The impact pressure which occured upon the bottom of a high speed hard chine deep V form small craft (YAMAHA STR-20HT) running in rough seas was measured. The vertical acceleration at bow and stern and the motion (pitching and rolling) of the craft were also measured. The full scale experiment was carried out at Imagire just out of the Lake Hamana, that is, the sea of Enshu between 1975 and 1977 by the Japan Craft Inspection Organization. Concluding remarks of this experiment are as follows, (1) Large impact pressures and accelerations were observed in condition of ahead and oblique seas. (2) The porpoising occured in heavy sea, and the maximum pressure (5kg/cm^2) was recorded at the stern of the craft. (3) Smaller the dead rise angle of the surface where the pressure gage is installed becomes larger the impact pressure gets. And these values belong to log-normal distribution as statistical characteristics. (4) The maximum expected impact pressure which acts upon a high speed small craft within its life seems to be a function of dead rise angle of the surface where the pressure gage has been attached.
