Vibration of high-speed ship frames

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 resulting from 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 antiphase. This leads to the structural strain energy in the two modes being slightly different, the mode with rather less strain energy having slightly lower frequency. Because the frequencies of the two modes are 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 cutouts in the web of the ship frames within the fuel tank areas of the hull.