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How to Calculate True Wind from Apparent Wind

On a boat that is moving, all we can feel is the apparent wind, which is the vector sum of the true wind (as it blows across the water) plus the headwind that is caused by our movement forward – just like you get a headwind in your face when you peddle your bicycle forward on a dead calm day.

So if all we can feel is the apparent wind, how do we know what the true wind is actually doing? The answer involves some basic trigonometry to resolve the vector subtractions. Essentially what you do is take the apparent wind speed and angle (the apparent wind vector) and subtract the boats forward motion (the headwind vector).

Bear in mind that you also have to take into consideration the leeway the boat makes. The apparent wind you feel is actually relative to your course over ground, not the boat’s heading or its course through the water. If you were to be pointing north, but drifting backwards, south, the ‘headwind’ would actually be a tailwind.

Or you can just use the calculator below. 

Calculate True Wind

Enter apparent wind speed:
Enter apparent wind direction in degrees relative to the boat's bow (+ for starboard tack or - for port tack):
Enter boat's heading in degrees magnetic:
Enter current magnetic variation (- for West, + for East):
Enter boat's speed over the ground (SOG)
(use the same units you used for wind speed):
Enter boat's course over the ground (COG) in degrees true:

 

Generate Polar diagram

Select NMEA Sentences txt file to upload:
Enter Boat Name: (name will be converted to lower case and blank spaces will be removed)
Enter Password:
Enter Magnetic variation for the date and location of the data: (+ for East and - for West)
Permit motoring data (see text above)?
Output polar for True or Apparent wind?
True   Apparent
Delete all database records for this boat?

3 Responses to “How to Calculate True Wind from Apparent Wind”

  • Bernie Marciniak:

    Just to let you know. Your calculation of true wind is not correct.
    You are in the wrong frame of reference. its the apparent wind vector added to
    the boat speed thru the water. I.e. same frame of reference.
    There is no wind on the bottom of the ocean i.e. Sog and Cog frame of reference.

    • Noel:

      I guess it depends on what you mean by TRUE wind. Is it the wind that you experience on the boat as you sit, not moving through the water, but drifting over land on the tidal current? Or is it the wind that you experience when you tie up to the mooring ball in that same current, and are now moving through the water, but not over land? To my mind the true true wind is the latter, so one then has to make adjustments for the movement OF the water, not just the movement through the water.

  • Frederic Endemann:

    Everyone should know the simple vector wind diagram. Draw a vertical line for Boat Speed, perhaps 6 knots, make it 6 inches or cms. label it BS.

    From the bottom end of that line, draw the apparent wind vector. This will be AW. You need to use an apparent wind instrument or guesstimate this angle for practice purposes.

    On a typical BEAT, the apparent wind will be about 35 degrees. This wind will be strong on a beat, perhaps 15 knots. Draw a 15 inch ( or cm ) line from the bottom of your boat speed line at an angle of 35 degrees. Get a protractor!

    Connect the two line ends and you have nice triangle. Measure the new short side and you have True Wind. Third side and third label, TW

    In this case the true wind is about 11 knots. To find the True Wind angle, extend your BS line with a dotted line and look at the angle formed with the TW line. In this case the angle is about 48 degrees.

    Do this excercise a few times with Apparent Wind beam reaches (90 degrees) and then figure out the technique for downwind angles. It should come to you easily if you start with the basic beating diagram and then widen your apparent wind angles. It’s fun!

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