Page 308 - Practical Ship Design

266 Chapter 9

9. I. 7 The ship’s requirement for electrical power and heat.

Because the main engine will generally be able to bum a cheaper fuel than is
required by the generators, the use of the main engine(s) to provide electrical
energy and/or heat for engine auxiliary plant and hotel services via shaft driven
alternator(s) and exhaust gas boiler(s) respectively can have an important influence
on running costs.
On passenger and other ships with high electrical loads this can lead to a
preference for diesel electric propulsion.

9.1.8 Reliability and maintainability

These aspects - very dear to all practical seagoing marine engineers - must be
considered on all ships, but become of outstanding importance on ships for which
the consequences of a breakdown may be particularly severe. Such ships include
passenger ships where not only are particularly high costs incurred in dealing with
the immediate emergency but future profitability may be prejudiced by attendant
publicity. On warships reliability is made the subject of very detailed studies and
redundancy is introduced to minimise the consequences of any loss of capability
whether this is caused by mechanical breakdown or enemy action.
Some marine engineers tend to favour the use of a slow-speed diesel because
this will have fewer cylinders, reducing the parts requiring maintenance, whilst
others prefer the lighter and more easily handled parts of a medium-speed engine.

9.1.9 The ship’s requirement for manoeuvring ability and/or slow-speed
operation

An ability to manoeuvre quickly and accurately can be an important factor in the
choice of main engines and, of course, their associated propulsors on ships which
berth or use canals or constricted waters frequently.
A need to be able to operate at slow speeds using low power, particularly if this
may have to be for protracted periods, can rule out certain machinery options,
unless this function is undertaken by an auxiliary system, such as a controlled
slipping clutch device or an auxiliary propulsion system such as a thruster.

9. I. 10 Ease of installation

This is probably a second-order criterion, but there is no doubt that some engines,
particularly of the slow-speed type have much simpler systems than others of the
same type and this may be taken into account when the choice is finely balanced.