Page 80 - Practical Ship Design

50 Chapter 2

Design spirals should not be thought of as showing the exact order in which the
different aspects of design should be tackled: this will depend on the type of ship
being designed. For example the cargo handling of a Ro-Ro ship must be consid-
ered at an early stage in the design process, whereas most aspects of the cargo
handling of a tanker can be dealt with quite late in the design process.
Some ship types require a near simultaneous assault on many of the features, or
failing this an interactive approach becomes desirable or even necessary. If,
however, the designer is able to recognise quickly which feature or features are the
main driving force for the design he can speed the design process. In the following
paragraphs an attempt is made to indicate the signs - some obvious and some less
so - that help a designer to spot the driving criteria.

2.9 THE CRITICAL CRITERIA

It is very helpful while setting the design requirements, and even more when
starting to convert these to a design, to be able to identify which of the require-
ments are likely to become the critical criteria. The following paragraphs consider
which criteria are most likely to be critical for various ship types and why.

2.9. I Ships for which weight is critical

Weight, coupled almost invariably with speed, is critical for the majority of ships,
although there are some surprising exceptions. Because at the end of a design, a
good designer will have brought weight and space into harmony with each other,
many designers think that the type of ship that they design cannot be categorised as
either weight or space critical.
However, at the start of a design when no holds are barred, one or other of these
will usually be dominant. Weight is clearly the critical factor when the cargo to be
carried is “heavy” in relation to the space provided for it. At one extreme, iron ore
loaded in alternate holds, and therefore using less than half the available space, will
take a bulk carrier down to its maximum draft even if this involves a B-60
freeboard (see Chapter 11).
The question of what is the critical cargo stowage rate - which decides whether
a design is weight or space critical - is an interesting one. The answer depends on
several factors such as the deadweight/displacement ratio, the proportion of the
cargo deadweight to total deadweight, the type of freeboard, and the ratio of cargo
capacity to the total volume below the upper deck. Some of these factors in turn
involve the ship’s speed, the power and type of machinery, the distance between
fuelling ports, and whether any space below the upper deck is required for any
other purpose such as passengers.