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What is an Outboard Propeller?

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  • Written By: Judith Smith Sullivan
  • Edited By: Susan Barwick
  • Last Modified Date: 19 September 2016
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An outboard propeller is the part of the lower unit of an outboard motor. It is typically fan shaped with two or more blades and powered by by gasoline. Unlike an inboard motor, the outboard motor is attached to the outside of a watercraft.

Outboard motors and propellers are quite common. They offer an advantage over inboard motors because they are easily accessible for repairs. Since the motor and propeller are self-contained, they can be upgraded or replaced with a different motor relatively easily. Inboard motors are typically more expensive and cost more time in labor to repair than outboard motors.

The type of outboard propeller varies by size, material, and design. Size is indicated by the diameter and pitch. The pitch is the calculated amount of movement the propeller creates with each revolution, and the diameter is found by measuring the blade from center hub to tip and multiplying by two. For instance, a 15 inch by 22 inch (38.1 cm by 55.88 cm) outboard propeller is 15 inches (38.1 cm) in diameter and theoretically moves the watercraft 22 inches (55.88 cm) with each turn.

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Common materials for outboard propellers include stainless steel, aluminum, bronze, and composites. Watercraft propellers must withstand a great deal of strain, so they are not typically made of any type of plastic. Composite propellers are created with a mixture of nylon and glass. They are typically lighter than traditional metal propellers and are not susceptible to corrosion.

The design of the outboard propeller makes a difference in the watercraft's performance. Pitch and diameter, as well as the number of blades, affects the rotations per minute (RPM) and acceleration. Usually four-blade propellers have superior acceleration and bow lift than three-blade designs, although three-blade propellers function well for all purpose use. An increase in pitch normally translates into a lower number of RPMs, as does an increase in diameter. How well the propeller functions also depends on the size and weight of the boat.

It normally requires some trial and error to match the correct outboard propeller to the motor and watercraft. In optimization of any one aspect of propulsion, there is typically some reduction in performance in other fields. It is not usually possible to achieve optimum efficiency, speed, hauling capacity and handling with the same motor and propeller. It is recommended that several propellers be purchased for different situations. This is the best way to maximize performance on a single watercraft.

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