Aerodynamic performance of a Japanese traditional square rig, “Bezai-ho”, and a Chinese lug rig, “Shinshi-bo” in Japanese, were studied by means of wind tunnel tests, sea trials and numerical calculations. Sail forces and sail shapes were measured in the wind tunnel tests. A sail dynamometer boat Fujin was employed for the sea trials, by which aerodynamic forces acting on sail, sail shapes, and sailing conditions of the boat can be measured at the same time. Using the measured sail shapes, sail forces are calculated by means of a vortex lattice method. Differences of sail performance of the above mentioned two types of rig were clarified in the wind tunnel tests and sea trials. The calculated sail performance shows good agreements with the measured data in upwind condition. Dynamic sail performance of the two types of rig during tacking and wearing operations was also clarified in the sea trials using the boat Fujin. Details of sail force variation in time during maneuvering can be investigated by the sail dynamometer system. For the “Bezai-ho,” the backward force acting on sail when the boat changes tacks (wind over the bow) was investigated. At this moment, the square sail falls into a “caught aback” situation, which makes the tacking operation difficult. On the other hand, “Shinshi-bo” showed good steady performance similar to that of the modern marconi rig, and good tacking performance. Obtained results of steady and dynamic sail performance in this paper provide useful information for sail trimming and maneuvering of boats equipped with the western square rigs and modern lug rig introduced by H.G. Hasler.

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