Page 298 - Introduction to Naval Architecture

VIBRATION, NOISE AND SHOCK 283

Table 11.1 Typical ship vibration frequencies

Shift type Length Condition Frequency <yf vibration
(m) of loading
Vertical f larizontal
2 3 4 :5 2 3 4
node node node nctde node node nodf.

Tanker 227 Light 59 121 188 2<IS 103 198 297
Loaded 52 108 166 2$10 83 159 238
Passenger ship 136 104 177 155 341

Cargo ship 85 Light 150 290 230
Loaded 135 283 200
Cargo ship 130 Light 106 210 180 353
Loaded 85 168 135 262
Destroyer 160 Average 85 180 240 120 200
action

frequency of free vibration, the frequency being higher for the
higher modes. If the ship were of uniform rigidity and uniform mass
distribution along its length and was supported at its ends, the
frequencies of the higher modes would be simple multiples of the
fundamental. In practice ships differ from this although perhaps not
1
as much as might be expected, as is shown in Table 11.1 . It will be
noted that the greater mass of a loaded ship leads to a reduction in
frequency.

Torsional vibration
In this case the displacement is angular and a one-node mode of
vibration is possible. Figure 11.4 shows the first three modes.

Coupling
It is commonly assumed for analysis purposes that the various modes of
vibration are independent and can be treated separately. In some
circumstances, however, vibrations in one mode can generate vibration
in another. In this case the motions are said to be coupled. For instance
in a ship a horizontal vibration will often excite torsional vibration
because of the non-uniform distribution of mass in the vertical
plane.