A new formulation is proposed for the analysis of the impact mechanics of ship collisions that can be applied to both 2D and 3D cases. It is assumed that the impact force is large, and all other forces except the impact forces are neglected. The equations of motion are solved in a local coordinate system, and a transformation matrix between the global and the local coordinate system is proposed. The mass and inertia properties are formulated in the local coordinate system. The orientation of the local coordinate system is determined by the hull shape of the struck ship at the contact point. A closed form solution of the external mechanics of ship collisions is derived. Excellent agreement with an alternative 2D formulation for ship–ship collisions is achieved. The features of the proposed 3D method are demonstrated by numerical examples. An application of the method to estimate the required energy dissipation in ship–iceberg collisions is included. Results and discussions are presented and finally, conclusions are made.