Figure 8-4.—Typical gas turbine reduction
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 propul-
sion 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.
gear module arrangement.
CONVERTING POWER TO DRIVE
The basic characteristics of a propulsion unit
usually make it necessary for the drive mechanism
to change both the speed and the direction of shaft
rotation. The engine in many installations includes
a device that permits a speed reduction from the
engine to the propeller shaft so that both the
engine and the propeller may operate efficiently.
This device is a combination of gears and is
called a reduction gear.
REDUCTION GEARS
Engines must operate at relatively high speeds
for maximum efficiency. Propellers must operate
at lower speeds for maximum efficiency.
Therefore, reduction gears are used to allow both
the engine and the propeller to operate within their
most efficient revolutions per minute (rpm)
8-3
