⚓ Prop Slip Calculator
Calculate Propeller Slip
How to Use This Calculator
Enter Propeller Pitch
Input the propeller pitch in inches. This is typically stamped on the prop hub.
Enter Engine RPM
Enter the engine RPM at which you measured the actual boat speed.
Enter Actual Speed
Input the actual boat speed you measured (preferably with GPS) in mph.
Calculate Slip
Click calculate to get the propeller slip percentage and theoretical speed.
Formula
Theoretical Speed = (Pitch × RPM × 60) / (12 × 5280)
Prop Slip = ((Theoretical Speed - Actual Speed) / Theoretical Speed) × 100
Where:
- Pitch = Propeller pitch (inches)
- RPM = Engine revolutions per minute
- Actual Speed = Measured boat speed (mph)
Example Calculation
If you have:
- Prop pitch: 19 inches
- Engine RPM: 5000
- Actual speed: 80 mph
Calculation:
Theoretical = (19 × 5000 × 60) / (12 × 5280) = 89.96 mph
Slip = ((89.96 - 80) / 89.96) × 100
Slip = (9.96 / 89.96) × 100 = 11.1%
About Prop Slip Calculator
The Prop Slip Calculator is an essential tool for boat owners and marine mechanics to evaluate propeller efficiency. Propeller slip is the difference between the theoretical forward distance a propeller should travel (based on pitch and RPM) and the actual distance the boat travels. Understanding prop slip helps diagnose performance issues and optimize propeller selection.
When to Use This Calculator
- Performance Evaluation: Assess propeller efficiency and boat performance
- Prop Selection: Compare different propellers to find the most efficient option
- Troubleshooting: Diagnose performance problems related to propeller condition
- Optimization: Fine-tune prop selection for better speed and fuel efficiency
- Maintenance: Monitor prop condition over time to detect damage or wear
Why Use Our Calculator?
- ✅ Instant Results: Get slip percentage calculations immediately
- ✅ Easy to Use: Simple interface requiring only pitch, RPM, and actual speed
- ✅ Accurate: Uses standard marine engineering formulas
- ✅ 100% Free: No registration or payment required
- ✅ Educational: Shows both theoretical and actual speeds for comparison
- ✅ Diagnostic Tool: Helps identify performance issues
Common Applications
Performance Boats: Evaluate propeller efficiency in racing or high-performance applications where every percentage point matters.
Fishing Boats: Optimize prop selection to balance speed, fuel economy, and hole-shot performance.
Recreational Boating: Check if your current propeller is providing optimal performance and fuel efficiency.
Marine Mechanics: Diagnose propeller-related performance issues during boat servicing and troubleshooting.
Tips for Accurate Results
- Use GPS speed for most accurate actual speed measurements
- Typical slip ranges: 8-15% for planing boats, 15-25% for displacement hulls
- Very low slip (<5%) may indicate over-propped condition (engine can't reach WOT RPM)
- Very high slip (>25%) may indicate under-propped condition or prop damage
- Measure speed in calm conditions for most consistent results
Frequently Asked Questions
What is a good prop slip percentage?
For planing boats, 8-15% slip is considered good. For displacement hulls, 15-25% is normal. Lower slip generally indicates better efficiency, but too low may mean the engine is lugging.
Why is my prop slip high?
High slip can be caused by: damaged or worn propeller, incorrect prop selection, hull fouling, engine not reaching WOT RPM, or hull design issues. Check these factors to improve slip.
Can slip be zero or negative?
No, slip cannot be zero in a real-world scenario because water is a fluid medium. Negative slip would mean the boat is traveling faster than theoretical speed, which is physically impossible with a propeller.
Does prop condition affect slip?
Yes! Damaged, bent, or worn propellers will have significantly higher slip. Regular inspection and maintenance of your propeller is essential for optimal performance.
How often should I check prop slip?
Check prop slip periodically, especially after prop repairs, when changing props, or if you notice performance degradation. Seasonal checks help track performance changes over time.