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There is lots of theory/text that sail twist use to adapt sail angle of attack because in top part wind is blowing stronger,so top of sail "feel" more true wind so must have twist...
I find some theory ( Frank Bethwaite high performance sailing) that say this is only myth.
"The wind gradient is not significant for sailboats when the wind is over 6 knots (because a wind speed of 10 knots at the surface corresponds to 15 knots at 300 meters, so the change in speed is negligible over the height of a sailboat's mast). According to the same source, the wind increases steadily with height up to about 10 meters in 5 knot winds but less if there is less wind. That source states that in winds with average speeds of six knots or more, the change of speed with height is confined almost entirely to the one or two meters closest to the surface.[39] This is consistent with another source, which shows that the change in wind speed is very small for heights over 2 meters["
But it is very easy to find out.Just buy 4 anemometers and fixed to bare mast on 1m,2m,3m and 4m height.Go on sea and read the velocity,if velocity are the same then this theory fall down.
So I dont understand why so much converstion about this ,if it so easy to find out...
I think that anemometes will show approx same veloctiy from 1m to 4m...
Hi Wing11.
I am curious as to why you posted real beginner questions not long ago, and now suddenly such technical stuff.
The twist post looks very familiar and there has been some talk of wing sails here (just a little.... 
) so what's the deal?
Surface where- surface on lake garda where it is mirror flat & 10 knots 1 metre up, surface on moreton bay with 0.5m of turbulant wind & 20 knots 1.5m up, or open ocean where you drop into a trough with no wind & 30 knots 5m up when you get back to the top of the swell.
The statement posted isn't very scientific, where did they measure, what was the surface like. Anyone that sails open ocean or in 50 knot winds will tell you there's a massive difference between low & just a bit higher, anyone that sails not far downwind of trees or dunes or hills can describe the feeling of wind buffeting high up on sail yet it's fine between your hands.
That same book should also describe the distance downwind that is affected by an object of x height.
For those that sail slow or in dead flat very open areas they probably dont need much twist, for the rest of us it is unless you love catapulting.
The tips of an Eagles wing twists away more than near its body. This isn't to compensate for the wind being greater further away from the birds body. I think there is more to a sail twisting off at the top than what you portray.
Airflow also travel up the sail as well as across it
anemometers on a bare mast wouldn't work as is it's the foil shape of the sail that affects the wind speed and direction, easiest way to see it is the old fashioned tell-tails stuck on the sail.
i do think that most have gone to far with the twist though and have just made under powered sails
This does seem a bit of a pointless discussion but I'll add my bit anyway.
Wind speed does increase as it hits the sail and therefore rises too.
Thats why we usually have battens at a higher angle higher up the sail, so the air flows over them at the same angle.
The twist does two things, it acts like a ramp for the air flow to accelerate off and its a buffer from gusts and makes the rig more reactive to handling........or easier to use when a gust hits.
Hope this helps a little.
Air speed at 1meter compaired to 4meters would be very very similar unless it's flowing over a sail.
Think of it as the air speed at the bottom of the sail vs air speed at top of sail rather then the air speed above the ground.
i agree that most sails have taken the twist to far and sails are loosing power but it does make easier to use.
I'm not that good , but I sail gusty .
To me it seems the more downhaul the more twisty the sail is . Meaning it's less powerful but can handle overpowering gusts. Less downhaul gives me more power in a top heavy catapulting way . I also kinda get that feel using the upper and lower clew options on my sails. Used that option yesterday that kinda saved my ass . Funny how a couple inches changes my ability to survive at the cost of upwind ability by a couple of degrees.
My measly opinion , that I thi nk I can feel , is the twist in a sail is for gust neutralising , for the average person.
Im sure in top speed aerodynamics it has some other kind of black magic.
My guess is the mast tip bends sideways, (off the wind) as well as backwards when overpowered by a gust. 
This twists the sail in a manner which affects something, which affects another thing, which in turn affects something else, which makes the real effect obscure from immediate recognition or explanation...
Pretty sure thats exactly how it works. This sail tuning stuff is a breeze.....
I recon with too much alcohol and stuff around a camp fire gazing at the stars we could work it out.
Im in , for scientific purposes .
What you're talking about is wind shear (IIRC) and it changes quite dramatically from time to time and place to place, so there are no hard and fast rules.
Sometimes you get a fairly steady stream of air flowing over a surface, like warm air flowing over a cool sea. The theory is that in such conditions, the air lower down is slowed by friction against the surface but the air up top is not slowed as much. If the air up top is doing 20 knots and the air down low is doing 5 knots, you will need lots of twist because the apparent wind is coming from a different angle due to the difference in vectors.
On the other hand, if you get get cold air flowing over a warm surface, the hot air will rise and mix with the cooler upper layers. That means that the airstream is mixed, therefore the upper air will not be moving much faster than the lower air. In such situations, there is less difference in the apparent wind angle at the top of the rig compared to the bottom of the rig and you need less twist.
This is really noticeable on yacht rigs, which are tall and where you have teams of people who just observe and trim the sails and have nothing else to worry about. It's also noticeable on dinghies on inland waters. If I'm sailing inland on a coolish day, where the lower air is very slowed by friction (so much that the water can be glassy even when there's wind at the top of the sail) I have to induce as much twist as a I can - much more than when sailing on the ocean.
Frank Bethwaite also did a test in light winds using a Tasar off Northbridge in Sydney Harbour. The ribbons on the mast showed enormous variation between the top and the bottom of the rig. At the same place on another day, when the wind is different, there's almost none.
With boards, we're stuck with little ability to control twist without affecting the rest of the sail, so we have to live with it. Boats have far more adjustments and good boat sailors allow for the way the wind shear affects sail trim all the time. The Kiwis even built a special wind tunnel with twisted windflow to allow for it, and I think the Italians and others have copied them.
This twists the sail in a manner which affects something, which affects another thing, which in turn affects something else, which makes the real effect obscure from immediate recognition or explanation... This sail tuning stuff is a breeze.....
I knew Frank for many years; I first watched his tests when I was at school and last saw him a few weeks before he died.
I really liked Frank as a person, but he was known to run away a bit with his theories. I had first-hand experience with it. I don't think he ever had the resources or the desire to measure twist as much as the America's Cup teams did.
I've sailed with America's Cup sailmakers and trimmers - they watch wind shear all the time. Sure, we're only talking a few degrees, but that is critical. This is a rather general piece that mentions their studies on shear;
people.eng.unimelb.edu.au/imarusic/proceedings/12/Flay.pdf
If we are talking about twist it may be better to consider that when heading upwind the apparent wind is high and the twist helps deal with gusts, when heading off the wind the head of the sail is bleeding less power when the apparent wind is less. There are other things going on here which I am sure a google search or seabreeze search will uncover. Here is one I found. It might be from a US forum. I have a very limited grasp of what is actually going on (with sails AND other stuff).
Ah, okay. That sounds good, but on the other hand guys like Grant Simmer (sailmaker, Australia II crewman and the head honcho of design for winning America's Cup teams) and Prof Richard Flay from TNZ and Auckland Uni say the opposite. Flay's paper on the first twisted wind tunnel was published in the Journal of Wind Engineering & Industrial Aerodynamics, which I assume to be an reasonable authority.
Tom Speer, Boeing wing designer and wingsail designer for America's Cup winners, also notes that the apparent and true wind both vary over a mast's height. If he, Richard Flay, Grant and the other guys involved in the AC efforts worth hundreds of millions of dollars are wrong, I would be very surprised.
Of course, if you're using a wind tunnel you won't get natural shear anyway, will you? Isn't that why TNZ and Auckland uni went to the trouble of building the twisted flow tunnel?
Marchaj plotted wind shear, although I admit I can't recall his source;
Yacht instrument makers like Nexus write about it;
www.blur.se/2009/02/09/wind-sheer-fact-or-fiction/
as does Ockam, who not only write about it here ( www.ockam.com/2013/06/02/wind-shear/ ) but show what appears to be wind shear (below) as does Frank with the unrigged Tasar pics in Higher Performance Sailing.
And a paper published in the journal of the Royal Meteorological Society, using physical measurements, concluded that "there is evidence that significant variations in mean wind direction do occur within a few tens of metres of the surface" although it did raise doubts about the conventional sailor's explanations which is interesting. However, that was a single test on a single day and it would seem likely that the multi-million dollar America's Cup efforts involving aerodynamics gurus were closer to the truth.
So, with respect, there seem to be plenty of people with significant experience who believe that wind shear does exist. It'd be interesting to hear about Sailrocket. Of course at that sort of boatspeed wind shear would seem to be be tiny so it seems logical that it wasn't a factor. However, it does seem to be significant at lower speeds.
@chris249
Nobody said the wind shear do not exist,just that wsurf sail is to small to feel sinigficinat differnece in wind speed,this 3-4m difference in height is to small...
@Madge
"The wind speed at the front of the mast will be exactly the same at the top or the bottom of the sail."
I agree with you...
Did you ever have measurment at higher altitudes ,30m etc, how much is there differnece in speed, compare to bottom?
@al planet
In this few senteces ,evertyhing about wsurf twist is well done explain.Straight to the point.
Where you find this text ,link?
