⚓ Prop Pitch Calculator
Calculate Propeller Speed
Typical slip: 8-15% for planing boats, 15-25% for displacement boats
How to Use This Calculator
Enter Propeller Pitch
Input the propeller pitch in inches. This is typically stamped on the prop hub (e.g., "19P" means 19 inches of pitch).
Enter Engine RPM
Enter the engine RPM at which you want to calculate speed. This is typically your cruising or maximum RPM.
Enter Prop Slip (Optional)
Enter the expected prop slip percentage. If unknown, use 10-15% for most boats. Leave at 0% to see theoretical speed only.
Calculate Speed
Click calculate to get theoretical and actual boat speeds in multiple units (mph, knots, km/h).
Formula
Theoretical Speed = (Pitch × RPM × 60) / (12 × 5280)
Actual Speed = Theoretical Speed × (1 - Slip/100)
Where:
- Pitch = Propeller pitch (inches)
- RPM = Engine revolutions per minute
- Slip = Propeller slip percentage
- 60 = Minutes to seconds conversion
- 12 = Inches to feet conversion
- 5280 = Feet to miles conversion
Example Calculation
If you have:
- Prop pitch: 19 inches
- Engine RPM: 5000
- Prop slip: 10%
Calculation:
Theoretical = (19 × 5000 × 60) / (12 × 5280)
Theoretical = 5,700,000 / 63,360 = 89.96 mph
Actual = 89.96 × (1 - 0.10) = 89.96 × 0.90 = 80.96 mph
About Prop Pitch Calculator
The Prop Pitch Calculator is a valuable tool for boat owners, marine mechanics, and engine tuners. It calculates boat speed based on propeller pitch, engine RPM, and propeller slip. Propeller pitch represents the theoretical distance a prop would move forward in one revolution if there were no slip. This calculator helps you understand the relationship between prop specifications and actual boat performance.
When to Use This Calculator
- Prop Selection: Determine the right propeller pitch for your boat and engine
- Performance Prediction: Estimate boat speed before purchasing a new propeller
- Engine Tuning: Verify that your current prop allows the engine to reach optimal RPM
- Speed Calculations: Calculate expected speeds at different RPM ranges
- Prop Sizing: Compare theoretical speeds of different pitch propellers
Why Use Our Calculator?
- ✅ Instant Results: Get speed calculations immediately
- ✅ Easy to Use: Simple interface requiring only pitch, RPM, and slip
- ✅ Multiple Units: Results displayed in mph, knots, and km/h
- ✅ 100% Free: No registration or payment required
- ✅ Accurate: Uses standard marine engineering formulas
- ✅ Educational: Shows both theoretical and actual speeds
Common Applications
Outboard Engines: Select the correct prop pitch to allow your outboard to reach its recommended WOT (wide open throttle) RPM range, typically 5,000-6,000 RPM.
Sterndrive Boats: Calculate speeds for I/O (inboard/outboard) systems to optimize prop selection for your boat's hull type and weight.
Performance Boats: Fine-tune propeller selection for maximum speed in racing or high-performance applications.
Fishing Boats: Balance between hole-shot (acceleration) and top speed by comparing different pitch options.
Tips for Accurate Results
- Prop slip typically ranges from 8-15% for planing hulls and 15-25% for displacement hulls
- Higher pitch = higher top speed but slower acceleration (hole-shot)
- Lower pitch = better acceleration but lower top speed
- Ensure your engine can reach its recommended WOT RPM with your prop selection
- Actual speeds may vary due to hull design, weight, water conditions, and engine condition
Frequently Asked Questions
What is propeller slip?
Prop slip is the difference between theoretical and actual boat speed, expressed as a percentage. It occurs because water is not a solid medium - the prop "slips" through the water. Typical slip is 8-15% for planing boats.
How do I find my propeller pitch?
Propeller pitch is usually stamped on the prop hub. Look for numbers like "19P" (19 inches pitch) or "21x14" (21 inches diameter, 14 inches pitch). The second number is the pitch.
What happens if I use the wrong pitch?
Too much pitch prevents the engine from reaching optimal RPM (lugging), reducing power and efficiency. Too little pitch allows over-revving, which can damage the engine. Always aim for the manufacturer's recommended WOT RPM range.
Does diameter affect speed?
Diameter primarily affects the amount of water the prop moves, not the theoretical speed. However, it can affect efficiency and actual speed through slip. Pitch is the main factor for speed calculations.
Why is my actual speed lower than calculated?
Actual speed depends on many factors: hull design, boat weight, water conditions, engine condition, trim angle, and propeller condition. The calculation gives theoretical speed; actual speed will always be lower due to these factors.