Wave drift force

The wave-drift force is the mean (and slowly varying) second-order force on a floating body in waves, proportional to the square of wave amplitude, that pushes the body in the direction of wave propagation. It arises from second-order pressure and from momentum transfer in the radiated and diffracted wave field, computed by near-field pressure integration or the far-field momentum (Maruo) method, and reported as a drift coefficient per unit amplitude squared. In irregular seas the mean drift follows by integrating the drift coefficient against the wave spectrum, and the difference-frequency part drives slow-drift surge and sway that mooring and dynamic-positioning systems must counter.