Props
How to Choose a Boat Propeller
The right prop lets your engine reach the top of its rated full-throttle RPM range with a normal load. Start from that number, then work out pitch, diameter, blade count and material.
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Find your engine's full-throttle RPM range
It is in the specifications of your owner's manual. As examples, Mercury lists 5,000 to 5,800 RPM for its 150 FourStroke, and Yamaha uses 5,000 to 6,000 RPM to illustrate a typical outboard. Everything else depends on hitting this range.
Read the size of your current prop
Mercury says the diameter and pitch are stamped or cast into the side or end of the hub: 14x19 means a 14-inch diameter and 19 inches of pitch. On a Yamaha prop, the number is the pitch and the letter is the series, so a 17M is a 17-inch pitch M-series prop for 150 to 300 hp outboards.
Run a full-throttle test with your normal load
Mercury says an accurate tachometer is important. Load the boat the way you usually run it; Yamaha says to count fuel, full livewells, batteries, passengers and gear, and to test with a normal-to-heavy load.
Adjust the pitch to land in the upper half
Mercury wants the engine in the upper half of its full-throttle range under normal load. If RPM is too high, go up in pitch; if it is too low, go down. Adding an inch of pitch cuts full-throttle RPM by 150 to 200, and removing an inch raises it by the same.
Choose the diameter
Mercury says the correct diameter is predetermined for your outboard's design. When a prop comes in more than one diameter for the same pitch, use the larger one for a heavy boat and the smaller one for a light boat.
Pick blade count and material
Three blades suit most boats; four add acceleration and lift at some cost in top speed, and stainless outperforms aluminum at a higher price. Solas says to drop the pitch by 1 or 2 inches when you go from three blades to four.
Pitch, explained
Pitch is the distance, in inches, a prop would travel forward in one revolution if it were turning through something solid. Mercury compares it to a car's gears: a lower pitch accelerates quickly but tops out lower, and a higher pitch usually runs faster but accelerates more slowly. The prop pitch calculator works out the change from your full-throttle RPM and rated range.
How pitch changes full-throttle RPM
Yamaha's own illustration, for an outboard rated at 5,000 to 6,000 RPM. Each inch of pitch moves RPM by about 150, give or take 50.
| Pitch | Full-throttle RPM |
|---|---|
| 25 in | 5,200 |
| 23 in | 5,600 |
| 21 in | 6,000 |
Start from a tested setup
Yamaha suggests starting with its Performance Bulletins: find the boat most like yours, note the prop listed, and look up the same series and pitch for your outboard. Then adjust for weight and altitude. A heavier boat or higher altitude calls for less pitch, while a light boat at low altitude can run the same pitch or slightly more.
Solas adds that a light boat and load will pull a higher pitch, while a heavy one needs a lower pitch to reach full-throttle RPM, and that makers measure pitch in slightly different ways, so the same numbers from two brands can perform differently.
Why the RPM range matters
Mercury warns that the wrong prop can significantly affect your boat's performance and could damage the outboard. A prop that holds the engine below its range is pitched too high, and one that lets it run past the range is pitched too low; both are fixed by changing pitch.
Rotation
Most single outboards and sterndrives use a right-hand prop, which Mercury says spins clockwise when pushing the boat forward. Left-hand props turn counterclockwise, and Mercury Racing says they are typically used on multi-engine boats to balance torque. Buy the rotation your gearcase calls for.
Common questions
The one that lets your engine reach the upper half of its rated full-throttle RPM range with your normal load. Start from the prop you have, test at full throttle, and change pitch until the RPM lands there.
Mercury says the first number is the diameter and the second is the pitch, so 14x19 is a 14-inch diameter with 19 inches of pitch. On a Yamaha prop, the number is the pitch and the letter is the series.
Mercury says 150 to 200 RPM at full throttle. Yamaha puts it at about 150, give or take 50. More pitch lowers RPM; less pitch raises it.
If your full-throttle RPM is above the rated range, go up in pitch. If it is below, go down. If it is already in the range, Mercury says to keep the same pitch when you replace or upgrade the prop.
Check full-throttle RPM with an accurate tachometer under normal load. If it falls outside the upper half of your engine's rated range, the pitch is wrong for your boat and load.
Yes. Yamaha says to pitch down for a heavier load or higher altitude, and to run the same or slightly more pitch on a light boat at low altitude.
Sources
Mercury, 150 FourStroke Owner's Manual and Propeller Guide; Mercury Racing, Prop School Part 5; Yamaha, propeller guide and Owner FAQs; Solas, Prop Q&A.