TECHNICAL FIELD

The present invention relates to a marine propulsion unit and a marine vessel with such a propulsion unit.

BACKGROUND

Known marine vessels comprising a propulsion unit in the form of a stern 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 stern drive via a transmission comprising shafts and gearing in order to drive a set of propellers on the stern drive.

Mounting a drive unit, such as an ICE or an electric motor, and the transmission required for such a drive unit within the hull of the vessel can require a significant amount of space. In operation, heat from the drive unit must be removed using a cooling system which as a rule employs water drawn in from the ambient marine environment. This often involves drawing in saline water from the sea and pumping it through the coolant system, which can cause problems with corrosion. Further, a vibration generated by rotary components in the drive unit and the transmission requires vibration isolation and dampers to be installed to avoid undesirable vibrations from being transmitted to the hull or other parts of the vessel. Finally, the transmission must pass through the transom of the vessel to reach the stern drive and the propellers. This requires a suitable sealing arrangement between an opening in the transom and a rotary transmission shaft to prevent water from leaking through the hull.

A possible solution to the above problems can be to provide an azimuthing propulsion unit or pod extending downwards beneath the hull. An example of such an azimuthing pod is shown in US 6 685 516. In this case the drive unit and its transmission can be mounted within a pod at one end of a leg extending downwards from the hull. However, this solution entails a significant draft and is mainly suited for larger vessels. In addition to the relatively large draft, it is not possible to tilt the propulsion unit out of the water when not in use. This will in turn increase the amount of marine growth on the submerged propulsion unit, which increases drag and reduces the efficiency of the propeller/-s. The invention provides an improved marine propulsion unit aiming to solve the above- mentioned problems.

SUMMARY

An object of the invention is to provide a marine propulsion unit for a vessel, which propulsion unit solves the above-mentioned problems.

The object is achieved by a marine propulsion unit and a marine vessel comprising such a propulsion unit according to the appended claims.

In the subsequent text, as a non-limiting example, the term “stern drive” may be defined as an assembly comprising an outdrive having two sub-units. An upper unit contains drive units and a transmission and is enclosed in a stern drive housing. A lower unit contains at least a portion of a driveshaft assembly, for instance a vertical driveshaft, receiving power from the transmission in the upper unit and a gearbox providing power to a propeller shaft for driving at least one propeller. The component parts of the lower unit are enclosed in a gearbox housing. The upper and lower units may be separated by a cavitation plate. A stern drive according to the invention is mounted to the transom of a marine vessel but differs from a conventional stern drive in that it does not comprise an inboard drive unit. The vessel is steered by pivoting the propulsion unit, or outdrive, relative to the transom. The propulsion unit can be pivoted up for trailer travel and between uses to avoid fouling. In the subsequent text, a stern drive according to the invention can be described as having a front portion or a rear portion. In this context, the front end of the stern drive will be the end arranged to be mounted on a transom on a marine vessel. This terminology is in line with the conventional definition of a front end and a rear end of a vessel in the main longitudinal direction thereof. Directional terms such as “forward”, “rearward”, “transverse" and “sideways" are to be interpreted according to this definition. The main longitudinal direction of the vessel also coincides with its direction of straight-ahead travel. Further, a plane defined as “passing through an axis” is considered to coincide with and contain the entire axis.

According to a first aspect, the invention relates to a marine propulsion unit comprising a stern drive with a front end for mounting to a transom of a marine vessel, which stern drive comprises an upper unit enclosed in a stern drive housing and a lower unit enclosed in a gearbox housing. The gearbox housing contains a gearbox arranged to drive at least one propeller rotatable about a first axis. The stern drive housing contains at least two electric motors, which electric motors are in driving connection with a drive shaft assembly comprising one or more drive shafts. Each one of the one or more drive shafts is rotatable about a common second axis and connected to the gearbox. The drive shaft assembly comprises a set of gear races comprising at least two gear races. The set of gear races is located in the upper unit. Each electric motor of the at least two electric motors is in driving connection with a gear race of the set of gear races.

The above features imply that a relatively compact marine propulsion unit can be obtained. For instance, since each one of the drive shafts of the above-mentioned drive shaft assembly rotates around a common axis of rotation, it may be possible to arrive at a space efficient marine propulsion unit.

Preferably, each electric motor of the at least two electric motors is in driving connection with an individual gear race of the set of gear races. The electric motors are preferably high-speed motors as defined below. In the subsequent text, the above-mentioned first and second axes will be referred to when defining the mounting positions of the electric motors within the stern drive housing. The number of electric motors can be selected depending on the desired output power of the stern drive. Optionally, the second axis and the first axis form an angle being greater than 60°, preferably greater than 80°. This implies that a relatively compact marine propulsion unit can be obtained which can be attached to a transom in an appropriate manner.

Optionally, the stern drive is adapted assume a use position in which it is partially submerged in a body of water having a still water surface, wherein the upper unit is adapted to be located above the still water surface and the lower unit is adapted to be located below the still water surface.

Optionally, at least one, preferably each one, of the at least two electric motors has an output shaft comprising a pinion gear, preferably the pinion gear meshes with the gear race with which the electric motor is in driving connection.

Purely by way of example, the transmission may have a gear ratio of at least 3:1 from the output shafts of the electric motors to the associated gear race of the drive shaft assembly. In order to achieve a configuration that allows the stern drive housing to be kept to a size that is the same or marginally larger than a conventional stern drive housing, the size of the electric motors should be selected accordingly. One way of achieving this is to use high speed electric motors. In this context, a high-speed electric motor is defined as a motor operable at speeds up to approximately 10.000 rpm or higher. A suitable maximum rotational speed of the drive shaft assembly connected to the gearbox can be selected between 3.500-4.000 rpm, when it is desirable to use a conventional gearbox for the propellers in a stern drive normally operated by an ICE. For electrical motors operable at speeds up to 10.000 rpm a suitable gear ratio for the transmission in the upper unit would be 3:1 , while a suitable gear ratio for electrical motors operable up to 25.000 rpm would be 6:1.

Optionally, the output shaft of at least one, preferably each one, of the at least two electric motors has an axis of rotation being offset from the second axis. Such an offset implies that that the at least one motor may be arranged in the upper unit in a space efficient manner.

Optionally, for at least one, preferably each one, of the at least two electric motors, a smallest distance between an electric motor line being coaxial with the axis of rotation of the output shaft of the electric motor and a second axis line being coaxial with the second axis is greater than zero.

Optionally, the smallest distance is equal to or greater than 20%, preferably equal to or greater than 40%, of an outer diameter of the gear race with which the electric motor is in driving connection.

Optionally, each output shaft has an axis of rotation forming an angle with the second axis being at least 45°. Such an orientation of the output shafts also implies that the electric motors can be stored in a space efficient manner.

Optionally, at least one of, preferably each one of, the gear race of the set of gear races is a crown gear race.

Optionally, at least one of, preferably each one of, the gear race of the set of gear races is a hypoid gear race. Generally, a hypoid gear is a type of spiral bevel gear whose axis does not intersect with the axis of the meshing gear. As such, a hypoid gear race is a spiral bevel gear race whose axis does not intersect with the axis of the meshing gear. A hypoid gear comprises a ring-shaped crown gear or crown wheel having a planar pitch surface and a planar root surface, both of which are perpendicular to the axis of rotation. The shape of a hypoid gear is a revolved hyperboloid, that is, the pitch surface of the hypoid gear is a hyperbolic surface. In comparison, the shape of a conventional spiral bevel gear is normally conical. The hypoid gear places the axis of the pinion gear off-axis or offset to the axis of the crown gear which allows the pinion gear to be larger in diameter and have more contact area. The helical design produces less vibration and noise than conventional straight-cut or spur-cut gear with straight teeth. In hypoid gear design, the pinion gear and crown gear are practically always of opposite hand, and the spiral angle of the pinion is usually larger than that of the crown gear. The hypoid pinion is then larger in diameter than an equivalent angled bevel pinion.

Optionally, at least two gear races of the set of gear races are located on individual gears, the centre planes of which are separated by a distance in a direction being parallel to the second axis. The distance is at least 1 %, preferably at least 3%, of the length of the drive shaft assembly in a direction parallel to the second axis. The use of at least two individual gears implies a versatility in the arrangement of the electric motors.

Optionally, the at least two gear races of the individual gears separated by a distance in the direction being parallel to the second axis have gear teeth facing in the same direction. In this way, the set of gear teeth on each gear race can face either in an upward direction or in a downward direction.

Optionally, the at least two gear races of the individual gears separated by a distance in the direction being parallel to the second axis have gear teeth facing in opposite directions. In this way, the gear teeth on one of a pair of gear races faces in an upward direction while the gear teeth on the other gear race faces in a downward direction.

Combinations of two, three or more crown gears which can be arranged with facing and/or opposing teeth can be employed within the scope of the invention.

Optionally, the at least two gear races of the set of gear races are arranged on opposite sides of a double-sided gear, preferably a double-sided crown gear. This implies a compact solution with a relatively low number of components. Optionally, at least two electric motors are arranged on the same side of the drive shaft assembly relative to a plane through the second axis and at right angles to the first axis. Alternatively, at least two electric motors are arranged on opposite sides of the drive shaft assembly relative to a plane through the second axis and at right angles to the first axis.

In this way, two, three, four or more electric motors can be arranged in front of, to the rear of or distributed on opposite sides of the vertical drive shaft relative to plane through the second axis and at right angles to the first axis. The location of the electric motors can for instance be selected in dependence of the available packaging space within the stern drive housing.

Optionally, the output shaft of at least one electric motor has an axis of rotation being parallel to the first axis. For instance, if it is desired to provide a stern drive housing having relatively limited dimensions in the transverse direction of the housing, then it is advantageous to locate the output shaft of at least one electric motor parallel to the first axis. In this way the electric motors will extend forwards and/or rearwards in the longitudinal direction of the stern drive housing, which helps in reducing the overall width of the stern drive housing. This arrangement can assist in balancing the mass of the electric motors on either side of the center of gravity of the stern drive housing and would also contribute to a reduced overall width of the stern drive housing.

Optionally, the output shaft of at least one electric motor has an axis of rotation arranged at an angle out of a plane formed by the first axis and second axis.

Optionally, the output shaft of at least one electric motor has an axis of rotation forming a reference angle, in a reference plane the normal of which extends parallel to the second axis, with the plane formed by the first axis and second axis. The reference angle is in the range of 5° - 45°, preferably 15° - 45°.

The axis of the at least one output shaft can be angled out of the plane formed by the first axis and second axis from a position located in front of or to the rear of the second axis depending on the location of the respective electric motor. In this way, the mass of the electric motors can be located relatively close to the center of gravity of the stern drive housing while the output shaft would be aimed outwards towards an annular toothed periphery of the corresponding gear race. Optionally, the output shaft of at least one electric motor has an axis of rotation arranged in a plane parallel to the plane of the gear race with which the electric motor is in driving connection.

Optionally, the output shaft of at least one electric motor has an axis of rotation angled up to 30°, preferably in the range of 10° to 30°, out of the plane of the gear race with which the electric motor is in driving connection. This would allow for a reduction in the longitudinal extension of the stern drive housing but would require additional height. An arrangement of this type could for instance be useful for solutions involving a double-sided gear, such as a double-sided crown gear, as it allows the electric motors to be mounted with a larger spacing. This will provide easier access for servicing and improves the cooling of the electrical motors. Also, it would allow larger electrical motors to befitted, which motors might not fit side-by-side if mounted parallel with a plane through the crown gear.

Optionally, the drive shaft assembly comprises a pair of concentric shafts connected to different electric motors via separate gear races. In this way, electrical motors connected to individual crown gears can drive separate propellers in a duo prop arrangement rotatable about the first axis. This arrangement allows individual propellers to be driven at different speeds or operation of one propeller only.

Optionally, at least one electric motor is permanently connected to the drive shaft assembly, while the remaining electric motors are freewheeling motors. Under certain operating conditions the propulsion unit can be arranged to drive the at least one propeller with at least one electric motor. The electric motors not being operated can be allowed to free wheel in order to improve efficiency. At least one electrical motor is permanently in driving connection with its associated gear race in order to allow one or more propellers to be reversed. One operating condition can be that the demanded or required power output from the propulsion unit can be achieved by operating fewer than the total number of available electric motors. The electric motors can drive the propellers together, independently or in variable combinations in response to different torque and power demands in order to improve the efficiency of the propulsion unit. The effect for instance of a hypoid gear is to allow the use of high-speed electric motors with a corresponding reduced output torque. In this way the cost is of the propulsion unit is lowered, while the electric motors can be operated in a high-efficiency area.

A further operating condition can be a so-called limp-home mode in which the propulsion unit is arranged to drive the at least one propeller when at least one or only one electric motor is operable. This arrangement provides a redundancy for the propulsion unit and ensures that the vessel can be operated even if one or more electric motors are inoperable.

Optionally, at least one gear race, preferably each gear race, of the set of gear races is rigidly connected to a drive shaft of the drive shaft assembly.

A second aspect of the present invention relates to a vessel comprising a transom and a marine propulsion unit according to the first aspect of the present invention.

In the examples described above, each set of annular gear teeth on a respective gear race can be driven by one or more electric motors, depending on the desired power requirement for the stern unit.

Examples of such arrangements will be described in further detail below.

Should multiple motors be used, they can be stacked in the vertical direction which would add to the height, or vertical dimension of the stern drive housing. As more than one electrical motor can be in driving connection with a single gear, the height is dependent on factors such as the number of electrical motors, the location of each electrical motor, the angle out of the plane formed by the first axis and second axis and in a plane parallel to the plane of the gear race, as well as the angle out of the plane of the gear race.

If the dimensions of the stern drive housing are less relevant to the overall design, the electric motors can be distributed around the drive shaft with the output shafts at any suitable angle out of a vertical plane through the first axis. The marine propulsion unit according to the invention solves the problem of providing a stern drive with electric propulsion without requiring significant modifications of existing units.

In most cases the marine propulsion unit can be advantageously be provided with a stern drive housing having the same or approximately the same shape and size as conventional stern drive housings. Further, the interface for mounting a stern drive and its steering gear connections can be maintained. As the inboard drive unit can be eliminated there is no need for an opening through the transom or for an associated sealing means for a drive shaft. The electric motors can drive the propellers together, independently or in variable combinations in response to different torque and power demands whereby the efficiency of the propulsion unit is improved. By allowing independent operation of at least a single motor the arrangement provides a redundancy for the propulsion unit and ensures that the vessel can be operated even if one or more electric motors are inoperable. Moreover, as an example, the use of hypoid gearings in a power transmission provides a more efficient gearing than a conventional worm or bevel gearing. Hypoid gears are considerably stronger in that any load is conveyed through multiple teeth simultaneously, which also makes it more silent compared to worm or bevel gearings.

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:

Figurel shows a side view of a schematically illustrated vessel comprising a marine propulsion unit according to the invention;

Figure2 shows a schematic side view of a propulsion unit according to a first example; Figure3 shows a schematic side view of a propulsion unit according to a second example; Figure4 shows a schematic side view of a propulsion unit according to a third example; Figure5 shows a schematic side view of a propulsion unit according to a fourth example; Figure6 shows a schematic plan view of a propulsion unit according to a fifth example; Figure7 shows a schematic plan view of a propulsion unit according to a sixth example;

Figure8 shows a schematic plan view of a propulsion unit according to a seventh example, and

Figure9 shows a schematic perspective view of a hypoid gear suitable for use in the propulsion unit.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Figure 1 shows a side view of a schematically illustrated marine vessel 100 comprising a marine propulsion unit 103 according to the invention. The marine propulsion unit 103 is mounted to a transom 102 on the vessel 100. Electric motors (see Figure2) in the marine propulsion unit 103 may be connected to an inboard battery pack 104 via suitable wiring 105. The battery pack 104 is indicated schematically in Figure 1 and is preferably located below the waterline of the vessel hull 101 where it can act as ballast and contribute to the stability of the vessel 100.

Purely by way of example, the marine propulsion unit 103 may be controllable by a control means such as a throttle lever 110 located at an operating position. The throttle lever 110 is connected to an electronic control unit (ECU) 111 via suitable wiring 112, which ECU 111 is connected to the battery pack 104 via additional wiring 113. The battery pack also may also comprise a power electronic controller (PEC) and an electronic controller for calibrating and charging the battery pack. Electronic controllers of this type are known in the art and will not be described in further detail here.

Figure 2 shows a schematic side view of a marine propulsion unit according to a first example. Figure 2 shows a stern drive 200 suitable for mounting to a transom of a marine vessel (see Figurel ; “102”). In Figure 2 the transom would be located at the left-hand side of the stern drive 200, which is defined as the front end of the stern drive. Similarly, the propellers are located at the opposite, rear end of the stern drive. The Figure 2 marine stern drive 200 comprises an upper unit 201 enclosed in a stern drive housing 203 and a lower unit 202 enclosed in a gearbox housing 204. The gearbox housing 204 contains a gearbox 205 arranged to drive at least one propeller 206, 207 rotatable about a first axis Xi.

Moreover, as indicated in Figure 2, the stern drive housing 203 contains at least two electric motors 211 , 212, which electric motors are in driving connection with a drive shaft assembly 210 comprising one or more drive shafts, each one of the one or more drive shafts being rotatable about a common second axis X2 and connected to the gearbox 205. In the embodiment illustrated in Figure 2, the drive shaft assembly 210 is constituted by a single drive shaft. However, as will be elaborated on hereinbelow with reference to e.g. Figure 3, it is also envisaged that the drive shaft assembly 210 may comprise two or more drive shafts.

As a non-limiting example, and as indicated in Figure 2, the second axis X2 and the first axis Xi may form an angle being greater than 60°, preferably greater than 80°. In the Figure 2 embodiment, the second axis X2 and the first axis Xi forms an angle of 90°. Furthermore, though purely by way of example, the stern drive 200 may be adapted assume a use position in which it is partially submerged in a body of water having a still water surface SWS and the upper unit 201 may be adapted to be located above the still water surface SWS and the lower unit 202 may be adapted to be located below the still water surface SWS.

Moreover, as indicated in Figure 2, the drive shaft assembly 210 comprises a set of gear races comprising at least two gear races 217, 218, the set of gear races being located in the upper unit 201. Furthermore, each electric motor of the at least two electric motors 211 , 212 is in driving connection with a gear race 217, 218 of the set of gear races.

Purely by way of example, and as indicated in Figure 2, each electric motor of the at least two electric motors 211 , 212 is in driving connection with an individual gear race 217, 218 of the set of gear races. However, as will be elaborated on hereinbelow with reference to Figure 7 for instance, it is also envisaged that two or more electric motors 211 , 212 may be in driving connection with the same gear race 217, 218 of the set of gear races.

As exemplified in Fig, 2, at least one, preferably each one, of the at least two electric motors 211 , 212 may have an output shaft 213, 214 comprising a pinion gear 215, 216. Preferably, and as also indicated in Figure 2, each pinion gear 215, 216 may mesh with the gear race 217, 218 with which the electric motor 211 , 212 is in driving connection.

As a non-limiting example, the least one of, preferably each one of, the gear race 217,

218 of the set of gear races is a crown gear race. Purely by way of example, at least one of, preferably each one of, the gear race 217, 218 of the set of gear races is a hypoid gear race.

Further, as illustrated as a non-limiting example in Figure 2, the output shaft of at least one electric motor has an axis of rotation X ₃ arranged in a plane parallel to the plane of the gear race 217, 218 with which the electric motor is in driving connection. As a non-limiting example, at least one electric motor 211, 212 may be permanently connected to the drive shaft assembly 210, while the remaining electric motors may be freewheeling motors. Furthermore, though again by way of example only, at least one gear race 217, 218, preferably each gear race, of the set of gear races is rigidly connected to a drive shaft of the drive shaft assembly 210.

In the Figure 2 example, the electric motors 211 , 212 are mounted horizontally with their output shafts 213, 214 extending towards a respective gear race 217, 218. Here, the term “horizontally” is intended to refer to an extension in a horizontal plane when the marine propulsion unit is in its intended use position.

To this end, though purely by way of example, as indicated in Figure 2, at least one, preferably each one, of the at least two electric motors 211 , 212 has an output shaft 213, 214 comprising a pinion gear 215, 216. As a non-limiting example, each pinion gear 215, 216 meshes with the gear race 217, 218 with which the electric motor is in driving connection.

In the Figure 2 embodiment, the gear races 217, 218 are located on individual gears, However, as will be elaborated on hereinbelow with reference to e.g. Figure 4, it is also envisaged that gear races 217, 218 may be arranged on opposite sides of a double-sided gear, preferably a double-sided crown gear.

Furthermore, as indicated in Figure 2, the centre planes of the individual gears may be separated by a distance h in a direction being parallel to the second axis X ₂ . As a non limiting example, the distance h may be at least 1%, preferably at least 3%, of the length H of the drive shaft assembly 210 in a direction parallel to the second axis X ₂ .

Additionally, in the Figure 2 example, the at least two gear races 217, 218 of the individual gears separated by a distance in the direction being parallel to the second axis X ₂ have gear teeth 227, 228 facing in the same direction.

As a non-limiting example, at least two electric motors 211 , 212 may be arranged on the same side of the drive shaft assembly 210 relative to a plane through the second axis X ₂ and at right angles to the first axis Xi. Here, the example in Figure 2 shows two electric motors 211 , 212 arranged axially separated in the vertical direction on the same side of and to the rear of the drive shaft assembly 210 relative to a plane through the second axis X ₂ and at right angles to the first axis Xi. The specific example in Figure 2 shows crown gears races 217, 218 with annular sets of gear teeth 227, 228 facing in the same direction. In Figure 2 the annular sets of gear teeth 227, 228 face upwards, but downwards facing teeth are also an option. An advantage with the arrangement shown in Figure 2 is that the downwards force applied to the crown gears races by the respective pinion gear can be taken up and supported by a bevel gear arrangement in the gearbox 205. The pinion gears 215, 216 and crown gears races 217, 218 in Figure 2 are preferably hypoid gears. A hypoid gear has the axis of the pinion gear offset to the axis of the crown gear. Hence, the electric motors 211 , 212 in Figure 2 can be arranged parallel and offset in the horizontal plane, or in the plane of the respective crown gear 217, 218. An example of such an arrangement is schematically indicated in Figure 6 below.

Within the scope of the invention, the marine propulsion unit can alternatively comprise two electric motors arranged on the same side of and front of the drive shaft assembly 210, or on opposite sides of the drive shaft assembly 210 relative to a plane through the second axis X2 and at right angles to the first axis Xi. Additional electric motors can be provided level with and in parallel to the electric motors shown in Figure 2. The number of motors is selected depending on the desired power output of the stern drive.

The gearbox 205 may comprise comprises bevel gears arranged to drive a first and a second drive shaft, respectively, to drive the pair of counter rotating propellers 206, 207. By mounting the electric motors 211 , 212 with their rotary axes in a horizontal direction and selecting high speed motors having a suitable size, the motors can be fitted within a stern drive housing 203. The interface for mounting the stern drive 200 to the transom and the connection to its steering gear (not shown) can be the same as the outdrive for a conventional stern drive.

Figure 3 shows a schematic side view of a propulsion unit according to a second example. In the Figure 3 embodiment, the drive shaft assembly 210 comprises a pair of concentric shafts 309, 310 connected to different electric motors 311 , 312 via separate gear races 317, 318.

Moreover, in contrast to the embodiment presented hereinabove with reference to Figure 2, Figure 3 illustrates an embodiment in which at least two gear races of the individual gears separated by a distance in the direction being parallel to the second axis X2 have gear teeth 327, 328 facing in opposite directions. As such, Figure 3 shows a stem drive 300 suitable for mounting to a transom of a marine vessel (see Figurel ; “102”). In Figure 3 the transom would be located at the left-hand side of the stern drive 300, which is defined as the front end of the stern drive. Similarly, the propellers are located at the opposite, rear end of the stern drive. The stern drive 300 comprises an upper unit 301 enclosed in a stern drive housing 303 and a lower unit 302 enclosed in a gearbox housing 304. The gearbox housing 304 contains a gearbox 305 arranged to drive a pair of coaxial shafts connected to counter rotating propellers 306, 307 rotatable about a first axis Xi. The propulsion unit in this example comprises two electric motors 311 , 312 arranged one above the other in the stern drive housing 303. The electric motors 311 , 312 are in the Figure 3 example mounted horizontally with their output shafts 313, 314 extending towards a respective crown gear 317, 318. Each output shaft 313, 314 comprises a pinion gear 315, 316 operatively connected to annular sets of gear teeth 327, 328 on its respective crown gear race 317, 318, which crown gear races 317, 318 are mounted fixed against rotation and axially separated on drive shaft assembly 210. Each shaft 309, 310 of the Figure 3 drive shaft assembly 210 is rotatable about a second axis X2 and is operatively connected to the gearbox 305.

The example in Figure 3 shows two electric motors 311 , 312 arranged axially separated in the vertical direction on the same side of and to the rear of the drive shaft assembly 210 relative to a plane through the second axis X2 and at right angles to the first axis Xi. Figure 3 shows crown gears races 317, 318 with annular sets of gear teeth 327, 328 facing in opposite directions.

An advantage with the arrangement shown in Figure 3 is that the opposing forces applied to the crown gear races by the pinion gears can be balanced. The pinion gears 315, 316 and crown gear races 317, 318 in Figure 3 are preferably hypoid gears. A hypoid gear has the axis of the pinion gear offset to the axis of the crown gear. Hence, the electric motors 311 , 312 in Figure 3 can be arranged parallel and offset in the horizontal plane, or in the plane of the respective gear race 317, 318. An example of such an arrangement is schematically indicated in Figure 6 below.

Within the scope of the invention, the propulsion unit can alternatively comprise two electric motors arranged on the same side of and front of the drive shaft assembly 210, or on opposite sides of the drive shaft assembly 210 relative to a plane through the second axis X2 and at right angles to the first axis Xi. Additional electric motors can be provided level with and in parallel to the electric motors shown in Figure 3. The number of motors is selected depending on the desired power output of the stern drive.

The gearbox 305 comprises bevel gears arranged to drive a first and a second drive shaft, respectively, to drive the pair of counter rotating propellers 306, 307. By mounting the electric motors 311 , 312 with their rotary axes in a horizontal direction and selecting high speed motors having a suitable size, the motors can be fitted within a stern drive housing 303. The interface for mounting the stern drive 300 to the transom and the connection to its steering gear (not shown) can be the same as the outdrive for a conventional stern drive.

Figure 4 shows a schematic side view of a propulsion unit according to a third example.

In the Figure 4 embodiment, at least two gear races 417, 418 of the set of gear races are arranged on opposite sides of a double-sided gear 419, preferably a double-sided crown gear.

The specific embodiment of Figure 4 shows a stern drive 400 suitable for mounting to a transom of a marine vessel (see Figurel ; “102”). In Figure 4 the transom would be located at the left-hand side of the stern drive 400, which is defined as the front end of the stern drive. Similarly, the propellers are located at the opposite, rear end of the stern drive. The stern drive 400 comprises an upper unit 401 enclosed in a stern drive housing 403 and a lower unit 402 enclosed in a gearbox housing 404. The gearbox housing 404 contains a gearbox 405 arranged to drive a pair of coaxial shafts connected to counter rotating propellers 406, 407 rotatable about a first axis Xi. The propulsion unit in this example comprises two electric motors 411 , 412 arranged one above the other in the stern drive housing 403. The electric motors 411 , 412 are mounted horizontally with their output shafts 413, 414 extending towards a double-sided crown gear 419. Each output shaft 413, 414 comprises a pinion gear 415, 416 operatively connected to a gear race 417, 418, comprising an annular set of axially separated gear teeth 427, 428, on its respective side of the double sided crown gear 419. The double-sided crown gear 419 can therefore be defined as axially separated crown gear races 417 and 418. The double-sided crown gear 419 is mounted on and fixed against rotation about a drive shaft assembly 410. As in the Figure 2 embodiment, the drive shaft assembly 410 in Figure 4 is constituted by a single shaft. The drive shaft assembly 410 is rotatable about a second axis X2 and is operatively connected to the gearbox 405. The example in Figure 4 shows two electric motors 411 , 412 arranged axially separated in the vertical direction on the same side of and to the rear of the drive shaft assembly 410 relative to a plane through the second axis X2 and at right angles to the first axis Xi. Figure 4 shows a double-sided crown gear 419 with annular sets of gear teeth 427, 428 facing in opposite directions. An advantage with the arrangement shown in Figure 4 is that the opposing forces applied to the double-sided crown gear by the pinion gears can be balanced. The pinion gears 415, 416 and the gear races 417, 418 of the double-sided crown gear 419 in Figure 4 are preferably hypoid gears. A hypoid gear has the axis of the pinion gear offset to the axis of the crown gear. Hence, the electric motors 411 , 412 in Figure 4 can be arranged parallel and offset in the horizontal plane, or in the plane of the respective gear race 417, 418. An example of such an arrangement is schematically indicated in Figure 6 below.

Within the scope of the invention, the propulsion unit can alternatively comprise two electric motors arranged on the same side of and front of the drive shaft assembly 410, or on opposite sides of the drive shaft assembly 410 relative to a plane through the second axis X2 and at right angles to the first axis Xi. Additional electric motors can be provided level with and in parallel to the electric motors shown in Figure 4. The number of motors is selected depending on the desired power output of the stern drive.

The gearbox 405 comprises bevel gears arranged to drive a first and a second drive shaft, respectively, to drive the pair of counter rotating propellers 406, 407. By mounting the electric motors 411 , 412 with their rotary axes in a horizontal direction and selecting high speed motors having a suitable size, the motors can be fitted within a stern drive housing 403. The interface for mounting the stern drive 400 to the transom and the connection to its steering gear (not shown) can be the same as the outdrive for a conventional stern drive.

Figure 5 shows a schematic side view of a propulsion unit according to a fourth example.

In the Figure 5 embodiment, the output shaft 513, 514 of at least one electric motor 511 , 512 has an axis of rotation X3 angled up to 30°, preferably in the range of 10° to 30°, out of the plane of the gear race 517, 518 with which the electric motor 511 , 512 is in driving connection.

Generally, the term “plane of a gear race” relates to a plane the normal of which extends in a direction parallel to the axis of rotation of the gear race, viz an axis around which the gear race is adapted to rotate. The specific embodiment of Figure 5 shows a stem drive 500 suitable for mounting to a transom of a marine vessel (see Figurel ; “102”). In Figure 5 the transom would be located at the left-hand side of the stern drive 500, which is defined as the front end of the stern drive. Similarly, the propellers are located at the opposite, rear end of the stern drive. The stern drive 500 comprises an upper unit 501 enclosed in a stern drive housing 503 and a lower unit 502 enclosed in a gearbox housing 504. The gearbox housing 504 contains a gearbox 505 arranged to drive a pair of coaxial shafts connected to counter rotating propellers 506, 507 rotatable about a first axis Xi. The propulsion unit in this example comprises two electric motors 511 , 512 arranged one above the other in the stern drive housing 503. The electric motors 511 , 512 are mounted with their output shafts 513, 514 at an angle a out of the plane of its respective crown gear 517, 518 with their output shafts 513, 514 extending downwards to a respective crown gear 517, 518. The angle a can be selected up to and including 30°. Each output shaft 513, 514 in the Figure 5 example comprises a pinion gear 515, 516 operatively connected to annular sets of gear teeth 527, 528 on its respective gear race 517, 518, exemplified as a crown gear race in Figure 5, which gears races 517, 518 are mounted fixed against rotation and axially separated on a drive shaft assembly 510. In the Figure 5 embodiment, the drive shaft assembly 510 consists of a single drive shaft 510. The Figure 5 drive shaft 510 is rotatable about a second axis X2 and is operatively connected to the gearbox 505.

The example in Figure 5 shows two electric motors 511 , 512 arranged axially separated in the vertical direction on the same side of and to the rear of the drive shaft assembly 510 relative to a plane through the second axis X2 and at right angles to the first axis Xi. Figure 5 shows crown gear races 517, 518 with annular sets of gear teeth 527, 528 facing in the same direction. In Figure 5 the annular sets of gear teeth 527, 528 face upwards, but downwards facing teeth are also an option. An advantage with the arrangement shown in Figure 5 is that the downwards force applied to the crown gear races by the respective pinion gear can be taken up and supported by a bevel gear arrangement in the gearbox 505. The pinion gears 515, 516 and crown gear races 517, 518 in Figure 5 are preferably hypoid gears. A hypoid gear has the axis of the pinion gear offset to the axis of the crown gear. Hence, the electric motors 511 , 512 in Figure 5 can be arranged parallel and offset in the horizontal plane, or in the plane of the respective crown gear race 517, 518. An example of such an arrangement is schematically indicated in Figure 6 below. Within the scope of the invention, the propulsion unit can alternatively comprise two electric motors arranged on the same side of and front of the drive shaft assembly 510, or on opposite sides of the drive shaft assembly 510 relative to a plane through the second axis X2 and at right angles to the first axis Xi. Additional electric motors can be provided level with and in parallel to the electric motors shown in Figure 5. The number of motors is selected depending on the desired power output of the stern drive.

The gearbox 505 comprises bevel gears arranged to drive a first and a second drive shaft, respectively, to drive the pair of counter rotating propellers 506, 507. By mounting the electric motors 511 , 512 with their rotary axes in a horizontal direction and selecting high speed motors having a suitable size, the motors can be fitted within a stern drive housing 503. The interface for mounting the stern drive 500 to the transom and the connection to its steering gear (not shown) can be the same as the outdrive for a conventional stern drive.

Figure 6 shows a schematic plan view of a propulsion unit according to a fifth example.

As indicated in Figure 6, in the embodiment disclosed therein, the output shaft 613, 614 of at least one, preferably each one, of the at least two electric motors 211 , 212 has an axis of rotation X3 being offset from the second axis X2.

In the Figure 6 embodiment, the output shaft 613, 614 of each one of the two electric motors 611 , 612 has an axis of rotation X3 being offset from the second axis X2.

To this end, though purely by way of example, for at least one, preferably each one, of the at least two electric motors 211 , 212, a smallest distance O between an electric motor line being coaxial with the axis of rotation X3of the output shaft 613, 614 of the electric motor 611 , 612 and a second axis line being coaxial with the second axis X2 is greater than zero.

As a non-limiting example illustrated in relation to Figure 6, the smallest distance O may be equal to or greater than 20%, preferably equal to or greater than 40%, of an outer diameter D of the gear race 627, 628 with which the electric motor 611 , 612 is in driving connection. Moreover, in the Figure 6 embodiment, the output shaft 613, 614 of at least one electric motor 611, 612 may have an axis of rotation X ₃ parallel to the first axis Xi. However, as will be elaborated on hereinbelow, for instance with reference to Figure 8, other orientations of the axis of rotation X ₃ of the output shaft 613, 614 are also envisaged.

The example in Figure 6 shows a stern drive 600 suitable for mounting to a transom 620 (indicated by a dashed line) of a marine vessel (not shown). The transom 620 would be located at the upper part of the stern drive 600 as shown in Figure 6, which is defined as the front end of the stern drive. The stern drive 600 comprises an upper unit 601 enclosed in a stern drive housing 603 and a lower unit enclosed in a gearbox housing (see Figure 2; “204”). The gearbox housing contains a gearbox arranged to drive a pair of coaxial shafts connected to counter rotating propellers, as described in connection with Figure 2. The propellers are rotatable about a first axis Xi. The propulsion unit in this example comprises two electric motors 611 , 612 arranged on opposite sides of and parallel to a plane through the second axis X ₂ and at right angles to the first axis Xi. The electric motors 611, 612 are further arranged one above the other in the stern drive housing 603 in the same way as schematically indicated in Figures 2-4. The electric motors 611, 612 in this example are mounted with their output shafts 613, 614 extending towards a respective first and second crown gear race 617, 618. Each output shaft 613, 614 comprises a pinion gear 615, 616 operatively connected to annular sets of gear teeth 627, 628 on its respective crown gear race 617, 618. The lower set of gear teeth 628 (indicated by dashed lines) is not visible as it is located below the uppermost crown gear race 617. The crown gear races 617, 618 are mounted fixed against rotation and axially separated on a drive shaft assembly 610. The drive shaft assembly 610 is rotatable about a second axis X ₂ and is operatively connected to the gearbox. The output shafts 613, 614 are rotatable about their respective axes X ₃ (one shown) which axes are parallel to the plane of the crown gear races 617, 618. The pinion gear 615, 616 are arranged with an offset O relative to the second axis X ₂ of the crown gear races 617, 618. As indicated above, the offset O may be defined as a smallest distance O between an electric motor line being coaxial with the axis of rotation X ₃ of the output shaft 613, 614 of the electric motor611 , 612 and a second axis line being coaxial with the second axis X ₂ .

Figure 7 shows a schematic plan view of a propulsion unit according to a sixth example. As may be gleaned from Figure 7, illustrated therein is an example in which at least two electric motors 711, 712, 721, 722 are arranged on opposite sides of the drive shaft assembly 710 relative to a plane through the second axis X2 and at right angles to the first axis Xi. In the Figure 7 embodiment, a first pair of the electric motors 711 , 712 is located on a first side of the above-mentioned plane and a second pair of the electric motors 721 , 722 is located on a second side of the above-mentioned plane.

Figure 7 shows a stem drive 700 suitable for mounting to a transom 720 (indicated by a dashed line) of a marine vessel (not shown). The transom would be located at the upper part of the stern drive 700 as shown in Figure 7, which is defined as the front end of the stern drive. The stern drive 700 comprises an upper unit 701 enclosed in a stern drive housing 703 and a lower unit enclosed in a gearbox housing (see Figure 2; “204”). The gearbox housing contains a gearbox arranged to drive a pair of coaxial shafts connected to counter rotating propellers, as described in connection with Figure 2. The propellers are rotatable about a first axis Xi. The propulsion unit in this example comprises four electric motors 711 , 712, 721, 722. A first pair of electric motors 711 , 712 are arranged in parallel on opposite sides of a first plane through the first and second axes Xi, X2 and to one side of a second plane through the second axis X2 and at right angles to the first plane. A second pair of electric motors 721, 722 are arranged facing the first pair of electric motors 711, 712. The second pair of electric motors 721 , 722 are also arranged in parallel on opposite sides of the first plane but on the opposite side of the second vertical plane. The first pair of electric motors 711 , 712 are further arranged above the second pair of electric motors 721, 722 in a direction parallel to the second axis X2 of the stern drive housing 703 with an axial spacing similar to the arrangement of electric motors schematically indicated in Figure 2-4. In Figure 7 the first pair of electric motors 711 , 712 are mounted horizontally with their output shafts 713, 714 extending forwards towards a first crown gear 717. Each output shaft 713, 714 comprises a pinion gear 715, 716 operatively connected to an annular set of gear teeth

727 on its crown gear race 717. The second pair of electric motors 721 , 722 are mounted horizontally with their output shafts 723, 724 extending rearwards towards a second crown gear race 718. The lower, second crown gear race 718 (not visible) is located below first crown gear race 717 and is indicated by dashed lines in Figure 7. Each output shaft 723, 724 comprises a pinion gear 725, 726 operatively connected to an annular set of gear teeth

728 on its crown gear race 718. The lower set of pinion gears 725, 726 and their associated gear teeth 728 are not visible as it is located below the uppermost crown gear race 717. These components are indicated by arrows in Figure 7. The crown gear races 717, 718 are mounted fixed against rotation and axially separated on the drive shaft assembly 710. In the Figure 7 embodiment, the drive shaft assembly 710 consists of a single drive shaft 710 being rotatable about a second axis X ₂ and is operatively connected to the gearbox. The output shafts 713, 714, 723, 724 are rotatable about their respective axes X ₃ (one shown) which axes are parallel to the plane of their respective crown gear 717, 718. The pinion gears 715, 716, 725, 726 are arranged with an offset O relative to the second axis X ₂ of the crown gear races 717, 718.

Figure 8 shows a schematic plan view of a propulsion unit according to a seventh example. In the Figure 8 embodiment, the output shaft 813, 814 of at least one electric motor 811, 812 has an axis of rotation X ₃ arranged at an angle b out of a plane formed by the first axis Xi and the second axis X ₂ . Purely by way of example, the output shaft 813, 814 of at least one electric motor 811 , 812 has an axis of rotation X ₃ forming a reference angle, in a reference plane the normal of which extends parallel to the second axis X ₂ , with the plane formed by the first axis Xi and second axis X ₂ . The reference angle may be in the range of 5° - 45°, preferably 15° - 45°.

Figure 8 shows a stern drive 800 suitable for mounting to a transom 820 (indicated by a dashed line) of a marine vessel (not shown). The transom 820 would be located at the upper part of the stern drive 800 as shown in Figure 8, which is defined as the front end of the stern drive. The stern drive 800 comprises an upper unit 801 enclosed in a stern drive housing 803 and a lower unit enclosed in a gearbox housing (see Figure 2; “204”). The gearbox housing contains a gearbox arranged to drive a pair of coaxial shafts connected to counter rotating propellers, as described in connection with Figure 2. The propellers are rotatable about a first axis Xi. The propulsion unit in this example comprises two electric motors 811 , 812 arranged to extend through a vertical plane through the first axis Xi. The electric motors 811 , 812 have output shafts 813, 814 operatively connected with a respective crown gear race 817, 818 mounted on a drive shaft assembly 810 rotatable about a second axis X ₂ . In the Figure 8 embodiment, the drive shaft assembly 810 consists of a single drive shaft 810 being operatively connected to the gearbox. The pinion gears 815, 816 are rotatable about a respective third axis X ₃ which axes X ₃ are arranged with an offset O relative to the second axis X ₂ of the crown gears 817, 818. The pinion gear 815, 816 of each output shaft 813, 814 is operatively connected to annular sets of gear teeth 827, 828 on its respective crown gear 817, 818. The lower set gear of teeth associated with the pinion gear 816 is not visible as it is located below the uppermost crown gear 817. The electric motors 811 , 812 are further arranged one above the other in the stern drive housing 803 similar to the arrangements schematically indicated in Figure 2-5.

The output shafts 813, 814 are mounted with their rotatable axes X ₃ (one shown) arranged at equal and opposite angles b out of a plane formed by the first axis Xi and the second axis X ₂ parallel to the plane of the respective crown gears 817, 818. The angle b can be selected up to and including 45° out of the vertical plane through the first axis if it is desired to provide a drive unit assembly that can allows the size of the stern drive housing to be maintained. Angles up to 90° are of course possible, but larger angle will result in an increase in width of the stern drive housing. It can be desirable to select an angle in the lower portion of the range as a larger angle can increase the offset O and causes the body of the electric motor to protrude in the transverse direction of the stern drive housing. A larger offset will also result in a reduction of mechanical efficiency and a consequent increase in the power requirement. However, the angle selection is balanced against the fact that a higher offset allows the gear to transmit higher torque.

The plan view in Figure 8 can illustrate two different examples. According to a first example, the output shafts 813, 814 of the electric motors 811 , 812 are rotatable about their respective axes X3 (one shown) which axes are arranged parallel to the plane of the respective crown gear race 817, 818. This example applies to the embodiments shown in Figures 2 and 5, where the gearing comprises two axially separated crown gear races 217, 218; 517, 518 having gear teeth 227, 228; 257, 258 facing in the same direction and the embodiment shown in Figure 3, where the gearing comprises two axially separated crown gear races 317, 318 having gear teeth 327, 328 facing in opposite directions. According to a second example, the output shafts 813, 814 of the electric motors 811 , 812 are rotatable about their respective axes X3 (one shown) which axes are arranged at an angle (see Figure 5; “a”) out of the plane of the of the respective crown gear race 817, 818. The output shaft axes of the electric motors can be angled up to 30° out of the plane of its respective crown gear. This example is applicable to a modification of the embodiment shown in Figure 4, where the gearing comprises a pair of separate crown gear races 417, 418 having gear teeth 427, 428 on opposite sides of a double-sided crown gear. In this case the axes can be angled to avoid mechanical interference between the electric motors. This arrangement of the electric motors as described in the above examples can contribute to minimizing the width of the stern drive housing 803. Figure 9 shows a schematic perspective view of a hypoid gear suitable for use in the propulsion unit. A hypoid gear is a type of spiral bevel gear whose axis does not intersect with the axis of the meshing gear. The hypoid gear in Figure 9 comprises a ring-shaped crown gear race 901 having a planar pitch surface and a planar root surface, both of which are perpendicular to the axis of rotation about the axis X2. The hypoid gear comprises a pinion gear 902 arranged on an output shaft 903 from a suitable motor (not shown). The output shaft 903 is rotatable about an axis X ₃ placed with an offset O relative to the axis X ₂ of the crown gear 901 which allows the pinion gear 902 to be larger in diameter and have more contact area compared to a conventional spur gear in al bevel gearing. Moreover, the crown gear race 901 forms part of a crown gear 904.

Finally, it should be noted that embodiments of the present invention may be represented by any one of the below items. Item 1. A marine propulsion unit comprising a stern drive (200) with a front end for mounting to a transom, which stern drive comprises an upper unit (201) enclosed in a stern drive housing (203) and a lower unit (202) enclosed in a gearbox housing (204); where the gearbox housing contains a gearbox (205) arranged to drive at least one propeller (206, 207) rotatable about a first axis (Xi) and where the stern drive housing contains at least two electric motors (211, 212), which electric motors are in driving connection with a vertical drive shaft (210) rotatable about a second axis (X2) and connected to the gearbox; wherein:

- each electric motor (211 , 212) has an output shaft (213, 214) comprising a pinion gear (215, 216), - the pinion gear (215, 216) of each output shaft (213, 214) is operably connected with a crown gear (217, 218) on the drive shaft (210);

-- the output shafts (213, 214) are arranged with their axes of rotation (X ₃ ) at an offset (O) from the axes of rotation (X ₂ ) of the drive shaft (210); and

- the drive shaft (210) comprises at least two crown gears (217, 218). Item 2. Marine propulsion unit according to item 1 , wherein the crown gears are hypoid gears. Item 3. Marine propulsion unit according to item 1 or 2, wherein the drive shaft comprises at least two separate crown gears (217, 218) which are axially separated.

Item 4. Marine propulsion unit according to item 3, wherein the at least two axially separated crown gears (217, 218) have gear teeth (227, 228) facing in the same direction. Item 5. Marine propulsion unit according to item 3, wherein the at least two axially separated crown gears (317, 318) have gear teeth (327, 328) facing in opposite directions.

Item 6. Marine propulsion unit according to item 1 or 2, wherein the drive shaft comprises at least one pair of separate crown gears (417, 418) arranged on opposite sides of a double sided crown gear. Item 7. Marine propulsion unit according to any one of items 1-6, wherein at least two electric motors are arranged on the same side of the vertical drive shaft relative to a plane through the second axis (X2) and at right angles to the first axis (Xi).

Item 8. Marine propulsion unit according to any one of items 1-6, wherein at least two electric motors are arranged on opposite sides of the vertical drive shaft relative to a plane through the second axis (X2) and at right angles to the first axis (Xi).

Item 9. Marine propulsion unit according to any one of items 1-8, wherein the output shaft (213, 214) of at least one electric motor has an axis (X ₃ ) parallel to the first axis (Xi).

Item 10. Marine propulsion unit according to any one of items 1-9, wherein the output shaft of at least one electric motor has an axis (X3) arranged at an angle out of a vertical plane through the first axis (Xi) and in a plane parallel to the plane of the crown gear.

Item 11. Marine propulsion unit according to any one of items 10, wherein the output shaft (813, 814) of at least one electric motor (811, 812) has an axis (X3) arranged at an angle up to 45°.

Item 12. Marine propulsion unit according to any one of items 1-11 , wherein the output shaft (513, 514) of at least one electric motor (511 , 512) has an axis angled up to 30° out of the plane of its crown gear.

Item 13. Marine propulsion unit according to any one of items 1-12, wherein the vertical drive shaft comprises a pair of concentric shafts (309, 310) connected to different electric motors (311 , 312) via separate crown gears (317, 318). Item 14. Marine propulsion unit according to any one of items 1-13, wherein at least one electric motor is permanently connected to the drive shaft, while the remaining electric motors are freewheeling motors.

Item 15. Vessel comprising a transmission with a marine propulsion unit according to item 1.

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 or the above items.