An investigation is made of the added mass moment of inertia induced by the rotational motion of the prism having hull section on water obtaining the information to estimate the natural frequency of the torsional vibration of ships. The special consideration to the effect of the draught upon the added mass moment of inertia is taken into account in the present study. The experimental studies by use of the prismatic models with the sectional forms of the main stations of the hull of a tanker are also carried out confirming the theoretical calculation. As the result of the present investigation, the convenient formula for estimation of the added mass moment of inertia induced by the torsional vibration of a tanker, as an example, is presented for practical use in design room.