Table 1.1 compares some of the features of propulsion plants in current use. The following notes are meant
to clarify the contents of the table:
The power range is given per unit for the steam turbines. One unit normally drives one propeller
shaft. As many as four units might be used in a large, fast ship, with four propellers, developing
about 300,000 shp, as on the big aircraft carriers.
low-speed diesel engines are usually arranged with each engine driving one propeller shaft. A
twin-screw ship would therefore have two engines.
Medium- and high-speed diesels, and aircraft-derivative gas turbines, are frequently fitted in
pairs, geared together, with each pair driving a single propeller shaft, as in the warship described
above.
Heavy-duty gas turbines could be arranged in pairs but, in fact, all of the ships that were built with
these engines had single-engine plants.
LNGCs are liquefied natural gas carriers; naval auxiliaries are non-combatant naval vessels
including tankers, stores ships, tenders, and many amphibious-force ships. '
The line for fuel quality shows the poorest quality fuel that is normally used in each type of plant.
Especially for the diesel engines, there is an inverse relation between fuel quality and maintenance,
so that, if a better quality fuel were used in a medium-speed engine, for example, maintenance
requirements would be lower.
Major maintenance of aircraft-derivative gas turbines is usually done by replacing the unit in service
with a rebuilt unit after several thousand hours of service. This practice reduces maintenance at sea
but adds to the shore-side support requirements.
For the nuclear plants, much of the machinery is inaccessible for maintenance when the plant is in
use.