On ship motions in harbors, a method for estimating diffraction forces from wave field calculations with the Boussinesq model is considered based on potential theory. Since the potential and its normal derivative on any arbitrary boundary enclosing the ship is obtained from the Boussinesq model as time histories of the water surface oscillation and the depthaveraged velocities at each grid point, the incident wave potential along the ship is calculated through Green's Identity Formula. Then a boundary value problem on the diffraction potential is formulated using the method of matched eigen function expansions. By solving the problem, the diffraction potentials are determined, and the ship motions are estimated through the retardation function method using thus obtained diffraction forces.
