Abstract
The maneuverabilities of two full ships are investigated by the model experiments. The principal particulars of the ships are almost the same except l_<cb> positions as shown in Fig. 1 and Table 1. From the general results of free running tests, the mean maneuvering characteristics of the two model ships resemble each other and seem to be unusual stable as frequently observed in recent full ships. The detailed analysis of free running tests and PMM (planar motion mechanism) tests, however, leads the following conclusions. 1) The motions of both model ships caused by some definite small rudder angle, can not be determined uniquely and involve small irregular yawing motions. 2) The hydrodynamic forces acting on the hull of ships in steady turning can be devided into two parts. One is the force proportional to the drift angle as an ordinaly ship has, and the other is some unusual force not depending on the drift angle but nearly constant. Anyway, this unusual force is not stable in small drift angle. 3) The conditions causing the unusual force are seemed to depend on the effective drift angle at the stern, the rudder angle and the propeller loadings. 4) To improve the maneuverability of the ship having such an unusual force, the enlargement of the rudder area may be useless, because it requires unrealistic large area. 5) The existence of an unusual force affects the thrust of the ship. 6) On ship A (fuller than ship B), two values of hydrodynamic forces are observed under a larger drift angle, and both values are seemed to be stable. This may be the reason of having two steady turning circles in a larger rudder angle.
