Impact testing of a typical high speed ship section has
shown that the mode of vibration most likely to fall in the
frequency range of excitation due to propeller or rotor blade
passing is that where the ship frame rocks in a fore and aft
direction about its base connection to the hull plate. This
vibration has significant amplitude to either side of the keel
and it is found that connection between the inboard and
outboard sides is weak. As a consequence vibration can
occur in two modes in which the opposite sides of the hull
either move together or in anti phase. This leads to the
structural strain energy in the two modes being slightly
different, the mode with opposite motion to either side
involving rather smaller strain energy. As a consequence the
two modes have slightly different frequency, the mode with
opposite motion to either side having rather less strain
energy and thus a lower frequency. Because the frequencies
of the two modes are thus close together the transient
response exhibits beating in which vibration energy is
exchanged between the two sides of the hull at the low beat
frequency. Vibration of the ship frames in this manner
appears to have been the cause of minor weld cracking
where the top of stiffeners pass through cut outs in the web
of the ship frames within the fuel tank areas of the hull.