Propeller Pitch & Slip Calculator ⚙️
Calculate propeller slip percentage and cavitation risk. Enter pitch, diameter, RPM, and gear ratio to analyze propulsion efficiency.
How to Use the Propeller Pitch & Slip Calculator
Enter propeller pitch (inches, from hub stamp), diameter, engine RPM, gear ratio, and GPS actual speed. The calculator outputs theoretical speed, slip %, and cavitation risk from tip speed.
Slip of 5–15% is normal for recreational planing boats. High slip (> 25%) may indicate hull fouling, prop damage, or incorrect pitch. Negative slip means actual speed exceeds theoretical — check GPS calibration.
Cavitation risk is based on tip speed (π × diameter × shaft RPM / 12). Below 9,000 ft/min is safe. Above 14,000 ft/min (critical) causes blade erosion and thrust loss — consider a larger diameter or lower RPM prop.
Boat & Marine Calculator - Introduction
Enter propeller pitch, diameter, RPM, gear ratio, and your actual GPS speed to calculate slip percentage and check cavitation risk. This helps you diagnose whether your current prop is a good match for your hull and engine, or whether re-pitching would improve fuel efficiency and top speed.
How It Works
Theoretical speed (knots) = pitch (in) × RPM / (gear ratio × 1215.2). Slip % = (theoretical − actual GPS) / theoretical × 100. Tip speed (ft/min) = π × diameter (in) × shaft RPM / 12. Cavitation thresholds: below 9,000 fpm is safe; 9,000–11,000 is caution; 11,000–14,000 is risk; above 14,000 fpm is in the danger zone. Example: 19-in pitch, 5000 RPM, 2.08:1 ratio → theoretical speed ≈ 37.6 knots; if GPS shows 33 knots, slip ≈ 12%, which is normal.
Usage Scenarios
- Post-season prop inspection: A runabout owner measures 18% slip—higher than last season's 12%. Entering the same pitch, RPM, and GPS speed confirms the gap, pointing to a bent blade or hull fouling rather than a prop-size mismatch.
- Checking cavitation before a race: A performance boat runs a 15-in diameter prop at 6000 shaft RPM. Tip speed = π × 15 × 6000 / 12 = 23,562 fpm—well into the danger zone. The owner swaps to a 14-in four-blade to bring tip speed below 11,000 fpm while holding similar pitch.
- Matching a new prop to a twin-engine setup: A twin-inboard owner re-engines and needs to verify both props run within 2% slip of each other for straight tracking. Entering each prop's pitch and comparing theoretical vs. GPS speed for each shaft confirms they are matched to within 1.4%.
Frequently Asked Questions
What is propeller slip and how is it calculated?
Propeller slip is the difference between theoretical speed (pitch × RPM / gear ratio / 1215.2) and actual GPS speed. Slip % = (theoretical − actual) / theoretical × 100. Normal recreational boats run 5–15% slip.
What causes high propeller slip?
High slip (> 25%) is caused by hull fouling, propeller damage, incorrect pitch for load, or heavy displacement. Clean hull regularly, inspect prop blades for nicks, and consider re-pitching.
What is cavitation and how does tip speed affect it?
Cavitation occurs when propeller tip speed creates low-pressure bubbles that implode on the blade surface, causing damage and loss of thrust. Tip speed = π × diameter × shaft RPM / 12. Below 9,000 ft/min is safe; above 14,000 ft/min is critical.
What is the ideal propeller slip for recreational boats?
5–15% slip is ideal for most recreational planing boats. Displacement hulls typically run 10–20%. Racing props can achieve 3–8%. Slip outside these ranges warrants inspection.