PURPOSE: To easily mount a propulsion unit on a boat by connecting a tilting composite arm to the boat at one point between each end through a universal joint, and rotating the arm with a shaft contained in a vertical plane as a center to perform steering displacement of the drive shaft housing.

CONSTITUTION: A hydraulic mechanism 35 performs as a role of steering. When hydraulic oil is supplied from a hydraulic pump 44 to a cylinder space 43 of a cylinder 34, a drive shaft housing 3 is displaced around a horizontal shaft for trimming or tilting a rig. This mechanism functions also as a trim cylinder and a tilt cylinder. A double trim cylinder and steering arm of a conventional propulsion unit can be replaced by the single piston-cylinder constitution 35. The driving mechanism having the unit 1 and the engine uses a short intermediate shaft 50 between the flywheel 51 of the engine and a drive joint 52 of the housing 3.