This invention relates to a sealable powertrain container for a boat, mounted to increase the buoyancy of the boat, and to the mounting of a powertrain to a boat.

There are two broad categories that describe powertrain placement in marine vessels: inboard and outboard. Inboard motors are mounted inside a boat, typically in the hull. They may be above a deck or hidden below a deck. The placement of an inboard motor affects the weight distribution and consequently the boat’s pitch and steerage behaviour when the boat is afloat. Outboard motors are placed outside of a boat, usually at the rear of the craft. Positioning a motor at the rear of a boat can increase power control and steering, as well as pushing down the stern and raising the bow. However, this additional weight at the stem can drag the back of the boat down so at low speeds the efficiency of the boat’s propulsion is reduced. This is particularly an issue with four-stroke outboard engines since they tend to be relatively heavy. Stem drag is particularly problematic in regulated-speed areas such as a harbour where the lower efficiency means that relatively higher power is required for a given speed, thereby consuming a larger amount of electric power or fuel.

Inboard motors typically have a higher upfront capital cost, they are relatively expensive to install and do not allow easy access for maintenance or servicing. Existing outboard motors can obstruct rescue operations by reducing accessibility from the stem to the water, they may also impede docking and the loading I unloading of cargo from the rear of the boat to a dock or land. For the inboard installation the powertrain is usually fixedly attached, leading to issues with servicing and replacement.

The tendency of boat propulsion systems to draw water from the surface can lead to cavitation, where air as well as water pass through the propellor, reducing the efficiency of the propulsion. Outboard motors and stem drives (used with inboard installations) may have a cavitation plate positioned to travel across the surface of the water when the boat is moving to prevent cavitation. There is a need for an improved propulsion system which can at least partially address one or more of the problems identified above.

SUMMARY OF THE INVENTION

According to a first aspect there is provided a rigid inflatable boat comprising: a hull; a stem; two inflatable tubes on opposing sides of the boat, each inflatable tube comprising a terminal portion at the aft end of the tube; and a buoyant structure being located at the stern and mounted relative to the hull so as to increase the buoyancy of the boat; wherein the buoyant structure comprises a sealable powertrain container housing a drive mechanism for driving the boat and side portions on opposing sides of the container, wherein each side portion envelops one of the terminal portions of the inflatable tubes.

Each side portion may fully or partially envelop one or the terminal portions. The buoyant structure may be configured so as to cover at least part of the terminal portions of the inflatable tubes. For one or more of the terminal portions (or at least part of one or more of the terminal portions), the following may be true. The terminal portion may protrude into the volume defined by the buoyant structure (or side portion thereof). The terminal portion may be received by the buoyant structure (or side portion thereof). The terminal portion may engage with the buoyant structure (or side portion thereof). The buoyant structure (or side portion thereof) may envelop the terminal portion when the buoyant structure is mounted at the stern of the boat. The buoyant structure (or side portion thereof) may mate with the terminal portion of the inflatable tube. The terminal portion may mate with a recess in the buoyant structure (or side portion thereof).

The sealable powertrain container may comprise an opening for providing access to an interior of the container and a closure movable to close the opening in a watertight manner.

The drive mechanism may be removable from the powertrain container through the opening. The container may define an exterior underside surface thereof, the container being located so that there is a stepped transition from a major underside surface of the hull to the underside surface of the container.

The exterior underside surface of the powertrain container may be located at a shallower draft than the exterior underside surface of the hull at the stem thereof.

The powertrain container may carry a propellor drivingly coupled to a powertrain located in the container, the propellor being located at a greater draft than the exterior underside surface of the powertrain container.

The drive mechanism may be a hybrid drive mechanism comprising an electric drive and an internal combustion engine. A first component of the drive mechanism may be housed in the sealable powertrain container and a second component of the drive mechanism may be housed in the hull of the boat.

Each terminal portion may extend aft of a transom of the boat.

The buoyant structure may comprise recesses each configured to receive a terminal portion of a respective inflatable tube.

The terminal portions may be convex. The recesses may be concave.

At least part of a lower surface of the buoyant structure may be angled relative to the longitudinal axes of the inflatable tubes. A lower surface of the side portions of the buoyant structure may be angled relative to the longitudinal axes of the inflatable tubes. A lower surface of the container may be angled relative to the longitudinal axes of the inflatable tubes.

The buoyant structure may comprise an upper surface defining a walk-on platform.

The buoyant structure may comprise a deployable walk-on platform. The deployable platform may comprise a substantially rigid surface element and at least one hinge whereby the surface element is attached to a body of the container.

The buoyant structure may increase the buoyancy of the boat when the boat is under way.

According to another aspect there is provided a boat having a hull comprising a stem; and a sealable powertrain container housing a drive mechanism for driving the boat, the sealable powertrain container being located at the stem of the boat and mounted relative to the hull so as to increase the buoyancy of the boat when the boat is under way.

There is provided a sealable powertrain container comprising an opening for providing access to an interior of the container and a closure movable to close the opening in a watertight manner.

The drive mechanism may be removable from the powertrain container through the opening.

The container defines an exterior underside surface thereof, the container may be located so that there is a stepped transition from a major underside surface of the hull to the underside surface of the container.

The exterior underside surface of the powertrain container may be located at a shallower draft than the exterior underside surface of the hull at the stem thereof.

The powertrain container may carry a propellor drivingly coupled to a powertrain located in the container, the propellor being located at a greater draft than the exterior underside surface of the powertrain container.

The powertrain container may further comprise an upper surface defining a walk-on platform.

The powertrain container may comprise a deployable walk-on platform. The deployable platform may comprise a substantially rigid surface element and at least one hinge whereby the surface element is attached to a body of the container.

The deployable platform may be inflatable.

The deployable platform may be telescopic.

The powertrain container may have a width equal to or less than the width of the stern of the hull of the boat.

The powertrain container may be removably mounted to the stern.

The boat may further comprise a mounting structure at a transom for attaching the powertrain container.

The drive mechanism may be removably attached to at least one internal surface of the container.

According to a further aspect there is provided a boat having: a hull comprising a stern and a base; a rigid brace attached between the stern and base of the hull for reinforcing the structure of the boat; and a hollow pole comprising an upper end and an opening coupled to an internal combustion engine to serve as an inlet.

According to a further aspect there is provided a sealable container for enclosing a powertrain, the container located at the stem of a boat and mounted relative to a hull so as to increase the buoyancy of a boat. The container may have a hydrodynamic underside which may improve the quality of the water flow to the propellor, reduce drag and improve performance.

The drive mechanism may be a hybrid drive mechanism comprising an electric drive and an internal combustion engine. A first component of the drive mechanism may be housed in the sealable powertrain container and a second component of the drive mechanism may be housed in the hull of the boat. According to a further aspect there is provided a boat comprising: a hull having a transom; and a force structure rigidly attached to the transom and extending aft thereof, the force structure being adapted for applying propulsion and/or buoyancy to the boat and having a forward-facing wall; and a mounting arrangement whereby the force structure is attached to the transom, the mounting arrangement comprising: a plurality of first holes distributed over and defined by and extending through the transom; a plurality of second holes distributed over and defined by the forward-facing wall; a plurality of bushes, each bush being fixedly mounted in a respective one of the first through holes; a plurality of threaded studs, each stud extending through a respective bush; the forward-facing wall of the force structure being held rigidly to the transom by the studs.

Each bush may extend aft of an aft face of the transom. Each bush may comprise a spacer element extending aft of an aft face of the transom.

Each bush may comprise a compression sleeve. The compression sleeve may be fixedly mounted in the respective hole. The compression sleeve may be internally threaded and configured to be threadedly coupled to the respective stud.

There may be a plurality of nuts, each such nut being threadedly coupled to the forward end of a respective one of the studs.

Each stud may be threadedly coupled to the respective bush.

Each stud may be threadedly coupled to the forward-facing wall.

There may be a plurality of nuts, each such nut being threadedly coupled to the rear end of a respective one of the studs.

There may be a sealing layer sandwiched between the forward-facing wall and the transom.

The sealing layer may be formed of an elastic material. The sealing layer may surround the rear rims of all the holes through the transom.

Each bush may comprise a part that extends aft of the aft surface of the transom for spacing the forward-facing wall from the aft surface (or aft-facing wall) of the transom.

The force structure may be configured to contain at least part of a drive mechanism for driving the boat. The force structure may be configured to contain one or more of an internal combustion engine and an electric drive.

The boat may be a rigid inflatable boat.

DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example with reference to the accompanying drawings. In the drawings:

Figure 1 shows a side view of a boat with a sealable powertrain container at the stern.

Figures 2a and 2b show a cutaway section of a boat and two closures of a sealable powertrain container.

Figure 3 shows a top-down view of a boat with a sealable powertrain container and deployable platform.

Figure 4 shows a cutaway section of a boat with a transom reinforcement.

Figure 5 shows a cross section of a rigid inflatable boat.

Figure 6 shows a series hybrid layout of motors and generators within a sealable container.

Figures 7a, 7b and 7c show different configurations of drive systems within sealable containers. Figure 8a, 8b and 8c show a mechanism for attaching, for example, a buoyant chamber to the transom of a boat. Figure 8a shows an elevation of the transom from the stem. Figure 8b is a cross-section through part of the attaching mechanism. Figure 8c shows a stud used in the attaching mechanism.

Figure 9a illustrates a further mechanism for attaching, for example, a buoyant chamber to the transom of a boat. Figure 9a shows a cross-section through part of the attaching mechanism.

Figure 9b shows an alternative mechanism for attaching a chamber to the transom of a boat.

Figure 10a illustrates a rigid inflatable boat and a further embodiment of a buoyant structure comprising a powertrain container.

Figure 10b illustrates the buoyant structure as shown in Figure 10a.

Figure 11 illustrates the buoyant structure of Figure 10b in position at the stem of a rigid inflatable boat.

Figure 12 illustrates a further embodiment of a buoyant structure comprising a sealable powertrain container.

Figure 13 illustrates an interface between a side portion of the buoyant structure and an inflatable tube of a rigid inflatable boat.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

In the present specification the term “engine” is used to mean a combustion engine, which bums fuel to generate power. The term “motor” is used to mean a device that uses electrical energy to generate power. A boat may be propelled by an engine or a motor. The terms “drive mechanism” are used to mean any form of drive for a marine powertrain. A drive mechanism may comprise one or more engines and/or motors. A "powertrain” may comprise one or more of an engine, a motor and a gearbox.

Figure 1 shows a boat 100 having a hull 101 comprising a stem 102. A powertrain container 103 is provided. The powertrain container houses a drive mechanism 104 for driving the boat. The powertrain container is sealable to exclude water from entering. The powertrain container is located at the stem 102 of the boat. The powertrain container is mounted relative to the hull so as to increase the buoyancy of the boat. The sealable container 103 has an opening for permitting access to its interior. The opening can be closed and sealed by a lid 105.

The drive mechanism 104 is connected to a driveshaft 106. The driveshaft is coupled to and transmits drive to a propellor 107. The propellor is located outside the container. Moulded protrusions in the container 103 serve as securing means 108 that support the drive mechanism 104 in the container and hold it securely. The drive mechanism 104 may be removable from the container 103. The two securing means 108 shown on either side of a drive mechanism are illustrative, but the number, configuration and position of securing means varies with the particular drive mechanism and additional components in the container. An attachment for the drive mechanism 109 is shown extending from an internal surface of the container to the drive mechanism 104. The attachment for the drive mechanism 109 may penetrate the transom of the boat and be cantilevered from inside the hull. The propeller may be mounted so that it is located below the base of the container. The propeller may be located under the container or rearward of and lower than the container.

An underside 110 of the sealable powertrain container is shown stepped relative to an underside of the boat. The forward edge of the underside of the powertrain container abuts the transom. The base of the powertrain container is offset from and above the underside of the hull where the underside of the hull meets the transom. There is a portion of the transom exposed to the exterior of the boat and located vertically between the underside of the hull and the base of the powertrain container. That portion of the transom extends in a generally upright direction when the boat is in its static floating attitude. When the boat is in its static floating attitude the underside 110 of the powertrain container is shallower in the water than the deepest part of the underside of the hull where it meets the transom. When the boat is in motion, the stepped transition from the underside of the hull to the base of the container can result in laminar flow of water from the base of the hull to the region under the container and rearward thereof. The propellor can be located in that region. This can result in improved efficiency of the propellor.

The powertrain container may contain multiple drive mechanisms. Each drive mechanism may drive a respective propeller. Therefore, there may be multiple propellors for propelling the boat.

Figure 2a shows a section of a boat 200 and a sealable powertrain container 201 . The container has an opening through which the interior of the container can be accessed, for example for servicing equipment located in the container. The container is provided with a movable lid or cover 202. The lid is configured so that it can be fitted to the opening so as to close the opening in a watertight manner. This can seal the container so that it is watertight. The movable cover 202 may be connected to a body of the powertrain container 201 , for example by hinges 203 or slidably on rails (not shown), or it may be entirely removable. The movable cover 202 acts as a lid to the container. Equipment for propelling the boat can be located in the container. That equipment may, for example, include any one or more of: a drive mechanism such as an engine or a motor; a cooling system for a drive mechanism; an energy source for a drive mechanism, such as a fuel tank or a battery; a control system for a drive mechanism such as an engine control unit (ECU) or a motor control unit (MCU); a gearbox, which may be coupled between an output shaft of the drive mechanism and a propellor output shaft; a fuel pump and an oil pump.

The propellor output shaft may extend through the container to convey drive to the propellor. A sealed bearing may be provided where the propellor output shaft penetrates the wall of the container.

Part of the internal volume of the container is not occupied by the drive mechanism 204 or other equipment as identified above, but is empty and filled with air. The container is mounted to the boat so that in at least an operational state of the boat, when the boat is being propelled forwards, the total weight of the container and its contents is less than the weight of the water displaced by the submerged volume of the container, meaning that the container provides buoyancy to the boat. For a typical powered boat there is a tendency for the stern to go lower in the water when the boat is under power than when it is in its static floating attitude. The container is mounted to the boat so that at least when the boat is in an under-way configuration the container provides additional buoyancy to the boat. The container may be mounted so as to provide additional buoyancy to the boat in most or all under way configurations or when the boat is in its static floating attitude. When the container provides additional buoyancy to the boat in this way, it can have the effect of reducing the drag to the stern of the boat in comparison to a boat with a similar powertrain mounted in a similar location. The buoyancy of the container can cause the stern of the boat to ride higher in the water than the stern of a comparable boat without the container.

The opening for providing access to the interior of the container may conveniently be located on an upper face and/or on a side face of the container. The cover for closing the opening me be constituted by a single solid piece for acting as a lid to the box, or by a plurality of pieces such as two doors. Such pieces may be connected by hinges. In all embodiments, the container is watertight when the opening is closed. Figure 2a shows a single lid 202 attached to the container with hinges 203 positioned on the rearmost side of the container. When the lid is closed, it may lie generally parallel with (i) a horizontal plane when the boat is in its static floating configuration and/or (ii) a deck of the boat. When the lid is closed, the forward edge of the lid may lie adjacent the transom. In a closed configuration, the lid 202 can extend the useable area of the hull. In a closed configuration the lid 202 can provide a step-on platform. A user of the boat can stand on the platform to embark or disembark the boat or to enter the water. The upper face of the container may serve as a bathing or swim platform. The upper face of the container may be equipped with a surface for increasing grip when a user of the boat steps on the platform. In this way the powertrain container can in effect extend the length of the boat.

The exterior of the container may be provided with protruding ribs extending generally along the longitudinal axis of the boat. These ribs may help to increase the stability of the boat in roll and pitch. The ribs may be located on the underside and/or the sides of the container.

The lid 202 may be moved away from the opening 205 defined by the container walls. When the lid is moved away, the opening is open so that the interior of the container can be accessed. An internal stay-hinge may be positioned in the container for supporting the lid in an open position. In the open position, the contained powertrain 204 may be accessed for servicing and repair. The lid 202 may be sized to exceed the external dimension of the opening 205. The lid may have a flexible gasket on an internal perimeter so that it fits tightly to seal the container. Alternatively the container may be provided with a flexible gasket that fits tightly to the lid.

Figure 2b shows an alternative closure for the container 201 with two doors 202 that are hinged about axes running generally along the longitudinal axis of the boat. In an open configuration, an opening 205 defined by the internal walls of the container is visible. In figure 2b attachment of the doors 202 to the container 201 is shown by hinges 203. A powertrain 204 is contained within the container 201 and surrounded by air. The powertrain or drive mechanism 204 may be removable from the container 201 . In one embodiment, the drive mechanism is removably attached to at least one internal surface of the container. The powertrain may comprise an electric motor, a cooling system, and drivetrain. The drivetrain may couple the drive mechanism to a propellor that is outside of the container. The electric motor may connect to one or more batteries in the hull of the boat by cables, suitably high voltage cables. Within the container components of the powertrain may be secured by the internal base having a moulded shape that corresponds to the shape of the component. Alternatively, removable attachments such as clips or hooks may be used to hold the powertrain in a fixed position within the container. The drive mechanism may be housed in the container such that access for servicing and repair is possible without removal of the drivetrain from the container.

The container 201 housing the powertrain 204 is readily removable from the main hull. The container 201 is external to the main hull. The container 201 is separate to the main hull. This removal of the complete unit allows replacement and allows remote servicing of the powertrain and container. Quick connects and/or dry break couplings are used between the container and boat. The removal and replacement of the container and powertrain is simplified by the use of these connections.

Figure 3 shows an aft section of a boat 300 with a sealable powertrain container 301 equipped with a deployable platform 302 for extending the hull. In a stowed (undeployed) configuration, the platform 302 is folded away and rests on the lid of the sealable powertrain container. The platform may extend the boat’s length, being deployable from the stowed configuration, shown by the dashed line in figure 3, to a deployed configuration where it is positioned above a propellor. The platform functions as a dive platform. It may facilitate easy access to the water for search and rescue operations as well as leisure activities. The platform may be equipped with a swim ladder which can extend into the water to assist a swimmer in boarding. In a further embodiment also shown in figure 3, a deployable platform 303 is attached to a side of the powertrain container such that in a deployed position it extends the boat’s beam. There may be two such platforms on opposing sides of the container. In one embodiment, the deployable platform 302 comprises a substantially rigid surface and at least one hinge 304 for attachment to an upper surface of the container. As an alternative, the deployable platform may be inflatable. In a stored configuration the platform is not inflated. The platform may be manually inflated. Alternatively, there may be an automatic inflation system that can be activated by a triggering mechanism accessible from the boat. The triggering mechanism may, for example, be a pull cord or button.

In another embodiment the deployable platform may comprise a plurality of hollow rigid units telescoped within each other. In an undeployed configuration each sub-unit is housed within a larger unit. The sub-units can be withdrawn from each other to extend the platform. In a deployed configuration each unit provides a rigid surface.

As seen in figure 3, the powertrain container has a beam less than the beam of the stem. The beam of the container may be less than that of the boat’s transom and/or less than the maximum beam of the boat’s hull. The part of the container below the boat’s waterline may be entirely within the projection of the transom and/or the hull on to a plane perpendicular to the longitudinal axis of the boat.

In an embodiment, the powertrain container is removably mounted to the stem of the boat. A mounting structure at a transom for attaching the container may be built into the transom board or provided as an additional component to an existing stem structure. A structural fixing system for supporting the container and its contents may be used.

Figure 4 shows a structural fixing system for reinforcing a transom. The structural fixing may be a rigid brace. The structural fixing may be a reinforcing beam for strengthening the transom of a boat. A central reinforcing beam 402 is attached to the main hull by bolts 409 into a plate and insert 407 and by bolts 409 at the transom 401 . There may be additional struts on either side of the reinforcing beam for increasing the structural support. The struts may be rigid braces. The struts may be made of a rigid, low-density material such as aluminium. The struts may be fitted to existing structural stringers in the main hull of a boat. The reinforcing beam 402 may comprise a horizontal brace plate 403, it may further comprise a vertical brace plate 404. Existing outboard engine mounting holes in the transom may be used to fasten the mounting plate 408 and structural support. The bolts 409 may be inserted through existing mounting holes in the transom 401. Alternatively, new holes may be drilled in the transom for fastening the reinforcing beam 402 to the transom. A mounting plate 408 may be disposed between the reinforcing beam 402 and the transom 408. The mounting plate may increase the grip between the bolted components. A container as shown in Figures 1 to 3 may be attached on the opposing side of the transom to the structural support, i.e. outside of the hull. The structural support may cantilever the weight of the container on the transom.

Figure 4 shows a pole 405 extending from a horizontal brace plate 403, mounted on a plate 406. The pole 405 may be hollow. The pole may have an upper end provided with an opening and be coupled to an internal combustion engine (ICE) to serve as an inlet. An ICE may be cooled by air drawn through the hollow pole 405. The pole may have an integrated ICE air intake system. The upper end of the air intake system is positioned away from water to prevent water entering and damaging the ICE. The air intake system may comprise a shut-off valve. The shut-off valve may be a float valve activated if the craft is inverted.

The pole 405 has a surface for mounting equipment such as a light, navigation equipment and for towing loads positioned outside of the boat. The pole may be used as a grab rail for access on and off of the sealable powertrain container.

The container may support one or more than one propellor. The propellor may be of any convenient form.

The boat may be a rigid inflatable boat (RIB) provided with inflated tubes disposed on opposing sides. The inflated tubes may be extended to accommodate the length of the container. The extended inflated tubes may be a separate tube attachable to each of the existing tubes on the RIB. Figure 5 shows a cross section of a RIB with side tubes 504 and hull 501 . There is a container 502 within the profile of the hull 501 . An internal combustion engine 503 is disposed in the container 502. The engine 503 may be fixed to the inside of the container by an attachment 506. The container is preferably positioned centrally within the cross section of the boat. The engine 503 may be removable from the attachment 506. The container 503 may be removable with the engine inside by lifting it up, this allows easy servicing of the drive components. A drive unit 505 may be positioned such that it is submerged below a water line 508 when the boat is in water. The drive unit 505 may be a linear jet propulsion system. An outrigger section of the hull 507 acts as a stabiliser and hydrofoil surface with minimal wetted surface area.

Figure 6 shows a series hybrid layout within a container 601 . The container 601 may have a subsection 607, this subsection may be removable from the container. The subsection 607 may contain a generator or plurality of generators 606 and an engine 603. The container hay house electric motors 605. The motor 605 may be coupled to a drive unit 602 by a driveshaft 604 for transferring mechanical drive. The drive unit 602 may be enclosed, for example it may be a linear jet propulsion system with high efficiency, it may be a conventional propellor. The series hybrid layout has a central engine 603 with twin drive systems 602 on either side, this configuration improves the steering performance, weight distribution and manoeuvrability. The hybrid layout is preferable to a linear configuration of the engine and drive unit as it is more compact. The electric motors 605 may be connected to batteries. In this configuration, the weight of the batteries may be optimally positioned in the main hull to reduce drag at the stem.

Figure 7 shows three alternative configurations of drive systems within a sealable container 701 . Figure 7a shows a mechanical hybrid configuration with an engine 703 positioned opposite a generator 702. Mechanical drive is transferred by the driveshaft 704 to the propellor 705. The length of the container 701 relative to the length of the boat is shorter than if a series drive system were used. In one embodiment, the length of the container 701 is less than 70cm. The short container allows increased manoeuvrability of the boat.

Figure 7b shows a hybrid configuration with an engine 703 and generator 702. The generators 702 may be connected to batteries positioned within the hull of the boat. Figure 7c shows a twin mechanical drive configuration with two engines 703 each connected to a drive shaft 704 powering a propellor 705. This configuration allows high power performance while maintaining a relatively short container 701. The container is removable from the boat for easy servicing and replacement of the drive components.

As indicated above, the drive apparatus may be mounted to the rear of the boat’s hull. In a typical light boat such as a dinghy or a RIB the drive apparatus can conveniently be mounted to the boat’s transom. The transom is typically an element of the hull that extends vertically, when the boat is in its resting attitude, at the rear of the enclosed part of the hull. The transom typically takes the form of an upstanding wall. The aft surface of the transom may be exposed to the water. It may be in or partially in the water when the boat is unloaded and at rest. The forward surface of the transom may be exposed to a load/passenger space of the boat.

In order to mount the drive apparatus, including for example a motor and/or a gearbox and/or a propellor or water jet, and any buoyant enclosure for it to the transom the following approaches may be taken.

In a first approach the drive apparatus and/or buoyant enclosure may be clamped to the transom by a clamp having a head on a threaded shaft that can be screwed tight to grip the transom.

A second approach is illustrated in figure 8. Figure 8a shows a view of the transom 800 from the rear. The transom is in the form of an upright wall. Multiple through- holes 801 are provided through the transom. The holes may be distributed over the area of the transom. Advantageously the holes are distributed with mirror symmetry about the centre plane of the boat. Collectively, the holes may lie on a first line 802 that runs horizontally and second lines 803 that converge downwards from the first line to the centre plane of the boat. This can help to distribute the force that will be applied through the holes in an effective manner.

Figure 8b shows a cross-section through one of the holes 801 with a forward face 804 of a buoyant structure attached thereto. The same arrangement as shown in figure 8b can be adopted for each of the holes 801 . To permit this, the forward face 804 of the buoyant structure can comprise holes in the same pattern as the transom. A bush 805 is received in the hole 801 . The bush has a cylindrical sleeve 806 which extends through the hole 801 and a flat spacer element 807 attached to the sleeve at its aft end. The bush may take a top hat form. The bush is fixed in the hole by adhesive, for example an epoxy.

A threaded stud 808 passes through the bush. The forward end of the stud protrudes from the bush and is threadedly engaged by a nut 809. Alternatively, it could threadedly engage with the bush itself. The aft end of the stud protrudes from the bush and extends through a hole in the forward wall 804 of the buoyant structure. A nut 810 threadedly engages the stud and is tightened to hold the buoyant structure against the transom. The stud may threadedly engage the wall of the buoyant structure.

A filling element 811 is sandwiched between the rear face of the transom and the forward wall of the buoyant structure. The filling element may, for example be a sheet of closed cell foam or a layer of gel sealant. The filling element can help to resist water ingress through the holes 801 . The filing element extends around the rims of all the holes when viewed from the rear. The filling element may be cut out so that it is not pinched between the bushes and the buoyant structure. That can improve the rigidity of the connection between the transom and the buoyant structure. The filling element may be of an elastically compressible material so that it can be compressed between the transom and the buoyant structure.

Before the bushes are inserted in the holes, it is advantageous to measure the unevenness of the rear surface of the transom, e.g. by scanning. The rear surface of the transom may be formed by a gel or other matrix material and may not be entirely planar. The thickness of the spacer parts of the bushes can then be selected so that they space the forward face of the buoyant structure from the transom over its whole area. The bushes may be supplemented by washers mounted for this purpose on the studs.

The centre parts of the studs may be waisted, as shown in figure 8c, to improve the elasticity of the studs. Put another way, a longitudinally central region of the stud may have a smaller diameter than the ends of the stud. This can help to improve the stiffness of the joints as a result of the stud being easier to stretch.

Figure 9a shows a similar arrangement in detail. In figure 9a like parts are numbered as in figure 8. Element 812 is a flanged washer located on the forward face of the transom 800 and through which the stud 808 passes.

Figure 9a shows that the forward wall of the buoyant chamber 804 is clamped by nut 810 against the aft face of the flange 807 at the aft end of the bush. That flange is in turn clamped by nut 810 against the aft face of the transom 800. The flanged washer 812 spreads the reaction force on the forward face of the transom 800. The flanged washer 812 has a stepped aft face to mate with the forward end of the bush 806 and allow variations in the thickness x of the transom to be accommodated.

The sealing layer 811 has a hole surrounding the aft flange 807 of the bush. This avoids the sealing layer being nipped between the forward face of the buoyant structure and the flange.

The sealing layer may be a sheet of elastically compressible material. In its uncompressed state it may be thicker than flange 807 so that when the nuts are tightened the sealing sheet is compressed. This can improve the seal provided by the sealing layer. Alternatively the sealing layer may be formed of a compound such as a silicone which can be injected into the space between the transom and the forward face of the buoyant structure after the two are clamped together. The sealing layer surrounds the hole in the transom when viewed from aft of the transom. This allows the sealing layer to resist the entry of water from around the buoyant structure into the hole.

Similar arrangements can be adopted for the other holes 801 .

Figure 9b shows an alternative mechanism for attaching the structure 902, for enclosing the drive mechanism and/or providing buoyancy, to the transom 901 . The transom has a plurality of holes distributed over and defined by and extending through the transom. The structure 902 has a plurality of holes distributed over and defined by a forward-facing wall of the structure. Each hole in the transom corresponds to a respective hole in the structure.

Each hole in the transom has a bore of larger diameter than the through hole at the forward and aft-facings walls of the transom.

In Figure 9b, the transom is shown at 901 . The wall of the structure is shown at 902. A stud is shown at 903. The stud extends through the transom 901 and the wall of the container of the structure 902. The bores at opposing end of the through hole in the transom can be drilled such that the dimension shown at x in Figure 9b is constant.

The stud 903 comprises at least one threaded portion. The threaded stud 903 extends through a respective bush, shown generally at 950. The bush 905 is fixedly mounted in the hole through the transom 901 . An interior surface of the bush may be threaded. The bush comprises a cylindrical sleeve 913 which is acts as a compression sleeve. The threaded stud 903 extends through the compression sleeve. The part 913 may be in the form of a top hat sleeve. The sleeve 913 may be bonded in place in the hole under compression.

The stud is threadedly engaged with the bush 950. Specifically, the stud is threadedly engaged with the interior surface of the compression sleeve 913. The stud is also threadedly engaged with a nut 904 at the aft-facing wall of the structure 902.

The part of the bush 950 shown at 910 is a washer, such as a top hat mating washer. The washer may mate with the compression sleeve 913. The washer can be used to apply a compressive force to the compression sleeve.

In this example, a transom brace 914 is also used. This can help to distribute the force applied across the transom 901 , which may reduce the possibility of damaging the transom, and I or losing clamp load through the main stud fixings. In the example shown in Figure 9b, the transom brace abuts the base flange of the bush and the forward-facing wall of the transom. In some implementations, the base flange of the sleeve 913 can provide a desired amount spacing between brace 914 and transom 901 if the base flange is not flush with the forward-facing wall of the transom. The stud 903 may have a collar, shown at 915. The collar is located towards the fore end of the stud. An aft-facing surface of the collar abuts the base flange of the bush. The transom brace 914 comprises a recess in which the collar 915 is accommodated. A forward-facing surface of the collar abuts the recess of the transom brace.

The stud 903 is also threadedly engaged with a nut 916 at the forward-facing wall of the transom 901 .

Along the surfaces indicated at 912 are bond joints where the sleeve is bonded to the interior surface of the hole through the transom.

In this way, the forward-facing wall of the buoyant structure 902 is held rigidly to the transom 901 by the studs 903. The forward-facing wall of the structure 902 and the transom 901 may not be in direct contact. There may be a space between the aft- facing wall of the transom and the forward-facing wall of the structure if the washer 910, or other spacer, extends aft of the aft-facing wall of the transom 901 . The washer 910 may therefore act as a spacer.

To fit the structure 902 to the transom, the through holes are drilled in the transom, which are then bored on other side to allow space for the base flange of the top hat compression sleeve 913 and the top hat mating washer 910. A jig can be used to accurately position the through holes and counter bores in the transom. The washer may be selected to give approximately 0.1 mm compression of the sleeve. The studs 903 can be torqued into the compression sleeves, with an O-ring seal at 911 . The transom brace 914 can then be fitted over the studs at the fore end, on the inside of the transom within the hull, and nut 916 torqued to hold it in place. The structure 902 can then be placed over the studs at the aft end of the transom and nut 904 can be torqued to secure the structure 902 to the transom 901 .

In the arrangements of figures 8, 9a and 9b, the strength of the forward wall of the buoyant structure can augment the strength of the transom. This can help in allowing the buoyant structure to be successfully mounted to pre-existing transoms that were, for example, not designed to be rigidly attached to outboard buoyant elements as shown herein. This may therefore allow the mounting structure to fix the structure 902 to the transom irrespective of how uneven the transom is.

Drive apparatus that does not comprise a buoyant outboard structure can be rigidly attached to a transom in an analogous way to that described above, with a wall of the drive apparatus being mounted to the studs.

In one particular implementation, as illustrated in Figure 10a, the boat 1000 is a RIB comprising inflatable tubes 1001 , 1002 disposed on opposing sides of the RIB. The inflatable tubes can be filled with air to inflate them when the boat is in use. In this example, the inflatable tubes 1001 , 1002 extend generally parallel to the longitudinal axis of the RIB, shown at 1006. In other implementations, the inflatable tubes may extend at an angle to the longitudinal axis of the RIB. Each inflatable tube may have a longitudinal axis. The longitudinal axis 1009 of the tube 1001 may be generally parallel to the longitudinal axis of the boat 1006 or may be angled relative to it. The same may be true for tube 1002. The transom 1005 may support the two inflatable tubes on either side of the boat. The deck of the RIB is shown at 1007.

Each inflatable tube 1001 , 1002 terminates at the aft end of the boat in a respective terminal portion 1003, 1004. The terminal portions 1003, 1004 may extend aft of the transom 1005 of the boat. In this example, the terminal portions are non-planar. However, in some implementations one or more of the terminal portions may be planar. The terminal portion may be a terminal face. The terminal portion may comprise an aft-facing surface. In the example shown in Figure 10a, the terminal portions of the inflatable tubes are conical in shape. The terminal portion 1003 may taper from the maximum diameter of the inflation tube 1001 (shown as d in Figure 10a) to the distal end 1008 of the terminal portion 1003. The terminal portions of the inflation tubes may have other profiles. For example, the terminal portion may have a domed or stepped shape. The terminal portion may be convex. The terminal portion may have a protruding shape. The terminal portion may protrude relative to a plane perpendicular to a longitudinal axis of the respective inflatable tube (axis 1009 of tube 1001 ). The longitudinal axis of the inflation tube may be parallel to the longitudinal axis 1006 of the boat (the axis that extends from the fore end to the aft end of the boat and bisects the boat). The terminal portion may itself be inflatable or not inflatable. The terminal portion of the inflatable tube may be made from a material such as polyvinyl chloride (PVC), ethylene propylene diene monomer (EPDM) rubber or glass- reinforced plastic (GRP), which are durable and hold their shape.

In some implementations, the terminal portions may be separate volumes from the volume defined by the main body of the tubes. In other implementations, the volume defined by the terminal portions may be part of the same volume as the main body of the inflatable tubes.

The sealable powertrain is shown at 1050. The container provides additional buoyancy at the stern of the boat. In the view shown in Figure 10a, the powertrain container 1050 has not yet been positioned at the stern of the boat 1000. At least part of the powertrain container 1050 may be configured so as to cover the terminal portions 1003, 1004 of the inflation tubes when the powertrain container 1050 is in place at the stern of the boat. For one or more of the terminal portions (or at least part of one or more of the terminal portions), the following may be true. The terminal portion may protrude into the volume defined by the powertrain container. The terminal portion may be received by the powertrain container. The terminal portion may engage with the powertrain container. The powertrain container may envelop the terminal portion when the powertrain container is mounted at the stem of the boat.

In the example shown in Figure 10a, the powertrain container 1050 comprises side portions 1051 , 1052. The side portions may be coupled to the main body 1053 of the powertrain container that houses the drive mechanism 104. The side portions may be integral with the main body or may be removably attachable. The side portions and the main body of the powertrain container may be integrally formed. The side portions may not contain any of the drive mechanism components, such as the internal combustion engine or hybrid drive. The main body 1053 and the side portions 1051 , 1052 may in some implementations define a single chamber, or may each define separate chambers. The drive mechanism may be housed in the chamber defined by the main body 1053 (or the single chamber in the case that the main body and side portions define a single chamber). For one or more of the terminal portions (or at least part thereof) and a corresponding side portion of the powertrain container, the following may be true. The terminal portion may protrude into the volume defined by a side portion of the powertrain container. The terminal portion may be received by the side portion of the powertrain container. The terminal portion may engage with the side portion of the powertrain container. The side portion of the powertrain container may envelop the terminal portion when the powertrain container is mounted at the stern of the boat.

The powertrain container may have corresponding recesses 1054, 1055 for receiving the terminal portions 1003, 1004 of the inflation tubes. The recesses may also receive other parts of the inflatable tubes proximal of the terminal portion. The recesses may have a concave shape. This may improve the connection between the inflatable tubes and the powertrain container when the powertrain container is mounted at the stern of the boat. In the example shown in Figure 10a, the terminal portions of the inflation tubes have a conical shape and the container has side portions 1051 , 1052 that comprise concave conical recesses 1054, 1055 that receive the terminal portions 1003, 1004 of the inflatable tubes when the powertrain container is mounted at the stern of the boat.

The terminal portions of the inflation tubes may abut an interior surface of the side portions of the container. The terminal portions of the inflation tubes may engage with an interior surface of the side portions of the container. Features on the outside of the terminal portions and/or distal ends of the inflation tubes may engage corresponding features on the interiors of the side portions of the container.

Figure 10b shows the powertrain container 1050 from another angle and separated from the RIB.

Figure 11 shows the powertrain container 1050 when it is in position at the stem of the RIB.

The powertrain container may be adhered to one or more of the inflatable tubes. For example, using a water resistant adhesive. Alternatively or additionally, the powertrain container may be fixed to the transom as previously described (the holes and fixings are not shown in Figure 10a or 11 ). The boat may comprise one or more adapters for connecting the terminal portions of the inflatable tubes to the powertrain container.

An outer surface of the powertrain container may be a continuation of the outer surface of the inflatable tubes, when inflated, and when the powertrain container receives the terminal portions of the inflatable tubes.

The inner surface of the side portions of the container may define a sleeve that extends over the distal ends of the inflatable tubes. The side portions may have recesses having an internal profile that matches the external profile of the terminal portions of the inflation tubes. When the container is coupled to the boat, the side portions may be extensions of the inflatable tubes. The total width of the powertrain container (including the side portions) may be equal to the width of the boat at the point of engagement between the powertrain container and the transom. When positioned at the stem of the boat, lateral outer surfaces of the powertrain container may be continuous with the outer surfaces of the inflation tubes. This means that the sides of the boat are a smooth continuation of the RIB’s inflatable tubes, which can improve the dynamics and stability of the boat.

The inflation tubes 1001 , 1002 of the RIB may have a rubbing strake (also referred to as a rub band or rub rail) that is continued on the side portions of the powertrain container. The continuation of this feature onto the powertrain container may help to prevent damage to the boat when bumping off or rubbing against objects and/or improve dynamics.

An upper surface of the powertrain container may extend over the terminal portions of the inflation tubes when the powertrain container is mounted at the stem of the boat. The upper surface of the powertrain container may therefore act as a continuation of the deck of the boat.

Figure 12 shows a further embodiment of a sealable powertrain container 1200. The main body of the container which houses the drive mechanism 104 (or a part thereof) is shown at 1203. The container 1200 comprises side portions 1201 and 1202 which are formed separately to the main body and attached to the main body thereafter. Together, the container and the side portions provide additional buoyancy at the stern of the boat. Each side portion 1201 , 1202 comprises a respective recess 1254, 1255 for receiving a respective terminal portion of the inflation tubes of a RIB.

In some embodiments, the side portions may be inflatable tubes. These tubes may act as extensions to the inflatable tubes of the RIB.

The powertrain container may comprise one or more deck elements 1251 , 1252 which extend over the terminal portion of the inflatable tubes of the RIB or the continuations of them (i.e. that extend over the side portions 1201 , 1202 of the powertrain container 1200). The deck elements of the powertrain container may be deployable. For example, as shown in Figure 12, the powertrain container 1200 may comprise side portions 1201 , 1202 which are attached on opposing sides of main body 1203 of the powertrain container. Deck elements 1251 , 1252 may be attached to the powertrain container (for example to the main body 1203) via hinges 1260 which allow the deck elements to fold out relative to the main body of the powertrain container. The deck elements may fold flat in the centre of the container, such that they do not cover the side portions of the powertrain container when the boat is in motion, but can then be deployed to swing out when it is desired to increase the deck space of the boat, for example to use as a swim platform. Alternatively, the deck elements they could be removed and then latched into place when not deployed. In this way, the powertrain container can in effect extend the length of the boat. Alternatively, the deck elements may be telescopic and extend laterally from the upper surface of the main body of the container.

As shown in Figure 13 for side portion 1202, the side portions of the buoyant structure can align with the inflatable tubes of the boat (such as tube 1002). In some implementations, both parts may have a planar face. The deployable parts of the of the powertrain container (such as deck elements 1251 , 1252) can fold down on top of the side portions, as described above. The deployable part may in some implementations cover the interface between the side portion and the inflatable tube when deployed. As discussed above, the terminal portions of the inflatable tubes of a RIB may have different profiles. The concave recesses of the side portions of the buoyant structure may be produced with a standard concave shape. Female adapters may be provided that can engage with the standard concave shape of the recesses and which also have an appropriately shaped recess for engagement with the terminal portions of the inflatable tubes for particular designs of tubes. For example, an adapter may have a domed recess for engaging with a domed terminal portion, or a conical recess having differently angled walls to the standard concave shape of the recess, in the case that the standard concave shape is also conical.

A lower surface of the side portions of the buoyant structure may be angled relative to the longitudinal axis of the inflatable tubes (for example the axis shown at 1009 in Figure 10a for inflatable tube 1001 ). For example, the lower surface may be angled by approximately 3 degrees. The lower surface may be angled by, for example 1-10 degrees. A longitudinal axis of the side portions of the buoyant structure may be angled relative to the longitudinal axis of the inflatable tubes. These features may help to improve buoyancy and hydrodynamics when the boat is in motion.

The buoyant structure is external to the hull of the boat. The buoyant structure is separate to the hull. The buoyant structure may increase buoyancy of the boat in normal operation of the boat. For example, when the boat is underway and/or stationary. This may help to prevent sinking if, for example, the inflatable tubes of the RIB become deflated or burst.

In all of the above-described embodiments, part of the drive mechanism may be contained within the main hull of the boat and part of the drive mechanism may be contained within the powertrain container of the buoyant structure at the stem of the boat. For example, in a hybrid drive system comprising an electric drive (such as a battery) and an internal combustion engine, the electric drive may be contained within the main hull of the boat and an internal combustion engine may be contained within the powertrain container mounted at the stern of the boat (for example, in the main body of the container). This may assist in distributing the weight of the hybrid drive system over the boat, which may improve efficiency. This may allow for increased range, particularly when the boat is operating in a mode where it is powered by the electric drive only.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description, it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.