Page 315 - Introduction to Naval Architecture

300 VIBRATION, NOISE AND SHOCK

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underwater explosion . The detonation of the explosive leads to the
creation of a pulsating bubble of gas containing about half the energy
of the explosion. This bubble migrates towards the sea surface and
towards the hull of any ship nearby. It causes pressure waves which
strike the hull. The frequency of the pressure waves is close to the
fundamental hull frequencies of small ships such as frigates and
destroyers, and can cause considerable movement and damage. A
particularly severe vibration, termed whipping, occurs when the
explosion is set off a litde distance below the keel. The pressure waves
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act on a large area of the hull and the ship whips . This whipping
motion can lead to buckling, and perhaps breaking, of the hull
girder.

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Figure 11.8 Underwater explosion (courtesy RINA )

Another major feature of any underwater explosion is the shock wave
containing about a third of the total energy of the explosion. This
shock wave is transmitted through the water, and so into and through
the ship's structure. It causes shock and may lead to hull rupture. The
intensity of shock experienced depends upon the size, distance and
orientation of the explosion relative to the ship. These factors are
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combined to produce a shock factor . The shock factor related to the
keel is: