Prop Slip Calculator

What Is Prop Slip?

Propeller slip expresses the difference between a propeller's theoretical pitch-based speed and the boat's actual measured speed as a percentage of theoretical speed. Because a propeller operates in water rather than a solid medium, its actual advance is lower than its theoretical pitch-based advance. Prop slip provides a useful way to compare theoretical and measured performance under specific operating conditions.

Prop Slip Formula

These equations are based on established propeller-slip calculation methods and are implemented independently in this calculator. The calculator uses the following equations:

Theoretical Speed = (RPM × Pitch) / (Gear Ratio × 1056)

Prop Slip % = ((Theoretical Speed − Actual Speed) / Theoretical Speed) × 100

How to Calculate Prop Slip

Enter your engine RPM, gear ratio, propeller pitch in inches, and actual GPS speed. The calculator first determines theoretical propeller speed and then compares it with your actual speed to calculate prop slip.

What Is a Good Prop Slip Percentage?

There's no single "correct" prop slip percentage. It depends on the boat, hull type, load, propeller design, and operating conditions. Use prop slip primarily as a comparison tool between consistent test conditions or known-good setups rather than as a universal pass/fail target.

How to Check for Prop Slip

To check prop slip out on the water, run your boat at a steady RPM appropriate for the test and note the RPM along with your GPS speed at that moment. Enter those two numbers here with your gear ratio and propeller pitch. Checking the same setup on different days, or before and after a prop change, is the most useful way to use this measurement.

Why Does Prop Slip Matter?

Prop slip is one useful indicator when evaluating a propeller and boat setup. Unusually high or low results can be a reason to investigate factors such as propeller pitch, propeller condition, hull characteristics, boat load, trim, RPM measurement, or operating conditions.

Can Prop Slip Be Negative?

A negative calculated slip means that the measured GPS speed is higher than the theoretical speed calculated from the supplied inputs. Before interpreting this result, check the RPM, gear ratio, propeller pitch, and GPS speed. Current, following seas, and measurement differences can also affect the comparison. A negative result should therefore be treated as a reason to verify the inputs and test conditions rather than as a direct measure of propeller efficiency.

Does Boat Load Affect Prop Slip?

Yes. Additional fuel, gear, passengers, or other load changes the operating conditions of the boat and can affect measured speed and prop slip. For meaningful comparisons, test the same setup under as similar a load and operating conditions as possible.

Can Prop Slip Determine the Correct Propeller Size?

No. Prop slip is one useful measurement when evaluating a propeller setup, but it should not be used alone to select propeller size. Engine operating range, manufacturer recommendations, hull characteristics, load, pitch, diameter, blade design, and sea-trial results should also be considered.

What Causes High Prop Slip?

High calculated slip can result from many factors, including propeller selection, hull characteristics, boat load, trim, propeller condition, ventilation, or measurement conditions. A high result is best used as a reason to investigate the complete setup rather than as a diagnosis by itself.

Does Propeller Pitch Affect Prop Slip?

Propeller pitch directly affects theoretical speed in the calculation, but a change in pitch can also change the engine's operating RPM and the way the propeller loads the boat. Slip should therefore be evaluated using measured RPM and speed for each propeller setup.