transmitted for a given propeller radius. The propeller efficiency is increased by recovering energy from the tangential (rotational) flow from the leading propeller. Tangential flow does not contribute to thrust, conversion of tangential to axial flow increases both thrust and overall system efficiency. Disadvantages The mechanical installation of coaxial contra-rotating shafts is complicated and expensive and requires more maintenance. The hydrodynamic gains are partially reduced by mechanical losses in shafting. Contra-rotating propellers are used on torpedoes due to the natural torque compensation and are also used in some motor boats. The cost of boring out the outer shafts and problems of mounting the inner shaft bearings are not worth pursuing in case of normal ships. See also Use in marine propulsion References External links Two Propellers in Tandem Turning in Opposite Ways Give Warplane New Speed'' Propellers