To provide a steering rudder capable of lifting according to a cruising speed for solving such problems as lowering steering performance due to reduced discharge of water-jet at the time of low-speed cruising and water flow resistance of a steering rudder at the time of high-speed cruising, in the case of a boat provided with a steering rudder for assisting the rudder function of a deflector.

This water-jet-propelled boat is provided with a deflector 8, on the side part of which a steering rudder 14 is disposed, and the deflector 8 is provided with a lifting drive 11 for the steering rudder 14. In this boat 2 provided with a cruising speed detector 18 and a rotational speed detector 20 for a impeller 6, the upper and lower limits of cruising speed determined by trial calculation of the rotational speed of the impeller 6 are preset, which are compared with the upper and lower limits of cruising speed set by the cruising speed of the boat 2 at the time of acceleration after start and the rotational speed of the impeller 6 at the time of deceliration from high-speed cruising. By a hydraulically-operating signal from a controller 21, the steering rudder 14 is submerged in water at the time of low-speed cruising and pulled up above the water surface at the time of high-speed cruising.