Page 120 - Introduction to Naval Architecture

SEAKEEPING 107

Because wave period is related to wavelength the abscissa can equally be
shown as the ratio of wave to ship length. The ordinates of the curve are
known as response amplitude operators (RAOs) or transfer functions.

MOTIONS IN IRREGULAR SEAS
Usually a designer wishes to compare the seakeeping behaviour of two
or more designs. If one design exhibited more acceptable response
operators in all waves and at every speed of interest, the decision would
be easy. Unfortunately usually one design will be superior under some
conditions and another will be better under other conditions. The
designer, then, needs some way of comparing designs in the generality
of wave conditions.
It was seen that the energy spectrum was a very useful means of
representing the nature of an irregular wave system. It is equally
valuable in the study of a ship's motions in irregular seas. Before
proceeding, the spectrum needs to be modified to reflect the fact that
the ship is moving through the waves, whereas the wave spectra so far
discussed are those recorded at a fixed point.

Period of encounter
As far as ship motions are concerned it is the period of encounter with
the waves that is important rather than the absolute period of the wave.
This is because the ship is moving relative to the waves and it will meet
successive peaks and troughs in a shorter or longer time interval
depending upon whether it is advancing into the waves or is travelling in
their direction. The situation can be generalized by considering the ship
at an angle to the wave crest line as shown in Figure 6.4.

Figure 6.4 Period of encounter