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Writer's pictureMike Hill

How to Precisely Measure Boat Propeller Pitch, without a Pitch Gauge?

When using a digital inclinometer to measure the pitch of a boat propeller, the "Blade Angle × Constant" method is a straightforward approach used by the Buckshot Racing #77 Formula 1 Tunnel Boat Racing Team.


The constant is typically related to the propeller's diameter and the point at which the measurement is taken, which in this case is at 75% of the radius.


We've included a proprietary Buckshot Racing #77 Calculator Sheet to make these instructions below much simpler, but included the detail for those who want to take a deeper dive into the math.


Tools You'll Need:

  • Digital inclinometer

  • Ruler or tape measure

  • Calculator (Print the Buckshot Racing #77 Calculator Sheet)


Steps to Calculate Pitch Using Blade Angle × Constant:

  1. Position the Propeller:

    • Place the propeller on a flat, stable surface.

  2. Set the Measurement Point:

    • Identify the point that is 75% of the way from the hub to the tip of the blade. This is done by measuring the propeller's radius (half of the diameter).

    • For example, if the propeller diameter is 14 inches, the radius is 7 inches.

    • 75% of this radius is 0.75 × 7 = 5.25 inches. Mark this point on the blade.

  3. Measure the Blade Angle:

    • Place the digital inclinometer flat against the blade at the 75% radius mark.

    • Read and record the blade angle shown on the digital inclinometer.

  4. Determine the Constant:

    • The constant is derived from the diameter and the position at which the angle is measured.

    • For simplicity, the constant can be approximated as: Constant=Propeller Diameter×π360∘\text{Constant} = \frac{\text{Propeller Diameter} \times \pi}{360^\circ} Constant=360∘Propeller Diameter×π​

    • This accounts for the propeller’s diameter and translates the angle into a linear distance.

  5. Calculate the Pitch:

    • Use the formula: Pitch=Blade Angle×Constant\text{Pitch} = \text{Blade Angle} \times \text{Constant}Pitch=Blade Angle×Constant

    • Substitute the angle and constant into the formula to calculate the pitch.


Example Calculation:

  1. Propeller Diameter: 14 inches

  2. 75% of Radius: 5.25 inches

  3. Measured Blade Angle: 20 degrees


Calculate the Constant:

Constant=14×3.1416360=0.1223 inches per degree\text{Constant} = \frac{14 \times 3.1416}{360} = 0.1223 \text{ inches per degree}Constant=36014×3.1416​=0.1223 inches per degree


Calculate the Pitch:

Pitch=20∘×0.1223=2.446 inches\text{Pitch} = 20^\circ \times 0.1223 = 2.446 \text{ inches}Pitch=20∘×0.1223=2.446 inches


The pitch of the propeller, based on these calculations, would be approximately 2.446 inches per revolution.


Notes:

  • Calibration: Ensure that the inclinometer is properly zeroed before measuring.

  • Average of Multiple Blades: For accuracy, measure the blade angle at the same point on each blade and take the average.

  • Blade Variations: Account for any variations in the blade angles by averaging them if necessary.


This method provides a simplified yet effective way to calculate the propeller pitch using a digital inclinometer.


Download our Free how-to-measure-prop-pitch-calculator-sheet, kindly reference Buckshot Racing #77 when sharing.


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