In the presence of incident waves and their interaction between two ships, the waves may scatter and excite the adjacent moving ship. On the assumption of a slender ship and linearized boundary condition, the velocity potentials are expanded by taking into consideration the radiation and diffraction potential due to the motions and scattering waves of the adjacent ship, respectively. By combining this technique with the strip method all the motion equations are derived except for surge. The excitation forces are numerically calculated and evaluated with results from the present experiment of two ships in waves. In this research, the interaction effects of two ships have been found significant in views of heave and sway force. The change of the incident wave angle also affects the increases/decreases of force/moment acting on the hull form of the ship. © Springer International Publishing AG 2018.