A method for opening, closing and rotating a rigid marine sail, which comprises the steps of: selectively opening, closing and rotating, in response to a wind velocity and a wind direction, a rigid sail comprising at least two sail portions which are fitted in front of a mast rotatable around the vertical axis thereof substantially in parallel with said axis, and which are selectively pivotably movable between the opened position and the closed position relative to the mast: the method being characterized by: smoothing a plurality of wind velocity signals and a plurality of wind direction signals from a wind velocity/wind direction meter, by a calculating device at prescribed intervals of time; determining opening and closing of the at least two sail portions by the calculating device on the basis of smoothed wind velocity signals and smoothed wind direction signals; automatically selectively opening or closing the at least two sail portions by an opening/closing device in response to the determination; determining by the calculating device an optimum sail angle relative to the horizontal reference line of the ship, the optimum sail angle providing the maximum propulsion to the at least two sail portions when the sail portions are opened, and minimizing the wind resistance acting on the at least two sail portions when the sail portions are closed; and, selectively rotating the mast together with the at least two sail portions, by a mast rotating mechanism, in response to the deviation of an actual sail angle relative to the horizontal reference line of the ship from the optimum sail angle, thereby always keeping the at least two sail portions at the optimum sail angle.