To know the force and displacement in the case of head on collision is quite important in designing safe ship. In this paper, the Yang & Caldwell's technique of deriving simplified equation of mean collapse force based on the Wierzbicki's collapse mechanism is improved and applied to the ship bow structures. The ship bow structure is divided into simple elements of L shape, T shape and + shape. The absorbed plastic energy in crushing of the these elements is derived. The method is extended to the case in which plates of several different thickness exist. The collapse modes are chosen so that the collapse force becomes minimum, while maintaining the consistency between elements. The absorbed energy of each elements are summed up to be the total absorbed energy in the crushing of the bow structure. The number of waves between frames is determined to make the collapse force minimum. The crushing force is computed as the absorbed energy divided by the crushing length. Also extension are made to the non-prismatic structures. The finite element analysis using MSC/DYTRAN is applied to verify the validity of this approach. The comparisons are made with experiments by Nagasawa et al. The good agreements of this simplified equation with numerical analysis and experiments are observed.