Tuesday, April 04, 2006

Wind Shadows II

My post on wind shadows sparked off a stream of comments from Tillerman. At first I thought he was trying to trick me into saying something incorrect, but it seems he was just trying to figure it out for himself.

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?

4:30 PM

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.

5:11 PM

Tillerman said...

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

5:17 PM

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.

5:47 PM

Tillerman said...

Straight down true or straight down apparent wind?

6:11 PM

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.

6:33 PM

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"?

7:38 PM

Litoralis said...

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

What does everyone else think? How do you visualize wind shadows? Am I correct about wind shadows extending directly downwind?


Tillerman said...

Well at first I was just trying to figure it out too but it looks like, in the process, I did trick you into getting it wrong.

Litoralis said...

Yeah...I think you are right...vector calculus and all that.

I wonder if anyone else will figure it out?

Adrift At Sea said...

I'd say that the wind shadow isn't affected by the speed of the boat, and must extend directly down true wind of the boat.

That said, I would also like to point out that the wind shadow a boat casts is affected by the speed of the boat and moves along with the boat.

For a fast enough boat casting the wind shadow, would the wind shadow appear to lag a bit, and would it appear to be slightly behind where the boat is? Would this would vary depending on the distance between you and the boat in question. Would the wind shadow appear to be farther behind the boat as you get further away?

Tillerman said...

I think you're on the right track Dan. The way I vizualize is that although the wind shadow close to the boat causing it is of course directly downwind of it, the shadow can only travel downwind as fast as the wind itself. Think of it as a stream of soap bubbles or smoke released from the boat causing the shadow.

So imagine I'm an anchored boat and a boat passes behind me on a broad reach say 90 feet directly upwind of me. And let's say the wind speed is 30 feet per second and the reaching boat is going at 15 feet per second. Then it will take 3 seconds before I feel the shadow, by which time the reaching boat will not be directly upwind of me, but will be 45 feet away from directly upwind. So it will appear that I feel his shadow when he is 45 feet forward of directly upwind.

Now if you draw the vectors of the boat's speed and true wind and the resultant apparent wind, you will see that I feel the wind shadow when I am directly opposite his apparent wind as he sees it. So the Dellenbaugh article found by litoralis was spot on. The shadowing boat's wind indicator points directly away from his wind shadow.

Of course in real life the other boat is not anchored but the principle is the same.

Tillerman said...

That comment should say that the reaching boat is on a beam reach - sailign at 90 degrees to the true wind direction.

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