Prop Pitch & Speed Calculator
Use this tool for boats and other prop-driven setups to estimate theoretical speed, real-world speed with slip, and the pitch required to hit a target speed.
What Is Prop Pitch?
Prop pitch is the theoretical forward distance a propeller would travel in one full revolution if there were no slip. A 19-inch pitch prop is designed to move forward 19 inches per turn in perfect conditions. In real life, water and air are not solid, so some energy is lost, which is why we account for slip.
How This Prop Pitch Calculator Works
This calculator uses common marine prop formulas and can help you answer practical questions quickly:
- How fast should my setup go with my current pitch?
- What is the difference between theoretical and actual speed?
- What pitch might I need to hit a specific target speed?
Core Formula
Theoretical Speed (mph) = (Prop Shaft RPM × Pitch in inches) / 1056
Actual Speed = Theoretical Speed × (1 − Slip%)
When target speed is entered, the calculator rearranges the formula to estimate required pitch for your current RPM, gear ratio, and slip assumptions.
How to Use It Correctly
1) Start With Real Numbers
Use observed wide-open-throttle RPM and confirmed gear ratio from your outboard or sterndrive manual. Incorrect gear ratio is one of the most common errors in pitch calculations.
2) Use a Realistic Slip Value
Slip is not a failure; it is normal. Different hull designs, load conditions, water temperature, trim angle, and prop style can change slip significantly. If you are unsure, start around 10% to 15% and test.
3) Compare Predicted and GPS Speed
Run the calculator, then compare with GPS speed over calm water. If the calculator is off, adjust the slip estimate until it aligns with your typical observed results. That becomes your baseline for future prop changes.
Choosing the Right Prop Pitch
If Pitch Is Too High
- Engine may not reach recommended RPM range.
- Holeshot and acceleration may feel sluggish.
- Engine can lug under heavy loads.
If Pitch Is Too Low
- Engine may over-rev and hit limiter too easily.
- Top speed may plateau early.
- Efficiency at cruise may drop.
A good prop choice keeps your engine in its manufacturer-recommended RPM window at wide-open throttle with normal load.
Practical Tuning Tips
- Change only one variable at a time (pitch, diameter, trim, or load).
- Track test runs in a small notebook or spreadsheet.
- Use consistent fuel load and passenger count when comparing props.
- Remember that cup and blade design can affect RPM similarly to pitch changes.
Important Notes
This calculator provides estimates, not guarantees. Water conditions, prop condition, altitude, hull cleanliness, and drivetrain efficiency all impact real performance. Use this tool as a planning guide and validate with on-water testing.