Page 113 - Introduction to Naval Architecture

6 Sea keeping

In their broadest sense the terms seakeeping and seaworthiness cover all
those features of a vessel which influence its ability to remain at sea
in all conditions, for which it has been designed, and carry out its
intended mission. They should, therefore, embrace stability, strength,
manoeuvrability and endurance as well as the motions of the ship
and related phenomena. In this chapter only those aspects of a ship's
performance directly attributable to the action of the waves are
considered. Other aspects are discussed in later chapters.
Considered as a rigid body, a ship has six degrees of freedom.
They are the three rotations of roll (or heel), pitching (or trim) and yaw,
together with the three translations of heave, surge and sway. For a
stable ship the motions of roll, pitch and heave are oscillatory and
these are the three motions dealt with here. The other three degrees
of freedom will be excited in a seaway but are of lesser importance.
As the ship is flexible other degrees of freedom will be excited but
these are dealt with under strength and vibration.

SEAKEEPING QUALITIES

Motions Excessive motions are to be avoided if possible. They make
for discomfort of passengers and crew, make the crew less efficient
and make some tasks difficult, perhaps impossible. Apart from their
amplitudes the phasing of motions can have significance. Phasing
generally creates an area of minimum motion about two-thirds of the
length from the bow. This becomes a 'desirable' area and in a cruise
liner would be used for the more important passenger spaces.

Speed and powering In waves a ship experiences a greater resistance
and the propulsor is working under less favourable conditions. These
combined, possibly, with increased air resistance due to wind, cause
a reduction in speed for a given power. The severity of motions,
slamming and wetness can usually be alleviated by decreasing speed
and a master may reduce speed voluntarily for this reason on top of

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