In 1935 the Forth Fleet of the Imperial Navy encountered a typhoon in the occasion of the naval grand maneuvers in the Pacific Ocean off the northern coast of Honshu Island of Japan, and many naval vessels of the fleet were damaged. Destroyers “HATSUYUKI” and “YUGIRI” lost their fore bodies being splitted in front of bridges with 51 crews on board. They were designed on the basis of the theories and experiments of those days ; they had long forecastles with large flare for high performance in rough seas, and their section moduli were designed to be in proportion to the estimated longitudinal bending moment for the reduction of hull weight. In this paper the authors attempt to analyze the disaster of the two destroyers theoretically with the aid of the knowledge available today. The wave bending moments acting on the destroyers are calculated for several wave conditions by TSLAM based on the nonlinear theory of ship motions, and they are compared with the ultimate moment. It is shown from the calculated results that the sagging moments are increased by slamming to the flare and buoyancy of the long forecastle dipped into a wave. If the longitudinal strength were designed in such a manner that the section modulus had an almost constant value sufficiently far towards the fore body of the vessel, the disaster of the two destroyers could have been avoided.