3.1 Straight Running Test

　

 Ship speed ratio U/Uo, oblique angle β and counter rudder angle δc in straight running test are shown in Fig. 4. Model ship was controlled by autopilot system using parameters that had been determined to make the model ship head to intended direction by preliminary running in waves. Pitch, roll and yaw amplitude;θa,ψa,Ψaare shown in Fig.5.

　

 Ship speed ratio U/Uo becomes smallest in head wave condition of which wavelength ratio λ/L is equal to 1.0. This fact is mainly attributed to large pitching motion and presumable large heaving motion as is often pointed out relating to large added resistance of ships in head waves. On the other hand, ship speed ratio U/U0 in beam, bow and quartering wave conditions of which wavelength ratio λ/L is equal to 0.4 and 0.6 is mainly attributed to combined effect of resistance increase due to reflected wave on hull surface and oblique motion including rudder force.

　

 Oblique angle β and counter rudder angle δC is large for short wave range especially in beam wave condition. This is because wave drift force and moment of short waves is large comparing with that of long waves. Positive oblique angle β means drifting motion toward wave propagating direction. The fact that almost of all data for counter rudder angle δC show negative value means that wave effect tends to make ship bow direct to wave propagating direction. However, two conditions in beam wave of 1.0 wavelength ratio in full load condition and in bow wave of 0.6 wavelength ratio in ballast condition show slightly large positive counter rudder angle δC.This implies that drift moment due to wave and oblique motion in these two conditions has opposite polarity to that in other conditions. This fact is related to results of stopping test as mentioned later.