(Archived article - Flash animation no longer supported)

For starters lets assume you have a bit of an understanding of propeller terminology and pitch. If not, have a look through some of my other articles under the heading 'Propeller Info'. My Simple Pitch Measuring flash interaction is for educational purposes and will give you a lead in to some of the things I will be explaining on this page.

On a more professional level, pitch is measured across the blade face and at several diameters or radii. (radius = diameter divided by 2) As most of this web site concentrates on smaller outboard and stern-drive propellers I'll just be talking about basic pitch measurements at the one diameter. However, with this simple concept and multiple pitch measurements you can use it on just about any size propeller from a tender to an ocean liner.

Lets begin.

image1a

image1b

The images above shows the pitch gauge used with my repair tools. The upper probe holder is locked at the chosen measuring diameter (1a) Each probe is pressed against the propeller blade then clamped (1b) The 2 probes are measured from the back of the pitch holder to the rear end of the probes (1a) Subtracting the shortest probe measurement from the longest probe measurement will be the difference between the probes tips as shown in the image 1c below.

image1c

There are 3 variables used above to determine pitch.

1. The measuring diameter (radius)
2. The probes distance apart.
3. The probe length difference.

1. The measuring diameter (radius)

For this experiment, lets say we are measuring a 15 inch diameter 3 blade propeller. We need to choose a diameter to measure it at. As a general rule of thumb a 3 blade propeller is measured at two thirds of it's diameter and a four blade is measured at three quarters of it's diameter. Seeing we are using a three blade propeller for our calculations our measuring diameter will be 15 divided by 3 multiplied by 2. If my sums are correct, that should equal 10 inches.

image2

We need to measure the propeller at 10 inch diameter so we can set the gauge to suit. See image1a above. When we come to the actual formula required we will need to convert this measuring diameter to a radius using the formula Radius = diameter divided by 2. Therefore the measuring radius will be 5 inches.

2. The probes distance apart

The two probes are set at a fixed distance apart on the measuring diameter's circumference. They have to be square to the circumference and will touch at two points on that circumference. In the example below I'll use a random distance apart measurement of 3 inches.

image3

3. The probe length difference

The dimension of C in the next image is the difference of length between the two probes. Lets say the probe length difference = 1.5 inches for this experiment.

image4

Propeller Pitch Calculation

Our probe distance apart is 3 inches (76.2mm)

Our measuring radius is 5 inches

Our probe length difference is 1.5 inches (38.1mm)

Here you can see how our measurements look drawn on a sheet of paper. The probe distance apart is 3 inches and the measuring radius is 5 inches. We need to know what the degree is of the lower angle ? . Once we have worked out that angle we can then assume the following: If a propeller goes forward 1.5 inches in ? degrees than in a full circle (360 degrees) it will travel X inches of pitch.

image5

Putting those dimensions into a calculator below will give us the ? angle as being 34.915206247

The resulting pitch over 360 degrees equals 15.46604067 inches

I've left all the measurements in imperial in this article to simplify things. The Simple Pitch Measuring flash interaction uses a mixture of both imperial and metric measurements. In the end the results are in inches of pitch.

Notes- Please note the concept and dimensions shown here are similar to our simple propeller tool system but should not be used with the tools.