Page 128 - Introduction to Naval Architecture

SEAKEEPING 115

will be able to operate from a frigate in conditions which might prove
dangerous for a lesser pilot. The criteria are usually obtained from
careful questioning and observation of the crew. Large motion
simulators can be used for scientific study of human performance
under controlled conditions. These can throw light upon how people
learn to cope with difficult situations. The nature of the usual criteria
has already been discussed.

SHIP FORM AND SEAKEEPING PERFORMANCE

It is difficult to generalize on the effect of ship form changes on
seakeeping because changing one parameter, for instance moving the
centre of buoyancy, usually changes others. Methodical series data
should be consulted where possible but in very general terms, for a
given sea state:

(1) increasing size will reduce motions;
(2) increasing length will reduce the likelihood of meeting waves
long enough to cause resonance;
(3) higher freeboard leads to a drier ship;
(4) flare forward can reduce wetness but may increase slamming;
(5) a high length/draught ratio will lead to less pitch and heave in
long waves but increase the chances of slamming;
(6) a bulbous bow can reduce motions in short waves but increase
them in long waves.
Because form changes can have opposite effects in different wave
conditions, and a typical sea is made up of many waves, the net result
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is often little change. For conventional forms it has been found that
overall performance in waves is little affected by variations in the main
hull parameters. Local changes can be beneficial. For instance fine
form forward with good rise of floor can reduce slamming pressures.

A ship's rolling motions can be reduced by fitting a stabilization system.
In principle pitch motions can be improved in the same way but in
practice this is very difficult. An exception is the fitting of some form of
pitch stabilizer between the two hulls of a catamaran which is relatively
shorter than a conventional displacement ship. In this section attention
is focused on roll stabilization. The systems may be passive or active.