Propeller pitch determines the speed and power that a propeller will produce. The amount of propeller pitch refers to the angle of the propeller blades as compared to the propeller hub or a horizontal line drawn through the center of the propeller. By altering the propeller pitch or the angle of the blades, the propeller can be tuned to deliver more top speed or more slow speed power or torque. This is only part of the equation, however; propeller pitch is used hand in hand with propeller blade cupping as well as material used in its production to produce the proper propeller for any given application.
Performance propellers are typically made of stainless steel, while the typical pleasure boat is equipped with an aluminum or composite propeller. Due in part to cost, the aluminum propeller blades are used because they can be easily replaced in the event of damage from striking an underwater object. Many times the aluminum propellers will bend instead of breaking. This allows an experienced repair person to reset the propeller pitch and straighten the bent propeller. In the case of a composite propeller, more often than not, the propeller blades will break off when encountering an obstacle.
Stainless steel propellers are much thinner than composite or aluminum types. This thin design coupled with the proper propeller pitch makes for a very high-performance propeller. Producing more speed at top end as well as being able to push the boat on plane much faster, the typical stainless steel propeller is engineered with the propeller pitch and cupping to extract the top level of performance from the outboard motor. This performance does not come cheap, and most stainless steel propellers are purchased at double the price of a comparable aluminum unit.
The amount of cupping designed into a propeller has as much to do with its level of effectiveness or performance as the propeller pitch does. The cupping affects the manner in which the water spins off of the propeller blade. Much in the same manner as a baseball is controlled by the placement of the pitcher's fingers as it is thrown, the cupping controls the manner in which the water is actually driven off of the propeller blades. By increasing the speed at which water is propelled off of the propeller blades, the speed at which water can enter the area occupied by the propeller is also increased. A properly tuned propeller is actually pulling water from underneath the entire length of the boat's hull.