In order to investigate the effect of the amount and distribution of the load upon the natural frequencies of tanker's hull vibration, various loading conditions were provided on the trial trips of tankers of D.W.18,000 ton class which were built during the last several years. The measured frequencies of the 2-node vertical vibration of tanker's hull gave good coincidence with the values estimated by the method published by the author in the Journal of the Society of Naval Architects in Japan, vol.86. As for the 2-node horizontal vibration the estimated frequencies did not show the same degree of coincidence as in the case of 2-node vertical vibration, presumably due to the unsolved problems such as the ambiguities of the amount of virtual mass and the effectiveness of the load for horizontal direction. But suitable correction factors being introduced, the 2-node horizontal frequencies can be estimated in practically sufficient degree of coincidence with the measured values. The 2-node natural frequencies of tanker's hull at full load condition are given by the following formulae: for 2-node vertical [numerical formula] per minute, and for 2-node horizontal [numerical formula] per minute, where I_V & I_H are the vertical and the horizontal moments of inertia at midship section in m^4 respectively, ΔF is the full load displacement in K.T., and Lp is the length between perpendiculars in m. With the view of avoiding the resonance with natural frequencies of hull the revolution of propeller should be selected in the range of (1.75〜1.90) times the 2-node vertical frequency at full load condition.
