In the propulsion plants of some diesel-driven ships, there are no mechanical connections between the engine(s) and the propeller(s). In such plants, the diesel engines are connected directly to generators. The electricity produced by such an engine-driven generator is transmitted through cables to a motor, which is connected to the propeller shaft directly, or indirectly, through a reduction gear. When a speed reduction gear is included in a diesel-electric drive, the gear is located between the motor and the propeller. The generator and the motor of a diesel-electric drive may be of the direct current (dc) type or the alternating current (ac) type. Since the speed of a dc motor varies directly with the voltage furnished by the generator, the control system of an electric drive is arranged so that the generator voltage can be changed at any time. An increase or decrease in generator voltage is used to control the speed of the propeller. Generator voltage may be changed electrically, by changes in engine speed, and by a combination of these methods. The controls of an electric drive may be located remotely from the engine, such as in the pilot house.

In an electric drive, the direction of rotation of the propeller is not reversed by a reverse gear. The electrical system is arranged so that the flow of current through the motor can be reversed. This reversal of current flow causes the motor to turn in the opposite direction. Thus, the direction of rotation of the motor and of the propeller can be controlled by manipulation of the electrical controls.

DIRECT DRIVES

In some marine engine installations, power from the engine is transmitted to the drive unit without a change in shaft speed; that is, by a direct drive. In a direct drive, the connection between the engine and the driven unit may consist of a solid coupling, a flexible coupling, or a combina-tion of both. There may or may not be a clutch in a direct drive, depending upon the type of installation. Some installations have a reverse gear.

Solid Couplings

Solid couplings vary considerably in design. Some solid couplings consist of two flanges bolted together (fig. 12-1). In other direct drives, the driven unit is attached directly to the engine

Figure 12-1.-Flange-type solid coupling.

crankshaft by a nut, as in the P-250 centrifugal fire pump.

Solid couplings offer a positive means of transmitting torque from the crankshaft of an engine; however, a solid connection does not allow for any misalignment between the input and output shafts, nor does it absorb any of the torsional vibration transmitted from the engine crankshaft.

Flexible Couplings

Since solid couplings do not absorb vibration and do not permit any misalignment, most direct drives consist of a flexible coupling, which uses a flexible member or element to connect two flanges or hubs together. Connections of the flexible type are common to the drives of many auxiliaries, such as engine-generator sets. Flexible couplings are also used in indirect drives to connect the engine to the drive mechanism. The two solid halves of a flexible coupling are joined by a flexible element. The flexible element may be made of rubber, neoprene, a steel spring, or gears. An example of a grid-type flexible coupling that uses a steel spring (view A) and a gear-type coupling (view B) are shown in figure 12-2.