MasterTool

Boat Prop Slip Calculator

Calculate propeller slip from pitch, RPM, gear ratio and your GPS speed.

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What this tool does

Calculate propeller slip from pitch, RPM, gear ratio and your GPS speed. Uses the stamped nominal pitch. Cupping, blade design and tachometer/GPS error all shift the result a few points.

How to use the Boat Prop Slip Calculator

  1. Enter or select engine rpm.
  2. Enter or select gear ratio (engine : prop, e.g. 1.5).
  3. Enter or select propeller pitch (inches).
  4. Enter or select actual speed (mph, gps).
  5. Read the calculated result; change any measurement to compare alternatives.

Formula

prop RPM = engine RPM ÷ gear ratio; theoretical mph = pitch × prop RPM ÷ 1056; slip % = (theoretical − actual) ÷ theoretical × 100
rpm
Engine RPM
gearRatio
Gear ratio (engine : prop, e.g. 1.5)
pitch
Propeller pitch (inches)
actualSpeed
Actual speed (mph, GPS)

Uses the stamped nominal pitch. Cupping, blade design and tachometer/GPS error all shift the result a few points.

Worked example

For boat prop slip calculator, the following measurements illustrate the exact method: Engine RPM: 5000; Gear ratio (engine : prop, e.g. 1.5): 1.5; Propeller pitch (inches): 19; Actual speed (mph, GPS): 45.

Inputs

  • Engine RPM5000
  • Gear ratio (engine : prop, e.g. 1.5)1.5
  • Propeller pitch (inches)19
  • Actual speed (mph, GPS)45

Result

  • Prop slip (%)24.97
  • Theoretical speed (mph)59.97
  • Propeller RPM3,333.33

Results explained

Prop slip (%)
Prop slip (%) from the formula above. Uses the stamped nominal pitch. Cupping, blade design and tachometer/GPS error all shift the result a few points.
Theoretical speed (mph)
Theoretical speed (mph) from the formula above. Uses the stamped nominal pitch. Cupping, blade design and tachometer/GPS error all shift the result a few points.
Propeller RPM
Propeller RPM from the formula above. Uses the stamped nominal pitch. Cupping, blade design and tachometer/GPS error all shift the result a few points.

Frequently asked questions

Theoretical speed = pitch (in) × prop RPM ÷ 1,056 (mph). Slip = (theoretical − actual) ÷ theoretical × 100. A 19-inch prop at 3,333 prop RPM theoretically makes about 60 mph; at a real 45 mph the slip is about 25%.

Unit conversion: pitch in inches per revolution × revolutions per minute gives inches per minute; dividing by 1,056 (63,360 inches per mile ÷ 60 minutes) converts to mph.

Roughly 5–15% for a well-matched planing setup at cruise/WOT; very high slip suggests wrong pitch, damage, ventilation or a heavy load, while near-zero or negative readings usually mean the tachometer or pitch figure is off.

The prop turns slower than the crankshaft by the gear ratio. Enter engine RPM and the ratio (e.g. 1.5 : 1) and the tool divides before applying the pitch formula.

The maths allows it if actual speed exceeds the nominal-pitch prediction — some props' effective pitch exceeds their stamped pitch. Treat small negatives as measurement noise.