To improve rudder control and turning performance in advancing with inertia even when an engine is stopped or in the idling state.

In advancing with inertia when the engine 15 is stopped or in the idling state, if the rudder handle 13 is operated, for example, to the right, a left flap 30L is oscillated downward D by driving mechanisms 27, 29 and 32 and projected into the water, a lift force is generated, the portside of a boat body 10 is lifted, and the boat body 10 is turned to the right, while in high-speed navigation of the boat body 10, the flap 30L keeps upward oscillation by water pressure due to a contracting action of a telescopic member 33.