Marine propeller is an axial flow impeller. Standard propellers have three blades, but can be two-bladed, four-bladed, or encased by a circular guard. A revolving propeller traces out a helix in the fluid. One full revolution moves the liquid a fixed distance. The ratio of this distance to the propeller diameter is known as the pitch. Marine type is often used as a side-entering mixer in large tanks and as a top-entering mixer in small tanks. It can be designed with a different pitch to change the combination of pumping rate and thrust.