The Drift Force and Moment on Ships in Waves

TLDR

The authors derive second‑order steady horizontal forces and vertical moments for a freely floating ship in regular waves using ideal‑fluid, linearized motion and momentum relations, expressing general results via the Kochin function or far‑field potential and providing explicit slender‑ship formulas through slender‑body theory. Computations for a Series 60 hull confirm the theory, showing that the predicted vertical moment yields stable heading angles only in beam waves unless the waves are very short, and the results agree with experimental data.

Abstract

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.