The drive mechanisms of most engine-powered ships and of many boats are the indirect type. With this drive, the power developed by the engine(s) is transmitted to the propeller(s) indirectly, through an intermediate mechanism that reduces the shaft speed. Speed may be reduced mechanically, by a combination of gears, or by electrical means (for example, a diesel electric drive).

Mechanical Drives

The mechanical drives discussed in this chapter include devices that reduce the shaft speed of the driven unit, provide a means for reversing the direction of shaft rotation in the driven unit, and permit quick-disconnect of the driving unit from the driven unit.

Propellers operate most efficiently in a relatively low rpm range. The most efficient designs of diesel engines, however, operate in a relatively high rpm range. In order that both the engine and the propeller may operate efficiently, the drive mechanism in many installations includes a device that permits a speed reduction from engine crankshaft to propeller shaft. The com-bination of gears that brings about the speed reduction is called a REDUCTION GEAR. In most diesel engine installations, the reduction ratio does not exceed 3 to 1. There are some units, however, that have reductions as high as 6 to 1. The propelling equipment of a ship or a boat must provide astern power as well as forward power. In some ships, backing down is accomplished by reversing the pitch of the controllable pitch propeller; in other ships and boats, however, backing down is accomplished by reversing the direction of rotation of the propeller shaft. In mechanical drives, the direction of rotation of the propeller shaft is reversed by use of REVERSE GEARS. The drive mechanism of a ship or boat must do more than reduce speed and change direction of rotation. Most drive mechanisms have a CLUTCH. The clutch disconnects the drive mechanism from the propeller shaft and permits the engine to be operated without turning the propeller shaft.

The arrangement of the components in an indirect drive varies, depending upon the type and size of the installation. In some small installations, the clutch, the reverse gear, and the reduction gear may be combined into a single unit. In other installations, the clutch and the reverse gear may be in one housing and the reduction gear in a separate housing attached to the reverse-gear housing. Drive mechanisms arranged in either manner are usually called TRANSMISSIONS. In large engine installations, the clutch and the reverse gear may be combined; they may be separate units, located between the engine and a separate reduction gear; or the clutch may be separate and the reverse gear and the reduction gear may be combined.

In most geared-drive multiple propeller shaft ships, the propulsion units and their drive mechanisms are independent of each other. In others, the drive mechanism is arranged so that two or more engines can drive a single propeller. In one type of installation, the CODOG (combination diesel or gas turbine) system, each propeller is driven by a diesel engine or both propellers are driven by one gas turbine. The diesel engines are used for normal cruising and maneuvering in confined waters. Each diesel drives a propeller shaft independently of the other. The gas turbine is used for high-speed operation. The single gas turbine drives both propeller shafts through the drive mechanism. This combination permits a large cruising range along with high speed whenever it is needed.