The primary function of any marine engineering plant is to convert the chemical energy of a fuel into useful work and to use that work in the propulsion of the ship. A propulsion unit consists of the machinery, equipment, and controls that are mechanically, electrically, or hydraulically connected to a propulsion shaft. After reading this chapter, you will have a basic understanding of how a ship's propulsion unit works. You will learn about the three main types of propulsion units used in the Navy. You will also learn how power is transmitted from the propulsion unit to the ship's propeller through the use of gears, shafts, and clutches.

PRINCIPLES OF SHIP PROPULSION

A ship moves through the water by propelling devices, such as paddle wheels or propellers. These devices impart velocity to a column of water and move it in the direction opposite to the direction in which it is desired to move the ship. A force, called reactive force because it reacts to the force of the column of water, is developed against the velocityimparting device. This force, also called thrust, is transmitted to the ship and causes the ship to move through the water.

The screw-type propeller is the propulsion device used in almost all naval ships. The thrust developed on the propeller is transmitted to the ship's structure by the main shaft through the thrust bearing (fig 8-1). The main shaft extends from the main reduction gear shaft of the reduction gear to the propeller. It is supported and held in alignment by the spring bearings, the stern tube bearings, and the strut bearing. The thrust, acting on the propulsion shaft as a result of the pushing effect of the propeller, is transmitted to the ship's structure by the main thrust bearing. In most ships, the main thrust bearing is located at the forward end of the main shaft within the main reduction gear casing. In some very large ships, however, the main shaft thrust bearing is located farther aft in a machinery space or a shaft alley.

The main reduction gear connects the prime mover (engine) to the shaft. The function of the main reduction gear is to reduce the high rotational speeds of the engine and allow the propeller to operate at lower rotation speeds. In this way, both the engine and the propeller shaft rotate at their most efficient speeds.

TYPICAL PROPULSION UNITS

Various types and designs of prime movers are currently in use on naval ships. The prime movers

Figure 8-1.-General principle of geared ship propulsion.

Figure 8-2.-Geared steam turbine drive.

may be either a geared turbine (steam or gas) or diesel engine.