PURPOSE: To improve steerability during crawling forward propulsion and backward propulsion by forming a joint section between at least one of rudder angle adjusting arms and a connecting bar positionally changeably so that the steering ratio between both rudders at the front and back side of a propeller can be changed.
CONSTITUTION: The steering force of a hydraulic steering gear 7 is transmitted from an arm 41 to a rudder shaft 40 to rotate a rudder 4 and is also transmitted from a connecting bar 8 to a rudder shaft 60 via an arm 61 to rotate a rudder 6 to the same angle as the rudder 4. Accordingly, during forward propulsion, the accelerated stream from a propeller 2 hits the rudder 4 to generate a turning force, and the inflow stream of the propeller 2 is deflected by the rudder 6 to generate a turning force, thus steerability is improved even during crawling forward propulsion. During backward propulsion, the inflow stream of the propeller 2 is deflected by the rudder 4, and the accelerated stream from the propeller 2 hits the rudder 6, thus steerability is improved. In case steering angles of the rudders 4, 6 are to be changed during forward propulsion and backward propulsion or due to the ship shape, ship speed, etc., steering at a predetermined steering ratio can be performed by changing the joint position between the connecting bar 8 and the arm 41, i.e., the arm length.