Understanding the basics of prop and outboard performance
By
Todd Spurgeon
Have you considered what your prop has done for you lately? Most people take their prop for granted, without realizing that the shiny silver accessory on the back of their outboard has a direct impact on their boatıs performance, speed, fuel efficiency, handling, engine life and safety.
There are many things to consider when selecting a prop. Some of the general terms are as follows:
Diameter: The distance across the imaginary circle made when the prop rotates. Generally, the heavier the boat, the larger the prop diameter need
Pitch: The theoretical distance, in inches, that a prop travels forward during one revolution. The higher pitch prop the engine can turn, the faster the boat can usually go
W.O.T. Wide open throttle
RPM: Revolutions per minute
Prop size differences have a big impact on boat performance. For example:
- A 1² increase in pitch will result in a reduction of W.O.T. RPM by 150 to 200
- A 1² decrease in pitch will result in an increase of W.O.T. RPM by 150 to 20
- If at W.O.T. the RPM is too high, you will need to try a prop with a lower pitch or higher diameter
To keep it simple, remember that pitch = speed. Increase the pitch to increase the boat speed
Peak boat performance equals a combination of hole shot, mid range performance and top-end speed but you canıt have the best of both worlds propping is a compromise. You canıt have ear ripping speed and an incredible hold shot using one prop. For this reason, if your budget will allow it, running 2 props is recommended.
A 3 Blade prop for top-end speed on calm water situations and a 4 Blade prop for better hole shot in rougher water . Remember that a 4 blade prop generally turns 50 to 100 RPM less than a 3 blade prop with the same pitch, therefore decreasing speed.
Always test your boat on the water after a new prop is installed. Due to differences in hull designs, engines and weight, finding the correct prop for each boat will vary. Whether you are running a 2-stroke, 4-stroke or Optimax, the science of propping remains the same.
For more detailed information about propping, contact your Mercury authorized
dealer. Mercury offers a prop-lending program through some of their dealers.
Always operate your outboard at the manufacturerıs recommended RPM, because
operating the engine outside the recommended RPM can cause severe damage to
your engine and may void your warranty. Recommended RPM's, outboard RPM ratings
for your outboard motor and other information can be found in the Mercury
Outboard Owners Manual, your local Mercury dealer or see the prop selector
at www.mercurymarine.com
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