Calibration of Masthead Fly
Calibrating Wind Direction

You can still use this tool without calibrating and just use my numbers or eyeball what you have. Because you will be using the same masthead fly for determining what course you sail as you did in making the measurement, some of the potential errors cancel. Being off a couple of degrees one way or the other doesn't matter that much. You may also want to make sure that your masthead fly is centered correctly. I will show how I did both of these measurements below using Google SketchUp.

Next I imported the picture into Google SketchUp (free program) as follows.
1) On a blank page, draw a rectangle. This will be the base for your work.
2) Click File, then Import, then enter the name of your photo and select "Use as Texture".
3) Click on the lower right corner of the rectangle to place your image, then move the cursor to the upper right and click again.
You can now use the SketchUp Protractor tool to make your measurements. It may be helpful to draw lines to the various points you want to measure from the center of the bolt on the masthead fly but that is optional as you can use the protractor directly to make the measurements.
On this picture, what we are interested in is the difference between the two flags as compared to the vertical. In SketchUp, you can place the first point of the protractor in the center of the bolt, then click on the vertical (blue axis) then click move the protractor to each flag reading off the angle. They should be the same.

1) The width of the flags. In this case, 9.5 degrees.
2) The width of the tail. I measure 15 degrees.
3) Half the amount of the angle between the two flags. I measure 55.75 degrees so my flag spacing is 27.875.
It should be noted that I measured a Windex at West Marine and the tail was 15 degrees and the flag 10 degrees. The standard setup is to have the flag spacing at 60 degrees or +- 30 degrees. It doesn't make a significant difference in the calculated target boat speed so feel free to use the standard numbers.
