A small cargo boat of 48 m suffered from serious damages among rough seas, and its main hull was bent at S. S. 7 by hogging moment. The ship under consideration was in ballast condition, and was carrying a large amount of ballast water in the fore-peak tank. If slamming impact pressure occurs at S. S. 7, a large hogging moment may appear at the section. In the first report, the analysis based on the hydroelasticity shows that this damage is caused by hogging moment due to bottom slamming, although, in the case of conventional ships, slamming causes a large sagging moment in the mid-part. Elastic responses are calculated step by step for the ship idealized as a Timoshenko beam, taking account of slamming impact force, which are determined with respect to instantaneous configurations. It is shown that slamming response gives hogging moment enough for the damage in an unlucky condition of waves corresponding to the sea state reported. In the second report, the collapsing strength of the ship will be estimated by the help of experiments.