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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:
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:

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