Boat Propeller Slip & Pitch Calculator
Enter a GPS speed, engine RPM, gear ratio, and propeller pitch to calculate measured propeller slip. Then compare another pitch or work backward from a target speed.
Measured propeller slip17.3%
- Propeller-shaft RPM
- 2,688
- Current theoretical speed at zero slip (mph)
- 48.37
- Candidate-pitch estimated speed (mph)
- 48.11
- Estimated speed change (mph)
- 8.11
- Pitch required for target speed (rounded to 0.25 in)
- 19.75
- RPM required with candidate pitch
- 4,677
- Measurement check
- No measurement warning.
Candidate calculations hold engine RPM and planning slip constant; a pitch change can alter both.
Target pitch is calculated without rounding, then displayed to the nearest quarter-inch.
How to use this calculator
- Choose mph, knots, or km/h, and use that same unit for both speed entries.
- Enter the engine RPM, reduction ratio, current stamped pitch, and steady GPS speed from one test run.
- Review measured slip and the measurement check before using it as a planning assumption.
- Enter a candidate pitch to estimate its speed at the same engine RPM and assumed slip.
- Enter a target speed to see the required pitch and RPM estimate.
How propeller slip and pitch are calculated
A propeller's pitch is its nominal forward travel in one revolution through a solid medium. Water moves, bends, and escapes around the blades, so a boat travels less than that no-slip distance. Propeller slip is the difference between the calculated no-slip speed and the speed your GPS records.
The calculator first finds propeller-shaft RPM, then applies the selected speed-unit constant:
N = R / G
V0 = (R x P) / (G x C)
Sm = 100 x [1 - (Va x G x C) / (R x P)]
- R is engine RPM and G is engine revolutions per propeller-shaft revolution.
- P is propeller pitch in inches, N is shaft RPM, and V0 is no-slip speed.
- Va is actual GPS speed and Sm is measured slip as a percentage.
- C is 1,056 for mph, 1,215.2 for knots, or 656 for km/h.
For the candidate-propeller estimate, the tool applies your planning slip: Vc = [R x Pc x (1 - Sp / 100)] / (G x C). It holds RPM and slip fixed so you can isolate the effect of pitch. For a target speed, it reverses the same relationship: Pt = (Vt x G x C) / [R x (1 - Sp / 100)] and Rt = (Vt x G x C) / [Pc x (1 - Sp / 100)].
What moves the result most
Use RPM and GPS speed taken together, ideally in a repeatable wide-open-throttle or cruise test. The gear ratio must be entered as engine revolutions per one propeller-shaft revolution, such as 1.86 for a 1.86:1 reduction. A reversed ratio makes the theoretical speed and slip misleading. Pitch also has a direct, proportional effect on calculated speed: at unchanged RPM and slip, a 21-inch propeller predicts about 10% more speed than a 19-inch propeller.
Negative calculated slip does not mean the boat is creating free speed. It usually points to an input mismatch, current, GPS or tachometer error, or a nominal pitch that differs from the propeller's effective pitch. Very high slip can be normal before planing, but it is not a useful top-speed comparison.
What this calculator leaves out
This is a kinematic estimate, not a full propeller-selection model. It does not model diameter, blade count, cup, rake, material, ventilation, cavitation, hull condition, load, trim, current, wind, or elevation. Changing pitch changes engine load and may change both attainable RPM and slip, so treat the candidate result as a what-if estimate and confirm acceptable wide-open-throttle RPM with the engine maker or a qualified propeller shop.
Worked example
Suppose the engine turns 5,000 RPM through a 1.86:1 gear ratio with a 19-inch propeller, and GPS speed is 40 mph. The propeller shaft turns 5,000 / 1.86 = 2,688.17 RPM. Its no-slip speed is (5,000 x 19) / (1.86 x 1,056) = 48.37 mph, giving measured slip of 100 x (1 - 40 / 48.37) = 17.30%.
With a 21-inch candidate pitch and 10% planning slip, estimated speed is [5,000 x 21 x 0.90] / (1.86 x 1,056) = 48.11 mph, or 8.11 mph faster than the entered run. For a 45 mph target at the same RPM and slip, required pitch is 19.64 inches, displayed as 19.75 inches to the nearest quarter-inch. With the 21-inch candidate, the target calls for about 4,677 RPM.
Common questions
What is propeller slip?
Propeller slip is the percentage difference between a propeller’s calculated no-slip speed and the boat’s GPS speed. It describes how much less distance the boat covers than nominal pitch would suggest; it is not the same thing as overall propeller efficiency.
How do I calculate prop slip from GPS speed and RPM?
First calculate no-slip speed from engine RPM, pitch, gear ratio, and the selected speed-unit constant. Divide GPS speed by that no-slip speed, subtract the result from one, and multiply by 100. Use GPS speed and tachometer RPM recorded at the same steady condition.
What gear ratio should I enter?
Enter the engine-to-propeller reduction ratio shown in the engine or drive specifications. For a 1.86:1 ratio, enter 1.86 because the engine makes 1.86 revolutions for each propeller-shaft revolution. Do not enter the reciprocal unless the manufacturer states the ratio in that convention.
Why can calculated propeller slip be negative?
Negative slip means the entered GPS speed exceeds the calculated no-slip speed. Check the units, GPS speed, tachometer, gear-ratio convention, current, and pitch marking first. Effective pitch can also differ from the stamped nominal pitch, especially with cup, repair, or modification.
How does increasing propeller pitch affect speed and RPM?
A higher-pitch propeller travels farther per revolution in the simple calculation, but it also increases engine load. If the engine can hold the same RPM and slip, speed rises roughly in proportion to pitch. In a real boat, RPM often falls with more pitch, so verify wide-open-throttle RPM after any change.
Should I use cruise-speed or wide-open-throttle measurements?
Either can calculate slip if RPM and GPS speed come from the same stable run, but compare like with like. Wide-open-throttle observations are commonly used to check propeller selection because engine makers specify an acceptable RPM range there. Cruise readings can be useful for efficiency and load comparisons.