Propeller pitch and cupping affect a propeller’s performance. Stainless steel propellers are high-performance but expensive, while aluminum ones are cheaper and easier to replace. Cupping affects water spin-off, and a properly tuned propeller can pull water out from under the boat.
Propeller pitch determines the speed and power a propeller will produce. The amount of pitch of the propeller refers to the angle of the propeller blades in comparison to the propeller hub or a horizontal line drawn through the center of the propeller. By altering the pitch of the propeller or the angle of the blades, the propeller can be tuned to deliver more top speed or more low-speed power or torque. However, this is only part of the equation; The propeller pitch works hand in hand with the cupping of the propeller blade, as well as the material used in its production to produce the right propeller for any application.
Performance propellers are usually made of stainless steel, while the typical recreational boat is equipped with an aluminum or composite propeller. Due in part to cost, aluminum propeller blades are used because they can be easily replaced if damaged by hitting an underwater object. Many times aluminum propellers will bend rather than break. This allows an experienced repair person to reset the propeller pitch and straighten the bent propeller. In the case of a compound propeller, most of the time, the propeller blades break when they meet an obstacle.
Stainless steel propellers are much thinner than composite or aluminum ones. This slim design together with the proper pitch makes it a very high performance propeller. Producing more top end speed, as well as being able to push the boat on the plane much faster, the typical stainless steel prop is designed with prop pitch and suction cups to extract the maximum level of performance from the outboard motor. This performance doesn’t come cheap, and most stainless steel propellers buy for twice the price of a comparable aluminum unit.
The number of suction cups designed into a propeller has as much to do with its level of effectiveness or performance as the pitch of the propeller. Cupping affects the way the water spins off the propeller blade. In the same way that a baseball is controlled by the placement of the pitcher’s fingers as it is thrown, the cup controls the way the water is ejected from the propeller blades. As the rate at which water is forced out of the propeller blades increases, the rate at which water can enter the area occupied by the propeller also increases. A properly tuned propeller is actually pulling water out from under the entire hull of the boat.
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