GB2503510A

GB2503510A - A boat with a low-profile opposed piston engine

Info

Publication number: GB2503510A
Application number: GB1211620.8A
Authority: GB
Inventors: Andrew Mcleod-Ross; Christopher Downton
Original assignee: BLUEWATER WESLAKE MARINE Ltd
Current assignee: BLUEWATER WESLAKE MARINE Ltd
Priority date: 2012-06-29
Filing date: 2012-06-29
Publication date: 2014-01-01
Also published as: GB201211620D0; WO2014001821A1

Abstract

A boat 10 comprises a main deck 18 located at the level of the bilges of the boat and a low-profile inboard engine 30 located at, or towards the centre of the vessel. The engine is at least partially located below the main deck of the boat. The boat may be a rigid inflatable boat and may be a jet tender in which the engine is operatively connected to a jet pump 36 located below the level of the main deck. A fuel tank 34 may also be located below the level of the main deck while a passenger or cargo area 16 may be located above the level of the main deck. Also disclosed is an opposed piston marine engine (Fig. 3) and a cylinder liner (80, Fig 6) for an internal combustion engine of the opposed piston type.

Description

Title: Watercraft

Description:

This invention relates to watercraft and in particular, but without limitation, to Rigid Inflatable Boats (RIBs) and jet tenders.

There is a large market for small powerboats for use as pleasure craft, tenders to larger boats, safety boats and so on and many such small powerboats are of the "rigid inflatable" type known as RIBs.

A RIB generally comprises a hull having a rigid bilge portion and an inflatable bulwark defining an outer periphery of the vessel. RIBS are particularly suited for use as tenders, safety boats and for use in confined areas.

Many RIBS are powered by outboard engines, which are normally mounted on brackets on the transom of the boat. However, outboard engines can be unsightly and noisy. Higher-specification RIBs, therefore, generally comprise an inboard engine, which is concealed beneath a quarter deck or seat of the boat.

Some RIBs are of the "jet tender" type, which have an inboard jet pump engine, rather than a conventional screw propeller engine. This has a number of advantages including a vastly reduced likelihood of prop fouling, a shallower draft for the vessel and also a reduced likelihood of injury because the propeller (actually, an impeller) is contained within the jet pump mechanism, rather than being located externally of the boat.

Known RIB jet tenders often comprise a central seat that the driver straddles and below which seat the inboard engine is located. Although some jet tenders have the driving position aft of the engine, rather than on top of it, the seat normally doubles as a driver's seat and an engine cover. The known type of jet tender, that is, having a central driver's seat position suffers from a number of disadvantages in as much as the engine compartment/driver seat is located in the centre of the vessel and thus presents an obstruction to users of the vessel as they move around it. In addition, the presence of the seat restricts the number of additional passenger seats or cargo areas that can be provided.

It is therefore desirable to try to minimise the size of the engine compartment to the greatest possible extent -to free up space within the vessel to enable seats and cargo areas to be positioned with greater flexibility.

A known way to achieve this objective is to locate the inboard engine towards the rear of the vessel, such as below a quarter deck adjacent the boat's transom. This solution, however, tends to shift the boat's centre of gravity rearwards, which can adversely affect its trim or handling. Such adverse effects can be reduced by locating the driver's seat towards the front of the vessel to counterbalance the weight of the engine, or by the provision of ballast, but this can lead to an unsatisfactory design and can increase the weight of the boat.

A need therefore arises for an improved and/or alternative type of RIB that addresses one or more of the above problems, including enabling the engine to be located at, or towards, the centre of the vessel, whilst still allowing a great deal of flexibility in the positioning of seats and cargo areas within the vessel.

A first aspect of the invention provides boat comprising an inboard engine, wherein the inboard engine is located at, or towards, the centre of the vessel, and wherein the engine is at least partially, or wholly, located below the level of the main deck of the vessel.

By locating the engine at or towards the centre of the vessel, the balance and trim of the boat can be optimized without the need for ballast to counterbalance the weight of an engine at an off-centre location. Such a design avoids the need to provide counterbalancing ballast, which can thus enable the boat to be made light enough to be lifted using a hand-powered, or an electric winch.

Moreover, by locating the engine at, or below the level of the main deck (that is the main walkway surface) of the boat, the boafs centre of gravity is lowered thereby improving the stability of the boat. Further, by locating the engine at or below the deck level, that is to say, partially or wholly within the bilges of the boat, the engine compartment can be minimally invasive into the main passenger orcargoarea of the boat.

The main passenger or cargo area of the boat may comprise one or more seats, which may arranged to suit particular applications. It will be appreciated that by locating the engine low down in the boat, or below the main deck of the boat, such a design affords much greater choice and flexibility in the location of the seats and cargo areas.

io where the boat is a small RIB, such as a jet tender, it may be necessary to use a compact engine.

According to a second aspect of the invention, there is provided a compact, inboard marine engine of the opposed piston type, the engine comprising a cylinder liner for receiving a pair of pistons configured to synchronously reciprocate within the cylinder in opposite directions to form a combustion chamber within the cylinder between the pistons as they move together, and further comprising an injector and a heating/sparking plug in communication with the combustion chamber, wherein the injector and heating/sparking plug are located less than 180 degrees apart when viewed along a longitudinal axis of the cylinder.

Preferably, the injector and heating/sparking plug are located less than 90, 45 or 20 degrees apart when viewed along a longitudinal axis of the cylinder and are most preferably located substantially side-by-side.

Preferably, the configuration of the engine is such that it has a low-profile design, that is to say, having the smallest possible height, to facilitate its location at, or below, the level of the deck of the vessel and/or at least partially, or wholly within the bilges of the vessel.

In a preferred embodiment of the invention, the inboard engine has flat piston configuration, to enable the engine block to be relatively wide but flat. By locating the combustion chambers of the engine horizontally, the overall height of the engine can be reduced.

In most preferred embodiment of the invention, the engine configuration is of the opposed piston type, that is to say, having two reciprocating pistons per cylinder that come together during the compression stroke to form a shared combustion chamber towards the centre of the cylinder, and which move apart during the power stroke. The engine can be a two-stroke engine or a four-stroke engine, depending on individual requirements. Of course, a two-stroke engine will be of the ported type as described herein, whereas a four-stroke engine will have valves and a timing mechanism as will be apparent to the skilled reader.

Because the engine is located in the bottom of the boat, its design should ideally be optimized to permit servicing and inspection from above only, or from above and either side. This is because access to the engine from below, when installed, will normally be blocked or inhibited by its proximity to the underside of the boat. A new type of engine design is therefore required, in which the cylinders are located at the bottom of the engine and in which access ports, for injectors, sparking plugs, and so on, extend from one side of the cylinder only, most preferably an upper side of the cylinder.

A third aspect of the invention provides an opposed piston engine comprising at least one cylinder and a cylinder liner for the cylinder, each cylinder liner: being adapted to sealingly receive a pair of pistons configured to reciprocate synchronously within the liner, the cylinder liner comprising: at least one inlet port and at least one exhaust port, which ports are selectively opened and closed by side of a piston moving past it, an injector port and a sparking plug (for a petrol engine) or heater plug port (for a diesel engine), characterised in that the injector port and the sparking plug/heater plug ports are located at positions less than 180 degrees apart when viewed along a longitudinal axis of the cylinder liner.

A fourth aspect of the invention provides a cylinder liner for an internal combustion engine of the opposed piston type, the cylinder liner comprising a central bore forming the cylinder and being adapted to receive, in use, a pair of pistons, the cylinder liner comprising at least one air intake port and at least one exhaust port located on opposite sides of a substantially central combustion chamber of the liner and a sparking/heat plug receiving port and an injector receiving port in communication with the combustion chamber, wherein the sparking/heat plug receiving port and the injector receiving port are arranged less than 180 degrees apart when viewed along a longitudinal axis of the cylinder.

The engine or cylinder liner may be able to accommodate a plurality of injectors and/or heater/sparking plugs per cylinder, in which case the ports will preferably be clustered together such that they are arranged less than 180 degrees apart when viewed along a longitudinal axis of the cylinder.

Preferably, the injector and heating/sparking plug ports of the cylinder liner are located less than 90,45 or 20 degrees apart when viewed along a longitudinal axis of the cylinder and are most preferably located substantially side-by-side.

By locating the sparking plug/heater plug ports less than 180 degrees apart, they can both be accessed from one side of the engine, most preferably from above the engine. Preferably still, the sparking plug/heater plug ports are located at positions less than 90 degrees apart when viewed along a longitudinal axis of the cylinder liner, more preferably they are located at positions less than 90 or 45 degrees apart when viewed along a longitudinal axis of the cylinder liner, and most preferably they are located at positions less than 25 0120 degrees apart when viewed along a longitudinal axis of the cylinder liner.

A preferred two-stroke opposed piston type engine of the invention has a central injector that sprays the fuel into the shared combustion chamber. The fuel can ignite in the conventional manner and the pistons can be forced apart along the cylinder whereupon exhaust ports can be uncovered to allow combustion products to escape. The cylinder can then be recharged with clean air through ports at the opposite side of the cylinder, which ports are re-closed as the pistons move together again at the start of the next compression cycle.

The preferred two-stroke engine has a cylinder liner having radial inlet ports approximately halfway up one side of it that are uncovered when the piston on that side of the engine is at the bottom of its stroke. The liner preferably has radial exhaust ports approximately halfway up the other end of the liner, which are uncovered when the piston on that side is at the bottom of its stroke.

The pistons are preferably synchronized via crankshafts and a series of gears so that the pistons are at inner dead centre together to form the common combustion chamber. In a preferred engine design, the pistons are not perfectly symmetrical and may have a very slight angular lead with the exhaust side piston so that the exhaust ports open first, allowing enough time for the pressure in the cylinder to drop so that when the inlet ports open the inlet charge can refill the cylinder. Both the inlet and exhaust ports preferably close at a similar timing.

The cylinder liner is preferably made from a thin wall steel tube and its bore is preferably nitrided to provide a hard wearing surface.

The inlet ports of the cylinder liner are preferably arranged tangential to the longitudinal axis of the cylinder to induce swirl as air enters the liner, which can aid fuel/air mixing.

Preferably, at least part, and preferably, the central part of the liner is cooled by a liquid coolant, such as a water/glycol mix. The outer parts of the cylinder liner may be cooled with the engine oil. TO facilitate cooling of the engine and the cylinder liner, the centre section of the liner preferably has radially outwardly projecting rib formations which increase the surface area therefore aiding cooling, and which also to provide additional strength to overcome the high internal pressure in this area during combustion. The spaces between the outwardly projecting rib formations preferably define channels through which coolant liquid can pass to cool the engine. The liquid cooled-parts and the oil-cooled parts of the cylinder liner are preferably separated by 0-ring seals, as too may be the areas between the inlet air, exhaust ports.

Preferred embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a plan view from above of a rib in accordance with the invention; Figure 2 is a schematic side view of a rib in accordance with the invention; Figure 3 is an isometric view of an embodiment of an engine in accordance with the invention; Figure 4 is a side view from the front of the engine shown in Figure 3; FigureS is a plan view from the above of the engine shown in Figures 3 and 4; Figure 6 is an isometric view of a cylinder liner for the engine of Figures 3,4 and 5; and Figure 7 is a plan view from above of cylinder liner shown in Figure 6.

In Figures land 2, a Rigid Inflatable Boat (RIB) 10 in accordance with the invention comprises a rigid hull portion 12 and an inflatable tube 14 extending around the rigid hull portion 12 to form gunwales. The RIB 10 as a central cargo/passenger area 16 with a main deck 18 upon which users of the RIB can stand or upon which cargo can be placed. The RIB 10 shown in Figures land 2 has seven seats which are arranged in three rows of one, four (two back-to-back) and two, respectively (from left to right in the drawing). A driver for the boat 10 sits in one of the seats and has a control console 22 with a steering wheel 24 and a throttle control (not visible) for controlling the boat 10.

The throttle control includes a paddle throttle for controlling the boat 10. The steering wheel 24 and/or the console 22 comprises a gauges and/or one or more display screens (not visible), such as an LED or LCD display, for displaying relevant information to the driver, such as boat speed, GPS position, throttle and rudder setting, draught, engine speed, water temperature, etc. The gauges and displays receive information from a series of sensors and other on-board equipment (not shown for clarity), and

S

a user interface, such as a keypad or touch screen, may also be provided to enable the driver to configure and/or select information to be displayed.

The boat 10 has an inboard engine 30, which is partially located within the bilges of the boat 10 and underneath the central bank of seats 32. It will be noted, in particular from Figure 2, that the engine 30 is located mostly below the level of the deck 18 thereby lowering the centre of gravity of the boat 10.

The engine 30 can be accessed from above by removing the central bank of seats 32 for servicing, repair and maintenance. The fuel tank 34 for the engine 30 is similarly located within the bilges of the boat 10 to keep the centre of gravity as low as possible. The engine 30 is connected to a jet pump 36 of a known type via a prop shaft 38 that turns an impeller 40 within the jet pump assembly 36. The jet pump 36 has a water inlet aperture 42 through which seawater can enter the jet pump 36 and be pumped by the impeller 40. The outlet of the jet pump 36 has a movable nozzle 44, whose position can be adjusted electronically, for example, by a steering servo 46 that is operatively connected to the driver's steering wheel 24, or by cabling, pneumatic or hydraulic lines. The boat 10 can therefore be propelled forwards or backwards in the water by the jet pump and steered by vectoring the thrust of the jet pump using the movable nozzle 44.

The engine 30 as air inlets 51 that comprise fairings 49 affixed to the top of the inflatable tube 14. The fairings 49 have inlet apertures 51 that communicate with the air intake of the engine 30 via tubes (not visible). The engine's exhaust gases are similarly vented via further fairings 57 with exhaust ports 43 that direct the exhaust gases rearward from the back of the boat 10.

It will be noted that by locating the engine 30, fuel tank 34 and jet pump 36 low down in the boat 10 it is possible to provide a substantially flat deck 18 upon which users of the boat 10 can walk around and place cargo. Additionally, because the engine 30 openly protrudes minimally into the main passenger/cargo area 16 of the boat 10, it is possible to place the seats 20 at various locations within the boat 10 thereby affording greater flexibility in the layout.

The particular configuration of the boat shown in Figures 1 and 2 is merely exemplary of the invention and it will be appreciated that other layouts and seating positions could be provided depending on particular requirements.

The engine 30 of the invention is shown in particular in Figures 3, 4 and 5. The engine 30 is of the opposing piston type and has a generally flat design with two horizontal cylinders (not visible) located between a pair of crank case covers 53 located on opposite sides of the engine 30. A pair of pistons (not visible) reciprocate horizontally within each cylinder (not visible) with the combustion chambers being formed in the space within the cylinders between the pistons. The engine of Figures 3, 4 and 5 is a two-cylinder engine and so has two injectors 54 and two heater plugs 55, each entering each cylinder from above. The injectors 54 are connected to a fuel supply line 56 from a high pressure pump in the usual manner and are accessible from above the engine 30.

Air enters the engine 30 via air intake ports (not visible) of each cylinder, via an air inlet manifold which are located to one side of the injectors 54. A supercharger 58 is fitted to the top of the engine and is driven continuously by a drive belt 60 connected to the output shaft 62 of the engine. An air filter 64 is fitted to the inlet of the supercharger 58 and is also accessible from above the engine.

Exhaust gasses leave the engine 30 via a bifurcated exhaust manifold 64 having separate tubular portions extending above and below the engine 30. The tubular portions connect to the exhaust ports of the engine, which ports are located on the opposite side of the injectors 54 to the air intake ports.

The exhaust manifold 64 is also bolted to, and is accessible from, the top of the engine 30.

The pistons (not visible) are connected to crank shafts (not visible) located inside the crank case covers 52 at either side of the engine 30, which crank shafts rotate synchronously via an odd integer gear train (in this case, a five-gear gear train), the central gear of which train is connected to the output shaft 62. As the output shaft 62 rotates, therefore, the pistons reciprocate in opposite directions -coming together during the induction/compression stroke and moving apart during the power/exhaust stroke (for a two-stroke cycle engine). As the pistons move within the cylinder one piston opens and closes the inlet ports and the other piston opens and closes the exhaust ports of the cylinder depending on their instantaneous positions, in a known manner.

It will be noted from Figures 3, 4 and 5, that the axis of the output shaft 62 is located low down on the engine 30 to enable it to connect to the jet pump (not shown) located within the bilges of the boat. It will also be noted that access to all the major components of the engine 30, that is to say: the crank case covers 52, the supercharger 58, the inlet and upper exhaust manifolds 64, the injectors 54, the sparking/heater plugs 55, the air filter 64, the oil filter 66, the fuel pipes 56, the water pump, alternator and fuel injector pump, is possible from above the engine 30. As such, the engine 30 can be mounted close to a floor and is fully accessible from above or from either side for most servicing and maintenance tasks.

One of the cylinder liners 80 of the engine 30 shown in Figures 3,4 and 5 is shown in Figures 6 and 7 and comprises generally tubular main body portion 82 having a central bore 84 forming the engines cylinder, which receives, from opposite ends, a pair of pistons (not shown) that reciprocate in opposite directions to form a combustion chamber in a central part 86 of the bore 84.

Located to one side 88 of the central part 86 of the cylinder liner 80 is the induction side, which comprises a plurality of induction ports 90, which are selectively opened and closed by the side wall of one of the pistons (not shown) as it reciprocates. The induction ports 90 comprise a plurality of through holes whose axes 92 are aligned at an angle to the radius and longitudinal axis of the cylinder to induce a helical swirl as the air enters the cylinder. Located on either side of the induction ports are a series of ribs 94 with grooves 96 therebetween that receive 0-ring seals (not shown) for providing aseal between intake air and cooling water on one side and the oil cooling the other. The liner is fully enclosed within the crankcase such that the 0-rings seat between the liner and crankcase. The manifolds attach to the outside of the crankcase via an inlet cuff.

Located on the opposite side of the central part 86 of the liner 80 to the intake area 88 is an exhaust portion 100, which also comprises a number of outlet ports 102 that are selectively opened and closed by the side wall of the other of the pistons (not shown) as it reciprocates. Again, the outlet ports 102 are milled as slots set at an angle 104 to the radius and longitudinal axis of the cylinder liner 80 to facilitate exhausting the combustion products from the cylinder 84 after each power stroke. The exhaust portion 100 is also bounded on either side by a series of ribs 94 and grooves 96 for receiving 0-ring seals (not shown) for sealingly isolating the exhaust portion 100 in a similar manner to that described above for the inlet ports.

The central part 86 of the cylinder liner 80 comprises a plurality of longitudinal ribs 106 with grooves 108 therebetween, which grooves 108 provide channels for the passage of coolant. The coolant is prevented from entering the cylinder bore 84 by the 0-ring seals (not shown) seated in the grooves 96 previously described. As such, the central part 86 of the cylinder liner 80 is cooled separately from the remainder of the cylinder liner 80.

Importantly, it will be noted that the central part 86 of the cylinder liner 80 has a pair of internally tapped bosses 110, 112 for screw-threadedly receiving the injectors 54 and sparking/heater plugs 55, respectively. The bosses 110, 112 are located side-by-side allowing the cylinder liner to be oriented with both the injectors 54 and sparking/heater plugs 55 located upwardly, that is to say, pointing in substantially the same direction, to enable the engine 30 to be serviced/maintained from one side.

The invention is not restricted to the details of the foregoing embodiments) which are merely exemplary of the invention. For example, the layout and configuration of the boat could be changed, the specific placement, shape and configuration of the various engine and cylinder liner components could be changed, all without departing from the invention.

Claims

Claims: 1. A boat comprising an inboard engine, wherein the inboard engine is located at, or towards, the centre of the vessel, and wherein the engine is at least partially, or wholly, located below the level of a main deck of the boat.
2. A Rigid Inflatable Boat according to claim 1.
3. A jet tender according to claim 2, wherein the engine is operatively connected to a jet pump located at or below the level of the main deck of the boat.
4. Further comprising a fuel tank located below the level of the main deck.
5. A boat according to any preceding claim, further comprising a passenger or cargo area located above the main deck.
6. A boat according to claim 5 comprising a seat located within the passenger or cargo area, which seat forms part of a removable engine cover for the engine.
7. A boat according to any of claims 2 to 6, comprising an air inlet or an exhaust outlet formed as a fairings affixed to the top of an inflatable tube forming the gunwales of the boat, the air inlet or exhaust outlet being in fluid communication with an air intake or an exhaust pipe of the engine.
8. A marine engine of the opposed piston type, the engine comprising a cylinder liner for receiving a pair of pistons configured to synchronously reciprocate within the cylinder in opposite directions to form a combustion chamber within the cylinder between the pistons as they move together, and further comprising an injector and a heating/sparking plug in communication with the combustion chamber, wherein the injector and heating/sparking plug are located less than 180 degrees apart when viewed along a longitudinal axis of the cylinder.
9. A compact marine engine according to claim 8.
10. An engine according to claim 8 or claim 9, wherein the injector and heating/sparking plug are located less than 90 degrees apart when viewed along a longitudinal axis of the cylinder.
11. An engine according to any of claims 8,9 or 10, wherein the injector and heating/sparking plug are located less than 45 degrees apart when viewed along a longitudinal axis of the cylinder.
12. An engine according to any of claims 8 toll, wherein the injector and heating/sparking plug are located less than 20 degrees apart when viewed along a longitudinal axis of the cylinder.
13. An engine according to any of claims 8 to 12, wherein the injector and heating/sparking plug are located substantially side-by-side.
14. An engine as claimed in any of claims 8 to 13, in which the cylinder or cylinders are located, in use, at the bottom of the engine and in which the injector and heater/sparking plug ports enter the cylinder from above.
15. An engine according to any of claims 8 to 14, further comprising a supercharger in fluid communication with an air inlet of the engine, the air inlet and supercharger being located, in use, on top of the engine.
16. An engine as claimed in claim 15 further comprising an air filter fitted to the inlet of the supercharger, the air filter being accessible, in use, from above the engine.
17. An engine according to any of claims 8 to 16, further comprising an exhaust manifold in fluid communication with an exhaust port of the engine, the exhaust manifold being located, in use, on top of the engine.
18. An engine according to any of claims 8 to 17, wherein the pistons are connected to crank shafts located on opposite sides of the engine, the crank shafts being adapted to rotate synchronously via a gear train, and wherein one of the gears of the gear train is connected to an output shaft of the engine.
19. An engine according to claim 18, wherein a longitudinal axis of the output shaft is located at, or towards, in use, the bottom of the engine.
20. A cylinder liner for an internal combustion engine of the opposed piston type, the cylinder liner comprising a central bore forming the cylinder and being adapted to receive, in use, a pair of pistons, the cylinder liner comprising at least one air intake port and at least one exhaust port located on opposite sides of a substantially central combustion chamber of the liner and a sparking/heat plug receiving port and an injector receiving port in communication with the combustion chamber, wherein the sparking/heat plug receiving port and the injector receiving port are arranged less than 180 degrees apart when viewed along a longitudinal axis of the cylinder.
21. A cylinder liner according to claim 20, wherein the injector and heating/sparking plug ports are located less than 90 degrees apart when viewed along a longitudinal axis of the cylinder.
22. A cylinder liner according to claim 20 or claim 21, wherein the injector and heating/sparking plug ports are located less than 45 degrees apart when viewed along a longitudinal axis of the cylinder.
23. A cylinder liner according to any of claims 20, 21 or 22, wherein the injector and heating/sparking plug ports are located less than 20 degrees apart when viewed along a longitudinal axis of the cylinder.
24. A cylinder liner according to any of claims 20 to 23, wherein the injector and heating/sparking plug ports are located substantially side-by-side.
25. A cylinder liner according to any of claims 20 to 24, comprising a plurality of radial inlet ports whose axes are arranged tangentially to a longitudinal axis of the cylinder.
26. A cylinder liner as claimed in any of claims 20 to 25, wherein a centre section of the liner comprises radially outwardly projecting rib formations. is
27. A cylinder liner as claimed in any of claims 20 to 26, further comprising 0-ring receiving radial grooves located on either side of the inlet and outlet.
28. An engine according to any of claims 8 to 19 comprising a cylinder liner according to any of claims to 27.
29. A boat according to any of claims ito 7, comprising an engine according to any of claims 8 to 19.
30. A boat according to any of claims 1 to 7, comprising an engine according to any of claims 8 to 19 and a cylinder liner according to any of claim 20 to 27.
31. A boat, engine or cylinder liner substantially as hereinbefore described, with reference to, and as illustrated in, the accompanying drawings.Amended claims have been filed as follows:-Claims: 1. A boat comprising a main deck located substantially at the level of the bilgesof the boat and a low-profile inboard engine located at, or towards, the centre of the vessel, the engine being at least partially) or wholly, located below the level of the main deck of the boat.2. A Rigid Inflatable Boat according to claim 1.3. A jet tender according to claim 2, wherein the engine is operatively connected to a jet pump located at or below the level of the main deck of the boat.4. A boat as claimed in any of claims ito 3, further comprising a fuel tank located below the level of the main deck.(\,J 5. A boat according to any preceding claim, further comprising a passenger or cargo area located above the main deck.N-o 6. A boat according to claim 5 comprising a seat located within the passenger or cargo area, which (Q seat forms part of a removable engine cover for the engine.7. A boat according to any of claims 2 to 6, comprising an air inlet or an exhaust outlet formed as a fairings affixed to the top of an inflatable tube forming the gunwales of the boat, the air inlet or exhaust outlet being in fluid communication with an air intake or an exhaust pipe of the engine.8. A marine engine of the opposed piston type, the engine comprising a cylinder liner for receiving a pair of pistons configured to synchronously reciprocate within the cylinder in opposite directions to form a combustion chamber within the cylinder between the pistons as they move together, and further comprising an injector and a heating/sparking plug in communication with the combustion chamber, wherein the injector and heating/sparking plug are located less than 180 degrees apart when viewed along a longitudinal axis of the cylinder.9. A compact marine engine according to claim 8.10. An engine according to claim 8 or claim 9, wherein the injector and heating/sparking plug are located less than 90 degrees apart when viewed along a longitudinal axis of the cylinder.11. An engine according to any of claims 8,9 or 10, wherein the injector and heating/sparking plug are located less than 45 degrees apart when viewed along a longitudinal axis of the cylinder.12. An engine according to any of claims 8 toll, wherein the injector and heating/sparking plug are located less than 20 degrees apart when viewed along a longitudinal axis of the cylinder.13. An engine according to any of claims 8 to 12, wherein the injector and heating/sparking plug are located substantially side-by-side.14. An engine as claimed in any of claims 8 to 13, in which the cylinder or cylinders are located, in use, at the bottom of the engine and in which the injector and heater/sparking plug ports enter the cylinder from above.15. An engine according to any of claims 8 to 14, further comprising a supercharger in fluid communication with an air inlet of the engine, the air inlet and supercharger being located, in use, on top of the engine.C'J 16. An engine as claimed in claim 15 further comprising an air filter fitted to the inlet of the supercharger, the air filter being accessible, in use, from above the engine.17. An engine according to any of claims 8 to 16, further comprising an exhaust manifold in fluid communication with an exhaust port of the engine, the exhaust manifold being located, in use, on top of the engine.18. An engine according to any of claims 8 to 17, wherein the pistons are connected to crank shafts located on opposite sides of the engine, the crank shafts being adapted to rotate synchronously via a gear train, and wherein one of the gears of the gear train is connected to an output shaft of the engine.19. An engine according to claim 18, wherein a longitudinal axis of the output shaft is located at, or towards, in use, the bottom of the engine.20. A cylinder liner for an internal combustion engine of the opposed piston type, the cylinder liner comprising a central bore forming the cylinder and being adapted to receive, in use, a pair of pistons, the cylinder liner comprising at least one air intake port and at least one exhaust port located on opposite sides of a substantially central combustion chamber of the liner and a sparking/heat plug receiving port and an injector receiving port in communication with the combustion chamber, wherein the sparking/heat plug receiving port and the injector receiving port are arranged less than 180 degrees apart when viewed along a longitudinal axis of the cylinder.21. A cylinder liner according to claim 20, wherein the injector and heating/sparking plug ports are located less than 90 degrees apart when viewed along a longitudinal axis of the cylinder.22. A cylinder liner according to claim 20 or claim 21, wherein the injector and heating/sparking plug ports are located less than 45 degrees apart when viewed along a longitudinal axis of the cylinder.C'J 23. A cylinder liner according to any of claims 20, 21 or 22, wherein the injector and heating/sparking plug ports are located less than 20 degrees apart when viewed along a longitudinal axis of the cylinder.24. A cylinder liner according to any of claims 20 to 23, wherein the injector and heating/sparking plug ports are located substantially side-by-side.25. A cylinder liner according to any of claims 20 to 24, comprising a plurality of radial inlet ports whose axes are arranged tangentially to a longitudinal axis of the cylinder.26. A cylinder liner as claimed in any of claims 20 to 25, wherein a centre section of the liner comprises radially outwardly projecting rib formations.27. A cylinder liner as claimed in any of claims 20 to 26, further comprising 0-ring receiving radial grooves located on either side of the inlet and outlet.28. An engine according to any of claims 8 to 19 comprising a cylinder liner according to any of claims to 27.29. A boat according to any of claims ito 7, comprising an engine according to any of claims 8 to 19.30. A boat according to any of claims 1 to 7, comprising an engine according to any of claims 8 to 19 and a cylinder liner according to any of claim 20 to 27.31. A boat, engine or cylinder liner substantially as hereinbefore described, with reference to, and as illustrated in) the accompanying drawings.N-(0