Two key measurements of a prop are diameter and pitch. Pitch is the theoretical distance (e.g., 19 inches) a propeller will move a boat with one revolution of the prop shaft. Generally speaking, the diameter of a four-blade prop is smaller than that of a three-blade prop with the same pitch, says Jared Reichenberger, brand manager for Mercury Propellers. This helps a four-blade spin up quickly and allows the motor to rev as high as it would with a three-blade of comparable pitch, Reichenberger points out. Nearly all saltwater props for outboards are made from stainless steel.