The objective of the study is to investigate the effects of sudden damage, and progressive deterioration due to corrosion, on the ultimate strength of a ship which has been collided by another vessel. Explicit finite element analyses (FEA) of collision scenarios are presented where factors are varied systematically in a parametric study, e.g. the vessels involved in the collision, and consideration of corroded ship structure elements and their material characteristics in the model. The crashworthiness of the struck ships is quantified in terms of the shape and size of the damage opening in the side-shell structure, and the division of energy absorption between the striking and struck ships for the different collision simulations. The ultimate strength of the struck ship is calculated using the Smith method and the shape and size of the damage openings from the FEA. In conclusion, the study contributes to understanding of how corroded, collision-damaged ship structures suffer significantly from a reduction in crashworthiness and ultimate strength, how this should be considered and modelled using the finite element method and analysed further using the Smith method.