Page 83 - Practical Ship Design

Setting Design Requirement.7 53

2.9.4 Ships for which speed and/or seakeeping are critical

It has already been noted that speed is a joint criterion along with either weight or
volume in the ship types discussed above. In some other ship types, however,
speed and/or sea keeping can be critical by themselves. This applies to the smaller
types of warship where a high speedhigh Froude number is required particularly if,
as is usually the case, there is arequirement for speed to be sustained in rough seas.
Even if the weight and space required could both be provided by a smaller ship,
these ship types should be built with a hull whose length enables the speed to be
obtained economically and the ship to meet the expected seas with acceptable
motions.
Whilst in theory this means building a ship with surplus space, it may be noted
that in practice uses for space are very quickly found. This, in fact, presents a
problem because although the provision of unused space costs very little it has
been found that very good project control is essential if the uses to which spare
space are put do not to result in a cost overrun - with all the uses found being, of
course, matters of high priority!
Other ships whose dimensions may be determined by speed andor seakeeping
include research vessels.

2.9.5 Ships for which tonnage is critical

There used to be several types of smaller cargo ships for which IMO or similar
rules created significant commercial advantages - usually by a reduced manning
requirement - for ships whose net tonnage was less than a critical number. Two
such numbers being 499 tons and 1499 tons. Owners and shipbuilders specialising
in these vessels become very expert in designing ships which met these criteria by
the smallest of margins and found ways of providing quite extraordinarily large
deadweights and cargo capacities within tonnage limits. Today these rules seem to
be of reduced significance.

2.9.6 Other critical criteria

Some ships in which particular limitations or specialist requirements have an over-
riding importance in determining the design are described in Chapter 16.

2.10 TRADE-OFFS BETWEEN OPERATING ECONOMY AND FIRST COST

Although these are not critical criteria in relation to the determination of the main
dimensions in the way that previous paragraphs in this section have been, this
seems an appropriate place to discuss what is in another sense a very critical