Propeller Pitch Speed Calculator
A propeller acts like a screw: in one revolution it tries to advance a fixed distance called the pitch. From engine RPM, the gearcase reduction ratio, and the prop pitch, you can estimate the theoretical forward speed, then subtract a realistic slip percentage to approximate real boat speed. This is a quick way to check propping before a sea trial or to predict the effect of a pitch change. Enter your engine RPM, gear ratio, pitch in inches, and slip, and the calculator returns propeller RPM, theoretical speed, and slip-adjusted speed.
Propeller speed formula
Prop RPM = engine RPM / gear ratio
Theoretical mph = prop RPM x pitch(in) x 60 / 63,360
Adjusted mph = theoretical mph x (1 - slip/100)
Knots = mph x 0.8690
There are 63,360 inches in a mile and 60 minutes in an hour. Pitch is the inches advanced per prop revolution; multiplying by prop RPM gives inches per minute, which converts to miles per hour. Slip reduces the result to a realistic figure.
Propping notes
- Slip near 10% suits efficient displacement hulls; 20% to 30% is common for planing and loaded boats.
- Lower pitch raises RPM and acceleration; higher pitch raises top speed if the engine can still reach rated RPM.
- Always confirm the engine reaches its wide-open-throttle RPM range with the chosen prop.
- Verify the gear ratio from the engine specification, not a guess.
- Compare the result with GPS speed to estimate your boat's true slip.
Propeller pitch speed: frequently asked questions
What is propeller pitch?
Pitch is the theoretical distance a propeller would travel forward in one full revolution if it moved through a solid, like a screw through wood. A 21-inch pitch prop would advance 21 inches per turn with no slip. Pitch is stamped on the prop hub alongside the diameter.
How is theoretical speed calculated?
Propeller RPM equals engine RPM divided by the gear ratio. Distance per minute equals propeller RPM times pitch. Convert inches per minute to miles per hour, then subtract slip. The formula is speed (mph) = (engine RPM / gear ratio) x pitch (in) x 60 / 63,360, reduced by the slip percentage.
What is propeller slip?
Slip is the difference between the theoretical distance a propeller should advance and the distance the boat actually moves, expressed as a percentage. Water is not solid, so a prop always slips. Typical slip ranges from about 10% for efficient displacement hulls to 30% or more for heavily loaded or planing boats.
Why is my actual speed lower than theoretical?
Actual speed is always lower because of slip, hull drag, wind, sea state, loading, and prop condition. The theoretical figure assumes a perfect screw with no losses. Use a realistic slip percentage for your hull type to get closer to real-world speed, then verify with GPS.
What gear ratio should I enter?
Enter the reduction ratio of your outboard or sterndrive gearcase, found in the engine specifications. A 1.85:1 ratio is entered as 1.85. The gear ratio reduces engine RPM to propeller RPM, so a higher ratio means a slower-turning, higher-thrust prop.
Official sources
- U.S. Coast Guard: Navigation Center.
- NOAA Office of Coast Survey: Nautical Charts.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.