Electric marine propulsion system

Abstract

There is provided an electric marine propulsion system having a variable relationship between the speeds of two contra-rotating and coincident propellers Here, by measuring the thrust applied from the propellers to the hull, the relationship between the speeds of the two propellers can be controlled and optimised to improve the efficiency of the marine propulsion system. Thus, the electric marine propulsion system comprises a first electric motor connected to a first (or primary) drive shaft and a second electric motor connected to a second (or secondary) drive shaft. Each primary and secondary drive shaft includes an associated propeller. Thrust from the primary and secondary drive shafts are transferred to the boat through a force sensor. The force sensor comprises an output signal for connection to a control means. The control means being configured to control the output rotational speed of the first motor and to control the output rotational speed of the second motor. Wherein the control means is configured to vary the relationship between the output speeds of the first and second motors.

Description

[0001] Electric Marine Propulsion System

[0002] Field of the Invention

[0003] The present invention relates to electric marine propulsion systems, and more particularly relates to a shaft-driven electric marine propulsion system.

[0004] Background

[0005] There are various different types of known marine propulsion systems. A first distinction can be the location of the engine. Outboard systems have an engine unit installed at the rear of the marine vessel (e.g. a boat) and entirely outside the boat. The engine unit is compact and combines the engine, transmission and propulsion device , such as a propeller, in a single unit. The engine unit is pivotable to provide an azimuth thrust and to therefore act as a rudder. Outboard systems installed on the transom are widely used on small boats.

[0006] An alternative to an outboard system is an inboard system, where the engine is installed inside a hold or engine room within the marine vessel (e.g. boat). The propulsion system comprises the propulsion device, such as the propeller, connected to the engine through the hull of the boat by a transmission, such as a drive shaft. Thus, resulting in a shaft-driven boat (or stated alternatively, a boat having or adapted to have a shaft-driven electric marine propulsion system).

[0007] One such inboard system for a shaft-driven boat is a so-called line-shaft system. Here, the propeller is installed at the end of a drive shaft directly connected to the engine in a straight line. Thus, straight shaft line inboard systems, comprise an inboard-mounted engine which is coupled to a straight drive shaft which extends aft, and often downwardly, from the engine. The shaft passes through the boat’s hull, and is supported towards its free end, in the region of the propeller which is mounted to the end of the shaft, by a bracket. A separate rudder is mounted to the hull at a position immediately aft of the propeller. The drive shaft passes through a sealed aperture (sometimes referred to as a “stern tube”) in the boat’s hull, and extends towards the engine for connection to a gearbox, which is conventionally mounted to the aft end of the engine. A development of the straight shaft line inboard system and which achieves a more compact arrangement has a V-configuration gearbox such that the engine is arranged to face the opposite direction and is mounted above the drive shaft.

[0008] Figure 1 illustrates a conventional straight shaft line drive arrangement of the type described above, installed in a boat having a hull 1. The engine 2 in this type of arrangement is typically mounted inboard, and significantly forward of the boat’s propeller 3. The propeller is mounted i to the aft end of a straight drive shaft 4. The drive shaft 4 extends forwardly from the propeller 3 and passes through a sealed aperture 5 in the boat’s hull, and then extends towards the engine 2. The drive shaft 3 is directly coupled to the engine via connection to a gearbox 6, which is conventionally mounted to the aft end of the engine 2. The aft end of the drive shaft 4 is supported by a propeller bracket 7, which is sometimes referred to as a P-bracket, and which is secured to the underside of the hull 1. As will be appreciated, the drive shaft is rotatably supported by bearings within a static tube, where the static tube is connected and rigidly secured to the hull. In Figure 1, the connections between the hull and static tube are referenced generally at A (to the P-bracket), B (at the stern tube) and C at the engine. As will be appreciated by those of skill in the art, the drive shaft 4 is conventionally connected to the gearbox 6 via a rigid coupling, illustrated generally at 8 in figure 1. The rigid coupling is typically connected to a thrust bearing arrangement.

[0009] Whilst conventional arrangements such as those described above, are relatively simple, and are widely used in the marine industry, they are not without problems. One problem is that the torque and thrust generated by rotation of the propeller are directed to the boat’s hull via the gearbox and engine, as denoted by the arrows marked T in figure 1, and hence also through the engine mounts (not shown) which are used to mount the engine to the structure of the hull 1. Whilst rigid engine mounts would provide the most efficient transmission of thrust to the hull, they would also transmit unacceptable levels of vibrations to the hull. This type of installation therefore usually comprises semi-rigid engine mounts (not shown), with the rigidity of the mounts being selected as a compromise between the effective transmission of thrust to the hull, and efficient vibration damping. The need to ensure that the engine mounts can absorb sufficient vibration, introduces a source of misalignment of the drive train and wasted power. The induced torque from the single propeller has to be countered by application of a rudder effect which again reduces the overall transmission efficiency for the boat.

[0010] Another problem arises from the need to accurately align various components of the driveline installation, which becomes exacerbated by the length (L) of the entire installation, and thus the axial spacing of the various components. As will be appreciated, the drive shaft 4, the static tube, the propeller bracket 7, the sealed aperture 5, various bearings associated therewith, the thrust bearing arrangement 8, and the gearbox must all be very accurately aligned with one another, over a very significant length (denoted L in figure 1). This can be very difficult and time-consuming to achieve when the propulsion system is installed in the boat, and can sometimes be difficult to maintain during service due to the fact that the boat’s hull 1 will inevitably flex and deform slightly when the boat is underway, particularly in a heavy seaway. Even very slight misalignment between these various components can result in the boat’s transmission suffering from substantial friction losses and a reduction in driving thrust, all of which will of course again reduce the energy efficiency of the boat in service.

[0011] Another problem which can arise in some conventional installations of the type described above, is the optimal angle of the propeller in the water, which means that the engine must be located quite a significant distance forward of the sealed aperture 5 through which the drive shaft 4 enters the hull 1 , in order to permit proper alignment of the gearbox 6 with the shaft 4. This can have implications for the overall arrangement of the boat, because space aboard is often limited.

[0012] GB2552644 discloses a marine propulsion system for a shaft-driven boat that addresses the above identified problems. The system is shown generally in Figure 2 and comprises a recess 10 formed in the hull 1 of said boat, and a drive cassette 12 wherein said cassette and said recess are mutually configured such that said cassette may be engaged within said recess to install said drive cassette within the drivetrain of the boat. Thus, when the cassette is mounted in the recess, the thrust from the cassette is transferred directly into the hull as shown by the indications T in Figure 2. This means that the thrust will not be directed through the boat’s gearbox and engine, as is the case with most conventional installations of the type illustrated in figure 1. This, in turn, helps to reduce the load applied to the engine and gearbox, and permits the use of more flexible engine mounts which can be optimised for their vibration absorption characteristics, given that they will not be required also to transmit thrust from the drive shaft to the boat’s hull as is the case with prior art conventional installations.

[0013] The cassette comprises: a housing 14; a static tube 16 extending rearwardly from said housing and fixed relative thereto; and at least one drive shaft 18 extending through said static tube 16 and into the housing; the drive shaft being supported for rotation within the static tube by at least one shaft bearing 20, and being rotatably supported within the housing by thrust bearings 22. Because the cassette is pre-assembled in a manufacturing environment; said static tube, said drive shaft, the or each said shaft bearing, and said thrust bearings are all coaxially aligned with one another. As will be appreciated, the pre-aligned and self-contained nature of the drive cassette therefore offers very significant advantages when it comes to installation in the boat.

[0014] It is known for boats to have twin propellers arranged in a side-by-side arrangement. Also, a known adaption to the single propeller is the contra-rotating propeller pair where each propulsion system has two, co-axially aligned propellers. Contra-rotating propeller arrangements can offer significant advantages over single propeller arrangements. When a single propeller rotates in water, a significant amount of energy is wasted to the creation of tangential or rotational water flow; effectively twisting the water around. Contra-rotating propellers, on the other hand, help to harness some of this energy by providing another propeller immediately downstream of the main propeller, and driving the downstream propeller in the opposite direction to the upstream one, which recovers the energy of the rotating water flow. Also by effectively dividing the load between two propellers acting in series they can be designed to improve the overall efficiency of the propeller system. Well-designed contrarotating propeller arrangements will produce negligible rotational water flow, and thus virtually no out-of-balance torque whilst cutting the drag on the boat. A typical contra-rotating design is achieved by a bevel gear driven by a main shaft connected to the engine, wherein two propeller shafts (one inside the other) have corresponding bevel gears at their ends and connected to opposed sides of the main shaft bevel gear.

[0015] The cassette disclosed in GB2552644 can be adapted to be a contra rotating arrangement using a pair of drive shafts arranged coaxially and for contra-rotation relative to one another. As shown in Figure 2, each drive shaft includes an associated propeller 24. The pair of drive shafts are disclosed as being a primary drive shaft and a secondary drive shaft, said primary drive shaft extending through said secondary drive shaft and the housing for connection to a source of propulsive power. The secondary drive shaft extends through said static tube and into the housing, wherein each said drive shaft is rotatably supported within said housing by respective sets of thrust bearings, and said housing contains a gear train which mechanically interconnects said shafts and which is configured to drive said secondary shaft in the opposite direction to said primary shaft.

[0016] Recently, there has been a drive towards the electrification of marine propulsion systems in order to move away from the diesel fuel used in mainstream combustion engines and the associated pollution and emissions. In general, the electrification of the marine propulsion system consists of swapping the diesel engine in the hull for a battery and electric motor. Although it is envisaged some applications may also include onboard electricity generation from fuels or other means to recharge the battery. When electrifying marine propulsion systems, either as a retro fit or as a new installation, the trend then has been to simply replace the engine with the electric motor, which ignores all the potential benefits revealed in the design of an adapted electric marine propulsion system whilst retaining all the disadvantages of the mechanical one. Thus, the transmission of the marine propulsion system has been largely left unaffected by electrification. That is, the above examples of marine propulsion systems have been adapted by simply connecting the drive shaft directly to an inboard electric motor.

[0017] Although, for example, whilst directly coupling the cassette of GB 2552644 directly to an electric motor would achieve a cleaner, marine propulsion system having the advantages as discussed above, there is an ongoing need to increase the efficiency of the complete marine propulsion systems by seeking out the potential benefits which only an electric drivetrain designed from scratch can access.

[0018] The present invention seeks to provide an improved marine propulsion system. In particular, there is a need to provide an improved marine propulsion system that address one or more of the above or other identified considerations. One particular aim is to provide a marine propulsion system utilising an electric motor and providing an improved efficiency.

[0019] Summary of the Invention

[0020] There is provided an electric marine propulsion system having a variable relationship between the speeds of two propellers. Here, by measuring the thrust applied from the propellers to the hull, the relationship between the speeds of the two propellers can be controlled and optimised to improve the efficiency of the marine propulsion system. Thus, the electric marine propulsion system comprises a first electric motor connected to a first (or primary) drive shaft and a second electric motor connected to a second (or secondary) drive shaft. Each primary and secondary drive shaft includes an associated propeller. Thrust from the primary and secondary drive shafts are transferred to the boat through a force sensor. The force sensor comprises an output signal for connection to a control means. The control means being configured to control the output rotational speed of the first motor and to control the output rotational speed of the second motor. Wherein the control means is configured to vary the relationship between the output speeds of the first and second motors. Suitably, the control means is configured to vary the relationship between the output speeds of the first and second motors based on the output signal of the force sensor (i.e. , the monitored thrust). Additionally, the speeds of the two propellers can be varied by a combination of the output signal of the force sensor (i.e., the monitored thrust) and a speed through the water of a craft (i.e., boat) to which the electric marine propulsion system is installed. That is, if the speeds of the two propellers are varied based on the output signal of the force sensor (i.e., the monitored thrust), this includes the speeds being varied based on a combination of the output signal of the force sensor (i.e., the monitored thrust) and a relative speed through the water of the craft.

[0021] The force sensor monitors the thrust transferred from the first and second propellers to the boat. Thus the output signal of the force sensor is representative of monitored thrust applied to the boat. For instance, the force sensor can be a torque flange in instances where the shafts pass through the hull of the boat. Alternatively, where a shaft does not pass through the hull of the boat, the force sensor (i.e., a thrust flange) is mounted between a housing and a hull of the boat to which the housing is mounted, wherein the thrust generated by the first and second propellers is transferred to the housing via thrust bearings. As will be appreciated, the first drive shaft and therefore first propeller is coincident with the second drive shaft and associated propeller. In exemplary embodiments, the first and second electric motors are arranged on the same side relative to the propellers. Thus, in exemplary embodiments, the secondary drive shaft is configured to extend partially or fully through the primary drive shaft as is known in the art. The first and second electric motors are connected to the first and second drive shafts repetitively as is known in the art. Likewise, the first and second propellers are connected to the first and second drive shafts also as is known in the art. Suitably, each electric motor includes an encoder for providing feedback signals to track the speed and position of the shaft for use by the control system to control the speed of rotation of the respective propeller.

[0022] According to exemplary embodiments, the marine propulsion system includes a housing in which the first and second drive shafts are suspended. The housing is connected to a hull of a boat. Thus, the housing comprises mountings for connecting the housing to the hull. The force sensor (i.e., thrust flange) is arranged at one of the mountings so that the force sensor is arranged between the housing and the hull. Here, the first and second drive shafts are suspended to rotate within the housing via at least one thrust bearing. Suitably, each of the first and second drive shafts are suspended within the housing by a thrust bearing. As will be appreciated, then each thrust bearing transfers the thrust generated by the propeller(s) to the housing. In turn, because the housing is connected to the hull, the thrust from the propeller is connected directly to the hull via the mounting where the torque flange is arranged. Thus, the force sensor can monitor the thrust transferred from the propeller into the hull.

[0023] In the exemplary embodiments having a housing, the housing is suitably provided with two mountings. Typically, it is envisaged one of the mountings would be a forward mounting, arranged toward a front of the housing, and providing the primary connection between the housing and the hull. Here, the force sensor can suitably be arranged between the hull and forward mounting. At a forward end, the housing can include a ‘swan neck’ for providing a primary mounting between the housing and the hull, the force sensor being provided at the primary mounting at the forward end of the housing. And the other of the mountings would be a rearward mounting arranged towards a rear of the housing (that is, rearward of the forward mounting). Suitably, one of the mountings to the hull can be adapted to engage the existing P- bracket (as described above). Here, the housing would be typically fixed to the P-bracket at a rear end of the housing with no intentional movement between the P-bracket and the housing. However, in rigidly mounting the rearward end of the housing to the hull, some of the thrust will be transferred to the hull via the rearward mounting, thus the torque flange might not accurately monitor the full thrust delivered from the propellers. Consequently, in a further embodiment, the rear end of the housing includes a mounting for connecting to the hull wherein the housing is restrained in two-dimensions, but allowed to move in-line with the developed thrust (i.e. forwardly). Thus, by the rear mounting providing the housing with a degree of freedom of movement in the forward direction, the thrust from the propeller is more entirely transferred through the forward mounting and torque flange. In exemplary embodiments, the rearward mounting is provided as a pair of brackets. Here, the pair of brackets comprises a port-side bracket and a starboard-side bracket spaced across the housing. Here the pair of brackets are spaced from the forward mounting, but do not extend past an axial zone defined by the propellers.

[0024] In exemplary embodiments where the marine propulsion system comprises a housing, the first and second electric motors are suitably arranged within the housing. Thus, unlike the conventional art, it is not necessary for a drive shaft to extend through the hull. Rather, electrical connection between the first and second motors and the battery and optionally the control system are the minimum connections required between the housing and the inside of the hull. Here, the marine propulsion system might be referred to as a ‘pod’. Advantageously, because the propeller and engine do not need to be directly coupled by a straight shaft, the propellers can be optimally orientated in the water (i.e. typically with the axis of the propeller aligned with the direction of travel). Advantageously, the pod can be fitted to a number of different hull designs and optionally retrofitted to existing hulls to upgrade the boat’s propulsion system to electric with minimal adaption.

[0025] In exemplary embodiments comprising a housing, the housing suitably provides a dominant portion of the pod. In the exemplary embodiments, the housing has a general outer profile representing a dolphin shape. For example, the dolphin shape of the housing forms a streamlined, torpedo-shape. That is, the front of the housing has a short front-nose section that expands to a large cross-sectional area, before tapering in a longer tail section. It is also envisaged that the housing may be substantially circular or oval in general cross-section. Here, the propellers may have a hub design to compliment the dolphin shape of the housing and the trailing propeller is suitably followed by a trailing nose that is smaller than the front nose. In one exemplary embodiment, the housing provides a dominant dolphin shaped portion, as explained above, wherein the dolphin shaped portion is attached to a foil or fin or the like. For instance, the housing comprises an outer profile generally representing an aircraft nacelle attached to a wing portion. Here the dolphin shaped portion forms the shape of an aircraft nacelle attached to a wing portion via a defined pylon or where the pylon is more integrated into the wing.

[0026] As will be appreciated, in the exemplary embodiments having a housing with a generally dolphin shape design, the housing may include extensions to provide any required mountings to the hull. Here, suitably a forward mounting is provided by an extension to the housing that extends forwardly from the nose of the housing. The extension also suitably extends upwardly to place the mounting (that is the connection between the housing and the hull) forward and upwards of the nose. It will be appreciated, the forward extension reduces the stress on the extension when transferring the thrust generated by the propellers and in the forward, surge direction. In addition, the forward extension can be arranged to eliminate any underhanging feature of the housing that would create a hook between the housing (e.g. pod) and hull. That is, if the mounting did not extend forward of the front nose of the housing, the front nose of the housing could act as a hook that could snag on debris under the water. By providing an extension to the front nose that extends forwardly to mate with the hull, the risk of damaging the pod or reducing the performance (i.e. from becoming snagged on debris that could drag in the water and / or act to rip the pod from the hull) is reduced.

[0027] In some exemplary embodiments, the first and second electric motors require cooling. In some embodiments, cooling fluid can be supplied from within the hull via one of the mountings (e.g. the forward mounting). However, in alternative embodiments, the first and second motors can be cooled integrally within the housing. For example, suitably, the electric motors can be oil cooled, wherein the housing provides conduits through which the oil can be pumped or otherwise cycled. Since it is envisaged the housing will be fully submerged, the surrounding water can act as a heat sink to provide a cooling effect to the oil as it circulates about the housing, thus the need for additional coolant to be provided from within the hull is reduced, or preferably eliminated, as are any associated accessories, pumps and systems.

[0028] In some exemplary embodiments, the first and second propellers each comprise a hub and a set of blades (i.e. primary blades). As is known, the set of blades comprise a plurality of blades. Here, the plurality of blades are spaced equidistant about the circumference of the hub. It will be appreciated that the exact design of the shape and configuration of the set of blades is a specialised process. However, in exemplary embodiments, it has been found advantageous to use a large, oversized hub (wherein oversized is relative to traditional designs). That is, in an oversized hub, the hub has a diameter that is around 50% of the diameter of the propeller. Here, the diameter of the propeller would be the circumference that a blade tip circumscribes in a full rotation of the hub. The large, oversized hub may have a diameter greater than 40% or greater than 45% or greater than 50% of the diameter of the propeller. Whilst the large, oversized hub is primarily selected for efficiency considerations, it has been advantageously found that the large hub also provides an opportunity to modularise the manufacture of the propeller. That is, it has been found that the large hub can be manufactured separately to the blades, where the blades are subsequently assembled to the hub. Here, damaged blades can be removed and replaced individually. In exemplary embodiments having a large, oversized hub with modular blades that are assembled to the hub to form the propeller, suitably the hub includes a plurality of slots formed in the outer circumference in which each blade is fitted. Here, each blade is formed with an integral base from which a fin portion of the blade extends. The base is configured to operatively correspond to the slot. Suitably, the base of each blade is bolted to the hub. For instance, the base of each blade includes apertures through which a bolt is inserted. The hub comprises corresponding threaded bores into which the bolts are screwed into so as to fixedly secure the blade to the hub. In some embodiments, the slots in the hub are arranged in an axial direction. That is, the slot has a width, a length and a depth, wherein one of the width or length is aligned with the axial direction of the hub and the depth is aligned with the radial direction of the hub. Here, for the blades to have an angle to the axial direction, the fins are envisaged as being arranged at an angle to the base. In alternative embodiments, the slots are arranged at an angle to the radial direction of the hub. Thus, the blades can be more economically formed with the fins aligned to the longitudinal direction of the base.

[0029] In some embodiments, at least the trailing propeller and optionally also the leading propeller is configured with individual blades having a raked distal tip. A raked distal tip is a distal tip that is bent forward in the direction of normal rotation (normal rotation being the intended rotation for forward surge motion, noting that in some instances, the propeller can be operated in a reverse rotation). Suitably, at least the trailing propeller has the raked distal tip as it has been found that raking the trailing propeller has a greater impact on the thrust generated than raking the distal tips of the leading propeller. Thus, the added complexity in the machining or forming process of each raked blade is targeted to the greatest increase in thrust. As will be appreciated, in a contra-rotating design, suitably the front propeller is raked towards the downstream side and the rear propeller is raked towards the upstream side.

[0030] In some exemplary embodiments the marine propulsion system is arranged to condition the flow of water prior (that is upstream or forward) to the forward most propeller. Suitably, here the system is arranged to condition the flow of water using a pre-swirl means arranged forward of the forward most propeller. The forward most propeller might also be referred to as the lead propeller (where the rearward most propeller would be termed the trailing propeller). That is, if the first propeller, for instance attached to the primary drive shaft, is arranged forward of the second propeller attached to the secondary drive shaft, the first propeller would be the forward most propeller. In the exemplary embodiments arranged to condition the flow of water prior to the forward most propeller, the forward most propeller comprises a hub and a number of primary blades that provide the desired surge motion. In one embodiment of the pre-swirl means, the hub then further comprises secondary blades that act on water flowing over the hub to introduce a rolling motion to control the flow of water. Here, the secondary blades are mounted to the hub forward of the primary blades. The secondary blades may have a smaller radial extent that the primary blades. The secondary blades may comprise a number of blades where the number of blades is less than the number of blades that form the primary blades. Here, each blade of the secondary blades suitably extends a greater extent about the circumference of the hub than the extent that each blade of the primary blade extends about said circumference.

[0031] In an alternative embodiment of the pre-swirl means, a stator ring is fixed in relation the forward most propeller, and suitably immediately adjacent the forward most propeller. Here, the stator ring includes a plurality of fins. Each fin being designed to act on water flowing past the fins, from upstream to downstream, to impart a rolling motion to control the body of water. That is, to impart a swirling motion to complement the action of the propellers. In particular, the stator is envisaged as being advantageous for boats that ‘plane’ (i.e. pitch due to part of the weight being supported by aerodynamic lift) where the stator can assist in recovering any loss in thrust due to the propeller thrust no longer being aligned to the line of travel).

[0032] In one exemplary embodiment, there is provided a boat having the electric marine propulsion system as described above and herein. Suitably, the boat comprises a hull to which the torque flange is connected. Thus, it will be appreciated the hull includes corresponding features as described above and herein for connecting the electric marine propulsion system. Additionally, the hull may typically comprise a keel as is known in the art. Moreover, the boat is envisaged as comprising the power source for the electric motors, for example batteries. The boat suitably comprises a rudder arranged downstream and behind the propeller of the electric marine propulsion system. That is, suitably the propellers of the marine propulsion system are fixed relative to a direction of travel of the boat and the direction of the boat altered via a downstream rudder as is known in the art. It is also beneficial to provide a function of the marine propulsion system that produces a crabbing motion to the boat. Here, a crabbing motion is the ability of a boat to move sideways without having any forward speed. For instance, during in-port manoeuvring (e.g. parking). Without bow thrusters, the boat having the electric marine propulsion system would be able to make a sway motion by realising repeated surge (forward) motions with a high rudder angle as is known in the art.

[0033] However, in a further exemplary embodiment, there is provided a boat having a first electric marine propulsion system and a second electric marine propulsion system, each system as described above and herein. Here, the propellers of the first and second marine propulsion systems are arranged either side of the boats centre line and can therefore also be termed port and starboard systems respectively. As will be appreciated, where the boat has a keel, the port and starboard marine propulsion systems are arranged either side of the keel. An unexpected advantage of the variable relationship between the rotation speed of the propeller of the primary drive shaft and the rotation speed of the secondary drive shaft is that the rotation speed of one of the propellers can be inverted (i.e. become negative) so as to be caused to rotate in the same direction as the other propeller. That is, as explained above, to provide the advantageous forward motion, the propellers of the primary and secondary drive shafts are driven in opposite directions. By rotating the propellers in the same direction, rather than provide forward motion, the system pushes the water sideways by the reaction of the two propellers and the water. That is, by variably controlling the speeds of the first and second motors, each of the port and starboard propulsion systems can be operated to produce radial flows and pressures.

[0034] Advantageously, it has been found that the port and starboard marine propulsion systems each operating with the respective first and second propellers rotating in the same direction as the other (i.e. the speed of one of the propellers controlled to operate backwards) can be controlled to generate a high and low pressure on respective opposed sides of the boat’s hull (and in particular any keel feature). Whilst a pure sway motion would require forces to be generated on both aft and fore-ends of the boat’s centre of gravity, it is believed the yaw motion created by the pressure imbalance at the aft of the boat will also create a substantial sway motion. Thus, the boat is provided with an improved manoeuvrability without the addition of bow thrusters. That is, the improved manoeuvrability is achieved with the marine propulsion system as described above or herein.

[0035] There is therefore provided a method of operating a marine propulsion system and therefore a method of propelling a boat, wherein the method comprises dynamically varying the relationship between the speed of rotation of a first propeller and the speed of a second propeller whilst monitoring the thrust transferred from the propellers to the hull of the boat through a force sensor. Here, the system is controlled to vary the relationship of speed based on the thrust transferred to the boat to optimise the efficiency of the propulsion system.

[0036] There is also provided a method of operating a marine propulsion system and therefore a method of propelling a boat, wherein the method comprises dynamically varying the relationship between the speed of rotation of a first propeller and the speed of a second propeller in order to generate a predetermined sound signature. Here the predetermined sound signature can be selected to be unique to the vessel, or to mimic the sound signature of an alternative vessel, structure or mammal.

[0037] There is also provided a method of operating a marine propulsion system wherein a port-side propulsion system and a starboard side propulsion system are driven to provide a sway motion to the boat. Here, each of the propellers in the port side system are driven in a common direction. And each of the propellers in the starboard side system are driven in a common direction. The propellers being controlled to generate a high-pressure volume to one side of the hull and a low-pressure volume to the other side.

[0038] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0039] Summary of the Figures

[0040] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0041] Figure 1. is a schematic view of a straight shaft line drive arrangement according to the conventional art;

[0042] Figure 2. is a schematic view of a straight shaft line drive arrangement according to the prior art;

[0043] Figure 3. is a schematic sectional view through an exemplary embodiment of a marine propulsion system;

[0044] Figure 4. is a perspective view of the marine propulsion system of Figure 3;

[0045] Figure 5. is a schematic sectional view through an exemplary electric motor for use with the exemplary embodiments;

[0046] Figure 6. show a schematic layout of an exemplary hub for use with the exemplary embodiments;

[0047] Figure 7. shows a schematic layout of an exemplary blade for use with the hub of figure 6;

[0048] Figure 8. show a schematic layout of an alternative exemplary hub for use with the exemplary embodiments;

[0049] Figure 9. shows a schematic layout of an exemplary blade for use with the hub of figure 8;

[0050] Figure 10. shows a perspective view of a blade adaption according to some exemplary embodiments;

[0051] Figure 11. shows a schematic layout of an exemplary embodiment of a marine propulsion system including a means for pre-swirl;

[0052] Figure 12. shows a perspective view of an alternative embodiment of an exemplary marine propulsion system; Figure 13. shows a schematic layout of a further exemplary embodiment of a marine propulsion system;

[0053] Figure 14. shows a perspective view of a boat including an exemplary marine propulsion system;

[0054] Figure 15. shows bottom view of Figure 14; and

[0055] Figure 16. shows a perspective view of an alternative design of an exemplary pod.

[0056] Detailed Description of the Invention

[0057] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0058] Referring to Figure 3, an electric marine propulsion system is 100 is shown. The propulsion system 100 comprises a first electric motor 110 and a second electric motor 210. Electric motors are known and are powered by electricity to rotate a shaft operatively connected to the motor. Thus, except where specifically adapted a general description of an electric motor is not given. The first electric motor 110 is operatively connected to a first propeller 120 and the second electric motor 210 is operatively connected to a second propeller 220. The first propeller and the second propeller are configured to be contra-rotating.

[0059] As is known in contra-rotating arrangements, the first and second propellers are coincident and arranged to be driven to generate a forward thrust by rotating the propellers in opposite directions. Because the propellers are directly driven by separate motors, unlike known systems that have a mechanical connection between the propellers, the first and second propellers are not linked. Thus, the first and second motors can be controlled independently to vary the relationship between the speed of rotation of the first propeller and the second propeller. That is, the speed of rotation of one of the propellers can be increased or reduced relative to the speed of rotation of the other. As will be explained below, in order to achieve some optional functionality, it is envisaged the speed may even be controlled to reverse the direction of rotation of one of the propellers to the other (that is to drive one of the propellers to rotate backwards whilst the other rotates forwards such that both propellers are rotating in the same direction). Consequently, the propulsion system further comprises a control means configured to control the speed of rotation of the first and second motors independently and configured to vary the relationship between the speed of rotation of the two propellers. Referring back to Figure 3, the propulsion system further comprises a force sensor. The force sensor is arranged to monitor the thrust transferred from the first and second propellers to the boat. Here, the control system receives signals from the force sensor to monitor the thrust being transferred. The control system is further configured to vary the relationship between the speed of rotation of the first electric motor and the speed of the second electric motor based on the monitored force. Thus, the propulsion system can be dynamically configured to operate at an optimised efficiency.

[0060] As will be appreciated, suitably the first electric motor 110 is shown as being operatively connected to the first propeller 120 via a first drive shaft 130. As shown, the first drive shaft is connected at a centre of the propeller and arranged at the centre of the electric motor. The connection between the first drive shaft and the first propeller and first electric motor is known in the art and not explained in detail here. In order to mount the first and second electric motors to the same side of the propellers (i.e. forward of the electric motors) the second electric motor is suitably connected to the second propeller through the first motor and propeller. Thus, the second electric motor 210 is connected to the second propeller 220 via a second drive shaft 230. The second drive shaft is arranged inside the first drive shaft. Thus, the second drive shaft extends through the first drive shaft and the first electric motor. Again, as shown, the second drive shaft is connected at a centre of the propeller and arranged at the centre of the electric motor. The connection between the second drive shaft and the second propeller and second electric motor is known in the art and not explained in detail here. Because the second drive shaft rotates within the first drive shaft, the first drive shaft may be referred to as the primary derive shaft and the second drive shaft may be referred to as the secondary drive shaft.

[0061] The marine propulsion system 100 is shown in figure 1 as a pod design. That is, the marine propulsion system 100 is designed as a pod that can be mounted to the underside of a boat as a pre-assembled unit. Here, the propulsion system includes a housing 400. The housing is mounted to the hull as herein described.

[0062] Advantageously, the first and second electric motors are shown arranged within the housing 400. Thus, the only required connection between the pod and the inside of the boat is an electrical connection to provide the power to the electrical motors. The control system can be a distributed system. That is whilst the exemplary embodiment is shown as a self-contained pod requiring electrical connection, the control system may be provided within the pod or within the boat or distributed between the two. Thus, although the system requires a controller to vary the relationship between the speed of rotation of the two propellers, the exemplary embodiments include a pod configured to be controlled by a controller to so operate. And references to a pod having a control means includes the control means being provided entirely on the boat and therefore the pod being configured for being so controlled by a control means.

[0063] Referring to Figure 1, the thrust generated by the first and second propellers is transferred to the housing 300 by a thrust bearing arrangement. In figure 3, the thrust bearing arrangement is shown as a first thrust bearing 140 connected to the first shaft and a second thrust bearing 240 connected to the second drive shaft. The first and second thrust bearings are mounted to the housing and transfer the thrust from the rotating propeller that is transferred to the respective drive shaft and to the housing whilst allowing the drive shafts to rotate. A bearing and seal arrangement 410 is shown at the rear end of the housing in order to support the rotating shafts and seal the housing. As is known in the art, further bearings may be provided, for instance between the first and second shafts to support the shafts to freely rotate whilst optionally, where necessary, also sealing the system.

[0064] As will be further explained below and referring initially to Figure 4, the housing 400 includes a main portion 410 that provides the predominant shape to the pod. In addition, to provide the at least one mounting to mount the pod to the hull of the boat, the housing 400 is shown as comprising a forward extension 420. That is, an extension from a forward end of the housing. Here, forward refers to the intended direction of travel of the boat. The forward extension 420 extends from a front of the main portion 410. Here, the forward extension can be designed to prevent the front of the main portion from producing a ‘hook’ in the water. That is, as the boat is propelled forwardly, there is a risk that debris or other obstructions under the water can become snagged on any hook created between the underhanging pod and the hull of the boat. By extending forwardly, the forward extension providing the mounting to the hull can be designed to eliminate and hook and thereby reduce the risk of damage or reduced performance (i.e. , drag) from debris under the water. The forward extension is also shown as extending upwardly. As will be appreciated, because the propulsion system (pod) does not require a mechanical connection between the propellers and inside the boat, the propellers can be arranged at an optimum alignment in the water (e.g., typically horizontally in the water). Thus, because the pod is hung under the streamlined hull, the forward extension extends forwardly and upwardly to provide the mounting to the hull. For instance, the forward extension resembles a swan neck design. The force sensor 300 is arranged on the forward mounting for being arranged between the forward mounting and the hull.

[0065] Still referring to Figure 4, the housing 400 suitably comprises a rear extension that extends from the main portion 410 of the housing at a location rearward from the forward mounting 420. The rear mounting 430 shown in Figure 4 is intended to mount to the ‘P-Bracket’ as herein described. Here, the mounting is envisaged as being a rigid coupling between the rear extension and the p-bracket mounting on the boat. The forward and rear extensions provide a suitably mounting of the housing (i.e. pod) to the boat.

[0066] The main portion of the housing is shown suitably configured to provide optimal characteristics of the water flow around the pod and over the propellers. Here, the main portion is shown as having an outer surface principally forming a dolphin shape. That is, the front end of the main portion (relative to the intended direction of travel) has a short front nose 412, and a tapered mid-portion 414. The front nose expands to a large cross-section, where the mid-portion tapers at a shallower angle than the front nose (i.e. over a loner distance). As shown, the main portion has a generally circular cross section shape. As will be appreciated, where the extension extend the housing can be configured to have fillets and deviations to streamline the shape. Moreover, it is envisaged that the main portion might be formed from multiple parts, for instance where infill sections are used to provide the final shape. The first and second propellers are arranged at the rear end of the housing. Here, as shown in Figure 4, the propellers can have hubs shaped to continue the dolphin shape of the main portion. Furthermore, as shown in Figure 4 an end cone 416 is provided at the rear end of the propellers.

[0067] Figure 16 shows a further pod design wherein the housing is shown as being adapted to be used with an underwater foil of a boat. Here, the housing 800 is shown as having a wing or foil portion 802 that has an aerofoil outer shape. Extending from the foil portion 802 is a nacelle portion that houses the motors and propulsion device (i.e., propellers). The nacelle portion and fin portion can be connected by a pylon similar to section 420 of figure 4, or the pylon may be more integrated into the fin portion as shown in Figure 16.

[0068] A typical sectional view of an electric motor to be used as one or both of the first and second motors is shown in Figure 5. As shown, the respective drive shaft 130, 230 extends through the motor. The motor typically requires cooling and lubricating. For instance, an oil bath 132 is shown. It is envisaged the motor can be in thermal contact with the housing so that the housing acts as a heat sink for the motor. Here, because the housing is immersed in the water during operation, the housing can be configured to transfer the heat to the water to act as a heat sink and to provide a cooling function to the motor. In some embodiments, it is envisaged the housing may include conduits through which coolant is pumped so as to cycle through the conduits and act as a coolant to the motor.

[0069] Referring to Figure 6 and 7 an exemplary propeller 500 is described. The propeller 500 may be adapted to provide either the first propeller 110 or the second propeller 120. Here the propeller 500 comprises a hub 510 and a set of blades formed by a plurality of individual blades 520. The hub is suitably shown as being an ‘oversized hub’, meaning the hub has a substantial diameter. That is, the oversized hub has a considerable diameter compared to the diameter of the propeller. For instance around at least 50% of the diameter of the propeller is formed by the diameter of the hub. As explained above, not only does the oversized hub 510 provide an improved flow dynamics and efficiency to the propulsion system, but it has also been found that by forming the hub ‘oversized’, the blades 520 can be formed separately and modularly connected to the hub. This allows damaged blades to be replaced individually. For instance, as shown in Figure 6, the hub 510 has a series of slots 512 formed about the circumference of the hub’s body 514. The slots 512 are formed in an axial direction and include fixing means 516, shown as bolt holes, for connecting a blade 520. For completeness, a centre of the hub’s body 514 is configured for connection to a respective drive shaft.

[0070] Referring to Figure 7, a blade 520 for use with the hub 510 is shown. The blade is suitably shown with a base portion 522 and a fin portion 524. The fin portion can be designed as is known in the art. The base portion is typically integrally formed with the fin portion and provides the mounting connection to the hub 510. For instance, as will be appreciated, the base portion 522 is arranged to locate in a respective slot 512 of the hub. Thus, the base portion 522 has corresponding fixings 526, shown as apertures through which bolts pass, to the fixings 516 of the hub. The blades can be machined from a single block of material, or, for instance, additively formed through a casting or 3D printing process.

[0071] Figures 8 and 9 show an alternative embodiment to the propeller 500 described in relation to Figures 6 and 7. The main difference is that the slots in the hub are formed at an angle, across the axis of the hub. This allows the blades to be machined from a smaller block of material as the fins do not need to be angled across the base portion. The figures have been shown with identical reference numbers to those used in relation to the description of Figures 6 and 7.

[0072] As mentioned and described herein, the exact design of the propeller, and in particular the shape of the blades is a complex process, that is known in the art. However, optimisation of the contra-rotating design with oversized hub as suggested that an improvement in efficiency can be achieved by bending the tip of each blade, and in particular the blades of the trailing propeller (that is the rearward most mounted propeller). Thus, as shown in Figure 10, the blades 520 of the exemplary propellers optionally include a bent distal tip. That is, the distal tip 528 of the blades is raked towards the downstream side for the front propellor and contrary for the rear propellor. In further optimisation of the marine propulsion system, an optional development has been identified as a means to condition the water prior to flowing over the forward most propeller to be directed to always flow at an ideal angle to the blades, for instance suitably by being urged into a rolling or corkscrew motion. That is, to provide a pre-swirl action. As shown in Figure 11, a pre swirl means 600 is suitably provided as a secondary set of fins 610 on the hub of the forward most mounted propeller (i.e. propeller 120). The secondary set of fins 610 rotate with the hub of the first propeller to act on the body of water flowing over the propellers and upstream (i.e. in front of the propellers) to impart or urge the water to first straighten. The secondary set of fins are shown as having a significantly reduced height to the primary blades of the first propeller. That is, to extend radially from the hub a much-reduced distance than the primary blades (i.e. blades 520). Also, the secondary set of fins are shown as having a reduced number as compared to the primary fins, or the secondary fins extend a greater distance about the circumference of the hub than the primary fins.

[0073] An alternative pre-swirl means is shown in relation to Figure 12, wherein the pre swirl means 600 is shown as a stationary stator 620. The stator 620 has a plurality of radially extending fins. Here, the stator is arranged about the housing 400 and adjacent the leading propeller 210. The fins are adapted to urge the body of water flowing along the housing 400 to be better conditioned prior to flowing over the forward propeller.

[0074] Figure 12 also provides an optional development to the rear extension and mounting. Here, the rear extension 430 is suitably shown as a pair of brackets 432, 434 that extend from the midportion of the housing. In contrast to the rear mounting described above that rigidly mounted the extension to the hull, in Figure 6, the rear mounting is shown as being adapted to provide a degree of movement in the forward direction. That is, the brackets allow the housing to move in a forward direction, whilst restricting relative movement between the housing 400 and the boat in the other degrees of movement. Thus, the thrust generated by the propellers is more completely transferred to the boat through the forward mounting, and therefore the force sensor.

[0075] Figure 13 shows an electric marine propulsion system according to a particularly exemplary embodiment. Here, the system 100 is shown as a pod. Here, the self-contained pod comprises a first electric motor 110 and a second electric motor 210. The first electric motor 110 is operatively connected to a first propeller 120 and the second electric motor 210 is operatively connected to a second propeller 220. The first propeller and the second propeller are configured to be contra-rotating. The first and second motors can be controlled independently to vary the relationship between the speed of rotation of the first propeller and the second propeller. That is, the speed of rotation of one of the propellers can be increased or reduced relative to the speed of rotation of the other. Consequently, the propulsion system further comprises a control means configured to control the speed of rotation of the first and second motors independently and configured to vary the relationship between the speed of rotation of the two propellers. The propulsion system further comprises a force sensor 300. The force sensor is arranged to monitor the thrust transferred from the first and second propellers to the boat. Here, the control system receives signals from the force sensor to monitor the thrust being transferred. The control system is further configured to vary the relationship between the speed of rotation of the first electric motor and the speed of the second electric motor based on the monitored force. Thus, the propulsion system can be dynamically configured to operate at an optimised efficiency. As will be appreciated, suitably the first electric motor 110 is shown as being operatively connected to the first propeller 120 via a first drive shaft 130. As shown, the first drive shaft is connected at a centre of the propeller and arranged at the centre of the electric motor. The second electric motor is suitably connected to the second propeller through the first motor and propeller. Thus, the second electric motor 210 is connected to the second propeller 220 via a second drive shaft 230. The second drive shaft is arranged inside the first drive shaft. Thus, the second drive shaft extends through the first drive shaft and the first electric motor. Again, as shown, the second drive shaft is connected at a centre of the propeller and arranged at the centre of the electric motor. The pod comprises a housing 400 having a forward extension 420 and a rear extension 430 that provides a degree of freedom of movement in the forward direction. A stator 600 is provided at the rear of the housing and adjacent to the leading propeller to impart a straightening motion to the flow of water before it enters (i.e, flows past) the propellers.

[0076] A shown in Figure 14, a boat according to the exemplary embodiments comprises a hull 700 with an exemplary electric marine propulsion system mounted to the underside thereof. Suitably, a second marine propulsion system is also mounted to the underside of the hull 700. Thus, as shown in Figure 15 a port-side propulsion system 100a and a starboard propulsion system 100b is provided. As will be appreciated, the propulsion systems are as herein described and are connected to a power source envisaged as being housed on the boat. Figure 15 shows an exemplary method wherein the boat 700 is driven to produce a sway motion (i.e. sideways motion). Here, the first and second propellers of each system are controlled so that one of the propellers in each system is driven in a backwards direction so that both the propellers in each system are driven to rotate in the same direction (though the first and second propellers in one system might rotate in the same or different common direction relative to the two propellers in the other). The radial flow of water generated when the propellers in each system operate in the same direction can be used to generate a high- pressure and low-pressure. That is, a high-pressure volume can be generated to one side of the boat’s hull and a low-pressure volume can be generated to the other side, wherein the effect of the high and low pressure is to urge the hull to move towards the low-pressure volume. Consequently, providing an electric marine propulsion system having a variable relationship between the speeds of two propellers enables the efficiency of the propulsion system to be optimised for the vessel and the operating conditions. Moreover, providing a variable relationship in a system with two contra rotating propellers provides an azimuth function. Yet further, by varying the relationship between the speeds of the propellers, a predetermined sound signature can be generated. Generating a predetermined sound signature allows the vessel to have a unique signature or to mimic other vessels or sound signatures such as to assist or camouflage identification of the vessel by the sound signature of the propulsion system.

[0077] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0078] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0079] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0080] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0081] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

Claims

Claims:

1. An electric marine propulsion system comprising; a first electric motor operatively connected to drive a first propeller; a second electric motor operatively connected to drive a second propeller; wherein thrust from the first and second propellers is transferred to a boat through a force sensor, the force sensor being configured to monitor the thrust applied to the boat; and the system including a control means configured to vary the relationship between the output speeds of the first and second motors based on the monitored thrust applied to the boat.

2. The electric marine propulsion system of Claim 1 , wherein the first propeller is operatively coupled to a first drive shaft and the second propeller is operatively coupled to a second drive shaft, wherein the drive shafts are coaxial, and the first drive shaft extends through the second drive shaft.

3. The electric marine propulsion system of Claim 2, wherein the first drive shaft extends through the second electric motor.

4. The electric marine propulsion system of any of Claims 1 to 3 further comprising a housing, wherein the first and second motors are mounted within the housing and such that the thrust generated by the respective propellers is transferred to the housing via thrust bearings, the housing having at least one mounting suitable to mount the housing to a boat, wherein the force sensor is arranged on the first mounting.

5. The electric marine propulsion system of Claim 4, wherein the housing has a main portion having an outer surface, and said outer surface has a predominant dolphin-shape.

6. The electric marine propulsion system of Claim 4 or Claim 5, wherein the housing includes a forward extension to a main portion that extends forwardly and upwardly from the main portion, wherein the forward extension provides said mounting.

7. The electric marine propulsion system of any of Claims 4 to 6, wherein the housing includes a rear extension that provides a second mounting suitable to mount the housing to said boat, wherein the second mounting is specifically adapted to allow movement of the housing in a forward direction.

8. The electric marine propulsion system of any of Claims 4 to 7, wherein the housing includes fluid passageways in thermal contact with an outer surface of the housing and through which coolant for the electric motors is arranged to be cycled.

9. The electric marine propulsion system of any preceding claim wherein one or both of the first propeller and the second propeller comprise a hub and a set of blades, wherein the hub is oversized and includes a plurality of slots, the set of blades comprising a plurality of individual blades formed separately to the hub and mounted and connected to respective slots in the oversized hub.

10. The electric marine propulsion system of any preceding claim wherein one or both of the propellers include blades having a raked distal tip.

11. The electric marine propulsion system of any preceding claim further comprising a preswirl means configured to condition the water, prior to a forward most of the first or second propeller, to introduce a rolling motion to a flow of water.

12. A boat comprising a hull, a power source, and at least one electric marine propulsion system of any of claims 1 to 11 electrically connected to the power source.

13. The boat of Claim 12, comprising a first electric marine propulsion system according to any of claims 1 to 11 and a second electric marine propulsion system according to any of claims 1 to 11 , wherein the first and second marine propulsion systems are mounted in a side-by-side arrangement.

14. A method of operating a marine propulsion system, the method comprising monitoring a force transferred from a first and second propeller to a boat, and controlling the relationship between the speed of the first propeller relative to the speed of the second propeller to vary based on the monitored force.

15. The method of claim 14, wherein the method comprises monitoring a second force transferred from a third and fourth propeller to said boat, and controlling the relationship between the speed of the third propeller relative to the speed of the fourth propeller to vary based on the monitored force, wherein the method further comprises: varying the relationship of the first and second propellers to rotate in a common direction; varying the relationship of the third and fourth propellers to rotate in a common direction; controlling the relationship between the first and second propellers and the relationship between the third and fourth propellers togenerate a low pressure volume to one side of the boat and a high pressure volume to the other side in order to generate a sway propulsive thrust.

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