As the manoeuvrability design is usually based on non-dimensional indices such as turning circles in ship's length and K', T' non-dimensioned by L/V, the figure of dimensional angular velocity in deg./sec decrease with the increase of ship's length when ship's speeds are similar.

 Therefore, the turning angular velocity is necessary to be indicated in the dimensional figures (deg./sec) and to be evaluated in view of operators human feeling. Fig. 3 shows the relationship between the angular velocity during course change motions and the operator's evaluations obtained by manoeuvring simulator studies for DW200,000T, 400,000T [10] and 1,000,000T which was assumed to appear in 1970's. An example of the time history of turning angle of DW200,000T is shown in Fig. 4. The author is to define and propose the two angular velocity figures in 30° course change as indices because it is known that 90% of the course change angles are less than 30 degrees in most of the voyages by Hara's study [11] and the author's investigations.

　

　

　

and the figures in Fig. 3 are classified as shown in Table 1 . Simply speaking, it will be judged that the ship is difficult to manoeuvre due to "Turning, too slow" if t=30 is smaller than Wagenaar's figure (0.017 deg./sec) [12] and max is smaller than Iwai's figure (0.15〜0.20 deg./sec) [13].

　

 The course keeping is also important although it is mostly done by autopilot. This performance can be evaluated by the estimated power increase caused by ship's yawing and check helm or energy indices J(%) which was proposed to evaluate the performance of autopilots [14], including the comparison of different autopilots on the same DW210,000T tanker. The ship when controlled by PID autopilot shows smaller check helms with longer period than the case by PD autopilot. Fig. 5 shows the relationships between the check helm and the loop width for different ships and different autopilots of which data were obtained in sea trials. The loop widths of 5°〜6°will be allowed in view of propulsive power increase [15] in case of PID autopilots, but so far as the course keeping concerned, larger figures up to about 10°might be accepted in case of the adaptive autopilots with superior performance which are commonly used in VLCC class vessels these days.