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 We Calculate This
This propeller calculator uses established formulas and industry-standard data to provide accurate estimates.
- Enter your specific values into the calculator fields above
- Our algorithm applies the relevant formulas using your inputs
- Results are calculated instantly in your browser — nothing is sent to a server
- Review the detailed breakdown to understand how each factor affects your result
These calculations are estimates based on standard formulas. For critical decisions, always consult a qualified professional.
How to Convert Oven Recipes to Air Fryer
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.
When Would You Use This Calculator?
This propeller calculator is designed for anyone who needs quick, reliable estimates without complex spreadsheets or professional consultations.
- When you need a quick estimate before committing to a purchase or project
- When comparing different options or scenarios side by side
- When planning a budget and need to understand potential costs
- When you want to verify a quote or estimate you've received from a professional
- When teaching or learning about the concepts behind these calculations
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.