PURPOSE: To perform the forward, backward, stoppage and course change of a ship freely without changing the rotational direction of a propeller by symmetrically halving a rudder body being set up rearward on a propeller shaft center, and mailing each halved rudder part steerable separately.

CONSTITUTION: A rudder body 1 being set up in a stem part 21 and in a rear position on a propeller shaft center is halved right and left, and a first split rudder plate 2 at the right is supported on a stern deck 22 via a hollow shaft body 4 free of rocking motion through a bearing. On the other hand, the second split rudder plate 3 at the left is supported by a support shaft body 6 being installed as inserted into the hollow shaft body 4, and these shaft bodies 4, 6 are made free of rotation with individual drive motors installed the stern part 21. At the time of rectilinear motion of a ship, both these split rudder bodies 2, 3 themselves are unitized as one body, and they are positioned straightly, while at the time of forward or backward movement, as shown in a virtual line, these split rudder plates 2, 3 are steered to the outside at an angle of about 115° with each other, thus a water current is directed in the stern direction.