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Calibrating Wind Direction

Using your masthead windex


Accurate measurements of apparent wind within a couple of degrees is close enought but that is still requires calibration. What I would suggest is that you take a photograph of your masthead fly from directly below it like the one to the right. You need to measure three things. Take your photograph and use a protractor, or import the photograph into Google Sketchup like I did. Photoshop will work as well as other photo image tools. I prefer Google Sketchup (free) over Photoshop ($$$), which I also have. You need to measure the angle between the flags. What you will want is half that angle for entry into the masthead tool. You will also need the angle the flags fill and the angle of the fly tail. All these angles are measured from the center of the masthead fly support. You can see the trace lines I used to make the measurements.

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.
fly_photo
flag


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