PURPOSE: To restrain the increase of rotary torque exerted between a casing and a support member, by holding the support member to a swivel bracket through the intermediary of bivration damping members, and as well by supporting the casing to the support member, rotatably over an angular range of 360°.

CONSTITUTION: Vibration damping members 19A, 19B are disposed under a pressed condition in recesses 29 formed in the support sections 28A, 28B of a swivel bracket 18, and recesses 27 formed in a support member 20 in its outer side surfaces orthogonal to the propelling direction of the engine. A vibration damping member 19c is disposed under a pressed condition between a recess 30 formed in the swivel bracket 14 and the front surface of the support member 20 facing in the propelling direction of the engine. One 28A of support sections of the swivel bracket 14 is removably attached to the body of the swivel bracket 14 by means of bolts so that vibration damping members 19A, 19B, 19C may be arranged in positions.