Propeller Size Calculator 🔩
Calculate the ideal propeller pitch and estimated diameter for your boat. Enter desired speed, engine RPM, and gear ratio to find the best prop.
How to Use the Propeller Size Calculator
Enter desired cruising speed (knots), engine horsepower (HP), engine WOT RPM, gear ratio, and typical slip %. The calculator uses Dave Gerr's formula to recommend propeller pitch and estimate diameter.
Slip is the difference between theoretical and actual speed. Planing hulls: 10–15%. Displacement hulls: 15–20%. Adjust slip % for your hull type to get a more accurate result.
Always verify prop selection with a certified marine mechanic. After fitting, confirm engine WOT RPM is within the manufacturer's recommended range.
Boat & Marine Calculator - Introduction
Enter your desired boat speed in knots, engine RPM, gear ratio, and estimated slip percentage to get the recommended propeller pitch and a diameter estimate. Typical slip is 10–15% for planing hulls and 15–20% for displacement hulls—adjust if you know your actual GPS vs. theoretical speed.
How It Works
Pitch (in) = (knots × 101.27 × 12) / (shaft RPM × (1 − slip/100)), where shaft RPM = engine RPM / gear ratio. Diameter is estimated from Dave Gerr's formula: diameter (in) = 632.7 × HP^0.2 / RPM^0.6. Example: 30 knots target, 4500 RPM engine, 1.84:1 gear ratio, 13% slip → shaft RPM = 2446, pitch = (30 × 101.27 × 12) / (2446 × 0.87) ≈ 17.1 inches.
Usage Scenarios
- Re-propping after an engine swap: A 24-ft center console gets a new 200 HP outboard replacing a 150 HP unit. The owner enters the new WOT RPM (5800), gear ratio (2.08:1), and target speed (42 knots) to confirm the existing 17-pitch prop is under-pitched and a 19-inch pitch would better use the power band.
- Optimizing a displacement cruiser for economy: A 36-ft trawler running a 120 HP diesel at 2400 RPM through a 2:1 reduction gear aims for 8 knots with 18% slip. The calculator returns a pitch of roughly 19 inches and 18-inch diameter—numbers to discuss with a prop shop before custom grinding.
- Diagnosing why the engine won't reach WOT: A 25-ft boat's engine peaks at 4200 RPM instead of the rated 5000. The owner enters current pitch (21 in), gear ratio (1.84:1), and target speed, confirming the prop is over-pitched by about 3 inches and the engine is lugging—a prop swap rather than a tune-up.
Frequently Asked Questions
How do I calculate propeller pitch?
Propeller pitch (inches) = (Speed_fpm × 12) / (Shaft_RPM × (1 − slip)). Where Speed_fpm = knots × 101.27, and Shaft_RPM = Engine_RPM / gear_ratio. Typical slip is 10–15% for planing hulls, 15–20% for displacement hulls.
What propeller pitch do I need for a 25-knot boat?
At 4000 RPM with 1.5:1 gear ratio and 15% slip: Shaft_RPM = 2667, Speed_fpm = 2532, Pitch = (2532 × 12) / (2667 × 0.85) ≈ 13.4 inches. Always verify with a marine propeller specialist.
How is propeller diameter estimated?
Diameter can be estimated using Gerr's formula: Diameter (inches) = 632 / √(Shaft_RPM). This gives a starting point for moderate displacement to planing hulls. Actual diameter depends on hull design and engine torque characteristics.
What is the difference between propeller pitch and diameter?
Pitch is the theoretical distance a propeller advances per revolution (like a screw thread). Diameter is the total width of the propeller blade circle. Higher pitch means more speed at the same RPM. Larger diameter means more thrust at lower RPM.