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Old 27th June 2011, 07:40 PM   #1
Farlo
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Default Where does the wind go?

Hi all, yesterday I was watching (thankfully from my bike) a few colleagues pumping in all directions and catching a gust occasionally for a ~100 meters run. You could see the gusts appear and disappear from the water surface and I started thinking: by the way, where does the wind go? If you look at it like a pressure wave, why does it stop? You know that feeling when you see people planing ahead but by the time the gust should hit you it's gone completely. Or sometime it will form from nowhere, blast for a couple of seconds in the middle of the spot, and then vanish. I suspect it is a complex 3D problem with lots of maths but has any of you heard about a meaningful model?
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Old 27th June 2011, 09:41 PM   #2
Ken
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Farlo,

It can be really weird, I agree. The hot winds of summer when they blow in Texas are amazingly gusty, swirly and variable. One thing I consistently notice where I usually sail (somewhat protected area from large chop) is the way the water looks as a gust approaches. A 3D image would be very interesting to see. The "black" water or small wavelets from the gust will pass by the board and under the sail for maybe 5 seconds before the wind powers the sail. It's seems like the gusts are shaped like a 20 degree wedge moving along the surface.

The other thing I notice is when planing through a hole and approaching a gust (tongue of black water ahead), the wind speed drops to almost 0 just before hitting the gust, almost like sailing through opposing vortexes.

You think someone with the right computer skills could produce a 3D image if there was a way to "see" what the wind was actually doing.

I am sure that part of the problem at my sailing site is the land geography and it's impact on the wind. Outside the protected area, the wind seems to be a bit steadier, but it's hard to see (read the water) with the larger waves and chop.
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Old 28th June 2011, 02:21 PM   #3
Farlo
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Hi Ken, admittedly last weekend was quiet but the one before was windy with gusts up to 25 knots. Despite a good orientation (West wind is usually rather steady here) it was panic on water. Most of the turbulence may be caused by ground or water roughness and carried over long distances. So be it, but what is the anatomy of a gust (or a lull)? Racers, notably formula ones because of their giant sails, are keen to negotiate gusts and keep planing in lulls. I just wonder if some model exist that could help everybody.

Last edited by Farlo; 28th June 2011 at 09:38 PM.
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Old 29th June 2011, 06:45 AM   #4
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Seems that you are seeing cellular bursts which come down rather than from across the landscape which as with gusty winds on a open ocean can be caused by a difference in speed or direction between high and low level wind movements. Also while all may appear to be calm on the water surface, refracted heat may cause a push back of thermal breeze activity and thus returning cellular bursts. These have different sailing angles as the WS sails through or around the edges or seeks a consistent path through these edge to edge. This is superimposed on the 'normal' wind generated as the differential between high and low pressure system gradients where the wind direction and velocity is vectored by the rotation of the globe and the friction of the larger scale surface media establishing the general wind direction and velocity. The direction of the gust lull sequences is subsequently magnified by the local landscape when flowing/ bending about nearby hills, forests or what ever obstacles. Anyway that is my rough interpretation not being a meteorologist by any means and much more accurate explanations are available from standard sailing text books in your local library probably.
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Old 29th June 2011, 02:32 PM   #5
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Hi Philip, thank you for your answer. I can figure out large portions of quiet air being pushed by faster portions and the resulting up/downwards movements. I will look in sailing literature as recommended. However you suggest that there is an optimal path when being hit by a gust (or lull) and probably common mistakes that could be avoided. I guess some would be specific to windsurf. Anyone with a clue of that?
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Old 29th June 2011, 03:56 PM   #6
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Hi Farlo, well the general rule if trying to optimise the mix of velocity made good and ground to windward depending upon the point of sailing is to head up in the gusts and head down in the lulls or the other way about for the reciprocal course. Watch the gust patterns and learn how to best exploit. Sailing fast in apparent wind, that is exceeding the true wind speed, does change strategies a bit though as it is often possible to punch straight through by sheeting in hard while driving the front hand forward. Gust shifting can be very useful for tacking or gybing to get a free ride half way around. For setting longer courses with cellular patterns the same applies but with a lot more opportunistic gybing and tacking or waiting for the next useful lift or knock to arrive.

Perhaps the Formula guys on another thread can offer some insights as I am sure they are more up with the tactics than my more limited slalom back and forth kind of sailing!

Last edited by Philip; 29th June 2011 at 04:01 PM. Reason: clarity and corrections
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Old 1st July 2011, 12:56 AM   #7
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Philip,

I have been racing formula and longboards and course slalom for 24 years. Tactics aren't too complex when it comes to wind changes.

If headed upwind, generally ride out a wind shift to avoid two additional tacks, unless you need to tack anyway. If on the lay line, avoiding two extra tacks is really important, but as you near the mark, you may find that you still have to tack twice to make the mark. The decision to tack is best made near the mark, since a lift (wind shift) could bring you up to the mark when it didn't look like you would make it.

If heading upwind and sailing into a hole, generally bare off to maintain speed unless you can "read the water" and see that a tack will keep you in the wind.

Downwind, wind shifts are pretty hard to identify (see), but if you are losing speed, head up a little to maintain your speed (and plane), then make little adjustments back downwind if your speed stays up. If speed continues to drop when you were heading up a bit, you are either in a hole or the wind shifted significantly, in which case you gybe.

There are always exceptions to the above, especially if you are near a mark

For us average "Joe" formula racers, avoiding extra tacks and gybes is a good idea. Tacking and gybing are where mistakes are generally made, especially in windy conditions. Drop your sail once, and you are usually toast.

If it is really windy (20+ knots) and it is really rough, it's very difficult to see any changes in the wind (reading the water). Generally, you just feel it in your sail and you adjust accordingly.
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Old 1st July 2011, 04:50 AM   #8
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Hi Ken,

Thanks for the tips. And I have noticed in upper wind speeds the 'black patches' on the water (when it is not that rough) that are a sign that things are about to get more interesting. All of which of course is why sailing is so much fun.
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Old 1st July 2011, 07:40 PM   #9
Farlo
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My last question is not so much about tactics but rather how to maintain speed when crossing a gust (or lull). Based on your answers I figure the best trajectory is a kind of S-shape, trying to keep the same sailing angle. Thank both of you.
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Old 1st July 2011, 09:31 PM   #10
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Farlo, Philip,

Basically it's pretty simple. If speed is dropping, regardless of the cause (wind shift or hole), you head off or up closer to the fastest point of sail (about 110-120 degrees off the wind).

In gusty conditions, keeping on a steady line is almost impossible if you want to maintain maximum speed. Yes, you will most likely sail a snake-shaped course. This is true upwind and downwind as well as a beam reach. The only variation would be a beam reach where heading above 90 degrees would only be done if overpowered, since it will slow you down a bit, assuming your goal is maximum speed.

With the black patches, the angle of the sun can fool you into thinking the gust is bigger than it really is. I see this in the late afternoon when the sun gets lower and you are heading into the sun.

One racing lesson that takes a long time to learn is how high you can point without sacrificing too much speed. In other words, what is the fastest line to the upwind mark. Point too high and you lose too much speed. Point too low and you go fast, but don't gain upwind distance. Somewhere in between there is the "best line" and only experience will teach you where it is.
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