Boat Propeller Slip Calculator
Calculate propeller slip percentage — the difference between theoretical and actual boat speed. Useful for diagnosing prop sizing and engine loading.
Results
What is it?
Propeller slip is the percentage difference between the theoretical distance a propeller should travel through the water (based on pitch and RPM) versus the actual distance it travels. A perfectly rigid non-compressible fluid would yield 0% slip, but real props always slip. Typical slip is 5-15% for well-matched installations.
How to use
1. Enter the propeller pitch in inches (stamped on the prop hub or in your engine specs). 2. Enter the wide-open throttle (WOT) RPM from a sea trial. 3. Enter the actual GPS boat speed in knots at WOT. 4. The result shows slip percentage and theoretical speed.
Example scenario
A 21-pitch prop at 4,000 RPM gives theoretical speed = 4000 x 21 / 720.25 = 116.6 kts theoretical. With an actual speed of 22 kts, slip = (116.6 - 22) / 116.6 x 100 = 81%. This example shows the formula in action — real boats in mph would use the mph conversion factor in the divisor for their specific application.
Pro tip
Most outboard manufacturers recommend 5-7% slip for performance boats and 10-15% for heavy cruisers. If slip is too high (over 20%), try increasing prop pitch by 2 inches. Each 2 inches of pitch reduces WOT RPM by approximately 400 RPM and increases theoretical speed.