How to Determine Propeller Pitch & Diameter

The pitch and diameter of a boat motor or airplane propeller affect the thrust the motor produces as it cuts its way through the water or air respectively. Blade pitch and diameter also affect the acceleration and top speed of the vehicle. Larger propellers, and props with greater pitch, "grab" more surrounding fluid or air with each revolution, and therefore generally produce greater speeds than smaller, less aggressive props.

Things You'll Need

  • Measuring tape
  • Calculator
Show More

Instructions

  1. Determining the Propeller's Diameter

    • 1

      Measure the distance from the center of the propeller's spindle shaft to the tip of one blade. This is the radius of the propeller's diameter.

    • 2

      Multiple the radius by two.

    • 3

      The answer is the propeller's diameter. Because many propellers have an odd number of propeller blades, it's impossible to measure from tip-to-tip for a diameter. For our example calculation below, we will use a diameter of nine inches as a sample diameter.

    Determining the Propeller's Pitch

    • 4

      Examine the propeller, as well as the area of the boat motor near the propeller shaft. Newer motors have the blade pitch stamped on the motor, propeller or engine housing. This measurement is in inches, and represents the amount of distance the propeller blade moves forward, when making one complete rotation, if it were cutting its way through a semi-solid material, such as a block of gelatin.

    • 5

      Remove the prop from the boat or airplane and lay it on a flat surface if the propeller is not clearly marked. The following example describes how to calculate the prop's pitch.

    • 6

      Measure the height of the propeller at the point at which it is highest from the flat surface. Also measure the distance between the lower edge of the blade and the table top at the same point. Calculate the difference between these measurements to find the overall height of the blade. Also measure the overall width of the blade at the same point at which you took the height measurements. For our example below, we will assume the height is 2.5 inches, and the width is 4 inches.

    • 7

      Measure the distance from the center of the propeller spindle shaft to the point at which you took the previous measurements. Calculate the ratio of this point along the propeller blade as compared with the total length of the blade. For example, if the propeller blade is five inches long, and you took your measurements at a distance of four inches from the center of the spindle shaft, then you took your measurements at a point that is four-fifths of the distance from the spindle shaft in relation to the total diameter of the propeller. This is the ratio fraction (RF), and is important for the next step.

    • 8

      Plug the variables into this equation: pitch = (RF * 3.1416) * diameter * height / blade width. Using the sample variables from above, the pitch = (4/5 * 3.1416) * 9 * 2.5 / 4, or the pitch = 14.14 inches.