Abstract: A marine propulsion unit providing for steerable legs for the propeller of a boat which are rotatable in steering direction through 360.degree.. A transmission is provided having spiral bevel gears arranged so that for a constant direction of rotation bearing, loading for parts below the hull can be significantly reduced. There are provided two separate legs having a common drive each being rotatable in steering direction through 360.degree. so that reverse thrust can be achieved without changing the rotating direction of the thrusting propellers relative to their support.