PURPOSE: To reduce the output of a main engine by the gain horsepower obtained by a sail for saving fuel oil by providing a main engine output control unit for reducing the output of a main engine by the amount corresponding to the brake horsepower of the sail calculated according to the output of the sail propulsive force arithmetic unit.

CONSTITUTION: When a sail post 2 and side sails 3 receive wind, bending force is imposed upon the sail post 2 and the stress distributing condition is shown by each deflectometer 6. When a sail is set at an angle and the sail post 2 is rotated at the same time, the stress distributing condition teaches the stress on a boat in the proceeding direction, and according to the stress, the propulsive force of the sail on proceeding of the boat is calculated by a propulsive force arithmetic unit 8, and the propulsive force thus obtained is converted into brake horsepower and transmitted to a main engine output control unit 9 to control reduction of the main engine output or others. Provision of such a main engine output control unit permits economic sailing of a sailing boat.