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.
FAQ
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.