PURPOSE: To reduce a high momentary torque pulse applied to steering component elements by using as a shock absorber a piston cylinder connected to drive units at one end at a position far away from a steering pivot shaft and to the fixed part of a boat at the other end.

CONSTITUTION: Two drive units 3 are provided at a boat transom beam 1, and an anti-cavitation plate 9 above a propeller 8 is synchronously rotatably connected thereto by a tie bar 10. The piston rod 12 of a piston-cylinder device 11 serving as a shock absorber is connected to the anti-cavitation plate 9 at a position far away from a right pivot shaft 16 via a ball joint 13, and a cylinder 14 is connected to the shield 4 of the left drive unit 3 via a ball joint 15. This constitution can reduce a momentarily very high torque pulse transmitted to the propeller 8 during travel at high speeds, and the steering component elements of the drive units 3 designed in a low-speed range are sufficient.