Taking pictures of the motion of a water surface at the impact of flat and concaved rectangular plates on the water, we obtained the phenomena which the air was entrapped and compressed between the water surface and plate surface, while blowing out through the contact part of both surfaces. In order to develop the equation of air motion reasonably, we took the energy dissipation due to volume change of entrapped air into consideration, hence, an unknown coefficient was herein introduced. It was suitably determined from measured data by the dropping test already done by one of authors. The aspect ratio of rectangular plate was chosen beyond about three, and plates have one-third or full scale when compared with the plate panels of both destroyers and torpedo boats. We finally obtained the experimental formula of impulsive water pressure covering M /ρ l ² from about 2 through 7, which might be considered most useful for practical purpose. To get the elasto-plastic response of concaved rectangular plates, the damping effect due to compressed air was also introduced into the theoretical treatment, and this damping coefficient was approximately determined so as to satisfy the critical damping condition of the plate, such as the case of flat plates. We therefore obtained the experimental formula of the response. Within plate scantlings used in our experiment, if the plate is repeatedly dropped on the water from the same height the induced concavity will finally converge to the certain magnitude. And hence, applying those two formulae obtained above to solve such phenomena, reasonable coincidences concerning concavities after each drop were obtained between the computed results by numerical analysis and measured data.