Apparent wind is the result of the true wind and your boat speed. Here is an explanation that you easily grasp because it relates to something you experience almost every warm summer's day.
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Put your hand outside the window of your car traveling at 60 miles per hour on a still warm summer day and your hand will feel a 60 mile per hour wind coming from the front of the car. 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. Whatever 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 apparent 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 forward speed in your car and a 20 mph cross wind, 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.
So now that you understand the difference – let’s put the practical application to work for you on a sailboat.
Collapse Excerpt from the course