Abstract: A rudder device for water-going craft or aircraft, where there is provided in the vessel a cavity (21) which is open to the outside of the vessel. In the cavity there is provided a body (2) which can rotate about an axis (3), which extends in the cavity (21) and out through the opening (22). The body (2) supports a rudder blade (7) which is fixed or movable in relation to the body (2) substantially in the direction of the axis of rotation (3) through the opening (22) and arranged to be secured by the body (2) after having been moved. The vessel is steered by turning the body (2) by the vessel's steering device.