In 1935, while the author was on duty in the Naval Technical Research Institute, some warships of Japan met a violent typhoon in their battle practice, and two destroyers were broken in two. By this accident, the necessity of research on the strength of ships running among waves was strongly impressed, and an experiment on the measurement of hull stress in an actual ship was planned. To obtain the conception, as to what phenomena should appear on ship running among waves, a model experiment in an experimental tank was thought effective as a preliminary test to the actual ship experiment. An electro-magnetic strain meter was designed to obtain the continuous record. In 1936, a 1.5 m. model having a rectangular section, the most part of which was made of celluloid, was towed in an experimental tank. In this experiment, the hull strain in waves changed with the ship speeds. Vibrational strain was also observed when a heavy blow was delivered to the foreward bottom of the model. The possibility of model experiment was shown. The project was modified and a new model experiment was designed, because the outbreak of Sino-Japanese hostilities made the actual ship experiment impossible. In 1939 and afterwards, a series of experiments was tried. The hull strain, pitching and heaving motion of the model and the relative position of the wave and the model were measured. The experiments demonstrated that model experiments of this kind would permit not only qualitative but quantitative analysis of the dynamical strength of ships. Actual ship experiment is necessary, indeed, to accomplish research concerning with the longitudinal strength of ships among waves. The actual ship experiment, however, has several disadvantages, such as expensiveness, difficulty in bringing ship in desirable experimental condition and difficult reproductivity, etc. Therefore, the author believes, research should first be made in detail with model, and the actual ship experiment may as well be considered as an experiment confirming the results of the model experiments.