The size of a propeller is usually described by two numbers. These two numbers indicate the sizes of diameter and the pitch respectively. Propeller diameter is twice the distance from the center of the hub to the tip of the blade as a propeller rotates.
Pitch is the theoretical forward movement that a propeller travels during one revolution.
The actual distance a propeller travels is about 10 to 20 percent less than the design pitch. Slip is the difference between actual and theoretical movement. It is said to have a cup if the trailing edge of the blades formed or cast with the edge curled. Cupped blades improve the grip of the propeller into the water, reduce cavitation and allow the boat to reach higher top speed. Cupping benefits are so desirable that almost all modern recreational, high performance propellers have some degree of cup.