Theoretical formulae are derived for stresses in the tank structures of a ship among the irregular waves under the following assumptions : i) The stresses in the tank structures are not affected by the deformation of the main structure of the ship. ii) The angular velocities about the center of gravity of the tank, that due to the rotational motions of the ship, do not affect the pressure which acts upon the wall of the tank. The formulae obtained under the above assumptions show that the stresses are non-linear functions of the three components of the acceleration at the center of gravity of the tank. The statistical characteristics of the irregular stresses are investigated by a numerical simulation method. The following results are obtained : i) The statistical distributions of the time serries of the stress are nearly equal to the normal distribution. Their means and variances can be estimated by equations (E・7), (E・8), (E・17) and (E・18) in Appendix E. ii) The statistical distributions of the maxima and amplitudes are nearly equal to Cartwright & Longuet-Higgins's and Ochi's truncated ones respectively. Based upon the above results, a method is proposed for the statical prediction of the irregular stresses.
