Known control devices, which comprise a servomotor or electric motor and, downstream therefrom, a toothed reduction gear system for moving a control element, often have the problem that there is play during the transmission of the control movement, particularly backlash play between the flanks of the gearwheels that mesh with one another. As a result, in its set position, i.e. when the servomotor is static, the control element is not stable. Such control devices are also used in boat drives that comprise a propulsion and steering unit which can pivot about a vertical axis, in particular an inboard drive. Control by means of such a control device has the particular disadvantage that as a result of the play the rudder or steering position concerned is unstable and the helmsman at the wheel has the sensation of indifferent and indirect steering.
From WO 2005/005249 an inboard propeller drive system is known, such that a propulsion unit comprises a propeller shaft with two tractor propellers rotating in opposite directions and an underwater casing arranged to pivot about a vertical axis in the hull. Thus, the steering action of the boat is obtained not—as conventionally—by a rudder, but by swiveling the propulsion vector that results from the propeller thrust. In this known propeller drive, the underwater casing with the propeller shaft is moved by a servomotor via a geared transmission. With this known control device as well, there is some play in the transmission of the steering movement, i.e. the helmsman's perception is that the rudder or steering mechanism operates inexactly and not directly.