The author carried out an experiment on the shipping water during the experiment of the propulsive performance of tanker models in regular head waves. The amount of shipping water per one cycle was measured and presented in Fig.4 versus model speed. After the calculation of relative motion of the bow to waves, it was found that water shipped on the forcastle deck even when the bow did not submerge into free ocean waves. This fact and further consideration show that when the free board at the bow is regarded as an allowance for shipping water, it must be reduced by the height of the statical swell-up of bow wave due to forward speed of the ship and the relative submergence of the bow must be increased by the height of the dynamical swell-up of water surface due to relative vertical velocity of the bow and waves. The amount of shipping water at the beginning is approximately proportional to forward speed, encounter period and to square of wave height and to 2.5th power of the ratio of excess of water surface over the top of bowchock to the free board at the bow. Then he shows a method to calculate the cumulative energy of the bow submergence in irregular heading seas and gives the time proportion of wet deck for a tanker of length L =190.5 m as an example.