Prop Slip Calculator
Calculate your boat's propeller slip in seconds.
How It Works
Enter Your Numbers
Type in your engine RPM, gear ratio, propeller pitch, and actual GPS speed.
Get Instant Results
Press Calculate Prop Slip and see your theoretical speed and slip percentage right away.
Share or Save
Generate a link to your result to save it or send it to your mechanic or dealer.
Who Uses This Calculator
Recreational Boat Owners
Dial in your outboard or sterndrive setup and compare results after a prop change.
Marine Mechanics & Techs
Quickly check a customer's setup or diagnose a prop-to-engine mismatch.
Boat Dealers & Surveyors
Document propeller performance as part of a sea trial or pre-purchase inspection.
Frequently Asked Questions
Straight answers about propeller slip, the formula behind it, and what your result actually means.
What Is Prop Slip?
Prop slip is the difference between your propeller's theoretical speed and your boat's actual speed, expressed as a percentage. A propeller doesn't move through water like a bolt through a nut — some slip is normal, and a propeller slip calculator like this one turns your RPM, gear ratio, and pitch into that theoretical speed so you can compare it against your real GPS speed.
Prop Slip Formula
The propeller slip formula used by this calculator is: Theoretical Speed = (RPM x Pitch) / (Gear Ratio x 1056). Prop Slip % = ((Theoretical Speed - Actual Speed) / Theoretical Speed) x 100.
Theoretical Speed = (RPM × Pitch) / (Gear Ratio × 1056)
Prop Slip % = ((Theoretical Speed − Actual Speed) / Theoretical Speed) × 100
How to Calculate Prop Slip
To use the prop slip calculator, enter your engine RPM, gear ratio, and propeller pitch in inches, then your actual GPS speed. The calculation runs on the server the moment you press Calculate Prop Slip, giving you the theoretical speed and prop slip percentage instantly — no sign-up, no extra steps.
What Is a Good Prop Slip Percentage?
There's no single "correct" prop slip percentage — it depends on the boat, hull type, load, and propeller design. Many recreational boats commonly land somewhere in a broad normal range, but a heavily loaded cruiser, a racing hull, and a pontoon boat won't share the same baseline. Treat prop slip as a comparison tool: track it over time or against known-good setups rather than chasing one fixed target number.
How to Check for Prop Slip
To check prop slip out on the water, run your boat at a steady RPM — often wide-open throttle or your normal cruising RPM — and note that 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 signal of how well your propeller matches your boat, engine, and load. Very low slip can mean the prop is over-pitched and lugging the engine, while unusually high slip can point to a prop that's too small, damaged, or poorly matched to the hull. It's a diagnostic clue to investigate further, not a pass/fail test on its own.
Can Prop Slip Be Negative?
Yes, occasionally. If your actual GPS speed comes out higher than the calculated theoretical speed, prop slip will show as a negative percentage. This can happen from GPS measurement variance, current or following seas pushing the boat, or a very efficient, low-slip setup — it doesn't necessarily indicate an error.
Does Boat Load Affect Prop Slip?
Yes. Added weight from fuel, gear, passengers, or a loaded livewell increases the load on the propeller, which typically increases slip. Comparing prop slip under a consistent, known load makes it a far more reliable reference point than checking it under different conditions each time.