Page 291 - Introduction to Naval Architecture

Vibration, noise and

11 shock

Ships must be designed so as to provide a suitable environment for the
continuous, efficient and safe working of equipment and crew. Also the
environment should be one in which crew and passengers will be
comfortable. Vibration, noise and shock are all factors in that
environment.
A ship responds to any applied force. For some responses, for
instance those of roll, pitch and heave in a seaway, it is acceptable to
regard the ship as a rigid body. In all cases however there will be some
flexing of the structure and the total response will include movements
of one part of the structure relative to others. These can be termed the
elastic body responses or degrees of freedom and they can be very
important. Even in ship motions slamming must be treated as a
dynamic, vibratory response. The vibratory stresses can increase the
overall hull stresses quite considerably and must be taken into account,
particularly in fatigue studies. Vibration, noise and shock are all
manifestations of the ship's elastic responses.

VIBRATION

In this chapter general vibrations of ships are considered. The most
common sources of vibration excitation are propellers and main
machinery. All vibration is undesirable. It can be unpleasant for people
on board and can be harmful to equipment. It must be reduced as
much as possible but it cannot be entirely eliminated. The designer of
systems and equipment to be fitted must allow for the fact that they will
have to operate in a vibratory environment, fitting special anti-vibration
mounts, if necessary, for especially sensitive items.
Vibration levels of machinery can be used to decide when a machine
needs attention and often vibration measuring devices are fitted as a
form of 'health monitoring'. If vibration levels start to rise the cause
can be investigated.
A ship is a complex structure and a full study of its vibration modes
and levels is very demanding. Indeed, in some cases it is necessary to
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