PURPOSE: To improve the efficiency of the propellers of an ordinary ship, by making the distance between two propeller shafts smaller than the diameter of the propellers and by prescribing the number of the blades of each propeller to be two, three or four and by synchronously rotating the propellers.

CONSTITUTION: The torque of a single main engine 4 is transmitted to propellers 1, 1 through a transmission mechanism 5 and propeller shafts 3, 3. The number of the blades 2 of each propeller 1 is prescribed to be two, three or four. The outside diameter of each propeller is made large but and the surface area of each blade is made small so that the blade is slender and the surface area of the developed propeller is small. The efficiency of the propellers 1, 1 is thus improved. The distance between the propeller shafts 3, 3 is made smaller than the outside diameter of the propellers 1, 1. The rotation loci of both the propellers are overlapped with each other. The propellers are synchronously rotated in the opposite directions to each other. According to this constitution, a danger of stern vibration is eliminated and a very high two-shaft propelling effect is produced.