To take the most suitable countermeasure according to navigation state and, particularly, properly cope with an unspecific factor such as sea weather condition in navigation, loading state, change with the lapse of time or the like by using a variable pitch propeller as a rear propeller.

The rear part of a propeller shaft 12 is rotatably supported by a bracket 14 suspended from a hull 10, and two propellers 16, 18 are mounted on the propeller shaft 12 in the front and rear of the bracket 14, respectively. The rear propeller 18 consists of a variable pitch propeller. When the front and rear propellers 16, 18 are rotated to generate a propulsive force, the pitch is changed if the performance of the rear propeller 18 is insufficient by a change of sea weather condition or loading state or a change with the lapse of time. The change of pitch not only increase the performance of the propeller 18 but also compensates the performance deterioration of the propeller 16.