Contra-rotating, also referred to as coaxial contra-rotating, is a technique whereby parts of a mechanism rotate in opposite directions about a common axis, usually to minimise the effect of torque. Examples include some aircraft propellers, 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 termed coaxial rotors, where similar issues and principles of torque apply.
Contra-rotating propellers should not be confused with counter-rotating propellers, a term which describes propellers rotating in opposite directions but sitting apart from each other on separate shafts instead of sharing a common axis. Tandem-rotor helicopters such as the CH-47 Chinook also use a counter-rotating arrangement.
The efficiency of a contra-rotating propeller 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 cost is not a primary concern.