  and a lower second bevel gear  415  arranged on the vertical drive shaft  410 . The first and second bevel gears  414 ,  415  are in driving connection with an intermediate third bevel gear  416  arranged on a first horizontal output shaft  420  connected to a main clutch  424  via a main clutch  424 . The first and second bevel gears  414 ,  415  are further in driving connection with an intermediate fourth bevel gear  419  arranged on a second horizontal output shaft  418 . The fourth bevel gear  419  is arranged opposite the third bevel gear  416  coaxially with the first horizontal output shaft  420 . The second horizontal output shaft  418  is connected to an optional further source of drive torque in the form of a second electric motor  417 . The first horizontal output shaft  420  is connected to a second source of drive torque in the form of an inboard ICE  421  located within the hull of the vessel (see  FIG. 1 ). The first horizontal output shaft  420  passes through a seal  422  in the transom  402  and is fixed in a vibration absorbing bushing  423  supported by the ICE output shaft. A clutch  424  is provided between the first horizontal output shaft  420  and the ICE crankshaft to control the rotation of the first horizontal output shaft  420 . The first and second bevel gears  414 ,  415  are freely rotatable about the vertical output shaft  412  and the vertical drive shaft  410 , respectively, in their disconnected state. Similarly, the third bevel gear  416  is freely rotatable about the first horizontal output shaft  420  in its disconnected state. The fourth bevel gear  419  is freely rotatable about the second horizontal output shaft  418  in its disconnected state. The upper and lower bevel gears  414 ,  415  are selectably connected to their respective shaft in order to transmit torque from the ICE  421  and/or from the second electric motor  417  to the vertical drive shaft  410  and the propellers. In this way, the vertical drive shaft  410  can be operably connected to the first horizontal output shaft  420  which extends out of the drive housing  404  through the transom  402 . 
     In operation, the driveline can be operated in electric mode using the electric motor  411  rotating the output shaft  412  and the vertical drive shaft  410  directly to drive the vessel in a forward direction, as described for  FIGS. 4 and 6 . The second electric motor  417  can be operated together with, or instead of the electric motor  411  in electric mode. In this mode, the third bevel gear  416  is allowed to rotate freely by disconnection of the main clutch  424 . This is achieved by maintaining the first bevel gear  414  disconnected. At the same time, or alternatively, the fourth bevel gear  419  is connected to the second horizontal output shaft  418  by actuation of the clutch  419 ′ and the second bevel gear  415  is connected to the vertical drive shaft  410  by actuation of the clutch  415 ′. Propelling the vessel in reverse direction is achieved by switching the direction of rotation of the electric motors  411 ,  417 . The provision of two electric motors provides a degree of redundancy in case one motor should malfunction. 
     Alternatively, the driveline can be operated in ICE mode, wherein the rotors (not shown) of the electric motors  411 ,  417  are demagnetized making the vertical output shaft  412  and the second horizontal output shaft  418  freely rotatable. In the gear unit  413 , the first bevel gear  414  is maintained disconnected while the second bevel gear  415  is connected to the vertical drive shaft  410  by actuation of the clutch  415 ′. At the same time, the third bevel gear  416  and the first horizontal output shaft  420  are operatively connected to the ICE  421  by actuation of the main clutch  424 . The ICE  421  can then be operated to transmit torque to the horizontal shaft  420  and the vertical drive shaft  410  via the third bevel gear  416  and the second bevel gear  415 , in order to propel the vessel in a forward direction. In order to propel the vessel in a reverse direction the main clutch  424  is deactuated. The second bevel gear  415  is then disconnected by deactuation of the clutch  415 ′, while the first bevel gear  414  is connected to the vertical output shaft  412  by actuation of the clutch  414 ′. Subsequently, the third bevel gear  416  continues to be driven by the horizontal output shaft  420  by actuation of the main clutch  424 . The ICE  421  can then be operated to transmit torque to the horizontal shaft  420  and the vertical drive shaft  410  via the third bevel gear  416  and the first bevel gear  414 . 
     According to a further example, the driveline can be operated in a hybrid mode using the electric motors  411 ,  417  and the ICE  421  together. In the hybrid mode, the gear unit  413  is operated in the same way as in the ICE mode described above, wherein the rotor of the electric motor  411  and/or the electric motor  417  is magnetized so that the motors can be operated to drive the vertical output shaft  412  to assist the ICE  421 . The direction of rotation of the electric motors  411 ,  417  is selected to correspond with the direction of rotation of the currently connected first or second bevel gears  414 ,  415  selected for forward or reverse operation of the vessel using the ICE  421 . 
     The propelling unit  405  contains a gearbox  408  operably connected to a lower end of the vertical drive shaft  410 , which can be rotated as shown by the arrow A 1  to drive the counter rotating propellers  407 . Gearboxes for driving counter-rotating shafts of this type are well known and will not be described in further detail. 
     The drive housing  404  further comprises a control unit and power electronics controller (PEC)  430  for the electric motor  411 . The combined control unit and power electronics controller (PEC)  430  is also used for controlling a steering arrangement  440  described below. The outer enclosure for the drive housing  404  provides a thermal mass to absorb the heat generated by the electric motor  411  and the PEC  430 . In operation, the drive housing  404  is immersed in water and the water provides effective convection cooling. The electric motor  411  is connected to the PEC  430 , which supplies current to the electric motor  411  from an inboard energy storage (not shown). Control means such as a throttle and a steering means (not shown) are provided at an operator station on-board the vessel. 
     The propelling unit  405  is arranged to be rotatable relative to the lower surface  406  of the drive housing by a steering arrangement  440  in order to steer the vessel. The steering arrangement  440  is located in the drive housing comprises a steering system with a control unit and a steering drive unit for rotating the propelling unit about its vertical axis. The steering drive unit can comprise an electric motor. The steering drive unit drives a steering transmission comprising a pinon gear that drives a gear fixed to the propelling unit  405  about the central axis X of the vertical drive shaft  410  as indicated by the arrow A 2 . 
     The drive housing  404  in  FIG. 4  further comprises a coolant and lubricant circuit of the same type as described with reference to  FIG. 2  above. 
     It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims. For instance, the electric motor can be connected operatively to the vertical drive shaft in different ways, such as at a position still above but closer to the gear unit, or below the gear unit but still inside the drive housing. The drive unit may comprise an additional separate gear unit for the electric motor.