PURPOSE: To completely absorb the rotation energy of a front propeller into a rear propeller and to improve the propelling performance by arranging an acceleration-type nozzle with a convex shape inside it on the stern of a ship with a counter-rotating propeller and housing the propellers in the nozzle.

CONSTITUTION: The torque of a main engine 9 is transmitted to large gears 7, 8 in opposite directions to each other through gears 6a, 6b. The torque 7 fed to the gear 7 is transmitted to a front propeller 2 through an outer shaft 4 to rotate it counter-clockwise when seen from the stern. On the other hand, the torque 7 fed to the gear 8 is transmitted to a rear propeller 3 through an inner shaft 5 to rotate it clockwise when seen from the stern. At this time, an acceleration-type nozzle 23 with a convex shape inside it is arranged surrounding the front and rear propellers 2, 3. Thereby, the back wash 22 of the front propeller 2 is prevented by the nozzle inside 24 from being dispersed and allowed to flow into the rear propeller 3, and also a negative pressure is induced in the nozzle inside 24 to generate thrust, thus improving the propelling performance.