The subject invention concerns a device in drive units for use in boats. The drive unit comprises an engine, a reversing gear, at least one propeller and a number of drive shafts interconnecting these components. The purpose of the device is to prevent reversal of the rotary direction of the engine.

The most common type of drive units in use today for boats utilize water as the engine cooling medium. As a rule, the cooling water is conducted from the cooling system of the engine to its exhaust duct and consequently the water is expelled through the latter together with the engine exhaust gases. In order to obtain the lowest possible noise level the exhaust duct usually opens below the water level. To allow reversal and to some extent deceleration the drive units are as a rule provided with a reversing gear, the latter usually operating by reversal of the rotational direction of the propeller. When the reversing gear is of the cone clutch type, for instance of the kind described in US-PS 3 269 497, it is possible to reverse the propeller rotational direction also at comparatively high forwards propulsion speeds.

When such reversals of the rotational direction of the propeller take place at high speeds the force of the water acting on the propeller might be of such a magnitude that it exceeds the power exerted by the engine. The result is reversal of the rotational movement of the engine rather than reversal of the rotational movement of the propeller. One consequence of such reversals of the rotational movement of the engine is that the engine operates in the manner of a piston pump having its suction side on the engine exhaust side. Because during operation the exhaust duct of the engine is largely filled with cooling water from the engine cooling system, the engine, working like a piston pump, will suck in not just exhaust gases but also considerable amounts of water. The volume of the water drawn in during one pumping stroke could exceed the compressed volume of the engine, and since it is not possible to compress a liquid, the result will be

an abrupt standstill of the piston with consequential serious damages to the engine. Examples of damages of this kind are bent crankshafts and cracked pistons.

In constructions where the exhaust duct mouth opens at a level below the water level, which usually is the case, a valve is provided in the duct to prevent water from flowing into the duct. However, some leakage past the valve often occurs, and reversal of the rotational direction of the engine therefore constitutes an added risk that sea water may be drawn into the engine. The purpose of the subject invention is to provide a device which in a simple and unexpensive manner solves the problem outlined in the aforegoing.

This purpose is achieved by means of a device consisting preferably of mechanical means which are positioned on the shaft connecting the engine with the reversing gear and which means are arranged to permit rotation of the shaft in one direction but to prevent rotation thereof in the opposite direction.

Further characteristics of the invention will appear from the dependent claims as well as from the following description of a preferred embodiment of the invention.

The prevention, in the manner mentioned, of the rotation of the shaft connecting the engine with the reversing gear in a direction counter to the normal direction of rotation of the engine ensures that the engine will never be able to operate in the manner of a pump as described in the aforegoing. Consequently, the risk that the cooling water of the engine will flow into the cylinders via the exhaust duct is wholly eliminated.

One preferred embodiment of the invention will be described in closer detail in the following with reference to the accompanying drawings, wherein

Fig. 1 is a partly sectionalized view of a shaft interconnecting the engine with the reversing gear and being provided with a device in accordance with the invention, and

Fig. 2 is a cross-sectional view along line I-I of Fig. 1.

The invention is advantageously applied on a drive unit of the type described in US-PS 3 269497. However, the drawings merely show the part of the shaft between the engine and the reversing gear, for which reason US-PS 3 269 497 is incorporated in the subject description for reference as far as the design and structure of the rest of the drive unit is concerned.

With reference to Fig. 1, a freewheel 4 of conventional type is positioned intermediate two shaft-supporting bearings 2, 3 around the shaft 1 connecting the engine with a reversing gear. In order to clarify the function of the freewheel 4 Fig. 2, however, shows a section through a freewheel 4 of a type which could be used in the device in accordance with the invention. When the shaft 1 rotates in the direction of arrow 5 blocks 6 which form part of the freewheel 4 assume a position allowing the shaft 1 to rotate freely. However, in case of a moment on the shaft tending to drive the latter in the direction opposite to that indicated by arrow 5, the blocks 6 will turn in the direction of arrows 7, thus locking the shaft 1 in position. It is also possible to replace one or both bearings 2, 3 by a combined bearing-freewheel unit. Finally should be pointed out that the invention is not limited to the embodiment described in the aforegoing but that a number of modifications are possible within the scope of the appended claims.