Patent Publication Number: US-11377187-B2

Title: Outboard motor

Description:
This application is a national phase of International Application No. PCT/EP2019/067388 filed Jun. 28, 2019 and published in the English language, which claims priority to European Application No. 18181924.4 filed Jul. 5, 2018, both of which are hereby incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an outboard motor. More specifically, the present invention relates to an outboard motor comprising a first power transfer arrangement, such as a first drive shaft, a second power transfer arrangement, such as a second drive shaft, a first propeller shaft and a second propeller shaft, wherein the second propeller shaft is arranged concentric with the first propeller shaft, and wherein the first propeller shaft is connected to the first power transfer arrangement to rotate the first propeller shaft in a first direction, and wherein the second propeller shaft is connected to the second power transfer arrangement to rotate the second propeller shaft in a second direction opposite to the first direction. 
     Outboard motors are self-contained propulsion and steering devices for watercrafts, such as boats, and are arranged to be fastened to the transom of a boat. One type of such watercrafts is boats that are designed to plane during operation, wherein the propeller shaft is arranged substantially horizontally and below a hull of the watercraft during operation. This type of outboard motors is generally used for driving dual counter-rotating propellers. 
     The present invention also relates to a watercraft with such an outboard motor. The present invention also relates to a method for driving a propeller shaft of an outboard motor. 
     PRIOR ART 
     Outboard motors are common for propulsion of watercrafts, such as boats. They have a powerhead with a motor, a midsection and a lower unit with a propeller shaft for driving a propeller connected to the propeller shaft. A power transfer arrangement is arranged for transferring output power from the motor to the propeller shaft. Further, a mounting bracket for mounting to the transom of the boat is common. A plurality of outboard motors is disclosed in the prior art. However, it is desirable to further improve such outboard motors. 
     One problem of such prior art outboard motors is that the efficiency is low. 
     Other problems with such prior art outboard motors are that they can be expensive, bulky or unreliable. 
     BRIEF DESCRIPTION OF THE INVENTION 
     One object of the present invention is to provide an efficient and reliable outboard motor. An outboard motor according to the invention can operate in an efficient manner to obtain straight tracking, faster acceleration and a favourable energy consumption. 
     The present invention relates to an outboard motor comprising a first power transfer arrangement, a second power transfer arrangement, a first propeller shaft and a second propeller shaft, wherein the second propeller shaft is arranged concentric with the first propeller shaft, and wherein the first propeller shaft is connected to the first power transfer arrangement to rotate the first propeller shaft in a first direction, and wherein the second propeller shaft is connected to the second power transfer arrangement to rotate the second propeller shaft in a second direction opposite to the first direction, characterised in that the outboard motor comprises a first electric motor having a first motor shaft, and a second electric motor having a second motor shaft, wherein the first motor shaft is connected to the first power transfer arrangement, and wherein the second motor shaft is connected to the second power transfer arrangement. The outboard motor can include a first propeller connected to the first propeller shaft, and a second propeller connected to the second propeller shaft. Hence, the outboard motor result in an outboard motor having dual counter-rotating propellers. The combination of dual counter-rotating propellers and two electric motors, wherein the first electric motor is for driving the first propeller shaft in one direction and the second electric motor is for driving the second propeller shaft in the opposite direction, result in efficient and reliable marine propulsion. The outboard motor according to the invention results in power transmission in opposite rotational directions in a simple and efficient manner to achieve favourable grip in the water by means of the first and second propellers, which also improves acceleration. Further, the outboard motor results in straight tracking of a watercraft and reduces lateral forces also when a plurality of outboard motors are used on a single watercraft. The first power transfer arrangement can be or comprise a first drive shaft or a first endless loop flexible drive coupling, such as a belt or a chain. The second power transfer arrangement can be or comprise a second drive shaft or a second endless loop flexible drive coupling. 
     The first motor shaft can be arranged in parallel to the second motor shaft. For example, the first and second motors can be arranged in a standing position, e.g. in a power head of the outboard motor, with the motor shafts directed downwards. Also, the first drive shaft can be arranged in parallel to the second drive shaft. Hence, a simple configuration is achieved for efficient and reliable drive of dual counter-rotating propellers. For example, both motor shafts can be directed vertically downwards, wherein both drive shafts also can be directed vertically downwards for efficient power transfer to the concentric propeller shafts in opposite rotational directions. 
     The first motor shaft can be offset from the first drive shaft, wherein the axis of rotation of the first motor shaft is displaced radially from the axis of rotation of the first drive shaft. Hence, a radial distance between the drive shafts can be smaller than a radial distance between the motor shafts, so that a favourable configuration and more powerful motors can be fitted in the outboard motor, wherein a smaller outboard motor can be achieved. The first motor shaft can be connected to the first drive shaft through a first power transfer device, such as a connection shaft, cogwheels, an endless loop flexible drive coupling or similar. Also, the second drive shaft can be offset from the second motor shaft in a similar manner. 
     The drive shafts can be connected to their propeller shaft by means of bevel gears, which can result in a simple configuration and cost-efficient manufacture of the outboard motor. 
     The present invention is also related to a watercraft, such as a boat, comprising a hull and the outboard motor as disclosed herein. The watercraft can be a planing boat. The outboard motor is arranged for both propelling and steering the watercraft. One or more batteries for supplying power to the electric motors can be arranged within the hull of the watercraft. 
     Disclosed is also a method for driving propeller shafts of an outboard motor, comprising the steps of 
     a) driving a first motor shaft of a first electric motor in a first direction, and driving a second motor shaft of a second electric motor in a second direction opposite to the first direction, 
     b) transferring rotational power from the first motor shaft to a first power transfer arrangement, and transferring rotational power from the second motor shaft to a second power transfer arrangement, and 
     c) transferring rotational power from the first power transfer arrangement to a first propeller shaft, and transferring rotational power from the second power transfer arrangement to a second propeller shaft arranged concentric to the first propeller shaft, and thereby rotate the first and second propeller shafts in opposite directions. 
     Further characteristics and advantages of the present invention will become apparent from the description of the embodiments below, the appended drawings and the dependent claims. 
    
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in more detail with the aid of exemplary embodiments and with reference to the accompanying drawings, in which 
         FIG. 1  is a schematic side view of a part of a watercraft with an outboard motor according to one embodiment, 
         FIG. 2  is a schematic side view of the outboard motor of  FIG. 1 , 
         FIG. 3  is a schematic and partial section view of the outboard motor, wherein an engine housing and a drive housing are illustrated in section to disclose a first electric motor and a second electric motor and a power transfer arrangement according to one embodiment, wherein the electric motors are in a standing position, 
         FIG. 4  is a schematic and partial section view of the outboard motor, wherein the engine housing and the drive housing are illustrated in section to disclose the electric motors and the power transfer arrangement according to another embodiment, wherein the electric motors are in a standing position, 
         FIG. 5  is a schematic and partial section view of the outboard motor, wherein the engine housing and the drive housing are illustrated in section to disclose the electric motors and the power transfer arrangement according to yet another embodiment, wherein the electric motors are in a standing position, 
         FIG. 6  is a schematic and partial section view of the outboard motor, wherein the engine housing and the drive housing are illustrated in section to disclose the electric motors and the power transfer arrangement according to an alternative embodiment, wherein the electric motors are in a lying position, 
         FIG. 7  is a schematic and partial section view of the outboard motor, wherein the engine housing and the drive housing are illustrated in section to disclose the electric motors and the power transfer arrangement according to yet another embodiment, wherein the electric motors are in a lying position, 
         FIG. 8  is a schematic and partial section view of the outboard motor, wherein the engine housing and the drive housing are illustrated in section to disclose the electric motors and the power transfer arrangement according to yet another embodiment, wherein the electric motors are in a lying position, 
         FIG. 9  is a schematic and partial section view of the outboard motor, wherein the engine housing and the drive housing are illustrated in section to disclose the electric motors and the power transfer arrangement according to yet another embodiment, wherein the electric motors are in a lying position. 
     
    
    
     THE INVENTION 
     With reference to  FIG. 1  an outboard motor  10  for a watercraft  11 , such as a boat, is illustrated according to one embodiment of the invention. The outboard motor  10  is a self-contained marine propulsion and steering device for propulsion and steering of the watercraft  11 . In  FIG. 1  a rear part of the watercraft  11  is illustrated. The watercraft  11  comprises a hull  12  and a transom  13 . For example, a lower part of the hull  12  is arranged to be below a waterline  14  when the watercraft  11  is in water and the watercraft  11  not is propelled, wherein an upper part of the hull is arranged to be above the waterline  14 . For example, the watercraft  11  is arranged to plane during operation at higher speed, wherein the hull  12  is arranged with a planing hull form. 
     With reference also to  FIG. 2  the outboard motor  10  comprises a power head  15 , a midsection  16  and a lower unit  17 . The power head  15  includes a motor housing  18 , such as a cowling. The lower unit  17  includes a first propeller  19   a  and a second propeller  19   b . For example, the lower unit  17  also includes a skeg  20  and other conventional parts, such as a torpedo-shaped part  21 . The midsection  16  is formed as a leg connecting the power head  15  and the lower unit  17 . Hence, the outboard motor  10  is arranged to be connected to the hull  12  of the watercraft  11 , so that the outboard motor  10 , or at least a major part thereof, is arranged outside the hull  12 . The midsection  16  is arranged outside the transom  13  and the lower unit  17  with the propellers  19   a ,  19   b  is arranged outside and below the hull  12 . When the outboard motor  10  is operated the propellers  19   a ,  19   b  are arranged below the water line  14  and also below the hull  12 . For example, the lower unit  17  is arranged below the hull  12  during normal operation of the outboard motor  10 . Hence, the outboard motor  10  is arranged to project a distance into the water when operated, so that the propellers  19   a ,  19   b , the lower unit  17  and optionally a part of the midsection  16  are immersed in the water, so that the water line  14  is arranged above the propellers  19   a ,  19   b  and above the lower unit  17 . Hence, the lower unit  17  is formed for efficient hydrodynamics. For example, the outboard  10  is arranged for a planing watercraft  11 . The propellers  19   a ,  19   b  are arranged for counter-rotating, wherein the propellers  19   a ,  19   b  are arranged to rotate in opposite directions in relation to each other for propelling the watercraft. Hence, one of the first and second propellers  19   a ,  19   b  is a right-handed propeller, which rotates clockwise as viewed from the stern when propelling the watercraft forward, wherein the other is a left-handed propeller, which rotates counter-clockwise as viewed from the stern when propelling the watercraft  11  forward. 
     For example, the outboard motor  10  comprises conventional fastening means for fastening the outboard motor  10  to the stern of the hull  12 , such as the transom  13 . The fastening means is, for example, arranged as a conventional mounting bracket  22 . For example, the mounting bracket  22  comprises or is provided with a trim/tilt system, such as a hydraulic or electric trim/tilt system. For example, the trim/tilt system is conventional. Hence, the outboard motor  10  comprises a laterally extending trim axis, such as a horizontal trim axis. The outboard motor  10  comprises a steering axis  23 , such as a vertical or substantially vertical steering axis (depending on trim). The entire outboard motor  10 , except for the mounting bracket  22 , is turned around the steering axis  23  for steering the watercraft  11 . Hence, the power head  15 , the midsection  16  and the lower unit  17  are pivotable around the steering axis  23 . For example, the power head  15 , the midsection  16  and the lower unit  17  are arranged in fixed positions in relation to each other and are turned as one unit around the steering axis  23 . 
     With reference to  FIG. 3  the outboard motor  10  according to one embodiment is illustrated schematically in section so as to disclose schematically some of the parts arranged therein. As illustrated in  FIG. 3  the outboard motor  10  comprises a first electric motor  24   a , a second electric motor  24   b , the first and second propellers  19   a ,  19   b  and power transmission arrangements for transferring output power originating from the motors  24   a ,  24   b  to the propellers  19   a ,  19   b.    
     The electric motors  24   a ,  24   b  comprise a motor shaft  25   a ,  25   b  for output power in the form of rotational power, also called torque herein. Hence, the first electric motor  24   a  has a first motor shaft  25   a , wherein the second electric motor  25   b  has a second motor shaft  25   b . For example, the electric motors  24   a ,  24   b  comprise, e.g. a stator and a rotor. For example, the electric motors  24   a ,  24   b  are AC electric motors, such as asynchronous motors. For example, the electric motors  24   a ,  24   b  are induction motors. Alternatively, the electric motors  24   a ,  24   b  are DC motors, such as brushed DC electric motors, permanent magnet DC motors, or brushless DC motors. The outboard motor  10  of the present invention can handle a variety of output powers and can be arranged smaller or bigger as desired within reasonable limitations, such as weight and volume suitable for outboard motors  10  and taking hydrodynamics into consideration. However, the outboard motor  10  according to the described structure can handle a variety of torques and still be hydrodynamic and efficient for use as an outboard motor  10 . For example, each of the electric motors  24   a ,  25  is able to develop at least 15 kW. For example, each of the electric motors  24   a ,  25  is able to develop at least 50 kW or at least 75 kW, such as 100 kW or 200 kW. For example, the electric motors  24   a ,  24   b  are conventional industrially produced electric motors, such as mass produced in series of at least thousands. The motors  24   a ,  25   b  are, e.g. mounted on motor support structures, which are not illustrated in the drawings. According to one embodiment, the outboard motor  10  comprises two similar electric motors  24   a ,  24   b . Alternatively, the outboard motor  10  comprises two different electric motors  24   a ,  24   b . For example, the first electric motor  24   a  is arranged for rotating the first motor shaft  25   a  in a first direction, such as clockwise, wherein the second electric motor  24   b  is arranged for rotating the second motor shaft  25   b  in a second direction opposite to the first direction, such as counter-clockwise. For example, the electric motors  24   a ,  24   b  are reversible, so that the motor shafts  25   a ,  25   b  can be driven in any rotational direction. For example, the speed of the electric motors  24   a ,  24   b  is adjustable, so that the motor shafts  25   a ,  25   b  can be driven in a selectable rotational speed. For example, the speed and rotational direction of the electric motors  24   a ,  24   b  are controlled by conventional control means, which is not illustrated in the drawings. 
     In the embodiment of  FIG. 3  the electric motors  24   a ,  24   b  are arranged in standing position, wherein the motor shafts  25   a ,  25   b  extend substantially downward when the outboard motor  10  is operated. Hence, the motor shafts  25   a ,  25  extend substantially vertically. In the illustrated embodiment, the first and second motor shafts  25   a ,  25   b  are arranged in parallel. The first motor shaft  25   a  has a first axis of rotation extending along the first motor shaft  25   a , wherein the second motor shaft  25   b  has a second axis of rotation extending along the second motor shaft  25   b . In the illustrated embodiment, the first electric motor  24   a  is arranged at the same level as the second electric motor  24   b , so that upper and/or lower surfaces of the electric motors  24   a ,  24   b  are arranged in a common plane. Alternatively, the first and second electric motors  24   a ,  24   b  are displaced in relation to the other, so that the upper and/or lower surfaces of one of the electric motors  24   a ,  24   b  is/are displaced in a direction along the motor shafts  25   a ,  25   b  in relation to the upper and/or lower surfaces of the other of the electric motors  24   a ,  24   b.    
     The outboard motor  10  comprises a first propeller shaft  26   a  and a second propeller shaft  26   b . The first propeller shaft  26   a  is arranged for driving the first propeller  19   a . Hence, the first propeller  19   a  is connected to or connectable to the first propeller shaft  26   a . The second propeller shaft  26   b  is arranged for driving the second propeller  19   b . Hence, the second propeller  19   b  is connected to or connectable to the second propeller shaft  26   b . For example, the outboard motor  10  comprises the first and second propellers  19   a ,  19   b  in the form of dual counter-rotating propellers, wherein the first and second propellers  19   a ,  19   b  are arranged to rotate in opposite directions. For example, the first propeller  19   a  is arranged to rotate in a clockwise direction, wherein the second propeller  19   b  is arranged to rotate in a counter-clockwise direction, or vice versa, to propel the watercraft  11  forward. The first and second propeller shafts  26   a ,  26   b  are arranged in the form of dual propeller shafts. The first and second propeller shafts  26   a ,  26   b  are concentric and arranged to rotate in opposite directions to rotate the first and second propellers  19   a ,  19   b  in opposite directions. In the embodiment of  FIG. 3  the first propeller shaft  26   a  extends through the second propeller shaft  26   b  and through the second propeller  19   b  to the first propeller  19   a . Hence, the first propeller shaft  26   a  is arranged with smaller diameter than the second propeller shaft  26   b . Further, the first propeller shaft  26   a  is longer than the second propeller shaft  26   b . The propeller shafts  26   a ,  26   b  are arranged in the torpedo-shaped part  21  of the lower unit  17 . In the embodiment of  FIG. 3  the propeller shafts  26   a ,  26   b  are arranged perpendicular to the motor shafts  25   a ,  25   b.    
     The outboard motor  10  comprises a first power transfer arrangement for transferring output power from the first motor shaft  25   a  to the first propeller shaft  26   a , and a second power transfer arrangement for transferring output power from the second motor shaft  25   b  to the second propeller shaft  26   b . For example, the first power transfer arrangement is arranged separately from the second power transfer arrangement, wherein the power from the first motor shaft  25   a  is only transferred to the first propeller shaft  26   a  and wherein the power from the second motor shaft  25   b  is only transferred to the second propeller shaft  26   b . In the embodiment of  FIG. 3 , the first power transfer arrangement includes a first drive shaft  27   a , and the second power transfer arrangement includes a second drive shaft  27   b . The first propeller shaft  26   a  is connected to the first motor shaft  24   a  through the first drive shaft  27   a  to rotate the first propeller shaft  26   a . Hence, the first drive shaft  27   a  has an axis of rotation. For example, the first propeller shaft  26   a  is connected to the first motor shaft  24   a  through the first drive shaft  27   a  to rotate the first propeller shaft  26   a  in a first direction, such as clockwise, for propelling the watercraft  11  in a forward direction. The second propeller shaft  26   b  is connected to the second motor shaft  24   b  through the second drive shaft  27   b  to rotate the second propeller shaft  26   b . Hence, the second drive shaft  27   b  has an axis of rotation. For example, the second propeller shaft  26   b  is connected to the second motor shaft  24   b  through the second drive shaft  27   b  to rotate the second propeller shaft  26   b  in a second direction opposite to the first direction, such as counter-clockwise, for propelling the watercraft  11  in a forward direction. In the embodiment of  FIG. 3  the first power transfer arrangement includes the first drive shaft  27   a  and any other elements for transferring output power from the first motor shaft  25   a  to the first propeller shaft  26   a , wherein the second power transfer arrangement includes the second drive shaft  27   b  and any other elements for transferring output power from the second motor shaft  25   b  to the second propeller shaft  26   b . In the illustrated embodiment, the first power transfer arrangement is arranged separately from the second power transfer arrangement, so that the first propeller shaft  26   a  is driven by means of only the first electric motor  24   a , and the second propeller shaft  26   b  is driven by means of only the second electric motor  24   b.    
     In the illustrated embodiment, the drive shafts  27   a ,  27   b  extend perpendicular to the propeller shafts  26   a ,  26   b . For example, the drive shafts  27   a ,  27   b  extend in parallel to the motor shafts  25   a ,  25   b  and, e.g. substantially vertically when the outboard motor  10  is operated. According to the illustrated embodiment, the drive shafts  27   a ,  27   b  extend from the power head  15 , through the midsection  16  and into the lower unit  17  of the outboard motor  10 . In the embodiment of  FIG. 3  the axis of rotation of the first drive shaft  27   a  is displaced to the axis of rotation of the first motor shaft  25   a . Hence, the first drive shaft  27   a  is offset in relation to the first motor shaft  25   a , so that they are not aligned and not coaxial. For example, the first drive shaft  27   a  is arranged in parallel to the first motor shaft  25   a . In the illustrated embodiment, also the axis of rotation of the second drive shaft  27   b  is displaced to the axis of rotation of the second motor shaft  25   b . Hence, the second drive shaft  27   b  is offset in relation to the second motor shaft  25   b , so that they are not aligned and not coaxial. For example, the second drive shaft  27   b  is arranged in parallel to the second motor shaft  25   b . Hence, at least one of the drive shafts  27   a ,  27   b  is displaced in relation to the motor shaft  25   a ,  25   b  it is connected to, so that a distance between the drive shafts  27   a ,  27   b , i.e. a distance between the axes of rotation thereof, is smaller than a distance between the motor shafts  25   a ,  25   b , i.e. a distance between the axes of rotation thereof. 
     In the illustrated embodiment, the first drive shaft  27   a  is connected to the first propeller shaft  25   a  through a first bevel gear  28   a  for transferring output power from the first drive shaft  27   a  to the first propeller shaft  26   a , wherein the second drive shaft  27   b  is connected to the second propeller shaft  26   b  through a second bevel gear  28   b  for transferring output power from the second drive shaft  27   b  to the second propeller shaft  26   b.    
     In the embodiment of  FIG. 3  the first motor shaft  25   a  is connected to the first drive shaft  27   a  through first power transfer device in the form of a first connection shaft  29   a  and bevel gears  30 ,  31 . The first connection shaft  29   a  is arranged perpendicular to the first motor shaft  25   a  and to the first drive shaft  27   a . For example, the first connection shaft  29   a  extends in the fore-aft direction, such as substantially horizontally in a longitudinal direction of the watercraft  11  when the outboard motor  10  is operated. For example, the second motor shaft  25   b  is connected to the second drive shaft  27   b  through a second power transfer device in the form of a second connection shaft  29   b  and bevel gears  32 ,  33  in a similar manner. In the illustrated embodiment, the first and second connection shafts  29   a ,  29   b  are arranged with different lengths. Alternatively, the first and second connection shafts  29   a ,  29   b  are arranged with similar lengths. In the illustrated embodiment, the first and second connection shafts  29   a ,  29   b  are arranged at the same level, such as in a common plane perpendicular to the drive shafts  27   a ,  27   b . For example, the first and second connection shafts  29   a ,  29   b  are coaxial, optionally with a gap between them. Alternatively, axis of rotation of the first and second connection shafts  29   a ,  29   b  are displaced in relation to each other. For example, one of the first and second connection shafts  29   a ,  29   b  is arranged below the other. 
     With reference to  FIG. 4  an alternative embodiment is illustrated, wherein the first motor shaft  25   a  is connected to the first drive shaft  27   a  through the first power transfer device in the form of an endless loop flexible drive coupling  34 , such as a belt or a chain. For example, the endless loop flexible drive coupling  34  is a toothed belt interacting with corresponding teeth on the first motor shaft  25   a  and the first drive shaft  27   a  or pulleys arranged thereon. Hence, the first drive shaft  27   a  is displaced in relation to the first motor shaft  25   a  as described above to reduce the distance between the first and second drive shafts  27   a ,  27   b  in relation to the distance between the first and second motor shafts  25   a ,  25   b . Hence, the endless loop flexible drive coupling  34  extends in a direction perpendicular to the first motor shaft  25   a  and to the first drive shaft  27   a . For example, the endless loop flexible drive coupling  34  is arranged substantially horizontally when the outboard motor  10  is operated. 
     For example, the second drive shaft  27   b  is displaced in relation to the second motor shaft  25   b  in a similar manner by means of the second power transfer device. In the embodiment of  FIG. 4 , the second motor shaft  25   b  is connected to the second drive shaft  27   b  through the second power transfer device in the form of at least first and second cogwheels  35 ,  36 . For example, the cogwheels  35 ,  36  are arranged substantially horizontally when the outboard motor  10  is operated. Alternatively, the second motor shaft  25   b  is connected to the second drive shaft  27   b  through an endless loop flexible drive coupling. According to alternative embodiments, the first motor shaft  25   a  is connected to the first drive shaft  27   a  through the first connection shaft  29   a , the endless loop flexible drive coupling  34  or cogwheels, wherein the second motor shaft  25   b  is connected to the first second shaft  27   b  through the second connection shaft  29   b , an endless loop flexible drive coupling or cogwheels. Hence, the first power transfer device is arranged for connecting the first motor shaft  25   a  and the first drive shaft  27   a  for transferring torque from the first motor shaft  25   a  to the first drive shaft  27   a . For example, the first power transfer device is arranged to transfer toque only from the first motor shaft  25   a  to the first drive shaft  27   a . The first power transfer device is, e.g. the first connection shaft  29   a , the endless loop flexible drive coupling  34 , a plurality of cogwheels or similar power transfer device. The second power transfer device is arranged for connecting the second motor shaft  25   b  and the second drive shaft  27   b  for transferring torque from the second motor shaft  25   b  to the second drive shaft  27   b . For example, the second power transfer device is arranged to transfer toque only from the second motor shaft  25   b  to the second drive shaft  27   b . The second power transfer device is, e.g. the second connection shaft  29   b , an endless loop flexible drive coupling, a the two or more cogwheels  35 ,  36  or similar power transfer device. 
     For example, the first and second drive shafts  27   a ,  27   b  are arranged in parallel or substantially in parallel. In the illustrated embodiment the first and second drive shafts  27   a ,  27   b  extend along the midsection  16  and into the lower unit  17 , wherein the first and second drive shafts  27   a ,  27   b  extend vertically or substantially vertically when the outboard motor  10  is operated (depending on trim) to transfer power in the same direction. The drive shafts  27   a ,  27   b  connect the motor shafts  25   a ,  25   b  and the propeller shafts  26   a ,  26   b , optionally through the connection shafts  29   a ,  29   b , endless loop flexible drive couplings or cogwheels, and transfers rotational power from the drive shafts  27   a ,  27   b  to the propeller shafts  26   a ,  26   b , e.g. through the bevel gears  28   a ,  28   b . In the embodiment of  FIGS. 3 and 4  the first drive shaft  27   a  is substantially equal in length as the second drive shaft  27   b . Alternatively, the first drive shaft  27   a  is longer than the second drive shaft  27   b . In the illustrated embodiment the first and second drive shafts  27   a ,  27   b  are arranged below the motors  24   a ,  24   b.    
     For example, the connection shafts  29   a ,  29   b  and the propeller shafts  26   a ,  26   b  are arranged in parallel or substantially in parallel. For example, the propeller shafts  26   a ,  26   b , the drive shafts  27   a ,  27   b , the connection shafts  29   a ,  29   b  and the motor shafts  25   a ,  25   b  are arranged in a common plane, such as a common vertical plane when the outboard motor  10  is mounted on the watercraft  11 . For example, the connection shafts  29   a ,  29   b  and the propeller shafts  26   a ,  26   b  are arranged horizontally or substantially horizontally when the outboard motor  10  is in a non-tilted operational position for propelling the watercraft  11  and the trim is neutral, wherein the motor shafts  25   a ,  25   b  and the drive shafts  27   a ,  27   b  are arranged vertically or substantially vertically. 
     According to one embodiment the outboard motor  10  does not comprise any clutch. For example, the outboard motor  10  does not comprise any gearbox for reversing the rotational direction of the output power. For example, the electric motors  24   a ,  24   b  are arranged for allowing seamless control from zero to maximum rpm of the output power of the motor shafts  25   a ,  25   b  in any of the selected clockwise or counter-clockwise rotational direction. For example, the output power from the electric motors  24   a ,  24   b  is reversible, such as fully reversible, wherein the propellers  19   a ,  19   b  can be driven in a forward mode as well as a reverse mode by the electric motors  24   a ,  24   b . Hence, the rotational power from the electric motors  24   a ,  24   b  can be transferred to the propeller shafts  26   a ,  26   b  in either rotational direction for full motor power forward or full motor power in reverse. 
     The outboard motor  10  comprises a drive housing  37  and the motor housing  18  for receiving the electric motors  24   a ,  24   b , the drive shafts  27   a ,  27   b , the bevel gears  28   a ,  28   b  and the propeller shafts  26   a ,  26   b  and optionally also the connection shafts  29   a ,  29   b . The drive housing  37  and the motor housing  18  provides functions of structural support, spacing and enclosing for other components of the outboard motor  10 , such as the electric motors  24   a ,  24   b , the drive shafts  27   a ,  27   b , the bevel gears  28   a ,  28   b  and the propeller shafts  26   a ,  26   b  and optionally also the connection shafts  29   a ,  29   b , and also supports the propellers  19   a ,  19   b  through the propeller shafts  26   a ,  26   b  being supported by the drive housing  37 . For example, the drive housing  37  extends from the motor housing  18  to the skeg  20 . According to one embodiment of the invention the drive housing  37  is formed with a water inlet or a water pickup for cooling. The drive housing  37  is, for example, formed in a composite material or any other suitable material. The propeller shafts  26   a ,  26   b  are positioned partially in the drive housing  37 , wherein outer portions thereof project out from the drive housing  37  for carrying the propellers  19   a ,  19   b.    
     The first and second electric motors  24   a ,  24   b  are connected to a battery  38 , which is illustrated schematically by means of dashed lines in  FIG. 1 . For example, both of the first and second electric motors  24   a ,  24   b  are connected to a single battery  38 . Alternatively, the first electric motor  24   a  is connected to a first battery dedicated to provide power to only the first electric motor  24   a , wherein the second electric motor  24   b  is connected to a second battery dedicated to provide power only to the second electric motor  24   b . The battery  38  is arranged outside the outboard motor  10 . In the illustrated embodiment, the battery  38  is arranged on the watercraft  11 . Hence, the watercraft  11  comprises a battery compartment, e.g. in the hull  12  or within the hull  12 . The electric motors  24   a ,  24   b  are, e.g. connected to the battery  38  through a cable  39  extending between the outboard motor  10  and the battery  38 . Alternatively, the first electric motor  24   a  is connected to the battery  38  or a first battery through a first cable, wherein the second electric motor  24   b  is connected to the battery  38  or a second battery through a second cable. 
     With reference to  FIG. 5  another embodiment is illustrated, wherein the first and second motors  24   a ,  24   b  are in standing positions and the first motor shaft  25   a  is directly connected to the first drive shaft  27   a  and the second motor shaft  25   b  is directly connected to the second drive shaft  27   b . Hence, the first drive shaft  27   a  is aligned to and coaxial to the first motor shaft  25   a , wherein the second drive shaft  27   b  is aligned to and coaxial to the second motor shaft  25   b . The motor shafts  24   a ,  24   b  and the drive shafts  27   a ,  27   b  are arranged substantially vertically when the outboard motor  10  is operated. 
     For example, the first and second drive shafts  27   a ,  27   b  are arranged in parallel or substantially in parallel. In the illustrated embodiment the first and second drive shafts  27   a ,  27   b  extend along the midsection  16  and into the lower unit  17 , wherein the first and second drive shafts  27   a ,  27   b  extend vertically or substantially vertically when the outboard motor  10  is operated (depending on trim) to transfer power in the same direction. The drive shafts  27   a ,  27   b  connect the motor shafts  25   a ,  25   b  and the propeller shafts  26   a ,  26   b , and transfers rotational power from the motor shafts  25   a ,  25   b  to the propeller shafts  26   a ,  26   b , e.g. through the bevel gears  28   a ,  28   b . For example, the propeller shafts  26   a ,  26   b  are arranged horizontally or substantially horizontally when the outboard motor  10  is in a non-tilted operational position for propelling the watercraft  11  and the trim is neutral, wherein the motor shafts  25   a ,  25   b  and the drive shafts  27   a ,  27   b  are arranged vertically or substantially vertically. 
     With reference to  FIG. 6  an alternative embodiment is illustrated, wherein the first and second motors  24   a ,  24   b  are in lying positions and the first motor shaft  25   a  is connected to the first drive shaft  27   a  through the bevel gears  30  and the second motor shaft  25   b  is connected to the second drive shaft  27   b  through the bevel gears  32 . Hence, the first drive shaft  27   a  is perpendicular to the first motor shaft  25   a , wherein the second drive shaft  27   b  is perpendicular to the second motor shaft  25   b . The motor shafts  24   a ,  24   b  are arranged substantially horizontally and the drive shafts  27   a ,  27   b  are arranged substantially vertically when the outboard motor  10  is operated. 
     In the embodiment of  FIG. 6 , the first and second motor shafts  25   a ,  25   b  are extending toward each other. For example, the first motor shaft  25   a  extends aftward while the second motor shaft  25   b  extends forward. In the illustrated embodiment, the first and second motor shafts  25   a ,  25   b  are aligned and coaxial, e.g. with a gap or bearing between them. The first and second drive shafts  27   a ,  27   b  are arranged in parallel or substantially in parallel and extend vertically or substantially vertically when the outboard motor  10  is operated. The drive shafts  27   a ,  27   b  connect the motor shafts  25   a ,  25   b  and the propeller shafts  26   a ,  26   b , and transfers rotational power from the motor shafts  25   a ,  25   b  to the propeller shafts  26   a ,  26   b , e.g. through the bevel gears  28   a ,  28   b . In the embodiment of  FIG. 6  the first and second motor shafts  25   a ,  25   b  are arranged in parallel to the propeller shafts  26   a ,  26   b.    
     With reference to  FIG. 7  another alternative embodiment is illustrated, wherein the first and second motors  24   a ,  24   b  are in lying positions and the first motor shaft  25   a  is connected to the first drive shaft  27   a  through the bevel gears  30  and the second motor shaft  25   b  is connected to the second drive shaft  27   b  through the bevel gears  32 . Hence, the first drive shaft  27   a  is perpendicular to the first motor shaft  25   a , wherein the second drive shaft  27   b  is perpendicular to the second motor shaft  25   b . The motor shafts  24   a ,  24   b  are arranged substantially horizontally and the drive shafts  27   a ,  27   b  are arranged substantially vertically when the outboard motor  10  is operated. 
     In the embodiment of  FIG. 7 , the first and second motor shafts  25   a ,  25   b  are arranged in parallel and extend in the same direction, such as aftward. For example, the first motor  24   a  is displaced vertically in relation to the second motor  24   b , wherein the first motor  24   a  is arranged above the second motor  24   b . Hence, in the embodiment of  FIG. 7  the first drive shaft  27   a  is longer then the second drive shaft  27   b . In the illustrated embodiment, the first motor  24   a  is displaced also aftward in relation to the second motor  24   b . Alternatively, the first motor  24   a  is arranged straight above the second motor  24   b , wherein the first motor shaft  25   a  is longer than the second motor shaft  25   b  or extended in another suitable manner, such as by a power transfer device. The first and second drive shafts  27   a ,  27   b  are arranged in parallel or substantially in parallel and extend vertically or substantially vertically when the outboard motor  10  is operated. The drive shafts  27   a ,  27   b  connect the motor shafts  25   a ,  25   b  and the propeller shafts  26   a ,  26   b , and transfers rotational power from the motor shafts  25   a ,  25   b  to the propeller shafts  26   a ,  26   b , e.g. through the bevel gears  28   a ,  28   b . In the embodiment of  FIG. 7  the first and second motor shafts  25   a ,  25   b  are arranged in parallel to the propeller shafts  26   a ,  26   b.    
     All embodiments disclose an outboard motor  10 . Hence, the motors  24   a ,  24   b , the motor shafts  25   a ,  25   b , the drive shafts  27   a ,  27   b  and the propeller shafts  26   a ,  26   b  are in a fixed configuration in relation to each other. 
     With reference to  FIG. 8  another alternative embodiment is illustrated, wherein the first and second motors  24   a ,  24   b  are in lying positions similar to the embodiment of  FIG. 6  and wherein the first power transfer arrangement includes a first endless loop flexible drive coupling  40   a , and the second power transfer arrangement includes a second endless loop flexible drive coupling  40   b . For example, the first and second endless loop flexible drive couplings  40   a ,  40   b  are arranged as one or more belts or one or more chains. For example, the first and second endless loop flexible drive couplings  40   a ,  40   b  are arranged as toothed belts. The first propeller shaft  26   a  is connected to the first motor shaft  24   a  through the first endless loop flexible drive coupling  40   a  to rotate the first propeller shaft  26   a  in a first direction, such as clockwise, for propelling the watercraft  11  in a forward direction. The second propeller shaft  26   b  is connected to the second motor shaft  24   b  through the second endless loop flexible drive coupling  40   b  to rotate the second propeller shaft  26   b  in a second direction opposite to the first direction, such as counter-clockwise, for propelling the watercraft  11  in a forward direction. 
     In the embodiment of  FIG. 8  the first power transfer arrangement includes the first endless loop flexible drive coupling  40   a  and any other elements for transferring output power from the first motor shaft  25   a  to the first propeller shaft  26   a , wherein the second power transfer arrangement includes the second endless loop flexible drive coupling  40   b  and any other elements for transferring output power from the second motor shaft  25   b  to the second propeller shaft  26   b . Such other elements can optionally include pulleys, cogwheels and similar for use in combination with belts or chains in a conventional manner. In the illustrated embodiment, the first endless loop flexible drive coupling  40   a  is arranged separately from the second endless loop flexible drive coupling  40   b , so that the first propeller shaft  26   a  is driven by means of only the first electric motor  24   a , and the second propeller shaft  26   b  is driven by means of only the second electric motor  24   b . The motor shafts  24   a ,  24   b  are arranged substantially in parallel to the propeller shafts  26   a ,  26   b.    
     In the embodiment of  FIG. 9 , the first and second motor shafts  25   a ,  25   b  are arranged in parallel and extend in the same direction, such as aftward, in a similar manner as described with reference to  FIG. 7 . In the embodiment of  FIG. 9  the first power transfer arrangement includes the first endless loop flexible drive coupling  40   a , and the second power transfer arrangement includes the second endless loop flexible drive coupling  40   b  in a similar manner as described with reference to the embodiment of  FIG. 8 . Hence, in the embodiment of  FIG. 9  the first endless loop flexible drive coupling  40   a  is longer then the second endless loop flexible drive coupling  40   b . The first and second endless loop flexible drive couplings  40   a ,  40   b  are arranged in parallel or substantially in parallel and extend substantially perpendicular to the motor shafts  24   a ,  24   b  and the propeller shafts  26   a ,  26   b . For example, legs of the first and second endless loop flexible drive couplings  40   a ,  40   b  extend vertically or substantially vertically when the outboard motor  10  is operated. In the embodiment of  FIG. 8 , the first endless loop flexible drive coupling  40   a  is arranged aftward of the second endless loop flexible drive coupling  40   b . The motors  24   a ,  24   b , the motor shafts  25   a ,  25   b , the endless loop flexible drive couplings  40   a ,  40   b  and the propeller shafts  26   a ,  26   b  are in a fixed configuration in relation to each other.