The cargo boat of LCU-Type, which was built at the Mukaishima Shipyard of the Hitachi Shipbuilding & Engineering Co. Ltd., had wide breadth and shallow draught in relation to her length. Moreover, the ship was fitted with twin propellers turning in the same direction (clockwise, from, stern towards bow).Therefore, the poor course-keeping quality of the ship was suspected when the initial design was started. Then a preliminary experiment with a small model was carried out in order to design the form of hull and skegs. After determination of the hull form, another self-propelled model, fairly large, was made for the experiments on the course-keeping quality at the Mitaka Model Basin of the Transportation Technical Research Institute. When the ship was completed, a special experiment called a spiral test was carried out on her. In this paper, details of these model and actual ship experiments are described, model-ship correlations on the turning and course-keeping qualities are examined, and the course-keeping quality of the ship is discussed. The conclusions thus obtained are as follows : (1) The self-propelled model has a good course-keeping quality. (2) The turning quality of the actual ship is very similar to that of the model when the helm angle is more than 5 degrees. (3) The characteristics of turning traces can easily show the unfavourable course-keeping quality both on the model and the ship. (4) The course-keeping quality which is believed very good in case of the model, showed riot the similar but rather worse tendency in case of the actual ship. However, it was confirmed on the sea trials and also in the actual services that the ship is satisfactory in her course-keeping quality.
