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.