The estimation of diffraction force components from the equivalent motion concept

Abstract

In many existing algorithms of strip theory the relative velocity concept is combined with sophisticated methods of determining added mass and damping coefficients in order to evaluate components of the diffraction force in waves. However, the concept is purely intuitive and it is shown in the paper to be not correct as an approximating technique.

Instead the concept of equivalent motion, which is valid from that point of view, is put forward. It is found that the velocities of the apparent motion as obtained according to the two concepts can differ very considerably even for long waves. The new concept can be easily employed in the algoritms based on relative velocity approach or in calculations where tabulated values of hydrodynamic coefficients are to be used.