PURPOSE: To enable to operate a main prime mover and a generator prime mover in parallel with each other by coupling the output shaft of the generating prime mover through a clutch and a power transmission to a propelling shaft.

CONSTITUTION: A main prime mover 4 capable of operating at a variable speed is coupled to the propelling shaft 2 of a marine propeller 1 at the output shaft 4a. A generating prime mover 6 capable of operating at a constant speed is coupled at one end of the output shaft 6a to a power supply generator 7 for loading and unloading equipments, and at the other end through a clutch 5 and a power transmission 3 to the shaft 2. Thus, the output 6a can be coupled to and decoupled from the shaft 2 by the engaging and disengaging operations of the clutch 5, but since the prime mover 6 necessitates only constant speed operation, and the prime mover 6 is coupled to the shaft 2 only when the shaft 2 is rotated at the prescribed rotating speed range. In this case, the power for driving the generator is very small. Accordingly, the prime mover 6 can energize the light load propeller 1 and can be sufficiently utilized for the capacities of both the prime movers 4, 6.