On the Characteristics of Unsymmetrical Rolling of Ships (No. 1)

Abstract

Experimental results on unsymmetrical rolling of two ship models, a fishing boat and a cargo ship, are presented.The model was supported freely, except yawing, in an experimental tank under the action of beam seas, beam wind or both.The heel angle, the amplitude of rolling and the velocity of drifting were obtained.In the fisher-boat experiment, observation of unsymmetrical rolling in regular waves due to shipping water was the main subject, while, in the cargo ship experiment, the wave characteristics were kept constant and the wind velocity was changed from zero to 8 metres per second so as to obtain the informations on the combined action of wind and waves.Unsymmetrical rolling due to shipping water with weatherside heel took place under wave action alone when freeboard was too small or/and effective area of freeing port was insufficient.It is noticeable that the principle of superposition seems to fail sometimes as to motions of ships under combined action of wind and waves. In other words, these experiments revealed that the rolling amplitude varied with increasing wind speed, the heel angle due to the same wind velocity increased more or less under the additive action of waves and the velocity of drifting was independent of the existence of waves except in case of very low wind speed.