Measure Prop Pitch, Diameter, and Rake

WORKER BY SHIP PROPELLER IN NORWAY
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On the surface, a propeller seems like a simple device. Once you learn to measure some common prop dimensions and ponder the nearly limitless combinations of these variables you see that it is very complex. Then at some point, after much studying, you will attain prop enlightenment and the propeller will become simple again.

There are no promises of prop enlightenment or other engineering magic here, just some basic terms and measurements to help you see how a prop interacts with the rest of the vessel and the elements. With this knowledge, you will be able to determine prop performance characteristics.

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 the 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 some 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.

Prop Markings

The easiest way to find prop diameter and pitch is to read the markings stamped or cast into the hub. These are two numbers separated by a dash. The first number is the diameter and the second is the pitch.

If you have enjoyed learning about the basics of propellers take a look at some of our other vessel engineering topics.