Page 296 - Introduction to Naval Architecture

VIBRATION, NOISE AND SHOCK 281

a relatively large area is equivalent to an increase in blade thickness and
the blade rate pressures increase accordingly. If cavitation is unstable
pressure variations may be many times greater. The number of blades
directly affects frequency but has litde effect on pressure amplitude.
The probability of vibration problems in single screw ships can be
reduced by using bulbous or U sections rather than V in the after body,
avoiding near horizontal buttock lines above the propeller, and by
providing good tip clearance between propeller and hull. Good tip
clearance is important for all ships although smaller clearances are
generally acceptable the greater the number of blades. Shallow
immersion of the propeller tips should be avoided to reduce the
possibility of air drawing. Generally the wake distribution in twin screw
ships is less likely to cause vibration problems. If A-brackets are used the
angle between their arms must not be the same as that between the
propeller blades or the propeller will experience enhanced pressure
fluctuations as the blades pass through the wake of the arms.
A ship in waves is subject to varying hull pressures as the waves pass.
The ship's rigid body responses were dealt with under seakeeping.
Some of the wave energy is transferred to the hull causing main hull
and local vibrations. The main hull vibrations are usually classified as
springing or whipping. The former is a fairly continuous and steady
vibration in the fundamental hull mode due to the general pressure
field. The latter is a transient caused by slamming or shipping green
seas. Generally vertical vibrations are most important because the
vertical components of wave forces are dominant. However, horizontal
and torsional vibrations can become large in ships with large deck
openings or of relatively light scandings such as container ships or light
aircraft carriers. The additional bending stresses due to vibration may
be significant in fatigue because of their frequency. The stresses caused
by whipping can be of the same order of magnitude as the wave
bending stresses.

The sMp responses
Having considered the various disturbing forces likely to be met, it is
necessary to consider the ship's oscillatory responses and their
frequencies.
Vibrations are dealt with as either local vibrations or main hull
vibrations. The former are concerned with a small part of the structure,
perhaps an area of deck. The frequencies are usually higher, and the
amplitudes lower, than the main hull vibrations. Because there are so
many possibilities and the calculations can be complex they are not
usually studied directly during design except where large excitation
forces are anticipated. Generally the designer avoids machinery which