January 14, 2015

Sailing the the Middle of the Fleet - Dirty Air

By Doug
Another question from Pam: how can I tell if I'm in dirty air? It's a simple question with a simple answer - the wind creates waves, so look at the direction of the waves to see where the wind is coming from.

The waves are coming straight towards us, so we must be looking directly into the wind. So white is creating dirty air for green, right?

Actually, the answer is no. In fact, green is in clear air and can easily hold this position. The reason is because both boats are moving, so what we call the apparent wind actually shifts forward. The faster the boats go, the more the wind shifts.

Here's the position of a boat sailing in dirty air.

And looking at the wind shadow, you want to be in green and not red.

So, what we see can be very misleading.

Here's a trick to accurately know where the apparent wind is and if you're in dirty air:
  • Look at the breeze and feel it on your face.
  • Now, close your eyes and move your head back and forth slowly. The sensation of pressure on your face will move slightly from side to side.
  • If the wind is light and you don't feel any difference, splash some water on both of your cheeks and move your head back and forth again to feel a slight change in the temperature.
  • When the pressure or temperature you feel on both sides of your face is the same, open your eyes. You're now looking directly upwind.
  • If you're looking directly at a boat, then you're in its dirty air. The closer it is, the more it will affect you. To get into clean air, tack or bear off.
When you get good at this, you'll also be able to feel when you're in dirty air because your boat will feel different, sort of like when a car is misfiring and not running smoothly. Sailing is similar because you'll feel the turbulence of the dirty air from the boat to windward of you.

So there you have it - rely on what you feel and not not what you see.

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