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.
FAQ
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.