CHAPTER 8
SHIP PROPULSION
The primary function of any marine engineer-
ing 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 velocity-
imparting 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.
8-1
