Calibration of Masthead Fly
Calibrating Wind Direction
These measurements depend on knowing what the apparent wind direction is fairly closely, within a couple of degrees is perfection 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. You can measure them directly off a paper printout using a protractor. Alternatively, you can import the photograph into Google SketchUp or Photoshop and use the protractor tools in the programs. 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 tools. 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. 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.
First, take a picture from the stern offset looking up at the masthead fly. On my boat, the masthead fly is offset slightly so I moved my camera away from center by approximately the same distance. You can see the backstay running up the picture from over at the right to the top of the mast.
Next I imported the picture into Google SketchUp (free program) as follows.
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.
The second picture you need is from directly under the masthead fly looking straight up. Import it the same way that is described above. In this case you need three measurements.
|The image to the right shows what you need to measure, in degrees.|
Please read website Cookie, Privacy, and Disclamers by clicking HERE.