Propeller Pitch Calculator
A 5,400 RPM outboard with a 2.0:1 gear ratio and a 15 inch prop runs about 34.5 mph at 10% slip. Predict speed, find the right pitch, or work out your prop slip.
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- Speed, pitch, and slip
- Outboards and sterndrives
- mph and knots
Prop pitch and speed chart
Estimated top speed in mph by propeller pitch and wide-open-throttle RPM, for a 2.0:1 gear ratio and 10% slip.
| Prop pitch | 4,500 RPM | 5,000 RPM | 5,500 RPM | 6,000 RPM |
|---|---|---|---|---|
| 13 in | 24.9 mph | 27.7 mph | 30.5 mph | 33.2 mph |
| 15 in | 28.8 mph | 32.0 mph | 35.2 mph | 38.4 mph |
| 17 in | 32.6 mph | 36.2 mph | 39.8 mph | 43.5 mph |
| 19 in | 36.4 mph | 40.5 mph | 44.5 mph | 48.6 mph |
| 21 in | 40.3 mph | 44.7 mph | 49.2 mph | 53.7 mph |
| 23 in | 44.1 mph | 49.0 mph | 53.9 mph | 58.8 mph |
| 25 in | 47.9 mph | 53.3 mph | 58.6 mph | 63.9 mph |
Assumes a 2.0:1 gear ratio and 10% slip. On a 1.85:1 gearcase, speeds run about 8% higher; on a 2.33:1 gearcase, about 14% lower. A prop only reaches these speeds if the engine can actually turn that RPM with it.
Prop slip chart
Prop slip is the share of theoretical speed a boat loses because the prop turns in water, not a solid. This chart shows top speed at 5,400 RPM on a 2.0:1 gear ratio for three common pitches.
| Prop slip | 15 in pitch | 19 in pitch | 23 in pitch |
|---|---|---|---|
| 0% | 38.4 mph | 48.6 mph | 58.8 mph |
| 5% | 36.4 mph | 46.2 mph | 55.9 mph |
| 10% | 34.5 mph | 43.7 mph | 52.9 mph |
| 15% | 32.6 mph | 41.3 mph | 50.0 mph |
| 20% | 30.7 mph | 38.9 mph | 47.0 mph |
| 25% | 28.8 mph | 36.4 mph | 44.1 mph |
| 30% | 26.8 mph | 34.0 mph | 41.2 mph |
| 35% | 24.9 mph | 31.6 mph | 38.2 mph |
0% is theoretical speed; no real boat gets there. To find your own slip, switch the calculator to Find slip and enter a GPS top speed. Slip = (1 - GPS speed / theoretical speed) x 100.
What your prop slip means
- Under 10%. Low. Normal for light, fast hulls; otherwise re-check the tach and use a GPS speed.
- 10 to 15%. Normal for a clean planing hull.
- 15 to 20%. High for a planing hull. Check the bottom, the prop, and the load.
- 20 to 30%. Typical for heavy boats, pontoons, and displacement hulls.
- Over 30%. High. Look for ventilation, a spun hub, a damaged or wrong-size prop, or an engine mounted too high.
How the propeller pitch calculator works
A propeller's pitch is the distance it would screw forward through a solid in one revolution. Theoretical speed is prop shaft RPM times pitch; real boats fall short of that by their slip percentage. The calculator divides engine RPM by your gear ratio to get prop RPM, multiplies by pitch, and discounts by slip. Run it backward with a GPS speed and it gives you slip.
- Where 1,056 comes from. A mile is 63,360 inches and an hour is 60 minutes, so inches per minute divided by 1,056 gives miles per hour.
- Prop slip is the gap between theoretical and actual speed, expressed as a percent. It is not a defect; every hull has it. Clean planing hulls run 10 to 15 percent, heavy boats, pontoons, and displacement hulls run 20 to 30. Measure it with a GPS, since speedometers read high.
- WOT RPM targeting. Your engine should reach the top of its rated wide-open-throttle range with a normal load. Each inch of pitch changes WOT RPM by roughly 150 to 200, and the calculator uses 200: under-revving (lugging) calls for less pitch, over-revving calls for more.
- Change pitch in small steps. If the math calls for a 3 inch or larger swing, something else is usually wrong: a fouled bottom, a damaged prop, an optimistic tachometer, or a mis-entered gear ratio.
Propeller pitch FAQ
What is propeller pitch?
Pitch is how far the prop would travel forward in one full turn with zero slip, measured in inches. A 19 pitch prop moves 19 inches per revolution in theory. Higher pitch trades acceleration and RPM for top-end speed.
How do I calculate prop slip?
Work out theoretical speed (engine RPM divided by gear ratio, times pitch, divided by 1,056), divide your GPS top speed by it, subtract from 1, and multiply by 100. At 5,800 RPM on a 1.86:1 gearcase with a 19 inch prop, theoretical speed is 56.1 mph, so a boat running 50 mph has 10.9 percent slip.
What is prop slip and what number should I use?
Slip is the percentage difference between theoretical speed (RPM times pitch) and what the boat actually does. Use 10 to 15 percent for a clean planing hull, and 20 to 30 percent for heavy boats, pontoons, and displacement hulls.
My engine will not reach its rated WOT RPM. What pitch should I run?
Drop about one inch of pitch for every 150 to 200 RPM you are short. An engine hitting 4,800 with a 5,400 target wants roughly 3 inches less pitch. Verify on the water and confirm the bottom and prop are clean first; fouling mimics over-propping.
Is it bad to over-rev or lug a marine engine?
Both. Exceeding rated WOT RPM accelerates wear, and lugging below the rated range loads the engine harder than it was built for at that RPM. Prop the boat so a normal load lands at the top of the maker's WOT band.
Does this calculator work for outboards and sterndrives?
Yes. The math is the same for outboards, sterndrives, and straight inboards; only the gear ratio differs. Find yours on the engine spec sheet, commonly 1.75 to 2.4 for outboards.