How to Measure a Propeller

Architecture of a Propeller

• Hub – This is the central portion of the prop that fits onto the prop shaft. It is a hollow cylinder where the bases of the blades are attached.
• Blades – These are the large, flat pieces that radiate out from the hub. This is what pushes the water making the boat move forward.
• Root – This is where the blade attaches to the hub.
• Leading Edge - This refers to the edge of a blade that is moving into the water.
• Trailing Edge – This is the edge of a blade that is opposite the leading edge.
• Blade Face – The wide part of the blade, often divided into fore and aft faces.

Propeller Variables

• Diameter – The diameter of a prop is the distance across the propeller. If you are viewing a prop from the rear of a boat and imagine the prop making a solid circle as it spins the diameter will be the distance across that circle. To measure this dimension measure one blade from the center of the hub to the tip of the blade then double that number to get the diameter.
• Pitch – This measurement is the mystery for many people but the definition is very simple. The pitch of a prop tells us the maximum distance a propeller will move a vessel forward through the water. Take note of the word maximum in this description. Pitch is often referred to as a theoretical measurement because no prop operates at one hundred percent efficiency. The laws of fluid dynamics tell us that there is a significant loss of power at the prop which can be as much as one-third of maximum efficiency. This means that a prop with a pitch of 21 inches will only move a boat forward fourteen inches in the real world.
  
  To measure pitch, you need to take several measurements. These measurements are going to be much more accurate if you have the prop off the shaft and can lay it flat on a table. Don’t worry if you need to do this while it is still attached to the vessel, it will be slightly less accurate but this is not a precision engineering measurement.
  
  First, find the widest part of one blade and draw a line across the face from edge to edge. Then measure the distance from the front of the hub to the points where your line meets each edge of the blade. You can do this best while viewing the prop from the side. Take the smaller measurement and subtract it from the larger.
  
  Next use a protractor, angle gauge, or carpentry square to measure the triangle formed by the two points at either end of the line drawn across the widest part of the propeller blade and the center of the hub. The narrow, pointy end should be at the center of the hub. Measure the angle between the two lines radiating out from the center of the hub.
  
  Now take the first measurement and multiply it by 360. Then take the result and divide it by the angle you found in the second measurement. The resulting number is the pitch of the prop.
  
  For example, a prop that has a three-inch difference between the leading and trailing edge at the center of a blade and has a thirty-degree angle between the leading edge and the trailing edge of the blade will have a pitch of ​36 inches. This is calculated as; 3 x 360 / 30 = 36.
  
  There are also inexpensive prop gauges available but where is the fun in that approach.
  
• Rake – Rake is the angle between the cylinder that forms the hub and an imaginary line from the blade root to the tip of the blade. This is best measured with a protractor or angle gauge since the measurement will be a fairly small number.

Alternate Method

1. Measure from the center of the propeller's hub--the center of the propeller to which the blades are attached--to the edge of the hub with a tape measure. Record this measurement.
2. Measure from the base of one blade to the tip of that same blade. Record this measurement.
3. Add the two recorded measurements together and multiply the result by 2 to determine the diameter of the propeller, which is the first element of the propeller's measurement. For example, If the distance from the center of the hub to the edge of the hub is 1 1/2 inches and the distance from the base of the blade to the tip of the same blade is 5 1/2 inches, then: 5 1/2 + 1 1/2 = 7 inches. This is the radius of the propeller. Multiply the radius by 2. 7 x 2 = 14 inches, so the first element of the this propeller's measurement is "14."
4. Use the grease pencil to draw a line around the propeller hub. Set the base of the protractor against the blade of the propeller and note the angle (on the protractor) between the blade of the propeller and the line drawn around the hub. Use your calculator's "TAN" or tangent function to determine the tangent of this angle.
5. Multiply the tangent of the angle by 6.2831. Multiply the result by the radius of the propeller. The result is an approximation of the pitch. Pitch is always a whole number; if you get a fractional answer, round the number off. For example, if the diameter of the propeller is 14 inches, the radius of the propeller is 7 inches. The angle of the blade is 20 degrees. The tangent of 20 is 0.36397. 0.36397 x 6.2831 x 7 = 16.0080. Rounded off to the nearest whole number, this means that the pitch of the propeller--the second element of this propeller's measurement--is "16."
6. Record your propeller's measurement by its diameter and its pitch. In the example, the diameter is "14" and the pitch is "16." The propeller measures "14 x 16."