Use this free propeller pitch calculator to estimate boat speed from pitch, or estimate the pitch you need for a target speed. It is useful for outboards, sterndrives, and many inboard setups when comparing prop options.
What is propeller pitch?
Propeller pitch is the theoretical distance (in inches) a prop would travel forward in one full revolution with no slip. A 21-pitch prop is designed to advance about 21 inches per turn in an ideal medium. In real water conditions, drag and turbulence reduce that advance, which is why slip is included in the calculation.
How this propeller pitch calculator works
1) Speed from pitch
If you know your current prop pitch, RPM, gear ratio, and estimated slip, this calculator predicts your boat speed in:
- Miles per hour (mph)
- Kilometers per hour (km/h)
- Knots
2) Required pitch from target speed
If you have a target speed, the calculator rearranges the same formula and solves for pitch. It also suggests a rounded whole-number pitch, which is handy because many props are sold in 1-inch increments.
Understanding slip and why it matters
Slip is the percentage difference between theoretical and real-world forward travel. It is not automatically bad; every propeller setup has some slip. The goal is to keep slip in a healthy range for your hull type and performance objective.
- Low slip (5%–10%): often seen on optimized performance setups.
- Moderate slip (10%–15%): common for many recreational boats.
- Higher slip (15%+): may indicate heavy load, ventilation, poor prop match, or setup issues.
Example calculation
Suppose your engine runs at 5800 RPM, gear ratio is 1.87, prop pitch is 21 inches, and slip is 10%. The predicted speed is approximately:
Speed = (5800 × 21 × 0.90) ÷ (1.87 × 1056) ≈ 55.5 mph
That gives you a practical baseline before water testing and GPS verification.
Tips for choosing propeller pitch
For better top speed
- Consider increasing pitch slightly (if your engine still reaches recommended WOT RPM).
- Confirm slip is not excessively high at speed.
- Track results with consistent load, fuel level, and water conditions.
For better acceleration / hole shot
- Consider dropping pitch 1–2 inches.
- Useful for towing sports, heavy crew loads, or quicker planing.
- Make sure engine RPM does not exceed manufacturer redline.
Important real-world factors
This calculator is a planning tool. Actual results can vary with prop design, blade count, rake, cup, diameter, hull condition, altitude, water temperature, current, and weight distribution. Use GPS runs in opposite directions and average results for best accuracy.
Quick FAQ
Is higher pitch always faster?
No. Higher pitch can reduce RPM too much, which may lower speed if the engine falls below its power band.
What if I do not know my slip?
Start with 10% to 12% for a rough estimate, then refine after comparing calculated speed with GPS speed.
Can I use this for airplanes or drones?
This specific formula and constant (1056) are for marine speed in mph using inches of pitch and marine drivetrain ratios. Aviation applications use different assumptions and performance models.