Boat prop calculator: speed, slip and pitch
Work out your boat speed from engine RPM, gear ratio and prop pitch, find your prop slip from a GPS reading, and see which pitch gets your motor into its best RPM range. Free, no sign-up, works on your phone.
1. Boat speed from your prop
Gear ratio is in your motor's manual or on the maker's spec sheet (for example 1.86:1 or 2.08:1). Prop pitch is the second number stamped on the prop, such as 14 x 21. If you don't know your slip, 10 to 15% is typical.
2. Find your prop slip
Use a GPS or phone speed, not the boat's pitot speedometer. Run both directions and average them to cancel out wind and current.
3. Which pitch should I run?
The rated full-throttle range is in your owner's manual (often something like 5,000 to 6,000 RPM). Test with a normal load and the motor trimmed for best speed.
4. Rough top speed from horsepower and weight
Uses Crouch's planing formula (speed in knots = C ÷ √(weight ÷ hp)) with the boat-type constants from Dave Gerr's Propeller Handbook. Loaded weight means boat, motor, fuel, gear and people. Treat it as a ballpark, not a promise.
How the math works
Speed = RPM × pitch ÷ (gear ratio × 1,056) × (1 − slip). The 1,056 turns inches per minute into miles per hour (63,360 inches in a mile ÷ 60 minutes).
Slip is the gap between how far the prop would move the boat in a perfect world and how far it really goes. Most runabouts and bass boats run about 10 to 20%. Heavy boats and pontoons often run higher.
Pitch is how far the prop would move forward in one turn, in inches. More pitch means more speed but lower RPM. Engine makers like Mercury say each inch of pitch changes full-throttle RPM by about 150 to 200. If your motor won't reach its rated RPM, drop pitch. If it runs past the top of the range, go up.
Questions
What is a good prop slip?
About 10 to 20% for most planing boats. Under 10% is very efficient. Over 30% usually means a damaged prop, ventilation, a spun hub or the wrong prop for the boat.
Why is my GPS speed lower than the calculator says?
Weight, trim, a dirty hull, waves and wind all add slip. Enter your real GPS speed in calculator 2 to see your true slip, then use that number in calculator 1.
Where do I find my gear ratio?
In the owner's manual or the engine maker's spec page for your model and horsepower. Common outboard ratios are between about 1.75:1 and 2.33:1.