Monday, April 03, 2006

Wind Shadows

(Following is a brief excerpt of a story by David Dellenbaugh posted on the Sailing Breezes online magazine.) Wind shadows, like the wind itself, are invisible, so you have to look around for clues in order to know where they are. Your best tool is undoubtedly the wind indicator on top of your mast (or on the mast of a boat to windward). This pennant shows the apparent wind direction and will give you a very accurate idea of which way the boat's wind shadow extends. In addition to determining the angle of the wind shadow, it's important to know its size. There are many factors that affect how large an area is covered by a boat's disturbed air. These include the wind velocity, height of the mast and speed of the boat. The length of a wind shadow is often measured in "mast-height" units rather than in boatlengths. For example, in heavy air a boat's wind shadow might extend four mast-heights to leeward. In light air, that same boat might cast a wind shadow for eight or more mast-heights. ... - David Dellenbaugh

10 comments:

Tillerman said...

So are you in another boat's wind shadow when its wind indicator is pointing at you, or when your wind indicator is pointing at them?

Litoralis said...

Wind indicators point to windward, but they are affected by apparent wind, so a more accurate description would be that you are potentially in someone's wind shadow when you are on the imaginary line drawn straight downwind from the windward boat's sail plan.

Tillerman said...

But is it a line straight down the true wind direction or straight down the apparent wind direction of the windward boat?

Litoralis said...

I think it is straight downwind. I visualize wind shadows as holes in the wind that extend downwind from the sail plan of the boat.

Tillerman said...

Straight down true or straight down apparent wind?

Litoralis said...

Straight downwind. The wind shadow is not shifted by the movement of the boat in my mental model.

If you imagine a fast boat like a catamaran sailing on a broad reach (with respect to the true wind direction) but with the sails trimmed in to his apparent wind direction. The wind shadow of the catamaran must still extend downwind. If it extends along the apparent wind angle then it would extend upwind. There will be a region of turbulent air following the path of the boat, but this is turbulence, not a wind shadow.

Also, imagine a large powerboat moving quickly in a direction perpendicular to the true wind direction. It will still have a wind shadow extending downwind from its beam even though the apparent wind angle on the boat is the same as its direction of travel.

Tillerman said...

Good points. So is Dellenbaugh talking out of his yahoo in saying that the wind indicator gives you a "very accurate idea of which way the boat's wind shadow extends"?

Dutch LeWombat said...

That's a big wind shadow sucking the air of that little boat. Great pic.

Litoralis said...

The wind indicator method suggested by Dellenbaugh is a good simplificataion for downwind sailing in slowish boats.

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