Prop Slip Calculator
Compute boat propeller slip % from RPM, pitch, gear ratio and actual speed — or run it in reverse to find the pitch, RPM or gear ratio for a target slip. Free, private, runs entirely in your browser.
How prop slip is calculated
Theoretical speed (mph) = (RPM × pitch in inches × 60) / (gear ratio × 63360). The 63360 converts inches per minute into miles per hour (63360 inches = 1 mile).
Prop slip (%) = (theoretical speed − actual speed) / theoretical speed × 100.
Example: 5500 RPM × 25″ pitch × 60 / (1.75 × 63360) = 74.40 mph theoretical. At 62 mph actual, slip = (74.40 − 62) / 74.40 × 100 = 16.67%.
Slip interpretation
| Slip % | Rating | What it means |
|---|---|---|
| < 0 | Negative | Impossible under steady conditions — check tach, speed and inputs |
| 0 – 5 | Unusually low | Possible over-pitched prop, lugging engine, or optimistic speed reading |
| 5 – 10 | Excellent | Well-matched prop and setup |
| 10 – 15 | Good | Normal for most recreational boats |
| 15 – 20 | Fair | Room for improvement — consider pitch, trim or load |
| > 20 | Poor | Investigate: worn/under-pitched prop, excess drag, ventilation |
Scope boundary
This tool covers propeller slip math: theoretical speed, slip percentage, and reverse-solving pitch, RPM or gear ratio from a target slip. It does not diagnose cavitation or ventilation, recommend a specific propeller model, account for blade area, cupping, rake or hull drag, or replace a sea trial. For Mercury and Yamaha owners, always confirm the gear ratio from the gearcase stamp or service manual — presets here are typical values, not guarantees for your model.
Frequently asked questions
What is prop slip?
Prop slip is the difference between a propeller's theoretical speed (how far the pitch should move the boat per engine revolution) and the boat's actual speed through the water, expressed as a percentage. Slip happens because water is fluid, so a propeller always travels less far than its pitch suggests. Typical slip is 5 to 20 percent; the calculator above computes it with the formula slip = (theoretical speed - actual speed) / theoretical speed x 100.
How do you calculate prop slip?
First compute theoretical speed in mph: (RPM x pitch in inches x 60) / (gear ratio x 63360). Then prop slip = (theoretical speed - actual speed) / theoretical speed x 100. Example: 5500 RPM, 25 inch pitch, 1.75 gear ratio gives a theoretical speed of 74.40 mph; if GPS shows 62 mph, slip is (74.40 - 62) / 74.40 x 100 = 16.67 percent.
What is a good prop slip percentage?
5 to 10 percent is excellent, 10 to 15 percent is good, 15 to 20 percent is fair, and over 20 percent is poor and worth investigating. Slip below 5 percent is unusual and often means the engine is over-propped (lugging), the tachometer reads high, or the speed reading is optimistic. These bands assume wide-open-throttle testing on calm water with a normal load.
What does negative prop slip mean?
Negative slip means the boat is going faster than the propeller's theoretical speed, which is physically impossible under steady conditions. It almost always points to a measurement error: a tachometer reading too high, a speedometer or GPS reading too high, an incorrect pitch stamp, or the wrong gear ratio entered. Double-check your inputs before trusting the result.
Why is my prop slip over 20 percent?
Common causes are an under-pitched or worn propeller, excessive weight or drag (fouled hull, wrong trim, heavy load), ventilation from a motor mounted too high, or testing in rough water or strong headwind. As a rule of thumb, each 1 inch of added pitch raises theoretical speed and can reduce slip by roughly 2 to 4 percentage points if the engine can still reach its rated RPM.
Does prop pitch affect slip?
Yes. Pitch is the dominant variable in theoretical speed, so changing pitch changes slip directly. A higher pitch raises theoretical speed; if the boat's actual speed does not rise proportionally, measured slip increases, and the engine may lug below its rated RPM. Use the reverse mode of this calculator to estimate which pitch hits your target slip while keeping the engine in its recommended RPM range.
How does gear ratio change the calculation?
The gear ratio sits in the denominator of the theoretical speed formula, so a numerically higher ratio (for example 2.33:1 versus 1.75:1) lowers theoretical speed and raises calculated slip for the same RPM and pitch. Common outboard ratios include Mercury 1.62:1, 1.75:1 and 2.00:1, and Yamaha 1.75:1, 2.00:1 and 2.33:1. Always use the ratio printed on your gearcase or service manual.
Should I enter speed in mph, knots, or km/h?
Use whichever unit you trust most, then pick it in the unit selector; the calculator converts internally using 1 knot = 1.15078 mph and 1 km/h = 0.621371 mph, and shows the theoretical speed in all three units. For consistency, take the speed reading at wide-open throttle on calm water with GPS, since pitot-tube speedometers are notoriously optimistic.
How do I find the right pitch for a target slip?
Switch to reverse mode, pick 'pitch' as the value to solve, and enter your target slip, actual speed, RPM, and gear ratio. The calculator rearranges the formula to pitch = (actual speed / (1 - slip)) x (gear ratio x 63360) / (RPM x 60). Example: a 12 percent target with 62 mph actual, 5500 RPM, and 1.75 gears needs about 23.7 inches of pitch. Round to the nearest available prop size and verify with a sea trial.
Do Mercury and Yamaha use different gear ratios?
Often, yes, which is why entering the correct ratio matters more than the brand name. Mercury outboards commonly run 1.62:1, 1.75:1, or 2.00:1, while Yamaha commonly runs 1.75:1, 2.00:1, or 2.33:1, with other ratios on specific models. The preset buttons above fill in these typical values, but always confirm against your gearcase stamp or owner's manual.
Is prop slip the same as cavitation?
No. Slip is a normal, continuous efficiency loss present in every propeller, while cavitation is water boiling on the blade surface from low pressure, and ventilation is air drawn down to the blades. Cavitation and ventilation cause sudden RPM spikes and can push slip far above 20 percent, so a very high slip reading can be a symptom of them, but the two concepts are different.
Why does slip change with load, trim, or altitude?
Slip rises when the boat is heavier or draggier because the prop must work harder for the same speed, and it changes with engine height and trim angle because both alter how cleanly water feeds the blades. At high altitude the engine makes less power, so RPM and speed fall while the pitch stays the same, which usually raises measured slip. Test in your normal loaded configuration for a meaningful number.
Can prop slip be zero?
Effectively never in real boating. Even a perfectly matched propeller loses some distance to water slip, blade drag, and hub turbulence, so zero percent usually indicates bad data rather than a miracle prop. Racing props with heavy cupping can dip into the low single digits, but anything at or below zero deserves a re-check of RPM, pitch, and speed inputs.
How accurate is this calculator?
The math is exact: it implements the industry-standard theoretical speed and slip formulas with no rounding until the final display. Accuracy of the result therefore depends on your inputs, mainly RPM (use a calibrated tachometer), pitch (stamped pitch can vary by plus or minus 1 inch between manufacturers), and speed (GPS on calm water). The model does not account for blade area, cupping, rake, or hull drag, so treat reverse-mode pitch estimates as starting points for sea trials.
Free tool by NRTechStudio