Find your pitch
Use the RPM from a full-throttle run with your normal load.
Calculators
Prop Pitch Calculator
The right prop lets your engine reach the upper part of its rated full-throttle RPM range with your normal load. Enter your pitch and RPM to see whether to go up, down or stay put.
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How to find the right prop pitch
Look up your rated range
Your owner's manual lists the full-throttle RPM range. Mercury's 150 FourStroke, for example, is rated at 5,000 to 5,800 RPM.
Load the boat the way you use it
Yamaha says to count fuel, full bait and live wells, batteries, passengers and all your gear, including safety gear such as anchors and ropes.
Run at full throttle and read the RPM
Mercury says an accurate tachometer is important when choosing a prop.
Compare it with the upper half of the range
Enter your numbers in the calculator. Mercury says the engine should run in the upper half of its range at full throttle under normal load.
Change the pitch, then test again
Less pitch raises full-throttle RPM and more pitch lowers it, about 150 to 200 RPM per inch. Test the new prop under the same load.
How much does one inch of pitch change RPM?
About 150 to 200 RPM. Mercury's 150 FourStroke manual says adding an inch of pitch lowers full-throttle RPM by 150 to 200, and taking an inch away raises it by the same. Yamaha's propeller guide gives the same 150 to 200 per inch, and shows it as 25 inches of pitch at 5,200 RPM, 23 inches at 5,600 and 21 inches at 6,000.
Blade count matters too. Mercury says going from a three-blade to a four-blade prop generally lowers full-throttle RPM by 50 to 100.
Where in the range to aim
Mercury says to run in the upper half of the rated range at full throttle under normal load: if RPM is above the range, choose more pitch, and if it is below, choose less. It warns never to use a prop that lets the engine exceed the range under normal full-throttle running.
Yamaha aims a little higher. Its guide says the right prop reaches the upper portion of the range without exceeding it, with a normal-to-heavy load: for an outboard rated at 5,000 to 6,000 RPM, about 5,700 to 6,000. It adds that a heavier load or higher altitude calls for less pitch, and a light load or low altitude for the same pitch or slightly more.
Prop pitch chart
Theoretical speed in mph by pitch and engine RPM, at a 1.92:1 gear ratio (the Mercury 150 FourStroke's), with no slip.
| Pitch | 4,500 RPM | 5,000 RPM | 5,500 RPM | 6,000 RPM |
|---|---|---|---|---|
| 13 in | 28.9 | 32.1 | 35.3 | 38.5 |
| 15 in | 33.3 | 37.0 | 40.7 | 44.4 |
| 17 in | 37.7 | 41.9 | 46.1 | 50.3 |
| 19 in | 42.2 | 46.9 | 51.5 | 56.2 |
| 21 in | 46.6 | 51.8 | 57.0 | 62.1 |
| 23 in | 51.0 | 56.7 | 62.4 | 68.1 |
| 25 in | 55.5 | 61.7 | 67.8 | 74.0 |
Pitch, gear ratio and speed
Pitch is the distance a prop would travel forward in one turn. Yamaha says a lower pitch gives a better hole shot and more pushing power but a lower top speed, and a higher pitch the reverse. Gear ratio works the same way: Yamaha says a higher ratio gives more pushing power and a lower one more potential top speed. The prop slip calculator compares the theoretical speed with your GPS speed.
Common questions
About 150 to 200 RPM at full throttle, according to both Mercury's 150 FourStroke manual and Yamaha's propeller guide. More pitch lowers RPM; less pitch raises it.
Go up if your full-throttle RPM is above the rated range, and down if it is below it, Mercury says. It also says full-throttle RPM should sit in the upper half of the range, so a reading in the lower half points to less pitch too.
The upper half of the range in its manual, Mercury says. Yamaha aims for the upper portion with a normal-to-heavy load, about 5,700 to 6,000 for a 5,000 to 6,000 range.
Usually a little. Mercury says going from three blades to four generally lowers full-throttle RPM by 50 to 100.
Mercury warns never to use a prop that lets the engine exceed its recommended full-throttle range, to avoid engine damage. Add pitch until it runs within the range.
Mercury says the diameter and pitch are stamped on the side or end of the hub, diameter first. A 14 x 19 prop is 14 inches across with 19 inches of pitch.
Sources
Mercury, 150 FourStroke Owner's Manual; Yamaha, propeller guide.