The present invention is based upon the premise that it would be possible to provide, as a retro-fit, an intermediate transmission unit replacing the standard intermediate unit of a stern drive and incorporating a gear box to enable the direct drive of the standard stern drive leg to be augmented by an optionally selectable geared drive to provide a step drive, that is to say either an overdrive or means of providing an alternative higher gear ratio.

The benefits of multiple gear ratios depend on whether a speed reducing or speed increasing ratio is installed. With a reduction ratio, low speed manoeuvrability is enhanced and acceleration on to the plane is improved.

With an overdrive ratip, top end speed can be increased or cruising speed can be obtained using less engine revolutions. Both options will reduce fuel consumption, wear emissions and noise.

There has also been proposed, see US-A-5,580,289 an intermediate unit, adapted to be incorporated in a stern drive leg of a stern drive or outboard drive of a watercraft, comprising an intermediate drive shaft which, in use, is arranged with its axis in an upright position and is adapted to be coupled at its upper end to a drive shaft of the stern or outboard drive, a first, upper gear arranged coaxially with said intermediate drive shaft, a second, lower gear arranged coaxially with said intermediate drive shaft, an auxiliary gear train providing a drive transmission between said upper and lower gears, and friction clutch means which is selectively actuatable to provide a driven output either via said upper and lower gears and said auxiliary gear train, or directly from said intermediate drive shaft. In such an arrangement, however the intermediate unit is designed as an integral part of the stern drive and therefore does not address the problem of providing a compact unit as a retrofit to replace a non-geared intermediate unit.

Thus, the invention is directed to solving the problem of incorporating a drive transmission for a gear box in a compact intermediate unit that can be retro- fitted to an existing stern drive or outboard drive mechanism. Essentially, the problem to be solved by the invention is that of incorporating an appropriate drive mechanism in the very limited vertical height available in the intermediate unit of a stern drive.

In accordance with the invention, an intermediate unit of the proposed kind referred to above is characterised in that said lower gear is arranged to be freely rotatable relatively to the intermediate drive shaft and to provide the drive output from the intermediate unit, that said friction clutch means includes a friction clutch having a driving portion non-rotatably keyed to said intermediate drive shaft and a driven portion non-rotatably coupled to said lower gear, that the said intermediate drive shaft is mounted for axial movement relatively to a housing of the intermediate unit, that an actuating means is provided for driving said intermediate drive shaft axially to cause frictional engagement of said driving and driven portions of said friction clutch, and that the upper end of said intermediate drive shaft is keyed or splined for sliding driven engagement with a coaxial drive shaft of a stern drive or outboard drive.

The arrangement in accordance with the invention takes advantage of the fact that in a conventional stern drive the input drive shaft of the intermediate unit is splined to a drive shaft transmitting power from the motor, so that translatory movement of the input drive shaft can be accommodated by the splined connection.

Advantageously, axial movement of the input drive shaft for actuation of the friction clutch is provided for by means of a hydraulically actuated piston arranged coaxially with the input drive shaft.

Further preferred features and advantages of the invention will become apparent from the following description and the accompany claims.

The invention is illustrated by way of example in the accompanying drawings in which: Fig. 1 is a sectional elevation of one embodiment of an intermediate unit in accordance with the invention, for incorporation in a stern drive, and Fig. 2 is a fragmentary view corresponding to Fig. 1 illustrating the clutch mechanism of the intermediate unit in greater detail.

Referring to the drawings, an intermediate unit for a stern drive comprises an outer housing 1 shaped to correspond with a standard intermediate unit for a conventional stern drive for which the intermediate unit according to the invention is to be a replacement fit. The housing 1 is closed at the upper end by means of a first cover plate 2 and a second sub-cover plate 3.

A lay shaft 4 is rotatably mounted in bearings respectively provided in the housing 1 and the cover plate 2 and carries, fixed with respect thereto, a first lay gear 5 of larger diameter and a second lay gear 6 of smaller diameter.

The sub-cover 3 provides an opening 7 allowing access to the upper end of a clutch drive shaft 8 that extends vertically within the housing 1 and is arranged coaxially with an input gear 9 meshing with the lay gear 5 and an output gear 10 meshing with the lay gear 6.

The input gear 9 and output gear 10 are mounted for rotation within the housing 1 via appropriate bearings indicated diagrammatically at 11.

The clutch drive shaft 8 is hollow, being splined internally at the upper end to receive a drive shaft from the drive head of a stern drive unit, and being open at the lower end to accommodate a drive shaft 12 that extends from the lower unit (not shown) of the stern drive, in known manner. The clutch drive shaft 8 is located within the input gear 9 and output gear 10 by means of bearings indicated diagrammatically at 13, the bearings 13 being such as to allow axial sliding movement of the shaft 8 relatively to the gears 9 and 10.

The lower end of the output gear 10 is splined to a hollow coupling shaft 14 (omitted from Fig. 2 for clarity) the coupling shaft extending coaxially with the output drive shaft 12 and being splined thereto in order to provide a direct connection between the output gear 10 and the drive shaft 12.

Referring now to Fig : 2, it will be seen that the clutch drive shaft 8 carries a regularly extending cottar 15 that is located upon an external spline of the clutch drive shaft 8 so that it is rotationally fixed with respect thereto, and is held in place against axial movement with respect to the clutch drive shaft 8 by means of circlips 16 that are recessed in the collar 15 and engaged within radial grooves in the splines of the clutch drive shaft 8.

The external splines of the clutch drive shaft 8 also engage within a series of friction clutch plates 17 that are located within an axial recess of the input gear 9 and inter-engage with friction clutch plates 18 that respectively engage within an internal spline on the input gear 9.

Likewise, within the output gear 10 are located a further series of clutch plates 19 splined to the clutch shaft 8 and clutch plates 20 splined within the output gear 10.

It will thus be seen that upon axial movement of the clutch drive shaft 8 in the upward direction the clutch plates 17 and 18 are compressed between an upper face 21 of collar 15 and a lower face 22 of input gear 9 so that the clutch drive shaft 8 becomes fixed with respect to the gear 9. The output gear 10 can then rotate freely, and thus a reduction drive is effected between the input gear 9, the lay gears 5 and 6 and the output gear 10. A low speed drive is thus provided between an input drive shaft 23 splined to the clutch drive shaft 8 and the output drive shaft 12 that is coupled to the gear 10 via the coupling shaft 14.

If, on the other hand, the clutch drive shaft 8 is moved axially downwards, the clutch plates 19 and 20 likewise become compressed between a lower face 24 of the collar 15 and an upper face 25 of the gear 10 whereby the clutch drive shaft 8 is directly coupled to the output gear 10 and the input drive shaft 23 is directly coupled to the output drive shaft 12 as in the conventional stern drive unit.

The aforesaid axial movement of the clutch drive shaft 8 is provided by means of a hydraulic piston and cylinder unit comprising a cylinder body 26 fixed between the housing 1 and the cover plate 2 and provided with hydraulic unions 27 and 28 incorporated within a conventional hydraulic circuit that is not further illustrated. The cylinder body 26 incorporates an upper piston 29 that can engage a thrust collar 30 of the clutch drive shaft 8 via a thrust face 31. The cylinder 26 also incorporates a lower piston 32 that can engage a thrust collar 33 of the clutch drive shaft 8 via a thrust face 34. Thus, hydraulic actuation of the clutch rive shaft 8 in each of two axial directions can be effected in a simple manner by means of a relatively compact mechanism.

The arrangement in accordance with the above described embodiment of the invention thus provides an extremely compact and simple hydraulically actuatable two speed drive that can be accommodated in an intermediate unit of a stern drive that is intended as a retrofit for a conventional non-geared unit.

It will be appreciated that various alterations and modifications may be made to the arrangement described above without departing from the scope of the invention. Thus, although the illustrated arrangement shows a clutch drive shaft 8 that is arranged to actuate each of two friction clutches by means of a double acting mechanism, it would be feasible to provide only a single acting mechanism with a single friction clutch for coupling the clutch drive shaft to the output gear 10, the input gear 9 being fixed with respect to the clutch drive shaft 8 and a further clutch arrangement being provided in association with the lay shaft 4. Such a construction might be desirable for the transmission of powers higher than that capable of being accommodated by the illustrated arrangement.