GB2401829A - A propulsion system for an amphibious vehicle - Google Patents

Abstract

Amphibious vehicle 10, may plane on water, and is operable in marine mode or land mode. Its propulsion system comprises prime mover 20, power transmission means 30, marine propulsion means 40, land propulsion means 50, and optional control means 60. Means 50 may be retractable, preferably by hydraulic suspension struts, which also provide road suspension. Power is transmitted to the marine propulsion means in marine mode; and to both marine and land propulsion means in land mode. The ratio of fluid inlet area to fluid outlet area in means 40 may be between 2.5 and 3.5. The maximum thrust may be 7000N, from engine power of less than 135kw, with a jet less than 860mm long. Water flow through means 40 may be reversible; or a reversing bucket may be fitted. Common controls may be operable on both land and water; the steering control travel may be the same in both modes.

Description

2401 829

A PROPULSION SYSTEM

FOR AN AMPHIBIOUS VEHICLE

The present invention relates to a propulsion system and, in particular, to a propulsion system for an amphibious vehicle.

The present invention also relates to a jet drive for use in the propulsion system.

In the case of dedicated land vehicles and marine vessels ]O known in the art, the apparatus employed for power generation, transmission and control thereof is well developed. However, an amphibious vehicle presents quite unique problems and considerations in this regard. There IS a need to employ both marine and land propulsion means, to optimise control and performance of the vehicle both in marine and land modes, and to cater for the transition between marine and land modes and vice versa. This presents conflicting requirements and dedicated prior art systems are poorly suited to the requirements of an amphibious vehicle, it not being possible to optimise both on water and on-land performance by simple incorporation of known dedicated systems.

In the past, designers of amphibious vehicles have focussed their efforts on optimizing either on-water or on-land performance. As a result of this focus, either on-water performance has been sacrificed in order to give satisfactory on land performance, or vice-versa. Conversely, the present applicant has preferred to design an amphibious vehicle having optimized on-land and on-water performance. To this end, the applicant has gone against conventional thinking in arriving at the propulsion system according to the present invention.

In particular, the applicant has found it desirable to have a propulsion system which generates and delivers power only to the marine propulsion means when the amphibious vehicle is operated in a marine mode, but which delivers power to both the marine and land propulsion means (equally or in a selectively variable ratio) when operated in a land mode (which mode includes entry into and egress from the water, i.e. both mode changes). t 2 -

This ensures that entry into water is quickly and controllably achieved, as both the marine propulsion (typically, jet thrust) and the land propulsion (typically, rotation of wheels) are available to power the vehicle as soon as it enters water.

Furthermore, both marine steering (typically by a jet drive steering nozzle) and road steering (typically by steered front wheels) are available to steer the vehicle both on entry into and egress from the water.

In prior art amphibious vehicles where marine drive has to be deliberately selected after entering water, there can be a delay in achieving availability of marine propulsion and steering, leading to uncertainty of control on entering water.

Thls can lead to difficulties for the vehicle operator in dealing with unexpected or strong water currents, wind effects, underwater obstacles, and/or other marine traffic.

Similarly when moving from water to land, if marine propulsion must be shut down before leaving water, only the land propulsion - typically, rotation of road wheels - is available to drive the vehicle forwards, and there is no marine steering. This severely limits driver control, and also results in slow progress. The availability of marine steering is particularly important when, for example, approaching a slipway on the bank of a fast flowing river or seafront. Marine propellers or jets can be beneficially employed to deliver a vectored thrust in water which can maintain vehicle station relative to a fixed point on the bank of a river, when steered front wheels have little effect.

Planing amphibious vehicles have considerably greater appeal than displacement vehicles, because of their greater speed over water. However, to plane effectively, they must have a rearward weight bias, to ensure that the bow sits up out of the water and the stern sits down in water. If a rearward weight bias is combined with front wheel drive, to ensure that the first wheels to touch a slipway can power the vehicle out of water, the resulting road handling characteristics can be somewhat 3 unexpected, as described in the applicant's co-pending application, published as WO 02/12005. Hence, rear wheel drive is preferred for a planing amphibious vehicle. The designer of such a vehicle must be aware that the first wheels to leave water are not driven, so availability of marine drive is particularly helpful in achieving controlled egress from water.

All of the above considerations are important in designing an amphibious vehicle having optimised on-land and on-water lO performance. Because such vehicles have never taken a significant share of the overall road vehicle market, it is likely that most owners and operators of such vehicles will never have driven such a vehicle before. Although they may already be skilled in operation of both road vehicles and marine vehicles separately, a IS vehicle designed to change over between these two modes presents quite novel aspects. Any modification to conventional amphibious vehicle control systems which helps new operators to feel at ease with the vehicle, particularly in the novel experience of driving from land into water and vice versa, is likely to have great value in helping said operators to feel at ease with their new means of transport. A vehicle operator who is calm and collected is more likely to be in safe control of their vehicle than one who is nervously adjusting to a new experience, and is uncertain as to how to proceed. The availability of land and marine vehicle control systems, which as far as possible require similar control inputs and have the same control effect in both land and marine mode, is a vital step in helping the vehicle operator to gain confidence in vehicle control. Furthermore, the "overlap" period when driving from land to water and vice versa, where most controls work in a similar manner to that established during recent operation, will assist the vehicle operator in confidently and safely controlling the vehicle.

It is suggested that the convention in prior art amphibious vehicles of providing full changeover from marine to land mode and vice versa, including decoupling of marine drive, is based on an engineer's understanding of separation of vehicle modes, rather than a consideration of ease of use by a novice operator, 4 - and that this has influenced prior art systems (as in US 3,903,831 to Bartlett and US 4,802,433 to Kovac, where it is mechanically necessary to retract the marine propulsion means before leaving water, to avoid grounding of the propeller).

It should also be noted that if, as in the present invention, marine drive is permanently engaged, the cost, weight, bulk, and complexity of a marine decoupler and its associated controls are avoided. These factors are drawbacks to an lO amphibious vehicle in both road and marine modes. The additional "flywheel effect" of a permanently engaged marine drive may be a drawback under certain conditions when driving on land, but may also be useful in curtailing any tendency to unexpected rapid acceleration on leaving water, when the greater hydrodynamic drag of passage through water is exchanged for the lesser aerodynamic drag of passage through air.

Jet drives are commonly employed in marine vessels as an alternative means of propulsion to shaft driven propellers, stern drives or outboard motors. A jet drive generates a propulsive thrust in a first direction as a result of a reaction force created upon forcing a jet of water in a second opposite direction.

Conventional jet drives have overall aspect ratios of at least 5:1, more typically 6:1; where the overall aspect ratio is calculated as the axial length of the fluid flow path through the jet drive divided by the mean diameter of the impeller. Such jet drives are designed to power marine vessels having smooth hulls from standing (i.e. fully displaced) up to speed, including speeds where sufficient hydrodynamic lift is generated due to speed through water that the hull rises up out of the water onto the plane. A conventional jet drive is designed for maximum thrust at high jet speeds to maximise the top speed achievable by the vessel. Furthermore, the aspect ratio of a conventional jet intake, calculated as its length over its width, varies from 1.9 to 3.3 or even more. l c

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Whilst conventional jet drives have proved to be ideal propulsion means for dedicated marine vessels, they have not done so for amphibious vehicles. An amphibious vehicle presents quite unique problems and considerations both in terms of performance S and packaging requirements. There is a need to optimise performance of the vehicle both in marine and land modes and to cater for the transition between marine and land modes and vice versa. This presents conflicting requirements and it has been found that conventional jet drives are poorly suited to the requirements of an amphibious vehicle, lt not being possible to optimise both on-water and on-land performance.

In the past, designers of amphibious vehicles have focussed their efforts on optimislng either on-water or on-land performance. As a result, either on-water performance has been sacrificed in order to give satisfactory onland performance, or on-land performance sacrificed to give satisfactory on-water performance.

Conversely, the present applicant has preferred to design an amphibious vehicle having optimized on-land and on-water performance. To this end, the applicant has gone against conventional thinking and overcome previous technical prejudices in arriving at the jet drive according to the present invention.

In particular, the applicant has found it desirable for the hull of an amphibious vehicle to have shallow deadrise angles (approximately 5 degrees) as compared to that of a conventional planing hull (lo to 25 degrees). This enables sufficient ground clearance when on land without compromising handling and performance of the vehicle in land mode by having to raise mechanical components and occupant seating to suit conventional deadrise angles. This would increase frontal area, and thus aerodynamic drag; and also raise the centre of gravity, thus increasing cornering roll and decreasing passenger comfort. The adoption of low dead rise angles is contrary to what has been previously accepted as being necessary for a planing marine hull.

Furthermore, the hull of an amphibious vehicle is not as smooth as that of a dedicated marine hull due to the need for wheel arch cut outs to accommodate retractable wheels. In the past, designers have expended great effort in providing complex S moving cover mechanisms for such wheel arch cut outs to re establish the smooth uninterrupted lines of a conventional marine hull when the wheels are retracted. This is done in order to minimise the through water drag of the hull, and to optimise on water performance. The higher the drag coefficient of a hull, the more difficult it is for that hull to be a planing hull.

Alternatively, in the past, one solution was to omit wheel arch cut outs completely. Instead, the road wheels were kept outboard of the hull in both the protracted or lowered (on-land), and the retracted or raised (onwater) condition. Indeed, in certain IS cases, road wheels are fixed in a lowered position both in land and marine modes. This arrangement also has consequences for both on-water and on-land performance due to the unusual weight distributions and increased wheel track dimension.

In the amphibious vehicle of the present applicant, however, wheel arch cut outs in the hull have been retained but optimised to minimise through water drag as far as possible, as described in the applicant's co-pending UK patent application entitled 'A Hull For An Amphibious Vehicle', reference AWP/PEH/P62458/000. No wheel arch covers are used. A net increase in the drag coefficient of the hull as compared to that of a dedicated marine hull has thus been accepted.

As such, the present applicant has been able to optimise on-land vehicle handling and performance at the expense of reduced dead rise angles and increased water drag of the hull.

This route has challenged conventional thinking, but has resulted in an amphibious vehicle which can still achieve sufficient through water speed and hydrodynamic lift to get the vehicle up onto the plane. As such, onwater performance is not compromised. This has been achieved using a jet drive which provides enhanced thrust at low jet speeds, to overcome the increased drag and reduced dead rise angle, and so to power the 7 amphibious vehicle up onto the plane. Conventional marine jet drives cannot provide this performance.

Furthermore, in order to obtain satisfactory on-water performance, and in particular to achieve satisfactory planing characteristics in an amphibious vehicle, it has been found that a mid- or rear-mounted engine is preferred, with driven rear wheels. This optimlses weight distribution on water, and also helps optimise on-road performance. This arrangement presents packaging problems in terms of the amount of space available for a jet drive and for the power transmission thereto. Conventional marine jet drives cannot be accommodated satisfactorily in an amphibious vehicle when the engine is mid- or rear-mounted and the rear wheels are driven, because the rear axle differential heavily restricts the space available for the water jet intake.

Again, the jet drive according to the present invention has been developed with an aspect ratio of less than 4:1, preferably less than 3. 2:1, and yet beneficially achieves the low speed increased thrust requirements. The short intake used results in an intake length to width ratio less than 1.5, preferably less than 1.2; or even more preferably, less than 0.95. Thus, the jet drive can be packaged within tight confines, to the extent that there is no overhang of the jet drive beyond the rear transom of the vehicle.

This is important from several aspects. Firstly, it prevents a sharp object such as a steering nozzle or reversing bucket protruding beyond the bodywork of the vehicle, where they can suffer damage through grounding, backing into herbs, etc. Secondly, it enables rear end crash protection to be improved, as there is clearance between the jet drive intake duct and the differential housing, allowing controlled displacement of the jet drive in an impact.

Another advantage of enhanced jet thrust at low engine speed is found when leaving or entering water. It is helpful, for example, to have good jet thrust available when landing on a slipway, to increase grip at the driven road wheels; particularly as marine drag is increased at this stage due to the road wheels being in their protracted positions. Furthermore, marine steering l 8 - is more effective as more thrust is available; steering control when leaving water being of course more crucial than in open water. If rear wheel drive is used, as is convenient and appropriate with a mid- or rear- mounted engine for good weight distribution for planing, the non-driven front wheels will hit the slipway first; so enhanced low-speed jet thrust is particularly helpful until the rear wheels touch ground. It is also clearly essential for marine drive to remain engaged until road wheel drive is fully effective; so the rotating speeds of these two systems must be compatible.

In order to provide enhanced thrust at low jet speeds, it is necessary to use a relatively large diameter impeller. Once this decision has been taken, two further benefits in particular flow from the unconventional proportions used.

First, the relatively large difference between the impeller housing diameter and the impeller hub diameter allows improvements in energy efficiency, corresponding to those found in high bypass ratio aircraft jet engines. This is due to three factors: one is the reduced influence of boundary layers, because a smaller proportion of the water flow loses energy through frictional contact with housing and hub. The second is lower frictional losses due to lower mean water velocity; and the third is that the kinetic energy input (one half of mass times velocity squared) falls faster than the momentum output (mass times velocity) at the necessarily reduced rotating speeds.

The second initially unexpected advantage of a large diameter jet is in the area of cavitation - the formation of "bubbles" of vacuum which implode adjacent to the impeller blades as they are powered through the water. Such activity damages the blades by pulling chips of metal off their surfaces - an effect known as spelling - and creates unwelcome noise and a concomitant loss in efficiency.

The onset of cavitation is strongly related to blade tip speed; so a larger diameter jet must run slower. However, small 9 - impeller blades are also particularly liable to induce cavitation, due to the large pressure difference between their pressure surfaces and their suction surfaces. If the load is spread over a larger blade area, the difference in pressure can be reduced, leading to a lower likelihood of cavitation.

Prior art displacement amphibious vehicles having marine jet drives require increased jet speed for increased thrust, but also consequentially increase wheel speeds when passing from water to land. This led to difficulties in obtaining grip when the wheels touch ground; and wastage of power before grounding by churning up water around spinning wheels. It is clear that if adequate jet thrust for effective marine to land conversion is available at a jet speed of say, lOOO rpm, rather than a typical 2500 rpm required by prior art jet drives, a much lower wheel speed will result than with a prior art jet drive, if the road transmission is driven at the same speed as the marine transmission. To express this conversely, once the road wheel transmission is engaged, the rotation of the wheels in the water will increase the load on the whole power train; thus limiting the attainable engine speed. Hence the jet speed is automatically reduced, and good jet thrust is essential.

The present invention thus addresses the limitations of prior art jet drives and provides a jet drive having particular utility in amphibious vehicle applications.

The present invention provides, in a first aspect, a propulsion system for an amphibious vehicle comprising: a prime mover; marine propulsion means; land propulsion means; and power transmission means, wherein: the amphibious vehicle is operable either in a marine mode or in a land mode and when the power transmission means transmits power from the prime mover then the transmitted power is transmitted always to the marine propulsion means whether the vehicle is operated in the marine or land mode; whereby the power transmission means can deliver power from the prime mover only to the marine propulsion means when the vehicle is operated in the marine mode; and the power transmission means can deliver power from the prime mover to both the marine propulsion means and the land propulsion means when the vehicle is operated in the land mode.

Preferably, when the vehicle is operated in the marine mode the marine propulsion means can power the vehicle to a speed where sufficient hydrodynamic lift is achieved for the vehicle to plane.

Preferably, the land mode includes entry of the vehicle into the water and egress of the vehicle from the water.

Preferably, when the amphibious vehicle is operated in the land mode the power transmission means can simultaneously deliver power from the prime mover to both the marine propulsion means and the land propulsion means in equal or selectively variable proportions.

Preferably, the propulsion system comprises decoupling means for selectively decoupling and/or controlling the delivery of power from the prime mover to the land propulsion means.

Preferably, the propulsion system comprises control means for controlling all adjustable parameters of each of the prime mover, marine propulsion means, land propulsion means and power transmission means. More preferably, the control means comprises electronic processing means and/or electrical, mechanical, hydraulic or electromechanical actuation devices, or any combination thereof. Preferably, the control means is at least in part made available to a driver of the vehicle to enable the driver to select or control at least the following: starting and stopping of the prime mover; marine or land mode; steering of the vehicle; gear selection; and speed of the vehicle. It is preferred that the speed of the vehicle both in marine and land modes is controlled by the driver using a single speed controller. Preferably, the direction of the vehicle both In À 1 marine and land modes is controlled by the driver using a single steering controller. Furthermore, it is preferred that the vehicle gearbox both in marine and land modes is controlled by the driver using a single gear change/selection controller.

Preferably, the prime mover of the propulsion system comprises any one or a combination of the following: a spark ignition internal combustion engine; a compression ignition internal combustion engine; an electric motor; a fuel cell; or a hybrid engine.

Preferably, the marine propulsion means of the propulsion system comprises one or more jet drives. Advantageously, the one or more jet drives comprise: a fluid inlet; a fluid outlet; a conduit extending from the fluid inlet to the fluid outlet and defining a fluid flow path therebetween; and a rotatable impeller housed within the conduit between the fluid inlet and fluid outlet, characterized in that: the ratio of the axial length of the conduit to the diameter of the impeller is less than 4.0.

Preferably, the land propulsion means of the propulsion system comprises one or more drivable wheels.

Preferably, the power transmission means is integral with the prime mover. Preferably, the power transmission means comprises a marine power transmitting means for transmitting power from the prime mover to the marine propulsion means and a land power transmitting means for transmitting power from the prime mover to the land propulsion means. Preferably, the marine and land power transmission means are of the same type.

Alternatively, the marine and land power transmission means are of different types.

Preferably, the marine and/or land power transmission means IS mechanical. Preferably, the mechanical power transmission l means is a manual or automatic gearbox or continuously variable transmission for providing drive (via drive shafts) to the marine and land propulsion means.

Alternatively, the marine and/or land power transmission means is hydraulic. Preferably, the hydraulic power transmission means includes one or more hydraulic pumps for generating hydraulic power (transmitted via hydraulic lines) and one or more hydraulic motors for providing drive to the marine and/or land propulsion means.

Alternatively, the marine and/or land power transmission means is electric. Preferably, the electric transmission means includes one or more generators, which may be alternators, for generating electric power (transmitted via wires/cabling) and one or more electric motors for providing drive to the marine and/or land propulsion means.

Preferably, the prime mover is located in the middle or the rear of the amphibious vehicle. More preferably, the prime mover is located such that its centre of gravity is positioned between 1.5m and 1.6m from the rear of an amphibious vehicle of 4.6m to 5.Om in length. Yet more preferably, the centre of gravity is substantially 1.54m from the rear of an amphibious vehicle of 4.82m in length.

Preferably the prime mover is arranged transversely in the amphibious vehicle in an East-West or West-East configuration.

Alternatively, the prime mover is arranged in line in the amphibious vehicle in a North-South or South-North configuration.

Preferably, the prime mover has an integral power take-off shaft which is used to provide power directly to the marine propulsion means. Preferably, the prime mover has an integral gearing arrangement such that the power take-off shaft rotates at a speed different to that of the prime mover.

Preferred embodiments of the present invention will now l described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic overview illustrating a propulsion system according to the present invention installed in an S amphibious vehicle; Figure 2 illustrates a first preferred embodiment of propulsion system according to the present invention; Figure 3 illustrates a preferred embodiment of driver interface for controlling the propulsion system of Figures 1 and 2; Figure 4 is a flow chart schematically illustrating a preferred control process of the propulsion system of Figure 2; Figure 5 is a perspective view of a jet drive and powertrain for an amphibious vehicle in accordance with a preferred embodiment of the present invention; Figure 6 is a schematic sectional view from one side of the jet drive of Figure 5; Figure 7 is a plan view from underneath of the jet drive intake, on line A-A' in Figure 6; and Figure 8 is a schematic illustration of the jet drive and powertrain of Figure 5 packaged in an amphibious vehicle.

Referring first to Figure 1, there is illustrated a schematic overview of a propulsion system according to the present invention installed in an amphibious vehicle 10. A prime mover 20 provides power for propelling the amphibious vehicle 10 when operating both in the marine and land modes. Power generated by the prime mover 20 is distributed via a power transmission means 30 to a marine propulsion means 40 and/or a land propulsion means 50. The delivery of power to the respective marine propulsion means 40 and land propulsion means 50 via the power transmission means 30 is controlled by a control means 60. The control means 60 acts at least in part on i) inputs from a driver of the vehicle 10; and ii) inputs from vehicle sensors (not shown), under the control of an electronic control module (ECM) (not shown) using control logic well known in the art (e.g. stored on EPROM, ROM and/or RAM).

An essential feature of the present invention is that when operating in the marine mode, the propulsion system generates and controls the delivery of power only to the marine propulsion means 40 in accordance with control inputs from the driver, and that while operating in the land mode, the propulsion system generates and controls delivery of power to both the marine propulsion means 40 and the land propulsion means 50 in accordance with control inputs from the driver. In the present invention, the amphibious vehicle 10 can operate in only one of two modes. The first is a marine mode where the vehicle 10 can be driven only on water either in a fully displaced mode or in a high speed planing mode where sufficient hydrodynamic lift is achieved by through-water speed that the vehicle 10 rises up out of the water and onto the plane. The second is a land mode in which the vehicle 10 adopts one of three phases. The first phase of the land mode is simply driving the vehicle 10 in normal land conditions, as in the case of a car. The second phase is entry of the vehicle 10 into the water from the land, and the third phase egress of the vehicle 10 from the water onto the land. In all three phases of the land mode, the propulsion system delivers power to both the marine propulsion means 40 and the land propulsion means 50 under the command of control means 60 acting on inputs from the driver.

It can be seen that the propulsion system according to the present invention is shown schematically and may take the form of many different embodiments. For example, it will be appreciated that the land propulsion means 50 may be effected by way of two wheel drive, either front wheel drive or rear wheel drive, or four wheel drive.

Referring next to Figure 2, there is illustrated a preferred embodiment of propulsion system according to the present invention which is installed in an amphibious vehicle 10 adopting the layout of two wheel drive (rear wheel drive) and having a mid-mounted engine (a rear mounted engine could also be used). In this context, a mid-mounted engine is conventionallymounted aft of the passenger seating area, but in front of the l rear axle. However, there is an exception to this rule, in that the French Hobbycar amphibious vehicle had an engine mounted in the centre of the vehicle, and individual passenger compartments at the four corners of the vehicle. The Hobbycar must be considered mid-engined, as the engine is at the centre of the vehicle; but the layout had various practical drawbacks, and the Hobbycar was not a commercial success.

Returning to Figure 2, the prime mover takes the form of an internal combustion engine 20 transversely mounted in the middle or rear of the amphibious vehicle 10. The power transmission means takes the form of an automatic gear box 30 and can transfer drive from the internal combustion engine 20 at a range of gear ratios to driveshafts 34 and a power take off (PTO) shaft 32.

Driveshafts 34 each deliver drive from the automatic gear box 30 to the land propulsion means, in the form of two rear wheels 50 mounted on and driven by driveshafts 34. Each driveshaft 34 includes a decoupled 36 in order to control the delivery of drive to each of the rear wheels 50. Although two decouplers 36 are shown, a decouples may be fitted to one driveshaft only, relying on the vehicle handbrake to lock the other driveshaft. Each decoupled 36 is preferably actuated by hydraulic means (not shown), and built into a constant velocity (CV) joint. A synchromesh mechanism may also be incorporated, as described in the applicant's copending application, published as WO 02/14092.

Drive from the gear box 30 is transferred to the marine propulsion means 40, via the power take off shaft 32 and coupling 33. In this preferred embodiment, the marine propulsion means 40 takes the form of a jet drive as described below and in the applicant's co- pending patent UK patent application entitled 'A Jet Drive For An Amphibious Vehicle', reference AWP/PEH/P61949/000.

As mentioned above, an electronic control module (ECM) (not shown) forms part of the control means 60 used to effect control of the propulsion system, and does so in part upon receiving inputs from the driver of the vehicle 10 via the driver interface illustrated in Figure 3. The driver interface 70 can be seen to take the form of a typical vehicle dashboard layout. The driver can be seen to have a central driving position with passenger/crew seats (not shown) located either side of the driver's seat. The seating arrangement and adjustment mechanism employed is disclosed in the applicants co-pending patent application no. GB0218604.7. The control means 60 may also include an ignition switch and an immobiliser system, and engine and gearbox/transmission management computers, as is known in the automotive engineering art. These additional items may be physically separate to, or integrated with, the ECM and/or the control means 60.

With reference also to Figure 4, which depicts a flow chart illustrating the functionality of the control means 60 l5 incorporating the ECM, a first preferred mode of operation of the amphibious vehicle 10 will now be described. The driver first activates the control means 60 of the amphibious vehicle 10 by inserting a key and turning on the ignition in a conventional manner, step 100. The engine starter interlock will only permit starting of the engine 20 provided that the gear selector or automatic transmission selector 74 is in the "neutral" or "park" position, step 110. If so, then the engine 20 starts, step 120, otherwise the engine 20 will not start, step 220.

The driver can control the power generated by the engine 20 using an accelerator pedal 76 as is known in the art.

Accelerator pedal 76 controls both the power generated by the engine 20 and the delivery of that power to the jet drive 40 and/or wheels 50 when power is being transmitted in either the marine or the land mode. Whilst in the preferred embodiment illustrated the transmission is an automatic gear box with associated gear selector 74, it will be appreciated that a manual transmission may be provided and include a clutch pedal for engaging drive as is known in the art.

A steering wheel 72 is provided for controlling the steering of the vehicle 10, again both when in the marine and land modes. Steering of the vehicle 10 is effected in both marine and land modes by the steering wheel 72 as is described in the applicant's co-pending patent application no. GB0309452.1.

The dashboard layout includes a series of dials 80 for informing the driver of the speed of the vehicle 10 (measured in mph or km/in when on land or on water), together with rpm or power output from the engine, and respective temperature and fuel gauges as is known in the art. A foot brake pedal 77 is provided for actuation of wheel brakes; and a parking brake pedal 78 is JO provided for actuating a parking brake when the vehicle 10 is parked In land mode.

Next, at step 130 (mode identification/selection), the control means 60 determines If marine mode has been selected via mode selector button 82. If marine mode has been or is selected, then as part of the land to marine mode change, drive to the road wheels 50 is decoupled. Hence, when gear selector 79 is moved to a "drive" position, step 170, power from the engine 20 is transmitted to the Jet drive 90 only, step 180. If the marine mode has not been selected, then the control means 60 determines at step 140 if land mode has been selected via mode selector button 82. If land mode has been or is selected, then as part of the marine to land mode change, drive to the road wheels 50 is coupled. Hence, when gear selector 74 is moved to a "drive" position, step 150, the engine 20 provides power to the wheels 50 via the gearbox 30 and driveshafts 34 (the de-couplers 36 being coupled) and also transmits power from the engine 20 to the jet drive 40 via the power take off shaft 32. In either mode, control of the vehicle 10 is then effected by the driver using the steering wheel 72, accelerator pedal 76, foot brake pedal 77, and gear selector 79. Next, the control means 60 proceeds to step 190 and if a mode change is detected, the system returns to the mode identification/selection routine at 130 and the control process continues. Alternatively, if no mode change is selected by the driver via the mode selector button 82, then the control means 60 proceeds to step 200 to determine whether or not the driver has selected to stop the engine 20. If not, then the control means returns to the start of the mode identification/selection routine at step 130 and continues as described above. However, if the driver selects to stop the engine 20, then the control means 60 moves to step 210 at which point the engine 20 is shut down with the vehicle control process terminating at step 220.

In an analogous process to the above, when the vehicle is converted from marine mode to land mode, the wheel decouplers 36 are coupled, so that drive to the marine drive only in marine mode is converted to drive to marine drive and to the wheels in land mode.

It will be appreciated that whilst in the preferred embodiment described above the prime mover 20 takes the form of an internal combustion engine, the prime mover 20 could alternatively take the form of a compression ignition internal combustion engine, an electric motor, a fuel cell, a hybrid engine or any combination thereof. Furthermore, whilst the power transmission means 30 takes the form of a conventional automatic gearbox delivering power via shafts and axles (including a power take off shaft), it will be appreciated that a number of alternative layouts could be implemented. For example, the power transmission means 30 could include or take the form of a continuously variable transmission for providing drive to the marine and land propulsion means. In particular, it is envisaged that if the prime mover is an electric motor or generates electricity via an alternator, that one or more electric motors could be provided to drive one or more wheels and/or the jet drive. Alternatively, the prime mover 20 could power a hydraulic pump or pumps such as a swash plate pump, and power could be distributed to the wheels and/or jet drive by means of hydraulic piping and pressurlsed hydraulic fluid which drives a hydraulic motor or motors coupled directly to each wheel 50 and/or jet drive 40.

Furthermore, it will be appreciated that different mechanical, electrical and/or hydraulic hybrid system combinations could be beneficially employed. For example, the prime mover 20 could take the form of an internal combustion engine which provides drive mechanically to a jet drive 40 via a gearbox and shaft combination, and which provides drive hydraulically to one or more wheels 50 via a swash plate pump, hydraulic lines and a hydraulic motor on the one or more of the wheels 50.

In all cases, the particular prime mover, marine propulsion means, land propulsion means and power transmission means, together with their associated layout, is adopted as best suits the particular application of amphibious vehicle, as will be readily understood by the skilled person in the art.

Furthermore, the prime mover may be mounted transversely as shown in the figures and as described in the applicant's co pending application published as WO 02/07999; or longitudinally, as is found convenient. An example of a power train comprising a longitudinally mounted prime mover may be found in the applicant's co-pending application published as WO 02/12005. The propulsion system described above may be particularly suitable to an amphibious vehicle incorporating road wheels which may be retracted above the water line for use on water, and protracted below the water line for use on land.

Whilst it is preferred that the marine propulsion means is embodied in the form of a jet drive 40, as will now be described below, lt will be appreciated that a traditional propeller and propeller shaft could alternatively be employed.

Similarly, whilst it has been described that the land propulsion means comprises road wheels 50, tracks could alternatively be employed.

Referring now to Figure 5, there is shown a jet drive and powertrain arrangement comprising a jet drive 40 and an internal combustion engine 20. A transmission 30 transfers drive from the internal combustion engine 20 to two wheel driveshafts 39 and to a power take off shaft 32 coupled to a drive shaft 333 of the jet drive 40. Numeral 15 denotes a central longitudinal axis of the vehicle. It is conventional, but not essential, to locate the axis of jet impeller 332 on such an axis.

In Figure 6, the jet drive 40 can be seen to comprise a housing 334 which defines a conduit 335 in which a rotatable impeller 332 is housed. The housing 334 defines a fluid inlet 336 located upstream of the impeller 332, a fluid outlet 337 located downstream of the impeller 332, and a fluid flow path therebetween. The fluid inlet 336 may be provided with a screen filter (not shown) to prevent the ingress of unwanted bodies into the fluid flow path of the jet drive 40. The fluid outlet 337 acts as a discharge nozzle for the exiting fluid jet. A first end of the drive shaft 333 passes through a wall of the housing 334 into conduit 335 and is supported by a bearing 347. The drive shaft 333 extends inside the conduit 335 and the impeller 332 is mounted on a second distal end of the drive shaft 333.

The drive shaft 333 acts as a cantilever, the drive shaft 333 and impeller 332 being cantilevered from the housing wall via bearing 347.

It should be noted that not only are some impellers tapered from front to rear, as shown; but also that some impeller blades have complex swept forms or curved edges. For the avoidance of doubt as to how to measure the impeller diameter, all ratios expressed within this specification which relate to the impeller diameter have been calculated in terms of the more easily defined mean internal diameter of the impeller housing (i. e. mean conduit diameter); which should of course be measured in the housing area surrounding the blades. Where the impeller and housing are tapered longitudinally, on a straight or curved taper, the mean diameter of the impeller housing in the area swept by the blades should be used for all calculations relating thereto.

A stator 331 may optionally be provided in the fluid flow path between the impeller 332 and the fluid outlet 337. The stator 331 is provided with stator vanes which are arranged so as to remove the rotational elements of the fluid flow exiting the impeller 332, thus presenting a uniform inline jet of fluid at the fluid outlet 337. A duct liner (not shown) may be fitted around impeller 332, to protect the jet housing against abrasion.

Its inside diameter would of course be considered as the effective internal diameter of the jet impeller housing 334 (i.e. the impeller diameter).

The dimensions of the jet drive 40 are important in terms of performance and packaging within an amphibious vehicle, as will now be described. The axial length of the fluid flow conduit is measured along the vehicle from point 342 at the front of ho intake 336 to point 345 at outlet 337. This dimension is preferably between 0.3m and 2.Om; more preferably between 0. 7m and 1.Om; and in a particularly preferred embodiment, substantially equal to 0.85m.

The mean impeller housing internal diameter is measured around the outside of the blades, and is preferably between O.lm and 0.66m; more preferably between 0.25m and 0.33m; and in a particularly preferred embodiment is substantially 0.30m.

The ratio between the axial length of the conduit to the mean impeller housing internal diameter is preferably less than 4.0; more preferably less than 3.2; and in a particularly preferred embodiment is substantially 2.9.

The width of the jet intake is measured between points 342 and 343 in Figure 7, as compared to the jet intake length, which is measured between point 342 and cutwater 344 in Figure 6. The ratio of intake length to width is preferably between 0.4 and 1.5; more preferably between 0.7 and 1.2; and in a particularly preferred embodiment is substantially 0.95.

It should be noted that In practice, both the front and rear of the jet intake may be of curved form in side view. Where the front of the intake Is curved, point 342 may be taken as the point at which the intake diverges upwards from the flat plane of the hull adjacent to arrow A at the front of the intake.

Similarly, cutwater 344 may be defined as the most forward point on a radius in vertical cross-section at the rearward end of the jet intake.

In order to improve the planing characteristics of the amphibious vehicle, a ride plate may be fixed below the jet intake area as described in the applicant's co-pending UK patent application entitled 'A Hull For An Amphibious Vehicle', reference AWP/PEH/P62458/000. Any reduction in jet intake area would be negated by the increase in the available planing area.

The ride plate would conventionally be a pressing, fastened to or through the jet housing casting. Hence for the purposes of calculations of areas and ratios, a ride plate is regarded as an accessory to the jet assembly, whose ratios according to design remain unchanged.

Is The fluid inlet area of a water jet is conventionally measured on a plane from the cutwater at right angles to the fluid flow through the jet. This is shown in Figure 4 as a dashed line joining cutwater 344 to point 346 on the housing. The outlet area is measured at outlet 337.

For the jet characteristics required for a planing amphibious vehicle, the ratio of fluid inlet area to fluid outlet area is preferably in the range 2.5 to 3.5; more preferably in the range 2.8 to 3.2; and in a particularly preferred embodiment is substantially 3.03.

Furthermore, the inlet area may preferably be between 0.020m2 and 0.400m2; more preferably between 0.040m2 and 0.150m2; and in a particularly preferred embodiment is substantially 0.081m2. The corresponding outlet area may preferably be between O.OlOm2 and 0.150m2; more preferably between 0.020m2 and 0.060m2; and in a particularly preferred embodiment is substantially 0.027m2.

The flow rate through the jet drive may vary from zero -at rest- to 1. 5m3sl at maximum operating speed. In a particularly preferred embodiment, the flow rate is substantially 0.2m3sl at an impeller speed of 600 rpm; and substantially 1.1m3s1 at an impeller speed of 3000 rpm.

The stator inlet diameter is substantially equal to the impeller outlet diameter, and is preferably between O.llm and S 0.66m; more preferably between 0.25m and 0.35m; and in a particularly preferred embodiment, is substantially 0.30m.

Packaging of the jet drive 40 and engine 20 in a vehicle 10 with retractable wheels 50 is illustrated in Figure 8.

Whilst in the preferred embodiment described the powertrain has been described as comprising an internal combustion engine and mechanical transmission 30, it will be appreciated that any prime mover and power transmission system may be used to implement the jet drive according to the present invention. For example, the prime mover may be a spark ignition or compression ignition internal combustion engine, an electric motor, a fuel cell, a hybrid engine or any combination thereof. The transmission of power from the prime mover to the jet drive may be effected using any conventional means such as direct mechanical drive via drive shafts, manual or automatic transmission or continuously variable transmission.

Alternatively, hydraulic or electric power transmission means may be implemented using hydraulic or electric pumps/generators and motors.

Furthermore, the prime mover may be mounted transversely as shown in the figures and as described in the applicant's co pending application published as WO 02/07999; or longitudinally, as is found convenient. An example of a power train comprising a longitudinally mounted prime mover may be found in the applicant's co-pending application published as WO 02/12005.

Drive shaft 333 may conveniently be mounted substantially parallel to the vehicle longitudinal axis 15, as is shown in the figures. However, this layout may result in significant transmission damage if the vehicle is struck from behind in an accident. It may therefore be preferable to introduce a lateral and/or vertical angle to the vehicle longitudinal axis in the mounting of the drive shaft 333 in the vehicle. Thls encourages the jet drive assembly to move laterally and/or vertically under rear impact conditions. In this case, one or more universal joints ("UJ's"); or preferably constant velocity ("CV") joints; may be fitted to at least one end of the drive shaft, preferably adjacent to coupler 33 and/or adjacent to the jet intake housing, to ensure smooth running. A suitable rearward joint position is shown at 348 in Figure 6. Alternatively, a joint and bearing may be provided just forward of the impeller. This has the advantage of providing improved support for the impeller, but obstructs water flow through the intake duct; and must have highly effective sealing against water ingress.

Such a skewed drive shaft may also accommodate the effects on the marine drive shaft of driveline shunt where a transverse engine is fitted. Driveline shunt is caused by a transverse engine rocking backwards and forwards on its mountings as power is applied and removed, and could be damaging to a substantially longitudinal drive shaft. Finally, as is known in the automotive engineering art, two universal joints could be fitted at opposite ends of the shaft; but ninety degrees out of phase with each other in a rotational sense, to dampen out transmission vibrations.

To allow the amphibious vehicle to brake or to travel in reverse on water, there are two possible solutions. One is to provide a reversing bucket downstream of the jet fluid outlet, as is known in the marine jet drive art. Reversing buckets provide efficient and controllable reverse drive, but are unsightly on a road vehicle. Where the jet is driven through the road wheel transmission, the alternative is to use the transmission reverse gear to drive the impeller in an opposite direction to that required for forward motion of the vehicle to effect a braking or a reversing function.

As an example of the efficient performance of the jet drive described above, it has been found that an amphibious vehicle travelling over water at a speed of 8.7 knots (4.5msl) at a trim angle between ten and fifteen degrees, will rise onto the plane despite having a through water drag coefficient greater than 0.27 under these conditions.

Furthermore, a jet drive according to the construction described above has been found to generate a peak bollard pull of at least 7kN from an engine peak power of less than 135kW, within a jet overall length of less than 860mm. At..

Claims (84)

1. A propulsion system for an amphibious vehicle comprising: a prime mover; marine propulsion means; land propulsion means; and power transmission means, wherein: the amphibious vehicle is operable either in a marine mode or in a land mode and when the power transmission means transmits power from the prime mover then the transmlLted power is transmitted always to the marine propulsion means whether the vehicle is operated in the marine or land mode; whereby the power transmission means can deliver power from the prime mover only to the marine propulsion means when the vehicle is operated in the marine mode; and the power transmission means can deliver power from the prime mover to both the marine propulsion means and the land propulsion means when the vehicle is operated in the land mode.

2. A propulsion system as claimed in claim l wherein when the vehicle is operated in the marine mode the marine propulsion means can power the vehicle to a speed where sufficient hydrodynamic lift is achieved for the vehicle to plane.

3. A propulsion system as claimed in claim 1 or claim 2 wherein the land mode includes entry of the vehicle into the water and egress of the vehicle from the water.

4. A propulsion system as claimed in any one of the preceding claims wherein when the amphibious vehicle is operated in the land mode the power transmission means can simultaneously deliver power from the prime mover to both the marine propulsion means and the land propulsion means in equal or selectively variable proportions.

5. A propulelon system as claimed in any one of the precedlog claims further comprising decoupllog means for selectively decoupling and/or controlling the delivery of power from the prime mover to the land propulsion means.

6. A propulsion system as claimed in any one of the preceding claims further comprising control means for controlling all adjustable parameters of each of the prime mover, marine propulsion means, land propulsion means and power transmission means.

7. A propulsion system as claimed in claim 6 wherein the control means comprises electronic processing means and/or electrical, mechanical, hydraulic or electromechanical actuation devices, or any combination thereof.

8. A propulsion system as claimed in claim 6 or claim 7 wherein the control means is at least in part made available to a driver of the vehicle to enable the driver to select or control at least the following: starting and stopping of the prime mover; marine or land mode; steering of the vehicle; and speed of the vehicle.

9. A propulsion system as claimed in claim 8 wherein the speed of the vehicle both in marine and land modes is controlled by the driver using a single speed controller.

10. A propulsion system as claimed in claim 8 or claim 9 wherein the direction of the vehicle both in marine and land modes is controlled by the driver using a single steering controller.

11. A propulsion system as claimed in claim 8, 9, or 10 wherein the power transmission means is a gearbox and the gearbox both in marine and land modes is controlled by the driver using a single gearchange controller.

12. A propulsion system as claimed in any one of the preceding claims wherein the prime mover comprises any one or a combination of the following: a spark ignition internal combustion engine; a compression ignition internal combustion engine; an electric motor; a fuel cell; or a hybrid engine.

13. A propulsion system as claimed in any one of the preceding claims wherein the marine propulsion means comprises one or more jet drives.

14. A propulsion system as claimed in any one of the preceding claims wherein the land propulsion means comprises one or more drivable wheels.

15. A propulsion system as claimed in any one of the preceding claims wherein the power transmission means is integral with the prime mover.

16. A propulsion system as claimed in any one of the preceding claims wherein the power transmission means comprises a marine power transmitting means for transmitting power from the prime mover to the marine propulsion means and a land power transmitting means for transmitting power from the prime mover to the land propulsion means.

17. A propulsion system as claimed in claim 16 wherein the marine and land power transmission means are of the same type.

18. A propulsion system as claimed in claim 16 wherein the marine and land power transmission means of different types.

19. A propulsion system as claimed in any one of claims 1 to 15 wherein the power transmission means is mechanical.

20. A propulsion system as claimed in any one of claims 16 to 18 wherein the marine and/or land power transmission means is mechanical.

21. A propulsion system as claimed in claim 19 wherein the mechanical power transmission means is a manual or automatic gearbox or continuously variable transmission for providing drive to the marine and land propulsion means.

22. A propulsion system as claimed in claim 20 wherein the marine and/or land power transmission means is a manual or automatic gearbox or continuously variable transmission for providing drive to the marine and/or land propulsion means.

23. A propulsion system as claimed in any one of claims 1 to 15 wherein the power transmission means is hydraulic.

IS

24. A propulsion system as claimed in any one of claims 16 to 18, 20 or 22 wherein the marine and/or land power transmission means is hydraulic.

25. A propulsion system as claimed in claim 23 wherein the hydraulic power transmission means includes one or more hydraulic pumps for generating hydraulic power.

26. A propulsion system as claimed In claim 24 wherein the marine and/or land hydraulic power transmission means includes one or more hydraulic pumps for generating hydraulic power.

27. A propulsion system as claimed in claim 23 or claim 25 wherein the hydraulic power transmission means includes one or more hydraulic motors for providing drive to the marine and land propulsion means.

28. A propulsion system as claimed in claim 24 or claim 26 wherein the marine and/or land hydraulic power transmission means includes one or more hydraulic motors for providing drive to the marine and/or land propulsion means.

29. A propulsion system as claimed in any one of claims 1 to 15 wherein the power transmission means is electric.

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30. A propulsion system as claimed in any one of claims 16 to 18, 20, 22, 24 or 26 wherein the marine and/or land power transmission means is electric.

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31. A propulsion system as claimed in claim 29 wherein the electric transmission means includes one or more generators, which may be alternators, for generating electric power.

32. A propulsion system as claimed in claim 30 wherein the JO marine and/or land electric transmission means includes one or more generators, which may be alternators, for generating electric power.

33. A propulsion system as claimed in claim 29 or claim 31 wherein the electric transmission means includes one or more electric motors for providing drive to the marine and land propulsion means.

34. A propulsion system as claimed in claim 30 or claim 32 wherein the marine and/or land electric transmission means includes one or more electric motors for providing drive to the marine and/or land propulsion means.

35. A propulsion system as claimed in any one of the preceding claims wherein the prime mover is located in the middle or rear of the amphibious vehicle.

36. A propulsion system as claimed in any one of the preceding claims wherein the prime mover is located such that its centre of gravity is positioned between 1.5m and 1.6m from the rear of an amphibious vehicle of 4.6m to 5.Om in length.

37. A propulsion system as claimed in claim 36 wherein the prime mover is located such that its centre of gravity is positioned substantially 1.54m from the rear of an amphibious vehicle of substantially 4.82m in length.

38. A propulsion system as claimed in any one of the preceding claims wherein the prime mover is arranged transversely in the amphibious vehicle in an East-West or West-East configuration.

39. A propulsion system as claimed in any one of the preceding claims wherein the prime mover is arranged in line in the amphibious vehicle in a North-South or South-North configuration.

40. A propulsion system as claimed in any one of the preceding claims wherein the prime mover has an integral power take-off shaft which is used to provide power directly to the marine propulsion means.

41. A propulsion system as claimed in claim 40 wherein the prime mover has an integral gearing arrangement such that the power take-off shaft rotates at a speed different to that of the prime mover.

42. A propulsion system as claimed in any one of claims 13 to 41 wherein each of the one or more jet drives comprise: a fluid inlet; a fluid outlet; a conduit extending from the fluid inlet to the fluid outlet and defining a fluid flow path therebetween; and a rotatable impeller housed within the conduit between the fluid inlet and fluid outlet, wherein: the ratio of the axial length of the conduit to the diameter of the impeller is less than 4.0.

43. A propulsion system as claimed in claim 42 wherein the ratio of the axial length of the conduit to the diameter of the impeller is less than 3.2.

44. A propulsion system as claimed in claim 42 or claim 43 wherein the ratio of the axial length of the conduit to the diameter of the impeller is substantially 2.9.

45. A propulsion system as claimed in any one of claims 42 to 44 wherein the axial length of the conduit is in the range 0.3m to 2.Om, and the diameter of the impeller is in the range O.lm to 0.66m.

46. A propulsion system as claimed in any one of claims 42 to 45 wherein the axial length of the conduit is in the range 0.7m to 1.Om and the diameter of the impeller is in the range 0.25m to 0.33m.

47. A propulsion system as claimed in any one of claims 42 to 46 wherein the axial length of the conduit is substantially 0.85m and the diameter of the impeller is substantially 0.295m.

48. A propulsion system as claimed in any one of claims 42 to 47 wherein the ratio of the length of the jet intake to the width of said intake is in the range 0.4 to 1.5.

49. A propulsion system as claimed in any one of claims 42 to 48 wherein the ratio of the length of the jet intake to the width of said intake is in the range 0.7 to 1.2.

50. A propulsion system as claimed in any one of claims 42 to 49 wherein the ratio of the length of the jet intake to the width of said intake is substantially 0.95.

51. A propulsion system as claimed in any one of claims 42 to wherein the ratio of fluid inlet area to fluid outlet area is in the range 2.5 to 3.5.

52. A propulsion system as claimed in any one of claims 42 to 51 wherein the ratio of fluid inlet area to fluid outlet area is In the range 2.8 to 3.2.

53. A propulsion system as claimed in any one of claims 42 to 52 wherein the ratio of fluid inlet area to fluid outlet area is substantially 3.03.

54. A propulsion system as claimed in any one of claims 42 to 53 wherein the fluid inlet area is In the range 0.020m2 to 0.400m2, and the fluid outlet area is in the range O.OlOm2 to 0. 150m2.

55. A propulsion system as claimed in any one of claims 42 to 54 wherein the fluid inlet area is in the range 0.040m2 to 0.150m2, and the fluid outlet area is in the range 0.020m2 to 0. 060m2.

56. A propulsion system as claimed in any one of claims 42 to 55 wherein the fluid inlet area is substantially 0.081m2, and the fluid outlet area is substantially 0.027m2.

57. A propulsion system as claimed in any one of claims 42 to 56 wherein the rate of fluid flow through the jet drive is in the range Om3s1 to 1.5m3Sl.

58. A propulsion system as claimed in any one of claims 42 to 57 wherein the rate of fluid flow through the jet drive varies from substantially 0.2m3s1 when the impeller is driven at 600rpm to substantially 1.1m3sl when the impeller is driven at 3000rpm.

59. A propulsion system as claimed in any one of claims 42 to 58 further comprising a stator housed within the conduit between the impeller and the fluid outlet.

60. A propulsion system as claimed in claim 59 wherein the stator has an inlet diameter in the range of O.llm to 0.66m.

61. A propulsion system as claimed in claims 59 or claim 60 wherein the stator has an inlet diameter In the range of 0.25m to 0.35m.

62. A propulsion system as claimed ln any one of claims 59 to 61 wherein the stator has an inlet diameter of substantially 0.305m.

63. A propulsion system as claimed in any one of claims 42 to 62 wherein the jet drive generates a peak bollard pull of at least 7kN from an engine peak power of less than 135kW, within a jet overall length of less than 860mm.

64. A propulsion system as claimed in any one of claims 42 to 63 further comprising a reversing bucket located downstream of the fluid outlet which can be used to redirect flow of fluid expelled from the fluid outlet to effect a braking or a reversing function.

JO

65. A propulsion system as claimed in any one of claims 42 to 64 wherein the impeller can be driven in an opposite direction to that required for forward motion of the vehicle to effect a braking or a reversing function.

66. A propulsion system as claimed in any one of claims 42 to wherein the drive shaft from the power take off to the jet input is skewed horizontally and/or vertically relative to the longitudinal axis of the vehicle.

67. A propulsion system as claimed in claim 66 further comprising at least one universal joint affixed to the drive shaft.

68. A propulsion system as claimed in claim 66 or claim 67 further comprising at least one constant velocity joint affixed to the drive shaft.

69. A propulsion system as claimed in any one of claims 42 to 68 which is fully contained within the amphibious vehicle such that no part of the jet drive extends out of the vehicle.

70. A propulsion system as claimed in any one of claims 13 to 41 wherein each of the one or more jet drives comprise: a fluid inlet; a fluid outlet; a conduit extending from the fluid inlet to the fluid outlet and defining a fluid flow path therebetween; and a rotatable impeller housed within an impeller housing in the conduit between the fluid inlet and fluid outlet, wherein: the ratio of the axial length of the conduit to the mean internal diameter of the impeller housing is less than 4.0.

71. A propulsion system as claimed in claim 70 wherein the ratio of the axial length of the conduit to the mean internal diameter of the impeller housing is less than 3.2.

72. A propulsion system as claimed in claim 70 or claim 71 wherein the ratio of the axial length of the conduit to the mean internal diameter of the impeller housing is substantially 2.9.

73. A propulsion system as claimed in any one of claims 70 to 72 wherein the axial length of the conduit is in the range 0.3m to 2.Om, and the mean internal diameter of the impeller housing is in the range O. lm to 0.66m.

74. A propulsion system as claimed in any one of claims 70 to 73 wherein the axial length of the conduit is in the range 0.7m to 1.Om and the mean internal diameter of the impeller housing is in the range 0. 25m to 0.33m.

75. A propulsion system as claimed in any one of claims 70 to 74 wherein the axial length of the conduit is substantially 0.85m and the mean internal diameter of the impeller housing is substantially 0.295m.

76. Use of a propulsion system as claimed in any one of the preceding claims in an amphibious vehicle.

77. Use of a propulsion system as claimed in any one of claims 1 to 75 to propel an amphibious vehicle operating in marine mode to a speed where sufficient hydrodynamic lift is achieved for the vehicle to plane.

78. Use of a propulsion system as claimed in any one of claims 13 to 75 to propel an amphibious vehicle having a through water drag co- efficient at a speed of 8.7 knots (4.5msl) and a trim angle between ten and fifteen degrees greater than 0.27, the jet drive propelling the vehicle when operating in marine mode to a speed where sufficient hydrodynamic lift is achieved for the vehicle to plane.

79. An amphibious vehicle incorporating a propulsion system as claimed in any one of claims 1 to 75.

80. An amphibious vehicle as claimed in claim 79, further incorporating road wheels which may be retracted above the water line for use on water, and protracted below the water line for use on land.

81. A propulsion system substantially as hereinbefore described with reference to or as shown in the accompanying drawings.

82. Use of a propulsion system substantially as hereinbefore described with reference to or as shown in the accompanying drawings.

83. An amphibious vehicle incorporating a propulsion system substantially as hereinbefore described with reference to or as shown in the accompanying drawings.

84. Use of a jet drive in a propulsion system substantially as hereinbefore described with reference to or as shown in the accompanying drawings.