A set of four characteristic figures-K, T1, T2 and T3-provides a full description of the response behaviour of a ship to steering [ref. 1]. Although a simplified analysis based upon this idea (so-called K-T analysis) has a wide popularity to measure and to assess manoeuvrability of ships, the four constants above-mentioned should be called upon to describe the response perfectly in the analysis and synthesis of steering control of a ship, taking an automatic steering device and/or a human

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... or into account [2]. This paper relates to a new procedure of defining these four characteristic constants using a radio-controlled, free-sailing model. This is also adoptable to a full-size experiment in principle. The procedure employs two kinds of manoeuvres ; a parallel shifting of ship's path and periodic steering with a fixed amplitude and a number of frequencies. Both manoeuvres can be performed in a conventional towing basin. The periodic steering results and harmonic analysis of the parallel shift manoeuvre give an enough information to assess the transfer function (frequency response function) of a ship, which, in turn, yields the characteristic constants K, T1, T2 and T3. These constants are composed of the eight stability derivatives (coefficients of the linear equation of motion), so that we can define these derivatives through the present procedure if four of these derivatives are given by any other sources, for example, oblique towing test. The data involved were obtained at the Osaka University Experiment Tank (80 mx7mx 3. 6m) employing two 4. 5m models out of Todd's 60 series.