In this paper we describe problems on a controllability of ships when very large vessels proceed through the middle-channel of the strait of Kurushima, and also as to whether it is safety enough. An output of irregular yaw motions of ships caused by a turblent flow was replaced with an input of Equivalent Rudder Angle δ_e, which is expected to generate the same yaw motion of ship, and the equivalent rudder angle was used as a new guidance to estimate the yaw angle of very large vessels in the turblent flow. In order to obtain the steering-pattern that eqauls the turblent flow effect, tests by full sized ship were carried out on a training ship (G. T. 360t) and passenger ships (G. T. 2, 900t). From the results simulated for very large vessels by using a digital computer and the experimental data, it can be considered that the ship-maneuvers in the turblent flow are not so difficult as it is imagined.
