Clutches may be used on direct-drive propulsion Navy engines to disconnect the engine from the propeller shaft. With small engines, clutches are usually combined with reverse gears and used for maneuvering of boats. In large engines, special types of clutches are used to obtain required coupling or control characteristics and to prevent torsional vibration.

Clutches are used on marine engines to reverse the direction or rotation of the propeller shaft for maneuvering the ship, without changing the direction of rotation of the engine. Reverse gears are used principally with relatively small engines.

Reduction gears are used to obtain low propeller-shaft speed with a high engine speed. Speed reduction gears resolve two conflicting requirements:

1. For minimum weight and size for a given power output, engines must have a relatively high crankshaft speed.

2. For maximum efficiency, propellers must rotate at a relatively low speed, particularly where high thrust capacity is desired.

Figure 12-2.-Flexible couplings.

There are many types of transmissions used by the Navy. This chapter covers, in general, the operation of transmissions that use friction, pneumatic, hydraulic, and electromagnetic clutches, which may be found on Navy marine installations. You will find additional informa-tion on a particular unit in the NAVSEA technical manual for that specific installation.