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Calcimator

Propeller Sizing Calculator

Calculate optimal propeller diameter and pitch from engine power, hull speed, and shaft RPM.

Inputs

HP
knots
RPM

Results

Propeller Diameter

66.8 in

≈ 11 smartphones

Pitch

8 in

≈ 2 credit cards

Diameter (metric)1,696 mm
Pitch Ratio (P/D)0.12
Propeller RPM1,250 RPM
Tip Speed364 ft/s
Estimated Slip15%
How to Use This Calculator
  1. Enter engine horsepower, target speed in knots, and engine RPM.
  2. Input the gear reduction ratio from your transmission specifications.
  3. Review the calculated propeller diameter in inches and pitch in inches.
  4. Check the pitch ratio (P/D) — values between 0.8 and 1.2 are typical for most vessels.
  5. Verify tip speed (ft/s) is under 140 ft/s to minimize cavitation and noise.

How the result changes with Gear Reduction Ratio

Gear Reduction RatioPropeller DiameterPitch
1.555.1 in6 in
2.7582.6 in10.9 in
4.25110.4 in16.9 in
5.5131.1 in21.9 in

What each input means

Engine Power
Shaft horsepower delivered to the propeller.
Target Speed
Desired cruising speed in knots.
Engine RPM
Engine RPM at cruising speed.
Gear Reduction Ratio
Transmission reduction ratio (e.g. 2:1). Enter 1 for direct drive.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Engine Power = 150, Target Speed = 8, Engine RPM = 2500, Gear Reduction Ratio = 2 = 4 input(s) provided
  2. Calculate Propeller Diameter
    Propeller Diameter
    66.8 = 66.8
  3. Calculate Pitch
    Pitch
    8 = 8
  4. Calculate Diameter
    Diameter
    1696 = 1696
  5. Calculate Pitch Ratio
    Pitch Ratio
    0.12 = 0.12

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