There are many kinds of tanks placed in the engine room of a large ship, and damages of these tanks caused by vibration are tending to increase as powers of engines grow. In order to prevent such troubles, it is necessary to estimate the natural frequencies of tanks precisely at the design stage and to avoid their coincidence with the frequencies of vibration exciting forces. The authors have carried out some theoretical and experimental investigations into the vibration of tank, and obtained a method of calculation of the natural freqencies taking into account effects of coupling vibration of side walls. This calculation method is applied to two tanks; a model tank and an actual on board a ship. The results are compared with the experiments, and the following features are observed. (1) For the model tank in empty condition, the calculated natural frequencies coincide well with the experimental. (2) The lowest natural frequency obtained by the tests for the model tank in full water condition is 61% lower than the one in empty condition, and virtual water coefficient derived from this experiment is rather higher than the calculated by the use of Prof. Kito's formula. (3) For the actual tank, the lowest natural frequency from the experiment is very much lower than the calculated. An analysis is made on this problem and it becomes clear that the observed lowest frequency is due to a rocking motion of the tank.
