True Wind versus Apparent Wind

Posted by Grant Headifen on April 3, 2010 under Crew, Skipper | Read the First Comment

We took this from a section in the Crew and Skipper Courses to easily explain the difference between true wind and apparent wind.

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Section 7.2 – True Wind versus Apparent Wind – Explained

Put you hand outside the window of your car traveling at 60 miles per hour on a still day and your hand will feel a 60 mile per hour wind. That’s apparent wind yet the true wind is zero. What if the car was driving into a 20 mile per hour head wind? Your hand would feel 80 mph. Or if the wind was blowing from behind at 20 mph, your hand would feel 40mph.

Now what about a cross wind of 20 miles per hour? Well we need to do a little Pythagorean theorem work on this. What is the square root of the sum of 60 squared plus 20 squared? Your hand would feel 63.24 mph and mostly from a direction in front of the car. If the car accelerated to 100 mph your hand would feel 102 mph again mostly from the front. If the car decelerates to 10 mph your hand would feel 22 mph mostly from the side of the car and if he car stopped you’d feel the full true wind of 20 miles per hour from the side of the car. What ever your hand feels is the apparent wind. The apparent wind equals the true wind when your car is not moving.

When determining direction of the wind, the faster the car goes the more the apparent wind direction comes from the direction of travel of the car. Again imagine the cross wind. At 1 mph the apparent wind feels almost like the true wind from across the car. As the car accelerates the wind feels more and more like it is coming from the front.

This is similar to a boat. The faster the boat sails into the wind, the more the apparent wind speed increases and the more it feels like it is coming from the front of the boat. As a general rule of thumb then, when sailing the true wind is about 15 degrees more towards the back of the boat. IE point to where you feel the wind is coming from then point 15 degrees further back and that is about where the true wind is coming from.

wind vectors, true versus apparent

wind vectors, true versus apparent

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