Fig. 6 indicates that drifting distance HD becomes large for short waves and ships in waves do not necessarily drift toward wave propagating direction. Large value of drifting direction μD for -35deg turn in wave of 1.0 wavelength ratio has little meaning because of its small drifting distance HD and the fact that the data is considered to be affected by wind. Note that drifting direction μD has negative value in +35deg turning condition and positive value in -35deg turning condition. This is inverted tendency comparing to almost of all experimental and calculated cases reported by Hirano et al. [2]. As a matter of fact, Nimura et al. [3] had indicated that drifting direction μD varies broadly from negative value to positive one depending on rudder angle S and wavelength ratio λ/L. Average yaw rate γave shows slight decreasing tendency in short wave region from that in calm water condition. Average ship speed ratio Uave/Uo shows smaller value in 1.0 wavelength ratio condition and larger in 0.4 wavelength ratio condition. Putting these tendencies for average yaw rate γave and average ship speed ratio Uave/Uo together provides that average non-dimensional yaw rate γave(L/Uave become larger in 1.0 wavelength ratio condition and smaller in 0.4 wavelength ratio condition than that in calm water condition. Average oblique angle βave during turning motion shows similar characteristics as that of presumable average non-dimensional yaw rate γave(L/Uave).

　

3.3 Zigzag Test

　

 First and second overshoot angle;Ψoa1,Ψoa2in -10deg and +20deg zigzag test for full load condition are shown in Fig. 8 and Fig. 9 respectively. In -10deg zigzag test, measurement was carried out two times for each wavelength and wave encounter angle condition.

　

 Most significant feature is that -10ded zigzag manoeuvre could not be done due to large wave drift forces and moment in beam wave of 0.4 and 0.6 wavelength ratio conditions. Corresponding values in these conditions indicate infinity in Fig. 7. Even in longer wave of 1.0 wavelength ratio condition overshoot angles for beam wave condition show large deviation from those in calm water condition.

　

　

　

　

 Overshoot angles in following waves seem to be larger than those in head waves. Absolute value of first overshoot angle Ψoa1 in following wave condition becomes smaller in short wave range and larger in long wave range. Wave effect on overshoot angles in +20deg zigzag test in beam wave condition are much smaller than those in -10deg zigzag test.

　

 First and second overshoot angle; Ψoa1,Ψoa2 in +10deg zigzag test in ballast condition in wave of 0.6 wavelength ratio are shown in Fig. 10. Abscissa stands for initial wave encounter angle χi in Fig. 10. Wave effect on overshoot angles in ballast condition is not so large as those in full load condition. This difference between full load and ballast conditions may originate in the difference of a directionally stable ship and an unstable ship.