Prop Slip Calculator — Measured Run Comparison
Calculate apparent propeller slip from pitch, gear ratio, RPM and measured speed. Keep multiple runs, compare setups and save your measurement record.
Check a measured run, then compare your propeller setups using actual RPM and speed observations.
Setup A
Run A1
Measured run
Apparent slip · setup A, run 1
- Observed speed
- 33 mph
- Geometric speed
- 36.22 mph
- Engine / propeller RPM
- 5,100 / 2,550
- Marked pitch
- 15 in
- Advance per turn from this speed
- 347.11 mm
Faster than geometric speed: . Check measured speed and RPM, gear ratio, current and actual effective pitch. A negative result is retained; it is not extra propulsion efficiency.
Dots are entered observations. Dashed lines show geometric speed at the entered pitch and ratio; they do not predict boat performance.
Compare at the same engine RPM
Add another measured setup to compare.
Slip here is apparent slip relative to the entered pitch. It is not propeller efficiency or a ranking of propeller quality. GPS measures speed over ground; current can change the result. Compare runs with similar load, trim and conditions, and check your tachometer, ratio and effective pitch when values look unusual.
Full measurement record
Setup A · 15 in · ratio 2:1
Run 1: 5,100 engine RPM · 33 mph observed · 36.22 mph geometric · 8.9% apparent slip
Calculation, scope and sources
Propeller RPM = engine RPM ÷ gear ratio. Geometric speed = pitch × propeller RPM. Apparent slip = 100 × (1 − observed speed ÷ geometric speed). Pitch and speed are converted to metres and metres per minute for the calculation.
Enter the engine-to-propeller ratio as a number: for 2:1, use 2. Supported input ranges: pitch 1–2000 mm, ratio 0.1–100, engine 1–50,000 RPM, speed 0–150 m/s. These are calculator input limits, not operating limits. Zero boat speed is valid; zero RPM, pitch or ratio is not.
Different RPM observations are kept separate. Matching RPM comparisons retain repeated runs and never average them automatically. This tool does not select a replacement propeller, predict the RPM change from a new pitch, remove current from GPS measurements, or assess safe equipment operation. Use the RPM limits in your engine manual.
