The radical growth in the transportation of crude oil has brought forth the enlargement of crude oil carriers, and the systematic studies on the economic potential have proposed the possibility to change the hull form by reducing length-to-beam ratio ( L / B ). As a consequence, it has become quite inevitable to investigate the manoeuverability at an early stage in the design of large tankers. Under such circumstances, one of the greatest concerns of the authors is the manoeuverability of a newly designed ship which is characterized by the stocky hull form with such a smaller L / B ratio as 5.0. In order to find out the scale effect upon manoeuverability and to confirm the accurate performances, two large 30 m models, one for a conventional V. L. C. C. now in service and the other for a newly designed U. L. C. C., have been subjected to various tests at sea. Systematic model tests by geosim small models, of course, have been conducted in an experimental tank. The result of these tests reveals that the new ship has rather superior performances to those of the conventional one, mainly due to the larger rudder fitted with the former. In addition, the scale effect on the normal force of rudder has been examined, and also the flow around rudder has been analyzed from the records of rudder force generated by a steering. These data contribute to refine a simulation program which has been developed so far for the prediction of the manoeuverability. A whole aspect of the research is outlined in this paper.