The Drift Force and Moment on Ships in Waves

The second-order steady horizontal force and vertical moment are derived for a freely floating ship in regular waves. The fluid is assumed to be ideal and the motion is linearized. Momentum relations are used to derive general results for an arbitrary ship or other body, in terms of the Kochin function or far-field velocity potential of the body. Explicit results are derived for slender ships, based upon the assumptions of slender body theory. Computations are made for a Series 60 hull and are compared with experiments. The analysis of the vertical moment permits the prediction of stable heading angles in oblique waves, and it is shown that unless the waves are very short, the ship will be stable only in beam waves.