“What is the best tacking procedure ?” has been long-pending problem for yachtsmen. In order to solve this problem, a numerical simulation of tacking motion was conducted, and the results were compared with data from the full-scale sea tests. The equations of motion were described in the horizontal body axes coordinate system, and the sailing parameters and the trajectory of the boat were calculated using the Runge-Kutta method. The full-scale sea tests were performed using a 10.6 m-LOA sailing cruiser, whose steady sailing performance and dynamic performance in waves were examined in the previous report. The trajectory of the boat was measured using differential GPS receivers. First, powering run turning test and zigzag maneuver test were conducted to confirm the adequateness of the equations of motion and the hydrodynamic derivatives of the hull. Then, the tacking test was performed under various wind conditions and steering processes. By the full-scale sea tests, the mechanism of tacking motion was revealed, and the results of the numerical simulation coincided well with the sea test data.