Page 306 - Practical Ship Design

264 Chapter 9

9.1.2 Required horsepower

The naval architect, when calculating the power to specify to the marine engineer,
has to make a number of assumptions. The most important of these assumptions
relates to the number and type of propulsors and to the propeller revolutions. All
these must be known to enable the quasi propulsive coefficient to be estimated and
this has, of course, a major influence on the required power. A secondary influence
on the power stems from the effect on the displacement of whatever assumption is
made in respect of machinery type with the influence this exercises on machinery
and fuel weights etc.
All these assumptions must be relayed to the marine engineer, who should feel
free to question them. If by changing any or all of the assumptions the marine
engineer can offer a technically better and/or cheaper solution, a dialogue with the
naval architect should ensue and the power estimate adjusted to suit what are then
agreed as the main technical features of the machinery.
Apart from adjustments of this sort the power is of course the fundamental
criterion.

9. I .3 Weight

This is not generally a very important matter for the majority of merchant ships,
although it undoubtedly plays quite a significant part in the selection of machinery
for ferries and similar relatively fast, fine lined ships, particularly if these are also
subject to a draft limitation.
In the design of warships, planing craft and catamarans, the need for a high
speed from a relatively small ship makes the powedweight ratio a matter of vital
importance.

9. I .4 Space

Much of what has been just been said about weight also applies to space. As far as
the main engines are concerned space and weight generally go together, but if a
trade-off between weight and space is possible, then ships designed on a dead-
weight basis should be fitted with the lighter machinery, even if this takes more
space, whilst those designed on a volume basis should be fitted with the less bulky
machinery even if this is heavier.
On warships space, like weight, is at a premium and the power/volume ratio is
very important.

9.1.5 Capital cost
The cost of the main engine itself must be considered along with any differential
costs which may arise from its installation. Such differential costs could include