In the steam turbine gear drive, the individual propulsion units consist of the main turbines and the main reduction gear fig 8-2 . These types of turbine drives are used on most types of naval ships. They provide a high power-to-weight ratio and are ruggedly constructed. When repairs are needed, they can usually be completed without removing the turbines from the ship. Steam turbine gear drives consist of one high-pressure turbine and one low-pressure turbine. They provide ahead propulsion. Smaller and simpler turbine elements inside the low-pressure turbine provide astern propulsion (fig 8-2).

DIESEL GEAR. DRIVE

In the diesel gear drive engine, the parts that make up the unit consist of the diesel engine, the reduction gear, and either the controllable-pitch propeller unit or the do motor/generator drive unit. The diesel gear drive engine is used on auxiliary ships, minesweepers, fleet tugs, patrol crafts, and numerous other yard craft and small boats. Standardization of fuels, cheaper fuel, and reduction in fire hazards are the chief factors why the Navy favors diesel engines.

Some diesel engines are directly reversible. The propeller shaft is connected directly to the diesel engine so that the speed of the propeller shaft is controlled by the speed of the diesel engine. When it becomes necessary to reverse the direction of rotation of the propeller shaft, the diesel engine is stopped, the cam shaft of the engine is shifted for reverse rotation, and then the engine is

Figure 8-3.-Diesel engine and reduction gear.

Figure 8-4.-Typical gas turbine reduction gear module arrangement.

restarted. This allows the engine to operate in the opposite direction. This operation takes time and presents a difficult situation if sudden changes in direction are required.

To eliminate this stopping-starting situation and to make a smoother transition from forward to reverse in less time, reverse-reduction gears, clutches, and controllable-pitch propellers are used. Figure 8-3 shows a typical diesel engine.

GAS TURBINE DRIVE

Gas turbines are used on patrol craft, destroyers, cruisers, frigates, amphibious craft, and auxiliary oilers. Compared to other propulsion units, they offer a high power-to-weight ratio.

Gas turbine gear drive units consist of gas turbines, reduction gears, and controllable-pitch propeller units. figure 8-4 shows a typical gas turbine reduction gear arrangement.