Diffraction and radiation forces result from the interaction between the ship hull and the moving fluid. These forces are typically simulated using added masses, a method that uses mass to compensate for not computing these forces directly. In this paper we propose simple mathematical model to compute diffraction force. The model is based on Lagrangian description of the flow and uses law of reflection to include diffraction term in the solution. The solution satisfies continuity equation and equation of motion, but is restricted to the boundary of the ship hull. The solution was implemented in velocity potential solver in Virtual testbed—a programme for workstations that simulates ship motions in extreme conditions. Performance benchmarks of the solver showed that it is particularly efficient on graphical accelerators.