and Kp can be measured from pure roll test. But pure roll test was not done successfully due to trouble of roll motor during tests. So, roll added mass and roll damping coefficients Kp were estimated from free roll tests. Fig. 6 shows the time history of the roll decay measured from free roll test at the speed of 1.2m/s. Using period of this curve, roll added mass can be estimated. By processing this data further[7], the roll extinction curve can be obtained as shown in Fig. 7. Linear damping coefficient kd can be estimated with the slope of this curve. The relation between Kp and kd is as follows[3].

　

　

 The linear damping coefficients obtained thus depends on speed as shown in Fig. 8. The linear damping coefficient decreases as speed decreases but it has a constant value below 0.8m/s.