Maker Label Studio

Boat Propeller Slip Calculator

Estimate marine propeller slip, theoretical speed, pitch, or speed using RPM, gear ratio, prop pitch, and measured boat speed.

Formula: theoretical speed = (RPM / gear ratio) x pitch(in) x 60 / 63360 (mph); slip% = (theoretical - actual) / theoretical x 100. Cited standard/category: Transport & Maritime.

Inputs

Choose the calculation mode and enter the known values.

required
required
required
required
for pitch/speed

Results

Report values are rounded for practical use.

Ready
Theoretical speed
-- mph / knots
Actual speed
-- mph / knots
Slip
-- typical healthy range: 10-30%
Prop pitch
-- inches

Enter values to calculate the report.

Input summary

Field Value

Result summary

Metric Value

Self-tests

Golden checks cover the core theoretical-speed and slip formulas.

Self-tests have not been run.

About the Boat Propeller Slip Calculator

Boat propeller slip compares theoretical advance from pitch, RPM, and gear ratio with actual GPS speed to show how efficiently a prop is moving the hull. Boaters, mechanics, and prop shops use it to diagnose setup, trim, ventilation, load, and pitch choices without treating one run as a complete performance test.

How it works

  1. Enter propeller pitch, engine RPM, and gear ratio.
  2. Add actual boat speed from GPS or a calibrated speed source.
  3. Choose units for pitch and speed.
  4. Review theoretical speed, actual speed, and slip percentage.

Frequently asked questions

What does boat propeller slip percentage mean?

It is the difference between theoretical propeller advance and actual boat speed, expressed as a percentage of theoretical speed. Some slip is normal because water is not a solid medium.

What is a normal prop slip number?

Normal slip varies widely by hull type, speed range, propeller design, load, and trim. Displacement boats, planing hulls, and high-performance setups should not be judged by one universal target.

Why can prop slip calculate as negative?

Negative slip usually means one input is wrong, such as gear ratio, tachometer reading, pitch, unit conversion, or speed measurement. It can also occur when effective pitch differs from nominal pitch.

Should I use speedometer or GPS speed?

GPS speed over ground is usually better for this calculation, but current and wind can still affect a test run. Make passes in opposite directions when accuracy matters.

Does changing prop pitch always improve slip?

No. Pitch changes also affect engine RPM, load, acceleration, and ability to reach the recommended wide-open-throttle range. Slip should be evaluated with the whole setup.

References