Propeller

A propeller (often called a screw if on a ship or an airscrew if on an aircraft) is a type of propulsor with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which exerts linear thrust upon a working fluid such as water or air when rotated. Propellers are used to pump fluid through a pipe or duct, or to create thrust to propel a boat through water or an aircraft through air. The blades are shaped so that their rotational motion through the fluid causes a pressure difference between the two surfaces of the blade by Bernoulli's principle which exerts force on the fluid. Most marine propellers are fixed pitch screw propellers with helical blades fixed to a boss, rotating on a propeller shaft with an approximately horizontal axis. Other types of marine propellers include controllable pitch, variable pitch, self pitching, and vertical axis propellers.