Arrangement for enhancing the steering effect of ship rudders (5) comprising at least one passageway for supplying seawater from a pump to ducts (11) in the rudder, and means (20) communicating with the ducts and being adapted to discharge seawater supplied through these at the rudder surface. Said means comprise a profile element (20) extending preferably vertically at the leading edge of the rudder (5) and being pivotable about a generally vertical axis to extreme positions at either side with respect to the central plane of the rudder. Moreover, there are provided distribution ducts (12A, 12B) which in association with the profile element (20) are adjustable so that the profile element by opposite pivot movement in relation to the angular deflection of the rudder causes seawater to be discharged (12X) along the rudder surface (5B) facing abaft at said angular deflection.