Dynamic stability of vessels with a suspended load

The transverse stability of a sample vessel with a suspended load in regular beam waves is investigated and quantified. Incorporating several series of experimental and numerical model tests in calm water and in waves, three aspects of influence from the suspended load are identified: GM (metacentric height) variation, lever effects, and swinging pull. As from the result of scale-model tests, a suspended load of 0.66% (nondimensionalised by the model's mass displacement) alters the GM value by about 2% corresponding to a change of the roll natural period of about 4%. In calm water, the suspended load alone at a position with the longest lever stretching out at side heels the model up to 8° and reduces the GZ (righting lever) value by about 40%. In waves, the suspended load with a longer connecting ‘rope’ does not seem to increase or decrease the roll amplitude of the vessel as opposed to the one with a shorter ‘rope’. But it changes the behaviour of the roll motion – from regular sinusoidal-like roll to irregular behaviour at higher wave excitation frequencies but not for lower ones.