When you're looking for good speed on a light-air run, the more wind you have the faster you will go. That's why you should avoid the bad air of other boats. Here's an interesting way to think of wind shadows: Areas of bad air are like the lulls on a puffy day. Areas of clear air are like puffs. If you sail in bad air or lulls, you will have less wind which means you will be sailing slower through the water and higher than the other boats. Neither is good if you want to beat them.
Light-air flow is turbulent longer
Whenever the wind blows up against an impenetrable object (like a spinnaker), it bends around that object, breaks into eddies and eventually re-establishes flow as it was before the wind was disturbed. For racing sailors, the key question is how long the wind takes to return to its uninterrupted flow. This determines the length of wind shadows that exist to leeward of every racing boat. In simple terms, the slower the air is travelling before it meets the object, the longer it will take to return to normal flow. When a strong flow is interrupted, it has enough energy to come back together quickly. But a weak flow takes longer to re-establish. That's one reason why you can feel the effects of bad air much farther away on light-air days.
Bad air goes farther in light wind
One reason why you have to be very careful about bad air when you're racing in light breeze is that wind shadows extend farther than most sailors think. How far from another boat (directly in line with their wind shadow) do you have to be before you feel completely comfortable that you are not being affected by them? In heavy air, your answer might be 4 or 5 boatlengths. In other words, you can be that close to them and not be slowed at all by their bad air. In light air, however, the answer might be 8 or 10 lengths! That's why, on a crowded light-air run, there is a much smaller area where you can find clear air.
Dave Dellenbaugh / Speed and Smarts / UK–Halsey
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