According to the invention there is provided a transmission system, for a marine craft of the surface drive type, having an input drive for connection to an engine; a transmission connected to the input drive; and a pair of output drives offset from the input drive and connected to the transmission for rotation in opposite directions, and to each of which, in use, is mounted a surface drive propeller.

Preferably, the output drives are pivotable relative to one another in a common plane transverse to the axis of the input drive. The output drives may be pivotable relative to one another angularly about a common axis which may be the axis of the input drive.

The transmission system may include one or more articulated drive shafts and a gearbox having a pair of output gears to each of which one end of a respective one of said articulated drive shafts is connected, the output gears being in fixed angular position relative to one another about the input drive axis.

The transmission system may include one or more belts or chains and a pair of pulleys or sprockets connected to said input drive and a pulley or sprocket connected to each of said output drives, and wherein said input drives are coupled to the respective output drives by means of said belts or chains.

Preferably, one or more actuators connected to move said output drives relative to one another. The one or more actuators can be connected to a respective output drive. A pair of actuators may be provided, each of which is connected at one end to a respective output drive.

The actuators may each be connected at one end to a respective output drive and at the other end to a mounting plate for mounting on a transom of a marine craft.

One of said output drives may be angularly fixed to said gearbox and the other angularly pivotable relative thereto and the output drives can be arranged to be translatable substantially along their axis.

The invention includes a marine craft having a transmission as defined above.

Each of said output drives can be rotatable parallel to the longitudinal axis of said craft and the longitudinal axis is normally substantially perpendicular to a transom of said craft.

Thus, the transmission system or drive transmission of the invention enables the drive from a single engine to be split between two propellers which are mounted, in use, external to a marine craft's transom. The arrangement also allows for lifting the propellers out of the water or clear of the bottom of the craft to prevent damage.

Four examples of transmission systems according to the present invention will now be described with reference to the accompanying drawings, in which :- Figure 1 is an isometric view of the rear of a marine craft having a transmission system according to the invention ; Figure 2 is a rear view of the craft; Figure 3 is a partially sectioned rear view of the transmission system of the craft shown in Figures 1 and 2; Figure 4 is a partly sectioned isometric view of an alternative transmission system; Figure 5 is a partial, sectional view of the transmission system of Figure 4; and, Figure 6 is a detailed sectional view of a main gearbox of the transmission system of Figure 4; Figures 7 to 9 show, schematically a third transmission system according to the invention; and Figure 10 shows, schematically, a modification of the third example.

The marine propulsion systems and drive transmissions shown in the drawings, split the drive from a single engine to two propellers and are mounted, in use, extemally to a marine craft's transom. The arrangement also allows for lifting the propellers out of the water.

The unit shown in Figures 1 to 3 comprises an input drive in the form of a drive shaft (not shown) driven by an engine 7 mounted inboard of the transom 6 of a marine craft 100. The drive shaft attaches to a central or main gearbox 1 which splits the drive to two pivotally movable output drives in the form of gearboxes 2 connected to the central gearbox 1 by corresponding drives in the form of individual drive-shafts 3. Each drive shaft 3 is articulated at a hinge point connection 4, between the central gearbox 1 and the respective movable output gearbox 2 and an actuator 5 acts as a positioning device to control the pivotal movement of the movable output gearboxes 2. Figure 2 shows a

rear view of the drive unit within the outline of the frame of the craft, with the port side lowered and the starboard side in raised position.

The central or main gearbox 1 can be either hard mounted to the transom 6 (preferred and as shown in Figures 1 and 2) or flexibly mounted through an opening in the transom to the engine 7, inside the craft 100. The drive shaft transmits power from the engine 7 into an input gear 8 of the central or main gearbox 1. The central gearbox 1 has a right angle drive from the input gear 8 to two output gears 9. Figure 3 shows two alternative output gear sizes at port and starboard, to illustrate how overall drive ratios can be varied as required. In use, each of the output gears will be the same. The right angle drive or gear set can be of any one of a number of different type, for example, bevel, spiral bevel, hypoid, convolute, Cylkro Tm, or any other type where one input gear can drive two output gears.

In the first arrangement illustrated in Figures 1 to 3, the angle between the two output gears 9 of the central gearbox 1 is fixed between 180° and 120° by fixed bearing locations of the central gearbox casing. This angle depends on the design of the craft, dead-rise angle of the hull, and engine size, etc. The drive shafts 3 are articulated using a Hooks or constant velocity joint 30 located at the hinge point 4. This enables the output gearboxes 2 to be swung out of the water without interference with the transom 6 while the propeller axis may swing further out of the water owing to the twisting action of the gearboxes on the articulated drive shafts 3.

The output shafts 10 of the output gearboxes 2 each carry a propeller (not shown) in use, preferably of the type shown in WO-A-99/39973 and arranged to rotate in opposite directions.

The actuator 5 which lifts the output gearboxes 2 out of the water can be a hydraulic cylinder and pistons 11, with pull rods 12 as shown, but could also be, for example, a mechanical screw jack-hand operated, or an electric linear actuator.

Additional frame arrangements (not illustrated) may be used to provide for greater structural strength, for example wishbone linkages and sliding tracks mounted to the transom 6 if desired for a given application.

An alternative arrangement for varying the angle between the two output shafts 3 is shown in Figures 4 to 6.

In the second arrangement illustrated in Figs 4 to 6, the hinge axes of the output gearboxes 2 are coincident with the input shaft axis of the central gearbox 1. In this case, during lifting action, the output gears 9 of the central gearbox roll about the face of the input gear 8. The inboard ends 13 of the output shafts 3 are fixed in bearings 14

formed in the casing of the central gearbox and able to slide arcuately around the casing to a predetermined limited degree.

The two output gearboxes 2 share a common casing design. The right angle drive to the propeller shaft is arranged to allow the final drive gear to be placed either ahead or in front of the input gear and this enables the propellers to contra-rotate and/or be set for alternative directions of rotation. The output gearboxes 2 may also contain other drive system components such as sensors and actuators (not shown).

Figures 7 to 9 show, very schematically, a third transmission system according to the present invention, with an engine 7 being shown connected to a pair of surface drive propellers 15 by means of an input drive shaft 16 which drives a pair of contra-rotating output shafts 17,18, by means of belts or chains 19,20. A gearbox 21 on the input shaft reverses the direction of rotation of the output shaft 18 relative to the input shaft 16.

Figures 8 and 9 show this example with the output shafts in relatively pivoted positions with the pivot angle shown at"a". This enables the propellers to be raised relative to the input drive shaft as might be required if the craft in which the transmission is installed were operating in shallow water. Figure 9, also shows the shafts not only pivoted, but also in a translated position relative to the input drive, separated by a translation distance"b". This may be achieved, for example, by means of a sliding or ball-spline within the gearbox and can enable the propellers to be raised relative to the input drive without the propellers or output drives contacting the transom 6 of the craft, in cases where the transom is not perpendicular to the input drive shaft axis.

Figure 10 shows a modification of the transmission system in which a gearbox 22 adjacent to the output shaft 17, reverses the direction of rotation of this shaft relative to the input shaft 16.