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 when the angle between the bottom surface and the surface of oncoming waves becomes nearly zero the water pressure is generally known to become very high. In order to get the fundamental phenomena on the elastic response of the bottom shell panel having large aspect ratio. the author is now concerned with the basic research of elastic response of high tensile steel plates developed by impact on water surface, from the view-points of both theory and experiment. The large elastic response of the long rectangular plate due to impulsive water pressure is first solved theoretically by taking the effect of stretching of the plate into consideration, but neglecting effects of higher modes of vibration. Introducing the measured data of impulsive water pressure into the theoretical formula above obtained, the calculated maximum elastic deflections at the center of the plate are compared with those of experiment, indicating good coincidences between them, and both results are further compared with those by theoretical results carried out by Greenspon, which show expanding discrepancies between experimental results corresponding to the pressure increment. The small elastic response of the stiffened plate is then solved theoretically by considering effects of higher modes of vibration, and the maximum elastic deflections obtained by introducing the measured data of impulsive water pressure into the formula are also compared with experimental results, which show good coincidences between them.