Contra-rotating, also referred to as coaxial contra-rotating, is a technique whereby propellers or fan blades mounted on a common axle rotate in opposite directions. It is used in aircraft engines, resulting in the maximum power of a single piston or turboprop engine to drive two propellers in opposite rotation. Contra-rotating propellers are also common in some marine transmission systems, in particular for large speed boats with planing hulls. Two propellers are arranged one behind the other, and power is transferred from the engine via planetary gear transmission. The configuration can also be used in helicopter designs, where similar issues and principles of torque apply.
Contra-rotating propellers should not be confused with counter-rotating propellers, a term which describes non-coaxial twin-propellered craft with each propeller on its own axle; one turning clockwise and the other counter-clockwise.
The efficiency of a contra-rotating prop is somewhat offset by its mechanical complexity. Nonetheless, coaxial contra-rotating propellers and rotors are moderately common in military aircraft and naval applications, such as torpedoes, where the added maintenance is not a primary concern to government budgets.
Contra-rotating propellers have benefits in providing thrust for boats for the same reasons. ABB have provided an azimuth thruster to ShinNihonkai Ferries in form of the CRP Azipod, claiming efficiency gains from the propeller itself and the more simple hull design. Volvo Penta have launched the IPS (Inboard Performance System), an integrated diesel, transmission and pulling contra-rotating propellers for motor yachts. Torpedoes have commonly used contra-rotating propellers to give the maximum possible speed within a limited diameter as well as counteracting the torque that would otherwise tend to cause the torpedo to rotate around its own longitudinal axis.
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.