In this paper, the authors deal with shipping of sea water onboard high-speed boats in connection with establishing a standard for reasonable amount of the freeboard and the corming height of highspeed pleasure boats. Shipping of water was observed and measured by model experiments on 1/6. 214 scale model in a model basin, and the critical condition of ship speed, wave length, and wave height for shipping of water onboard the model were obtained. Then the results were compared with computed critical condition based on theoretical calculation of relative motion of the bow of the boat and wave surface. Calculation of ship motion was conducted in the following three different ways : (A) ordinary strip method (OSM) applied for submerged part of the hull configuration at rest, (B) OSM applied for the hull configuration at specified speed, (C) measured hydrodynamic derivatives are to be used in solving the equation of motion. Compared with model experiments, method B gave closer result with model experiment data than method A, and was as close as method C which was supposed to give the closest result. This means that the strip method, if the change of the under water hull configuration with speed of advance was taken into account, applied fairly well even under drastic conditions which were much different from assumption of applicability of the strip method.