Choosing The Correct Propeller For Your Outboard
It is essential that a propeller is selected that allows the engine speed (when at full throttle) to stay within the rated rpm range which the engine is designed for (this can be found in the owners manual for your engine). Typically 4-Stroke engines run between 5,000 to 6,000 rpm.
We always recommend that you match your propeller to the brand of you outboard (i.e. if you have a Yamaha outboard, use a Yamaha propeller) as they are specifically designed to work with that engine.
Diameter - The distance across the imaginary circle that is made when the propeller rotates.
Pitch - The theoretical distance (in inches) that the propeller travels forwards in one rotation. The higher the pitch that the engine can turn, the faster the boat will usually go.
Determining the correct pitch
If your current propeller is at wide open throttle rpm within the specified rpm range for your engine, select a replacement with the same pitch.
If your current propeller isn't at wide open throttle rpm within the specified rpm range for your engine, select a replacement with a larger or smaller pitch using the following guide:
1. Adding 1 inch of pitch will reduce the wide open throttle rpm by 150 to 200.
2. Subtracting 1 inch of pitch will increase the wide open throttle rpm by 150 to 200.
3. If you are upgrading from a 3 blade to a 4 blade propeller, remember that a 4 blade propeller generally turns 50 to 100 rpm less than the 3 bladed equivalents.
How do I find the size of my current propeller?
The size of your propeller will either be stamped on the hub or on one of the blades (it varies by manufacturer). It will appear in the format of: 7 1/2 x 7. The first number (7 1/2) is the diameter and the second (7) is the pitch.
On Yamaha propellers there is often a letter following the pitch (e.g. 7 1/2 x 7 B), this indicates that the propeller is a Type B. So in this instance your outboard would require a Type B propeller, any of which would fit your outboard and it's just a case of finding the correct pitch/diameter.
What is a Dual or High Thrust Propeller?
These are propellers specially designed for sailboats and other large displacement craft. They are specially built to handle the job of pushing heavy loads through the water.
The hub is designed to redirect exhaust flow away from the blades, so Dual Thrust models cut through "clean water" for higher efficiency and better acceleration.
Transport & Workboats - Generally a smaller outboard with a boat of relatively heavy displacement or a semi-planing hull will need a propeller with a larger diameter and smaller pitch.
Cruising & Fishing - For best fuel economy, low noise and high engine durability, choose a propeller that allows the engine to operate at the lower end of the rated rpm range when at full throttle, usually a higher pitch propeller is required to achieve this.
Waterskiing & Wakeboarding - Maximum speed and acceleration is required here, so a propeller should be selected which allows the engine to run at the upper limit of the rated engine rpm at full throttle. However when no skier is being towed watch the rpm and ensure the limit is not exceeded - only operate with partial throttle under these conditions.
Sportsboats & Runabouts - Again maximum speed and acceleration are required, so a propeller should be selected which allows the engine to run slightly below the upper limit if the rated engine rpm at full throttle.