Page 82 - Practical Ship Design

52 Chapter 2

2.9.2 Ships for which volume, deck area, linear dimensions or stability are
critical

Volume, coupled once again with speed, becomes the critical criterion when the
cargo to be carried is light - with lightness being determined as shown above.
Particular factors that may make volume critical can be the need to provide space
within the normal cargo space area for some other need, e.g., for passenger
accommodation or the fitting of some special machinery.
Modern tankers in which a large segregated ballast capacity has to be provided
have moved from being the epitome of weight-based designs (which older ships
with class A freeboard were), to being volume controlled.
Most modern warships are volume controlled, the principal dimensions being
determined by the internal space required (with the length or the external deck area
needed sometimes being an additional factor), rather than by the need to provide
adequate buoyancy. This stems from the fact that most modern weapon systems
and modern machinery are of low density whilst armour and other heavy items are
now a thing of the past.
The requirement for these fairly small ships to achieve high speeds makes it
desirable to keep the displacement as low as possible to minimise the power
required. This, coupled to some extent with weight overruns in some construction,
has led to the adoption of weight reduction techniques and the imposition of strict
weight control measures becoming standard in warship design and building
practice. The desirability of constraining weight for hydrodynamic reasons should
not, however, be seen as making warships weight critical,
It may be noted that the fact that there are no freeboard rules for warships means
that the load draft can be adjusted when the ship is complete to take the as-built
weight into account. When this has been done the as-built data suggests that there
is a balance between weight and volume, although it may not be that originally
intended by the designer.
Ship types for which deck area is the most important criterion include car and
train ferries and possibly aircraft carriers, although it can be argued that linear
dimensions are equally important for these ship types and stability may in fact be
the real determining factor in fixing the dimensions in some cases.
Linear dimensions are very important for a container ship whose length, breadth
and depth should be tailored to maximise container numbers. On the other hand, if
maximising the carriage of tiers of containers on deck is important stability
becomes the ultimate criterion.
Stability is also the factor which determines how many superstructure decks can
be fitted on a cruise liner and therefore becomes the critical criterion for the other
ship dimensions as well (see 55.3.2).