Converting a boat to electric power

Abstract

A method (100 fig 2) comprises converting a sterndrive boat 1 powered by an internal combustion engine (7 fig 1B) located inside the boat 1 and having an outdrive unit 15 including a direction changin

Description

The present disclosure relates to a method of converting a sterndrive boat powered by an internal combustion engine inside the boat and an outdrive unit outside the boat, to use an electrical power train, and to a converted boat. Sterndrive (also referred to as inboard / outboard drive) is a form of propulsion used in boats. In a sterndrive boat, the power source (normally a petrol or diesel internal combustion engine) is provided inside the boat, and the drive (e.g. gearbox and propeller) is provided outside. Figures 1A and IB show an example of a sterndrive boat 1 according to the prior art. Figure 1A shows a side view of the boat 1, and Figure IB shows a sectional view in the aft portion of the boat 1. The boat 1 has a hull 3 defining an enclosed volume 3a. At the stern 5, the hull 3 includes a transom 5a. A transom 5a is vertical wall at the rear of the boat 1. An internal combustion engine 7 is mounted in the enclosed volume 3a, on an engine bed 9, adjacent the transom 5a. A first drive shaft 11 provides the output from the engine 7. The first drive shaft 11 extends horizontally through a drive shaft aperture 13 in the transom 5a into an outdrive unit 15. The outdrive unit 15 transfers the rotation of the engine 7 to the correct height to drive a propeller 21 to drive the boat 1. In the outdrive unit 15, a first gearbox 17 converts the horizontal drive from the engine 7 to a second drive shaft 23 which extends vertically. A second gearbox 19 converts the vertical drive of the second shaft 23 to a third shaft 25, extending horizontally and parallel to the first shaft 11, at the desired height for the propeller 21. The outdrive unit 15 has an upper casing (or upper housing or upper case) 27 enclosing the first gearbox 17 and a lower casing (or upper housing or gearcase) 29 enclosing the second gearbox 19. The upper casing 27 is a casting that has the first gearbox 17 formed integrally with it. The upper casing 27 is mounted to the transom 5a by a bracket 31, incorporating the mechanism for providing steering and trimming of the lower gearcase. The lower casing 29 is then secured to the upper casing 27. Therefore, the upper casing 27 acts as an interface between the transom bracket 31 and the lower gearbox casing 29. Telescopic rams 33 (pneumatic, hydraulic, or electric) extend between the bracket 27 and outdrive unit 15. Normally two rams are provided to trim the outdrive, whilst one is provided for steering, however these can be combined or additional rams used. Some or all of the steering rams may be inboard of the transom, or outboard of the transom. Actuation of the rams 29 changes the angle of the outdrive unit 15 relative to the transom 5a to allow steering and trim control. Manufacturers of sterndrive systems for boats with a transom 5a as discussed above include but are not limited to Mercury Marine ®, Volvo Penta ® and OMC (Outboard Marine Corporation) ®. Currently, all commercially available sterndrive boats are powered by internal combustion engines. With the increasing focus on reducing carbon emissions and other environmental issues, there is a desire to move towards environmentally friendly power sources. Current sterndrive boats require replacement engines through life as the engine life is less than the hull life. Conversion to electric drive trains is not only more environmentally friendly, but also reduces service and maintenance requirements and the motors would outlast the hulls and therefore not need to be replaced during the vessel’s life. Boats can have a useable lifetime of several decades, meaning there is a large legacy of boats that are powered by internal combustion engines. Currently, the only method of converting boats to electric power provide the electric motor directly in place of the internal combustion engine. This is complex to convert and does not take full advantage of using electric power. There is therefore a desire to easily change the power unit to an electrical power source and take full advantage of the electric power source. According to a first aspect of the invention, there is provided a method of converting a sterndrive boat powered by an internal combustion engine located inside the boat and an outdrive unit located outside the boat to use an electrical power train, the method comprising: replacing the internal combustion engine with a battery system; replacing a gearbox in the outdrive unit with one or more electric motors, the gearbox arranged to change a direction of rotation of mechanical drive through the outdrive unit; and connecting the battery system to the one or more electric motors through a drive shaft aperture via which the internal combustion engine drove the gearbox prior to the engine and gearbox being replaced. The method provides for a simple and easy way to replace an internal combustion engine in a boat with an electric power train. The method uses existing fixing points and openings, and replaces minimal components in the drivetrain, limited to a gear box and possibly an upper housing of the outdrive unit. No modifications are needed to the retained parts, and the parts that new parts are designed to using existing fixings made vacant by the parts removed. Putting the electric motors in place of a gearbox, with the output of the motors along the same direction as the output of the replaced gearbox removes a gearbox / bevel gear where the output direction from the engine is changed. This removes inefficiencies caused by the gearing, making the system more efficient and giving greater range, endurance and power from an equivalent system where the internal combustion engine is replaced by an electric motor inboard of the transom. The method also eliminates the drive shaft the extends through the transom, which is replaced by electric cabling (and possibly cooling conduits). Therefore, whereas the drive shaft needed a universal or constant velocity joint to accommodate changing the steering and trimming angle of the lower housing on the outdrive unit, the electrical cabling and cooling conduits are flexible, meaning no such joint is needed, eliminating another source of inefficiencies. Furthermore, the replacement of the engine allows for greater control of the weight and centre of gravity shift caused by the retrofit as the battery can be shifted forward and aft in the engine room whilst still attaching to the existing engine beds. This negates the requirement to fit ballast which increases vessel displacement and reduces performance. This control over the weight and centre of gravity could be dynamic to enable centre of gravity shifts whilst underway. Preferably, pancake motors are used. These are low profile and so do not use much space, and can also be stacked in series to provide the desired output power. The boat may comprise a transom formed at the back of the boat, the drive shaft aperture extending through the transom. The method may comprise: securing the battery system to mounting points used to mount the internal combustion engine. The battery system may include: a charger for charging the battery and / or an inverter for converting a DC output from the battery system to an AC output. The method may comprise: securing one both of the charger and / or inverter to mounting points used for the internal combustion engine. The gearbox that is replaced may be arranged to be driven by a first shaft extending from the internal combustion engine and to drive a second shaft arranged at ninety degrees to the first shaft, The method may comprise: arranging the one or more motors to drive the second shaft without a change in direction. The second shaft may be substantially vertical. The one or more motors may be pancake motors. The method may comprise replacing the gearbox of the outdrive unit with two or more pancake motors stacked in series along the direction of the second shaft. The outdrive unit may include a second gearbox arranged to be driven by the second shaft, and arranged to drive a third shaft, the third shaft parallel to the first shaft, and offset from the first shaft along the second shaft. The method may comprise: providing an upper housing arranged to enclose the one or more electric motors and to provide an interface between a bracket for securing the outdrive unit to the boat and a lower housing of the outdrive unit; securing the upper housing to the bracket and the lower housing; and fitting the one or more electric motors into the housing. The method may comprise: providing a cover to fit over the upper housing. The method may comprise: connecting a coolant system used to cool the internal combustion engine and outdrive unit to cool the battery system and one or more electric motors. The method may comprise: connecting conduits carrying coolant through the drive shaft aperture. The method may comprise: closing the drive shaft aperture with a blanking plate, the blanking plate arranged to allow passage of electrical connections from the battery system to the one or more electric motors, and coolant fluid from inside the boat to the outdrive unit. The method may comprise: retaining any one or more of the following: the lower housing of the outdrive unit; the propeller of the outdrive unit; one or more drive shafts of the outdrive unit; the bracket securing the outdrive unit to the boat; mounting points for the internal combustion engine; and the means for controlling the direction of the outdrive unit to provide steering and trim control. According to a second aspect of the invention, there is provided a converted sterndrive boat including: a battery system inside the boat; a drive shaft aperture extending through the hull of the boat, the drive shaft aperture for receiving a drive shaft extending from an internal combustion engine mounted at the same point as the battery system to an outdrive unit; and an outdrive unit having one or more electric motors arranged to drive the boat, wherein the electric motors are powered by the battery system, and are connected to the battery system through the drive shaft aperture. The converted sterndrive boat may comprise: a transom formed at the back of the boat, the drive shaft aperture extending through the transom. The battery system may be secured to mounting points for mounting the internal combustion engine in the boat. The motors may be arranged to drive a shaft extending vertically, at an angle of ninety degrees or substantially ninety degrees to drive shaft aperture. The one or more motors may be pancake motors. The converted sterndrive boat may comprise: a gearbox arranged to be driven by the shaft extending vertically, and arranged to drive a horizontal shaft, the horizontal shaft parallel to the drive shaft aperture, and offset from the drive shaft aperture along the shaft extending vertically. The second shaft may drive a propeller to drive the boat. The converted sterndrive boat may comprise: an upper housing arranged to support the one or more electric motors and to provide an interface between a bracket for securing the outdrive unit to the boat and a lower housing of the outdrive unit. The converted sterndrive boat may comprise: a freshwater and / or saltwater coolant system arranged to cool the battery system and outdrive, the coolant system including one or more conduits extending through the drive shaft aperture. The converted sterndrive boat may comprise: a blanking plate closing the drive shaft aperture, the blanking plate arranged to allow passage of electrical connections from the battery system to the one or more electric motors, and coolant fluid from inside the boat to the outdrive unit. According to a further aspect of the invention, there is provided a kit for converting a sterndrive boat driven by an internal combustion engine to converted boat of the second aspect, the kit including: a battery system (inclusive of the battery management system, inverters and frequency controllers etc); one or more electric motors in a casing to interface between the transom bracket and a lower housing of an outdrive unit, and cabling to connect the battery system to the one or more motors. Features discussed in relation to any aspect of the invention may be applied mutatis mutandis to any other aspect, unless mutually exclusive. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1A illustrates a schematic side view of a sterndrive boat powered by an internal combustion engine, according to the prior art; Figure IB illustrates a schematic section view through the boat of Figure 1A in the region of the outdrive unit; Figure 2 illustrates a flow chart for a method of replacing the internal combustion engine in the boat of Figures 1A and IB with an electrical power and drive system; Figure 3 illustrates the region of the outdrive unit of a sterndrive boat which has been converted to electrical power according to the method of Figure 2, in schematic sectional side view; and Figure 4 illustrates the boat of Figure 3, in top-down view, in the region of the outdrive unit. Figure 2 illustrates a flow chart of a method 100 for converting a sterndrive boat 1 powered by an internal combustion engine 7. such as shown in Figures 1A and IB, to use electrical power. The method 100 will be described below with reference to Figures 3 and 4, which illustrate the boat 1’ after conversion (referred to as the converted boat 1’). In the following description, it will be understood that where parts of the boat 1 are retained in the converted boat 1’ the same reference numbers will be used, whilst new parts will be referred to with new reference numbers. In a first step 102 of the method 100, the engine 7 is replaced by a battery system 37. The internal combustion engine 7 is secured to the engine bed 9 at a number of mounting points 35a-d, which are best shown in Figure 4. To replace the engine 7. the engine 7 is disconnected from the mounting points 35a-d and the output of the engine 7 is disconnected from the first shaft 11. The engine 7 is then lifted from the engine bed 9. The battery system 37 includes one or more batteries 39a-c mounted on a battery tray 41. The battery system 37 is sized such that it can be secured in place by connection to some or all of the engine mounting points 35a-d. Optionally, the battery system 37 may be selected to have a weight corresponding to the weight of the engine 7 to ensure the performance characteristics of the boat 1’ are unchanged. If the weight of the battery system is limited to be the same as the boat, the weight limiting may limit the range of the electric drive system. Alternative, if the battery system can be heavier than the engine, the range may be extended. If the desired range can be achieved with a battery system lighter than the engine 7. ballast (not shown) may optionally be provided on the battery tray 41 or elsewhere to match any shortfall in the weight. Alternative, the reduced weight may be acceptable or desirable. The battery system 37 includes a charger 43 to allow the batteries 39a-c to be connected to a power supply and recharged. The charge 43 may include a socket (not shown) for connecting to a power supply. The socket may be remote from the rest of the battery system and connected through a cable, it may be located at the fuel filler location for the fuel tank. The battery system 37 also includes an inverter 45 to convert the output of the batteries 39a-c to AC current. In addition to the charger 43 and inverter 45, the battery system 37 may include a battery management system (not shown). The battery management system is arranged to manage charge and discharge of the batteries 39a-c and enable coupling of different sized batteries in different locations in the boat. In the example shown in Figures 3 and 4, the charger 43 and inverter 45 are separate to the battery tray 41, and separately secured to engine mounting points 35a-d. The frequency controller and battery management system are integrated into the battery tray 41. The weight of the charger 43 and inverter 45 (and frequency controller and battery management system) may be considered in configuring the weight of the battery system 37. In a second step 104 of the method 100, the first gearbox 17 is replaced with one or more electric motors 47a,b. To replace the gearbox 17, the upper housing 27 is removed. The gearbox 17 is then disconnected from the vertical drive shaft 23, and the gearbox 17 removed. It will be appreciated that the first (horizontal) drive shaft 11 may be removed with the engine 7. the gearbox 17 or on its own. To fit the motors, 47a,b a new upper housing 61 is provided. The upper housing 61 is secured to the transom bracket 31 and the lower housing 29. Therefore, the upper housing 61 provides an interface between the bracket 31 and the lower housing 29. The original upper housing 61 is typically a casting. The new upper housing 61 may be moulded, cast or made from any suitable material in any suitable way. To secure the new upper housing 61 to the transom bracket 31 and lower housing 29, the parts of the rear and lower surfaces of the upper housing 61 that engage with the transom bracket 31 and lower housing 29 are sized and shaped in the same way as the original upper housing 27. The sides, front and upper surfaces are modified to accommodate the different size / shape of the motors 47a,b. The new upper housing 61 is secured to the transom bracket 31 and lower housing 29 using the same connection points and bolts (not shown) as the original upper housing 27. The rams 33 are also coupled to the new upper housing 61 in the same way as the old upper housing 27. This may be on the exterior of the housing 61 or inside the volume defined by the housing 61. The connection points for the rams 33 are also the same in the upper housing 61 in the converted boat 1’ and the upper housing 27 of the unconverted boat 1. The upper housing 27 includes mounting points for securing the motors 47a, b. Once the new upper housing 61 is secured into place, the motors 47a,b are then fitted in, or alternatively the motors may be fitted into the housing prior to fitment to the lower housing and transom bracket. The motors 47a,b are positioned so their output directly drives the vertical shaft 23. To achieve this, the motors 47a,b are arranged to rotate around the same axis as the vertical shaft 23. In the example shown in Figures 3 and 4, the motors 47a,b are pancake motors (also known as axial flux motors). The two motors 47a,b are arranged along the direction defined by the vertical shaft 23, with their outputs aligned, and arranged to drive the vertical shaft 23. In some cases, the existing vertical shaft 23 may be used. In other cases, the vertical shaft 23 may be replaced, modified or extended to interface with the motors 47a,b. The number of motors 47a,b and output of the motors 47a,b may optionally be selected to provide the same power output as the original engine 7. The choice of batteries 39a-c and inverter 45 may also be a factor in matching the previous power output. In selecting the size of the motors 47a,b the torque limits of the lower housing 29 is considered so as to not overload the housing 29. The battery system 37 may also include a frequency controller to regulate the frequency of AC current provided to the motor, to control the motor speed. This may also be performed by the inverter 45. In a third step 106 the battery system 37 is connected to the motors 47a,b, via the frequency controller, inverter 45 and cabling 49. The battery system 37, motors 47a,b and the cabling 51 form an electric power train 51. The electric cabling 49 is provided through the existing drive shaft aperture 13 in the transom 5a. The electric cabling may also include communications for control and telemetry for the motors 47a,b. It will be appreciated that the boat 1 may include a cooling system 53, using sea water, fresh water or other coolant fluid. The cooling system 53 may cool both the engine 7 and the outdrive unit 15. In the unmodified boat 1, coolant conduits 55 may be provided through an additional aperture (not shown) through the transom 5a, extending parallel to the drive shaft aperture 13. In the converted boat 1 ’, the coolant conduits 55 may pass through the drive shaft aperture 13, along with the cabling 49, to reduce the number of openings in the hull 3 of the boat 1’. Alternatively, the original coolant aperture may be used. The method 100 includes the step 108 of connecting the coolant system 53 to cool the electric drive train 51 and outdrive unit 15. Optionally, a fairing or cover (not shown) may be provided over the upper housing 61 once the outdrive has fully been connected to the boat 1. The cover may be decorative, and / or may protect the motors 47a,b from ingress of water. The upper housing 61 includes fixing points for the cover. The cover may be a moulded composite, or of any other suitable material. The method 100 further includes the step 110 of fitting a blanking plate 57 on the inner surface 59 of the transom 5a. The cabling 49 connecting the battery system 37 and motors 47a,b is smaller than the drive shaft 11 driven by the internal combustion engine 7. Therefore, the blanking plate 57 closes the driver shaft aperture 13 around the cabling 49, to ensure the transom 5a is watertight. The blanking plate is fitted to the transom including existing bolting points. The blanking plate 57 may be omitted if the outdrive unit 15 is already watertight to the transom 5a. The blanking plate 57 may provide a cable gland around the cabling 49. Alternatively, the blanking plate may include plugs on opposite sides to allow for connection of two separate lengths of cabling, or may simply include an aperture sized to the cable. As discussed above, the transom 5a may include an additional aperture for coolant. Further apertures (not shown) may also be provided for additional connections, between the boat 1 and the outdrive unit 15 (e.g. connections for telemetry, gearbox control and the like). If still necessary, these connections may optionally be rationalised by providing them all through the drive shaft aperture 13, and the blanking plate 57 may close these apertures. Alternatively, the blanking plate 57 may allow the other apertures to remain in use. It will be appreciated that when the boat 1 is powered by an internal combustion engine 7. fuel tanks (not shown) are required. However, these are not needed when using the electric power train 51. In one embodiment, the method 100 may include removing the fuel tanks. The fuel tanks may optionally be replaced with additional batteries, or ballast depending on the desired characteristics of the converted boat 1’. Alternatively, the space previously occupied by the fuel tanks may be converted for other uses. In another embodiment, the fuel tanks may be filled with ballast and sealed, or sealed empty. The fuel distribution system, including any manifolds and the like, which carries fuel from the tanks to the engine 7 may also be removed or left in place. When the boat 1 is driven by an internal combustion engine 7. the power generation (from the engine) and fuel store (fuel tanks) are inboard whilst the drive generation (propeller 21) is outboard. However, in the converted boat 1’, the power and drive are both outboard, and only the battery system 37 (which corresponds in function to the fuel tanks and fuel distribution system) is inboard. In the boat 1 driven by an internal combustion engine 7. a drive shaft 11 extends through the transom 5a. In the converted boat 1’, there is no drive shaft extending through the transom 5a. Instead, power cabling 49 extends through the transom 5a. The original boat 1, powered by the internal combustion engine 7, may include a peripheral battery (for example 12V) for powering various systems of the boat 1. In one example, this system may be maintained separately to the electric drive train in the converted boat 1’. In a second example, an inverter may be provided to allow the peripheral battery to be recharged by the batteries 39a-c in the drive train, or the charger 43. In a further example, the peripheral battery may be removed, and an inverter provided to allow all electrical systems to be powered by the batteries 39a-c in the drive train. In the method 100 discussed above, various components of the internal combustion engine sterndrive system are retained. This includes: the lower housing / gearcase 29; at least part of the drive train after the first gearbox 17 (e.g. the third drive shafts 23, the second / lower gearbox 19 and propeller 21, and possibly the second drive shaft 23); the bracket 31 used to mount the outdrive unit 15; the system for controlling the direction of the outdrive unit to provide steering and trim control; and the cooling systems. Within the boat 1, the mounts 35a-d for the engine 7 are also retained (and used to mount the battery system 37). It will be appreciated that some or all of the above components may be replaced. However, this replacement is not necessary for the conversion to electric power, and may be for aesthetic reasons, or maintenance or upgrading of parts. In order to change to electric power, the engine 7 and first / upper gearbox 17 (including the upper housing 27) are replaced. Over the lifetime of a boat 1, the internal combustion engine 7 may be replaced several times as part of servicing and maintenance. The conversion to an electric power train may be carried out as part of the usual servicing and maintenance schedule, or may be done at any other desired time. The embodiment discussed above is given by way of example only, and various modifications may be made. For example, the structure of the boat 1, 1’ is given by way of example only. The method 100 may be used in any sterndrive boat which has an internally mounted combustion engine 7 and an outdrive unit 15 driven by the engine 7. In the boat 1 described above, two gearboxes 17, 19 are provided in the outdrive unit 15, when power is provided by the internal combustion engine 7. It will be appreciated that the gearboxes 17,19 include bevel gears, arranged to change the direction of rotation to transfer rotation from the height of the output of the engine 7 to the height of the propeller 21. In one example, both of the gear boxes 17, 19 in the outdrive unit 15 have a 1:1 ratio, and are simply provided to change the direction of rotation. In other examples, one or both of the gearboxes may be a fixed ratio step up or step down gearbox, or a variable speed gearbox. In addition or alternatively, a fixed or variable speed gearbox (not shown) may be provided on the engine output, in the boat 1. Where a variable speed gearbox is provided in the boat 1, this is removed as part of removing the engine 7 or kept in place if in the lower gearcase. Where variable speed gearboxes are kept, they may optionally be fixed at a selected ratio, since control of the electric motor output can provide both speed variation and reverse without requiring gearing. Where gearboxes are retained in the drive train, the motors 47a,b can be tuned to suit any existing ratio, so the gear ratios would not need changing, As discussed above, the method 100 can be used with any sterndrive boat 1. Where the arrangement of drive shafts 11, 23, 25 is different to that described above, the motors 47a,b may be provided in place of any of the gearboxes where the direction of rotation is changed. In the example discussed above, the motors 47a,b are provided with their output along a different direction to the output of the internal combustion engine 7. In the example discussed above, this allows one gearbox 17 to be eliminated, by removing one change of direction of the drive shafts 11, 23, 25 in the outdrive unit 15. However, it will be appreciated that in some embodiments, the motors 47a,b may be oriented in such a way that the output direction of the motors 47a, b requires changing to engage the existing part of the drive train that is being used. In the above examples, pancake motors are used. However, this is by way of example only. Any suitable type of motor may be used, with any suitable output direction. Any suitable type of battery, supercapacitor, generator or other charge store may be used. In the examples discussed above, the charger 43 and inverter 45 are separate to the battery tray 41, but mounted at the same location as the batteries 39a-c, whilst the battery management system is integrated into the battery tray. In different examples, one or more of the battery management system, charger 43 and inverter 45 may or may not be integrated into the tray 41. Alternatively, the battery management system, charger 43 and / or inverter may be mounted in a completely different location, away from where the engine 7 was previously mounted. In one example, the inverter 45 may be mounted in the outdrive unit 15, with the motors 47a,b to reduce cable run length between the inverter and motors. As discussed above, ballast may be provided to ensure the weight profile of the converted boat 1’ is the same as the unconverted boat 1. This may accommodate changes in centre of gravity due to the removal of the engine 7 and fuel tanks. Ballast may also be provided in the outdrive unit 7 where the centre of gravity is changed by replacing the gearbox 17 with motors. It will be appreciated that in some examples, greater design freedom is achieved for placement of ballast in the whole boat 1’, by removing the weight of the engine 7 and optionally fuel tanks. In some cases, ballast need not be provided at the same locations as removed components, to match the weight of the removed components. An overall weight reduction can be desirable, and / or the overall weight profile of the boat can be optimised by placement of ballast to match or change the previous weight. Even if the boat with the electric drive train is heavier, placement of ballast and / or buoyancy aids can help the performance of the boat 1’. In the above description, the relative terms “horizontal” and “vertical” are used to describe directions. It will be appreciated that anything that is generally along the direction from the front to the rear of the boat is considered horizontal, and anything generally perpendicular to this is considered vertical. It will, in particular, be appreciated that “horizontal” and “vertical” are not necessarily exactly parallel to and perpendicular to a deck of the boat 1, 1’ or water surface, for example. The order of the steps in the method 100 discussed above are by way of example only. The steps may be performed in any suitable order. 20 06 25

Claims

1. A method of converting a sterndrive boat powered by an internal combustion engine located inside the boat and an outdrive unit located outside the boat to 5 use an electrical power train, wherein the internal combustion engine providesmechanical drive to a first drive shaft extending along a first direction through a drive shaft aperture in a hull of the boat, wherein the outdrive unit includes at least one propeller driven by a second drive shaft extending parallel to and vertically offset from the first drive shaft, and wherein the outdrive unit 10 comprises two gearboxes arranged to translate drive from the first drive shaftto the second drive shaft, the method comprising:replacing the internal combustion engine with a battery system and removing the first drive shaft;replacing a first gearbox of the two gearboxes in the outdrive unit with 15 one or more electric motors; andconnecting the battery system to the one or more electric motors through the drive shaft aperture,wherein the one or more electric motors are arranged to provide drive to a third drive shaft extending perpendicular to the first direction, and 20 wherein a second gearbox of the two gearboxes is retained to translatedrive from the third drive shaft to the second drive shaft.

2. A method as claimed in claim 1, wherein the boat comprises a transom formed at the back of the boat, the drive shaft aperture extending through the transom.

253. The method as claimed in claim 1 or claim 2, comprising:securing the battery system to mounting points used to mount the internal combustion engine.30 4. The method of any preceding claim, wherein the battery system includes:a charger for charging the battery and / or an inverter for converting a DC output from the battery system to an AC output.20 06 25securing one both of the charger and / or inverter to mounting points used for the internal combustion engine.

6. The method of any preceding claim, wherein the first gearbox is arranged to5 convert mechanical drive from the first direction to the third drive shaft.

7. The method of any preceding claim, wherein the third drive shaft is extendssubstantially vertically.10 8. The method of claim 6 or claim 7, wherein the one or more motors are pancakemotors.

9. The method of claim 8, comprising replacing the first gearbox with two or more pancake motors stacked in series along the direction of the third drive 15 shaft.

10. The method of any preceding claim wherein:the first gearbox has a one-to-one gearing ratio; and / or the second gearbox has a variable ratio .2011. The method of any preceding claim, wherein the outdrive unit includes an upper housing arranged to enclose at least the first gearbox, the upper housing arranged to provide an interface between a bracket for securing the outdrive unit to the boat and a lower housing of the outdrive unit, the method 25 comprising:replacing the upper housing with a new upper housing arranged to enclose the one or more electric motors and to provide the interface between the bracket for securing the outdrive unit to the boat and the lower housing of the outdrive unit; and30 securing the upper housing to the bracket and the lower housing; andfitting the one or more electric motors into the housing.

12. The method of claim 11, providing a cover to fit over the upper housing.20 06 25connecting a coolant system used to cool the internal combustion engine and outdrive unit to cool the battery system and one or more electric motors.5 14. The method of claim 13, comprising:connecting conduits carrying coolant through the drive shaft aperture.

15. The method of any preceding claim, comprising:closing the drive shaft aperture with a blanking plate, the blanking plate 10 arranged to allow passage of electrical connections from the batterysystem to the one or more electric motors, and coolant fluid from inside the boat to the outdrive unit.

16. The method of any preceding claim, comprising:15 retaining any one or more of the following:the lower housing of the outdrive unit;the propeller of the outdrive unit;the second and / or third drive shafts of the outdrive unit;the bracket securing the outdrive unit to the boat;20 mounting points for the internal combustion engine; andthe means for controlling the direction of the outdrive unit to provide steering and trim control.

17. A converted sterndrive boat including:25 a battery system inside the boat;a drive shaft aperture extending through a hull of the boat along a first direction, the drive shaft aperture for receiving a drive shaft extending from an internal combustion engine mounted at the same point as the battery system to an outdrive unit; and30 an outdrive unit having: at least one propeller driven by a propellerdrive shaft extending parallel to and vertically offset from the first direction and one or more electric motors arranged to drive the boat, wherein the electric motors are powered by the battery system, and are connected to the battery system through the drive shaft35 aperture;20 06 25wherein the one or more electric motors are arranged to provide drive to a further drive shaft extending perpendicular to the first direction; andwherein the outdrive unit further includes a gearbox to translate5 drive from the further drive shaft to the propeller drive shaft.

18. The converted sterndrive boat of claim 17, comprising:a transom formed at the back of the boat, the drive shaft aperture extending through the transom.1019. The converted sterndrive boat of claim 17 or claim 18, wherein the battery system is secured to mounting points for mounting the internal combustion engine in the boat.15 20. The converted sterndrive boat of any of claims 17 to 19, wherein the furtherdrive shaft is substantially perpendicular to the first direction.

21. The converted sterndrive boat of claim 20 wherein the one or more motors are pancake motors.2022. The converted sterndrive boat of claim 20 or 21, wherein the gearbox has a variable ratio.

23. The converted sterndrive boat of any of claims 17 to 22, comprising:25 an upper housing arranged to support the one or more electric motorsand to provide an interface between a bracket for securing the outdrive unit to the boat and a lower housing of the outdrive unit.

24. The converted sterndrive boat of any of claims 17 to 23, comprising:30 a freshwater and / or saltwater coolant system arranged to cool thebattery system and outdrive, the coolant system including one or more conduits extending through the drive shaft aperture.a blanking plate closing the drive shaft aperture, the blanking plate arranged to allow passage of electrical connections from the battery system to the one or more electric motors, and coolant fluid from inside the boat to the outdrive unit.20 06 25

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