How to Determine Outboard Motor Propellor Size and Pitch

by Tom Fritchman
To replace an old propeller, you will need to know its size.

To replace an old propeller, you will need to know its size.

Your outboard motor is installed at the rear of your boat. It provides the force that moves your boat forward through the water. It is also the mechanism you use to steer the boat. The motor does this by turning a propeller blade beneath the water. The propeller moves water past the boat, causing the boat to move. There are several different sizes of propellers, each with its own advantages. Determining the propeller size and pitch for your boat's outboard is fairly easy.

Measure the radius of your propeller with your measuring tape, from the tip of the blade to the center of your propeller. Record this measurement in inches.

Double the radius to find the diameter of your propeller. This number, also measured in inches, is the first half of your propeller's size.

Determine the blade's angle by laying the flat side the protractor against the central part of your propeller.

Determine your pitch using the mathematical formula (Tan a) * 2Pr, where a is the angle of your propeller, P is the mathematical number Pi, and r is the radius that you determined. Tan is the tangent function, indicated as "TAN" on most scientific calculators.

Round the result to the nearest whole number. This number is your pitch.

Write your propeller size in the standard format AxB, where A is the propeller's diameter listed in inches, and B is the propeller's pitch listed in inches.

Items you will need

  • Measuring tape
  • Protractor
  • Scientific calculator

About the Author

Tom Fritchman is a freelancer who has been writing professionally since 2009. His first writing credit was actually a stage play called "Window Watching" performed at the Northmont Auditorium in Clayton, Ohio. He holds a Bachelor of Arts in language and literature from Wright State University.

Photo Credits

  • old propeller image by Dimitar Atanasov from Fotolia.com