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» Online Classroom   »   » Public Discussion of Marine Weather   » True Wind Direction and Compass Heading

Author Topic: True Wind Direction and Compass Heading
John Kutschka

 - posted April 06, 2004 11:32 PM      Profile for John Kutschka           Edit/Delete Post 
How do we convert to true wind direction from apparent wind direction on a sail boat? I understand that we have to convert from apparent wind direction by taking into account boat speed, apparant wind angle and speed. Once we have that true wind speed and compass bearing we must convert that true wind compass bearing from magenetic to true so we can coorelate it to weather faxes and forecasts available to us. Please explain and show how we do the calculations
David Burch

 - posted April 07, 2004 11:47 AM      Profile for David Burch           Edit/Delete Post 
Thanks, you have raised a good point in that in this process we use the word "true" in two different ways. We first use apparent wind speed (AWS) and apparent wind direction (AWD) along with boat speed (S) to find the relative direction of the "true wind" (TWD). This is a relative direction because we started out with a relative direction (AWD), ie we observe that the AWD is, say, 30° on the starboard side, and we then learn that the TWD is 50° on the starboard side.

Next we note our heading, which is usually magnetic, such as 200 M, and then conclude that the TWD = 250M. If the local variation were say, 15° E, then we would have that the TWD = 265 T, where T = "true," meaning relative to due north. It is this latter direction that you need to record for keepiong watch on the weather and to plot on the weather map to compare with predictions from the map itself.

The process of converting (AWS, AWD + S) to (TWS, TWD) is given in great detail at this link Starpath TrueWind. Here you can download a free computer program and tutorial on the process, which includes step by step instructions on how to solve the problem with plotting. (Note the files are "zipped" so you will need Winzip on your computer to unzip them. There is a free evaluation version at www.winzip.com.)

From: Starpath, Seattle, WA

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