The DOLPHIN is a self-propelled semi-submersible vehicle that operates under but near to the sea surface; typically one-third of the mast projects above the sea surface, Fig. 1. As such, the hydrodynamic performance of the complete vehicle is affected by the hydrodynamic performance of the mast. The purpose of the mast is two-fold: to act as a streamlined fairing for the snorkel for the air-breathing Diesel engine, and to support the antenna that is used for communications with the vehicle. When the DOLPHIN operates in a choppy sea, the effective draft of the vehicle varies. As a result, the submerged length of the mast also changes, and this, in turn, causes the hydrodynamic side force and roll moment produced by the mast to vary. Moreover, the net side force will vary at the uppermost portion of the mast (i.e. where the mast pierces the water surface) where the rolling arm is the greatest. The net effect on the stability of the vehicle will depend on: the height of the oncoming waves; the encounter frequency of the waves; the natural roll period of the DOLPHIN; the amount of roll damping.
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