Page 140 - Practical Ship Design

Weight-Based Drsigns I07

4.5 MACHINERY WEIGHT

4.5.1 Machinery Qpe

The first step towards assessing the machinery weight is, of course, the calculation
of the required power and methods of power estimation appropriate to the design
stage are given in Chapters 6 and 7.
The second step involves taking a decision on the type of machinery best suited
to the service conditions of the ship under consideration. This subject is dealt with
in Chapter 9, but it may be helpful to give a simplified statement here:
- the almost universal choice for the machinery of most medium to large cargo
ships is a slow speed diesel engine;
- medium speed geared diesels are the general choice for smaller cargo ships,
ferries, tugs and supply boats;
- large cruise liners are frequently fitted with diesel electric installations as are
many specialist vessels such fishery research and oceanographic vessels.
Gas turbines andor high speed diesels are the choice for warships where the
need for a high powedweight ratio is all important. An unusual feature of warship
machinery is the fact that it usually has to provide both a high speed sprint
capability and a reasonable endurance at a slow to medium speed. The machinery
provided for these two roles may be arranged so that the two component parts
always operate separately (the “or” configuration) or combine together (the “and’
configuration) for the high speed role.
Obviously the aggregate power for both configurations must be used as the basis
for estimating the machinery weight.
As the weights per unit of power vary considerably between the extremes of
slow speed diesels and gas turbines, a decision on machinery type is a necessary
preliminary to the assessment of the machinery weight.
As with outfit weights, accurate machinery weights are best obtained by synthesis
from a number of group weights and a suggested system for this is given later in
this section.

4.5.2 Approximate machinery weight estimation

The simplest possible way of estimating the machinery weight is by the use of a
graph of total machinery weight plotted against total main engine power (MCR),
with a line for each of the four different main machinery types described in the last
section and preferably with the data spots on it identified with the ship types to
which they refer. This last suggestion stems from a recognition that the different
auxiliaries required by different ship types can exercise a considerable influence
on the total weight.