The Propeller Sizing Tool uses a combination of hydrodynamic and empirical formulas to estimate the optimal Propeller Diameter and Pitch based on engine specifications and vessel type coefficients.

I. Core Mechanical Calculation

The propeller shaft speed is derived directly from the engine’s maximum RPM and the gear reduction ratio.

Propeller RPM = Engine RPM / Gear Ratio

II. Sizing Calculations (Diameter and Pitch)

1. Theoretical Pitch (Speed-Based)

This establishes the minimum pitch required to achieve the Desired Speed, assuming an ideal 10% slip.

Theoretical Pitch (in) = (Desired Speed (Knots) x 101.3) / (Propeller RPM x 0.9)

• The constant $101.3$ converts the speed to inches per minute.

2. Final Diameter (HP-Based)

The diameter is scaled primarily by the engine’s horsepower (HP) and a vessel-specific coefficient.

Diameter (in) = sqrt(HP) x Boat Coefficient (Diameter)

3. Final Pitch (Adjusted)

The final pitch is adjusted using a specific coefficient for the vessel type.

Pitch (in) = Theoretical Pitch x Boat Coefficient (Pitch)

III. Performance Metrics (Estimates)

1. Slip Ratio (Estimated)

This is an estimate of the propeller’s efficiency loss, based on a 15% minimum base slip.

Slip Ratio (%) = 15% + (Efficiency Coefficient x 15%)

2. Thrust (Estimated)

A simplified, rule-of-thumb estimate for the propeller thrust (in pounds).

Thrust (lbs) = HP x 10 x Efficiency Coefficient

IV. Boat Coefficients (Empirical Data)

The core coefficients used to modify the sizing formulas based on vessel type are:

• Diameter Coefficient: 0.85 (Sailboat) to 0.65 (High Performance).
• Pitch Coefficient: 1.10 (Sailboat) to 1.40 (High Performance).
• Efficiency Coefficient: 0.55 (Sailboat) to 0.75 (High Performance).