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AC boats "wind shadow"


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Sorry if this has been discussed, a quick search brought no results. Please feel free to point me in the right direction.

Is there? where is? and how big is the "wind shadow" in the current AC boats?

I think traditional sailboats have a shadow or disturbed air zone extending roughly 5 mast heights dead downwind. Plus a little to windward of course when sailing upwind close hauled (leebow effect).

Upwind in a modern foiling AC boat I'm guessing disturbed air directly behind?

Downwind when the AC boats are foiling and sailing faster than the wind I'm not sure where the disturbed air is. I'm guessing behind the leading boat similar to upwind?

I would love to see diagrams if anyone knows of some? I suspect ice boaters might know?

Thanks

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Looking at the boats off the start line, the windward boat hasn't been able to live there for long before it has had to tack off, and once the get close the effect looks severe. The suggestion that I think i have heard made in commentary that the shadow is only immediately behind is optimistic/incorrect I think. 

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The wake always trails at the apparent wind angle.  This was really apparent when I was landsailing, because the landyachts would kick up a dust cloud that got entrained in the lower vortex trailing from the sail.  The yacht continually laid down vortex at the speed of the yacht, which then drifted sideways with the true wind. The result was the axis of the vortex was aligned with the apparent wind vector.

On these yachts, the apparent wind is on the order of 15 - 20 degrees upwind and down.  So even when going downwind, the leading boat can dump on the boat behind.

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Basil,

Excellent! This makes sense. Kind of sailing 101!

In my little boat when I'm on the run, I use the windex to see what direction my "wind shadow" goes so I can "point" it on the boat ahead.

The windex shows the apparent wind angle as well as the disturbed air direction, this should apply to the AC boats too. AC boats at speed always have the wind on the nose and their apparent going back. I guessed an even narrower angle than 15-20.

I see the big difference from my little slow boat is that the AC boat behind on the downwind does not have a "wind shadow" that goes forward or ahead to attack.

I wonder about when the AC boats are downwind, how long does the disturbance linger until the true wind takes over again?

Thanks good stuff!

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1 hour ago, Basiliscus said:

The wake always trails at the apparent wind angle.  This was really apparent when I was landsailing, because the landyachts would kick up a dust cloud that got entrained in the lower vortex trailing from the sail.  The yacht continually laid down vortex at the speed of the yacht, which then drifted sideways with the true wind. The result was the axis of the vortex was aligned with the apparent wind vector.

On these yachts, the apparent wind is on the order of 15 - 20 degrees upwind and down.  So even when going downwind, the leading boat can dump on the boat behind.

Kenny Was saying 10 degrees upwind in the heavy stuff a few days ago

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42 minutes ago, Wandering Geo said:

Kenny Was saying 10 degrees upwind in the heavy stuff a few days ago

Well, let's see.  A 10 deg apparent wind angle at 45 degrees to the true wind means the yacht is going 3.3 times the true wind speed.  Think they're doing that?

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Also, I'm guessing that the size/amplitude of the disturbance is relative to how much energy is extracted from the wind. So as these boats are so efficient at taking lots of energy from the wind, I would expect the shadow to be very "dark", but perhaps narrow due to their speed.

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2 hours ago, sfigone said:

Also, I'm guessing that the size/amplitude of the disturbance is relative to how much energy is extracted from the wind. So as these boats are so efficient at taking lots of energy from the wind, I would expect the shadow to be very "dark", but perhaps narrow due to their speed.

They are efficient but that both increases and decreases the effect. They may not cause as much turbulence as less efficient rigs but its still a lot, and the very efficiency means that the boat behind sees a bigger header because the wind has been "bent" a lot. Upwind that makes a very bad effect, but interestingly downwind could cause the boat behind to sail lower. Whether that means that boat behind has a nett gain or loss is harder to determine.

In one race (I can't remember which) I did see  Ineos sailing lower than LR downwind which could have been due to this.

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3 hours ago, sfigone said:

So as these boats are so efficient at taking lots of energy from the wind, I would expect the shadow to be very "dark", but perhaps narrow due to their speed.

I'm not sure they would take more energy from the wind than a similar-size leadmine sailing in the same apparent wind. Double-skin sails notwithstanding, the rigs are relatively conventional. The difference is what they do with the energy; the leadmine uses most of it to move large quantities of water around whilst going relatively slowly. The AC75 is presumably also dissipating all its energy into the water, but it needs to be going a lot faster to achieve that.

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Actually - thinking some more, there's only a certain amount of energy available in a given body of moving air, but by moving faster you can extra energy from a larger volume of air. That's why you can keep a kite flying by doing figure-of-eights, on a day that's too light to keep it flying in one place. That suggests that the AC75 probably does generate quite a big wind shadow, because it's extracting energy from a much larger body of air.

On the flipside, the same effect may mean the trailing boat needs a bigger wind shadow to slow it down, so the effects could cancel out.

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You can leave the true wind and boat speed out of the equation. It is all about the relative positions of the yachts and the apparent wind speed and direction. The disturbed flow area (wind shadow) for faster yachts is the same as for slower yachts provided they have the same sail configuration and they sail in the same apparent wind field (which would mean more true wind for the slower yachts to make up for their lack of boat speed). Upwind and downwind also have no specific meaning in the context of apparent wind. I am simplifying a bit but the bottom line is that it is all about the relative positions of the yachts and the apparent wind field they are sailing in.

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14 hours ago, Basiliscus said:

Well, let's see.  A 10 deg apparent wind angle at 45 degrees to the true wind means the yacht is going 3.3 times the true wind speed.  Think they're doing that?

10 knots TWS and 33 knots BS........not entirely beyond belief based on the telecasts numbers so far. Will keep a closer eye on next rounds

Not sure on the 45 degrees TWA tho.

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First hand experience: 

Was sailing the 12ft skiff and got lined up by LR, we were going upwind they were going down. Usually when a big boat passes in front we have to get off the wire before getting dunked. LR went by directly upwind of us, about 30m away, and felt absolutely nothing. Crazy. 

Must be slicing through the wind like a knife... I bet there is a bit of disturbance upwind sailing though if you're being covered/lee bow.

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