Page 297 - Introduction to Naval Architecture

282 VIBRATION, NOISE AND SHOCK

generate disturbing frequencies close to those of typical ship type
structures. Any faults are corrected as a result of trials experience. This
is often more economic than carrying out extensive design calculations
as the remedy is usually a matter of adding a small amount of additional
stiffening.
Main hull vibrations are a different matter. If they do occur the
remedial action may be very expensive. They must therefore be
looked at in design. The hull may bend as a beam or twist like a rod
about its longitudinal axis. These two modes of vibration are called
flexural and torsional respectively. Flexing may occur in a vertical or
horizontal plane but the vertical flexing is usually the more worrying.
Except in lightly structured ships the torsional mode is not usually
too important.

Flexuml vibrations
When flexing in the vertical or horizontal planes the structure has an
infinite number of degrees of freedom and the mode of vibration is
described by the number of nodes which exist in the length. The
fundamental mode is the two-node as shown in Figure 11.3.
This yields a displacement at the ends of the ship since there is no
rigid support there. This is often referred to as a free-free mode and
differs from that which would be taken up by a structural beam
where there would be zero displacement at one end at least. The
next two higher modes have three and four nodes. All are free-free
and can occur in both planes. Associated with each mode is a natural

Figure 11,3 (a) Two-node; (b) Three-node; (c) Four-node