Propeller Slip Calculator 🔄
Calculate propeller slip percentage by comparing theoretical speed to actual GPS speed. Diagnose hull fouling, prop damage, or incorrect pitch.
How to Use the Propeller Slip Calculator
Enter propeller pitch (in inches), engine RPM at cruise, gear ratio, and your actual speed from GPS. The calculator compares theoretical speed to actual speed to determine slip percentage.
Slip of 5–15% is normal for recreational boats. High slip may indicate a fouled hull, damaged propeller, or incorrect prop pitch for your application.
To reduce slip: keep the hull clean, check for propeller damage, and consider re-pitching or resizing the propeller. Consult a marine propeller specialist for persistent high-slip issues.
Boat & Marine Calculator - Introduction
Compare the speed your propeller should be producing with the speed your GPS actually reads, and you get propeller slip — the gap between theoretical thrust and water grip. A consistent baseline lets you tell whether a slow boat is fouled, mis-pitched, or just loaded heavy, instead of guessing.
How It Works
Theoretical speed (knots) = pitch (in) × engine RPM ÷ (gear ratio × 1215.2). Slip % = (theoretical − actual GPS speed) ÷ theoretical × 100. A 19" pitch turning 4,400 RPM through a 1.85:1 gearbox produces a theoretical 37.2 knots; if your GPS shows 32 knots, slip = (37.2 − 32) ÷ 37.2 × 100 ≈ 14%.
Usage Scenarios
- Routine end-of-season benchmark: Note slip at WOT in calm water. If it has crept from 12% in spring to 22% by late summer, schedule a hull clean — that delta is barnacles and slime, not engine wear.
- Diagnosing a new vibration: After a grounding, slip jumps from 14% to 28% at the same RPM and load. The math points to a bent blade or chunk missing from the trailing edge — pull the prop and have it scanned.
- Picking the right pitch after repowering: New engine reaches only 5,200 RPM (rated 5,800) and slip is just 4%. The prop is over-pitched; drop one inch of pitch to let the engine spin into its powerband and restore slip into the 10–15% sweet spot.
Frequently Asked Questions
What is propeller slip and why does it matter?
Propeller slip is the difference between your boat's theoretical speed (based on prop pitch and RPM) and actual speed through the water. Some slip is normal (5-15%), but excessive slip indicates wasted energy from hull fouling, prop damage, or incorrect propeller selection.
What is a normal propeller slip percentage?
For recreational boats, 5-15% slip is considered normal. Racing boats may achieve 3-8% slip. Displacement hulls typically show 10-20% slip. If your slip exceeds 20%, investigate potential causes like hull fouling, damaged propeller, or incorrect pitch.
How do I find my propeller pitch?
The pitch is stamped on the propeller hub, usually as two numbers like '13x19' where 13 is diameter and 19 is pitch (both in inches). Pitch represents the theoretical distance the propeller would advance in one revolution through a solid medium.
How do I reduce propeller slip?
Clean the hull regularly, inspect and repair propeller damage, ensure correct propeller pitch for your application, and verify gear ratio. Consider a different propeller if slip is consistently high — a lower pitch prop may be better for heavy displacement boats.