Page 309 - Practical Ship Design

Machinery Selection 267

9.1.11 Vibration

Any vibratory forces or couples that may emanate from a main engine under
consideration must be carefully assessed before it is accepted as suitable. An
engine which develops even a moderate couple should only be considered if it can
be clearly shown that the resulting vibration is within acceptable limits at all parts
of the ship where it could affect personnel or equipment. It is worth noting that the
relativity of the position in which the engine is to be fitted to the nodes and
anti-nodes of the ship’s vibration profile can have a significant effect. (See also the
next section on noise.)

9.1.12 Noise and other signatures
In some vessels such as fishery and oceanographic vessels and warships operating
submarine detection equipment such as towed array, the minimisation of the
under-water noise signature becomes a driving factor in the whole machinery
installation.
Where the noise targets are stringent, consideration must be given to raft
mounting the engine(s) and enclosing them in an acoustic enclosure. Both of these
requirements impose limits of both weight and space on the choice of engines.
Even if these measures are taken, the required performance will demand the choice
of an engine with minimum vibration and noise characteristics.
In mine hunters the magnetic signature becomes so important that all machinery
must be constructed of non-magnetic materials.

9.1. 13 Availability

This is a warship concept which is discussed in Chapter 14. It was a particularly
significant factor when the change from the general use of steam turbine machinery
to gas turbines and/or diesels was first being considered. Today, its greatest design
influence lies in the general move it has caused towards repair by replacement to
reduce out-of-service time (see also Q 14.6).

9.2 ALTERNATIVE MAIN ENGINE TYPES

9.2.1 Diesel engines
By far the majority of merchant ships in service or under construction today have
diesel engines. These are available in three main types:
(i) slow-speed diesels with a speed of rotation mainly in the range 60 to 150
revdmin: