The reason of the damage which would often appear on the bottom shell of forebody is simply because of the large impulsive pressure acted thereon, and it is known, in general, that when the angle between the bottom surface and the surface of oncoming waves becomes nearly zero the water pressure becomes very high, and the air is also entrapped between both surfaces. In order to get damage phenomena reasonably, which would often appear on the bottom shell plate panel surrounded by stiffeners at all sides, the author is now concerned with the elasto-plastic response of long rectangular plates developed by impact on the water surface, from the view-points of both theory and experiment ; where the aspect ratio of used plates was chosen beyond about two times by scaling similarly compared with the plate panels of both destroyers and torpedo boats. First, the damping effect being induced by air entrapped between the plate and the water surface is introduced into the theoretical treatment, and this will be approximately determined by putting the theoretical deflections of the plate equal to the experimental data, applying the try-and-error method to the nondimensional damping coefficient. After inserting the magnitudes of the damping coefficient determined as above into the theoretical equation again, we shall be able to obtain the so-called experimental formula of response due to only the first impact. Second, supposing that concavities developed on bottom shells of ship are the accumulation of each permanent set induced by the slam load, the experimental test due to repetition of the same loads is continued until reaching the saturated concavities. The multiplier is then determined by dividing the concavities developed due to only the first impact with the saturated concavities reached after many impacts. Applying such damping coefficient and multiplier determined as above to the actual cases, we discuss the comparison between computed results given by numerical analysis and measured data of concavities developed on bottom shells, by which reasonable coincidences are obtained between them. If we substitute concavities developed on plate panels, the impulsive loads with which ships would have encountered during slamming are reversely determined. Then, so as to resist obtained loads, the determination of plate scantlings is further pursued either from elastic or elasto-plastic view-point. Judging from our results, the plastic design which permits small amount of concavities is reasonably recommended rather than the elastic design, because we can prevent largely the increase of already developed concavities and also diminish newly spreading of concavities to the neighboring parts.