Page 317 - Practical Ship Design

Machinery Selection 275

and a conventional propeller, with all manoeuvring being done by adjusting the
engine speed and direction of rotation.

9.3.2 Geared drive

Geared drive can be associated with most of the other prime movers. Quite
frequently, gearing has more than one function, although the most common
requirement is the reduction of the revs from the engine output figure to that which
is required for the efficient operation of the propeller. It can, however, also be used
to combine the power of two prime movers onto one shaft or alternatively to divide
the power reaching it between two shafts or between shafting connected to the
propeller and a drive to a shaft alternator.
Reversing is a further function that gearing can be called upon to provide,
although this is an infrequent requirement as most diesels can be reversed fairly
easily whilst the reversing of ships with gas turbine machinery is generally
provided by fitting them with controllable pitch propellers.

9.3.3 Electrical propulsion

Electric propulsion has been used for many years, dating back to such famous
pre-Second World War passenger liners, as “Viceroy of India”, “Normandie” and
many others. The prime movers on these ships were all steam turbines and electrical
drive was adopted for a number of reasons, with a mistrust of the reliability of large
reduction gearboxes certainly figuring in a number of the decisions.
More recently, most electrical propulsion systems have had diesel engines,
either medium or high speed, as their prime movers.
The merits of electrical propulsion include the ease of control which it provides
giving an excellent manoeuvring capability together with an ability to operate
economically and for lengthy periods at reduced speed and power.
The principal disadvantage of electrical drive has always been that it is much
more expensive in first cost than the geared alternative. This economic disadvantage
is compounded by the fact that the mechanical efficiency is lower, leading to
increased fuel consumption and cost.
In early electrical propulsion systems D.C. motors were used and the ships
invariably had completely separate electrical systems for propulsion and other
purposes.
The development of marine-type thyristor converters has now made it possible
to equip ships on the power station concept with propulsion, engine room auxiliaries
and ship’s hotel load all drawing from a common energy pool, which is in turn fed
by whatever number of generators is needed with all engines therefore operating at
near peak efficiency.