What Is the Propeller Calculator?
The Propeller Calculator is a free online tool designed for boaters and sailors who need quick, accurate calculations in the boating and marine space. By entering your engine rpm, gear ratio, propeller pitch, you get instant results including theoretical speed, prop slip, prop rpm. No formulas to memorize, no spreadsheets to build — just enter your numbers and get the answer in seconds. Whether you're a beginner or experienced professional, this calculator saves you time and eliminates guesswork.
Why This Calculation Matters
Getting theoretical speed right can make the difference between success and costly mistakes. In boating and marine, small errors compound quickly. Manual calculations are error-prone and time-consuming, especially under pressure. This calculator applies proven formulas used by boaters and sailors worldwide, giving you confidence that your numbers are correct. Use it to navigate safely with precision and avoid common pitfalls that trip up beginners.
When Should You Use This Calculator?
This tool is most useful when you know your engine rpm and need to find the right theoretical speed. It's also great for quick estimates before committing to a decision, and to double-check manual calculations or professional quotes, and when comparing different scenarios side by side. Bookmark this page and come back whenever you need a fast, reliable answer — the calculator is always free and requires no signup.
Boat Propeller Calculator
Propeller Pitch vs. Theoretical Speed
Theoretical speed at various RPMs and pitches (gear ratio 1.92:1).
| Pitch | 4000 RPM | 4500 RPM | 5000 RPM | 5500 RPM |
|---|---|---|---|---|
| 13" | 25.6 mph | 28.8 mph | 32.0 mph | 35.2 mph |
| 15" | 29.6 mph | 33.3 mph | 37.0 mph | 40.6 mph |
| 17" | 33.5 mph | 37.7 mph | 41.9 mph | 46.1 mph |
| 19" | 37.5 mph | 42.1 mph | 46.8 mph | 51.5 mph |
| 21" | 41.4 mph | 46.6 mph | 51.8 mph | 56.9 mph |
| 23" | 45.4 mph | 51.0 mph | 56.7 mph | 62.4 mph |
| 25" | 49.3 mph | 55.5 mph | 61.6 mph | 67.8 mph |
How to Use This Calculator
- Enter Your Engine RPM (at WOT): Start by entering your engine rpm — this is the primary input for the calculation.
- Fill In Additional Details: Complete the remaining fields: gear ratio, propeller pitch, actual speed — for slip calc. Each value refines the calculation for greater accuracy.
- Click Calculate: Hit the Calculate button to run the numbers. Results appear instantly below.
- Review Your Results: Check your theoretical speed, prop slip, prop rpm. Use these figures to inform your next decision or compare against alternative scenarios.
How It Works
Propeller pitch is the theoretical distance (in inches) a prop moves forward in one revolution. Real-world performance is always less due to slip — the difference between theoretical and actual speed.
The basic rule:
- Prop RPM = Engine RPM / Gear Ratio
- Theoretical Speed (mph) = Prop RPM × Pitch / 1056 (1056 converts in/min to mph)
- Slip % = (Theoretical Speed − Actual Speed) / Theoretical Speed × 100
- Normal slip is 10-20%. Below 10% may indicate over-propping; above 20% suggests poor prop selection
Increasing pitch by 1 inch typically reduces WOT RPM by 150-200. Decreasing pitch raises RPM. Always ensure your engine reaches its recommended WOT RPM range for proper performance and longevity.
Tips & Considerations
- Double-check your engine rpm before calculating — even small input errors can significantly change your results.
- Run the calculator with different values to compare scenarios and find the optimal approach for your situation.
- Pay attention to both theoretical speed and prop slip — they work together to give you the full picture.
- Bookmark this page for quick access next time you need to navigate safely.
- If you're unsure about your actual speed — for slip calc, start with a conservative estimate and adjust from there.
Frequently Asked Questions
What does propeller pitch mean?
Pitch is the theoretical distance (in inches) a propeller advances in one complete revolution, assuming zero slip. A 19-pitch prop theoretically moves 19 inches forward per revolution. Higher pitch = more top speed but more load on the engine.
What is prop slip and what is normal?
Slip is the difference between theoretical and actual speed, expressed as a percentage. Normal slip ranges from 10-20%. Heavy boats have more slip, planing hulls have less. Damaged or wrong-sized props show abnormal slip.
How do I find my gear ratio?
Check your outboard or sterndrive owner's manual, or look up your engine model online. Common ratios: small outboards 2.0-2.33:1, mid-range 1.75-2.0:1, large outboards 1.75-1.86:1, sterndrives 1.47-2.0:1.
Should I go with higher pitch for more speed?
Only if your engine can still reach its recommended WOT RPM range with the higher pitch. Over-propping (too high a pitch) lugs the engine, reduces acceleration, and can cause premature wear.
How does diameter affect performance?
Larger diameter provides more thrust and is better for heavy boats. Smaller diameter allows higher RPM and suits lighter, faster boats. Diameter is usually matched to the engine's horsepower range.