Hydrodynamic effects in ship collisions are usually considered by increased inertia force in the form of the equivalent added mass. It is necessary to reasonably estimate the equivalent added mass to calculate energy absorbed by ships in collisions. In this paper, motion equations in the horizontal plane for the struck rigid ship are first established considering hydrodynamic effects in the form of added mass, linear, and quadratic damping. The equivalent added mass are obtained in three ways and analyzed. It is shown that the equivalent added mass for the sway motion depends on not only the duration of collision impact, impact force, but also the collision position, while the equivalent added mass for the yaw motion could be assumed to be independent of the collision position. In addition, a simple formula is proposed to relate the equivalent added mass for part of the vessel to that of the whole vessel, provided that the underwater area of the transverse section is known. As a consequence, it is possible to estimate rigid hull girder responses based on the simplified methodology, which could be used in design and probabilistic collision analyses. In the end, the hull girder responses are estimated considering both a flexible and rigid vessel. Comparisons are made between rigid and flexible hull girder responses.