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.

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mkseven said..
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.

Frank Bethwaite presents quite a lot of research in "high performance sailing" on wind/local effects etc, and how they influence high performance sail craft. I doubt you'll find another book that goes into greater detail both in theory and experimentation.

it is from a couple of decades back though, and relates more to high performance skiffs and dinghies.

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

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tryharder said..
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

You didnt understand me,I didnt think put anemometers on sail,just use mast instead stick .
(I well know that air speed on leeward side of sail is much faster then windward,this is basic aerodynamics)

Idea is just measure wind speed up to 4m height from sea(height where sail is working).
I think that will not be difference between 1m and 4m anemometer..but fore sure if wave is 2m than anemometer at 1m will show less air speed,logical thing,because wave then change/disturb wind flow.

One of you mentioned situation where waves are 5m height,normally than wave will reduce wind speed in lower part sail.But I think in normal slalom situation,maybe max 50cm chop etc...

Speaking sail twist in windsurf sail is because of wind change with height is little bit uncorrect,because in 4m there is not much difference in speed.

In my opinion twist in windsurf sail use most of all to depower sail when gust hit,and make this on two ways:
1)twist reduce pressure difference on upper parts ,that reduce sail overall power
2) twist lowerd center of effort so overturning moment is also reduce,thus make sail more easy to drive without catapult when gust hits..

also twist in head/leech make sail more efficient,better lift/drag ratio,because reduce air that wonts excape from high pressure sail side(windward) to low pressure side (leeward).
Birds have two ways to reduce wingtip escaping air from high to low pressure side;
1)"winglets" like eagle,condor etc..airbus a350
2)"pointed wings" like falcons,albatros,swift,seabirds,all migrations birds etc boeing 777x

air escape form high to low side pressure


Air ALLWAYS moving from high pressure to low ..
Lift is force caused by difference in pressure on sail/wing sides.
Bigger the lift = bigger difference in pressure= more air escaping on sail head if head has no twist


high pressure air stremline escape to low pressure side of wingtip/sail head,because there is no twist or winglet or pointed wingtip which will reduce this phenomena.

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.

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tryharder said..
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.

If velocity is same up to 4m,why then in all text book wright that twist on wsurf sail is because of wind gradient(increase velocity with height) ?
In my opinion twist main job is to depower upper sail ..

If the top of the sail doesn't twist how does that affect your boardspeed?

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mkseven said..
If the top of the sail doesn't twist how does that affect your boardspeed?

When sail dosent twist(not enough downhaul ) ,sail push nose to water too much,so on the end you are slower even you have more horsepower in sail...
+ it is hard to control sail in gusty condition with out twist,so that also reduce your ability to go fast.
+sail has more aerodynamic drag

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.....

As I said it's simple black magic 101

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.

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mkseven said..
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.

Well said.

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olskool said..
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.....

With the rate at which they are building new high rise appartments to the north east of GB, and the trees on Bribie to east, find clear air could be as hard as find fresh air in Bankok. Maybe it will be necessary to rig to taller masts to get above the turbulent air. Something around ten metres or more. You will need a lot of twist to hang onto that sail when a gust hits! The other answer would be to migrate to Western Australia where they have lots of clean wind. Do you need a passport to go there?

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Chris 249 said..
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.

Didnt Frank Bethwaite prove it that the wind gradient(shear) 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)?

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

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Chris 249 said..
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

But wsurf sail is only 4m(when rake even lower) height ,not 30m.
What can be difference in velocity between head and lower part?

You dont agree with me that twist in wsurf sail is mostly to depower when gust hit,make sail easy to sail?

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Wing 11 said..

Chris 249 said..
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

But wsurf sail is only 4m(when rake even lower) height ,not 30m.
What can be difference in velocity between head and lower part?

You dont agree with me that twist in wsurf sail is mostly to depower when gust hit,make sail easy to sail?

Your still just thinking about wind speed over water not wind speed over sail. They are very different!

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Wing 11 said..

Chris 249 said..
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

But wsurf sail is only 4m(when rake even lower) height ,not 30m.
What can be difference in velocity between head and lower part?

You dont agree with me that twist in wsurf sail is mostly to depower when gust hit,make sail easy to sail?

You're relying on one thing that one person wrote. You also appear to believe you and Frank are the only people who have ever looked at wind shear. You're not. Good sail trimmers aboard boats are looking at it all the time. We know about it because we see it on sails day after day. A Laser sail is about as tall as a windsurfer sail and you can clearly see varying amounts of wind shear over a Laser sail as conditions change from place to place, such as going from coastal to inland sailing.

You haven't given a reference to your quote from Frank, so I can't check it. As noted before, while I greatly respected him in many ways he was prone to blind spots.

Wind shear is a complex issue. It's also affected by mast pitch in boats, which is probably one reason even small boat sailors are very alert to it. And it appears that you may not be aware of the fact that tiny differences in wind shear can have major differences in sail performance. Even if there's only (say) four degrees difference from the boom to the head, that can still be critical. The upper leach of a sail is often operating right on the edge of a stall. If you get the angle of attack wrong by just four degrees it can cause the aft leach to stall and that can really slow you down. In sail trim, fractions matter. Unfortunately for windsurfers, while the board rig is efficient in many ways they are imperfect in other ways and there is no really good way to adjust twist without adjusting draft etc, so we tend to ignore wind shear in some ways.

I didn't actually say what I thought twist in a windsurfer sail is mostly for. Yes, in many cases it's probably to depower. In speed, FW and slalom boards the extreme twist may also have effects like that of washout, but I've never bothered to check the aerodynamics. For personal reasons I sail boards that have fairly poor gust response in some ways. That means that I'm not as fussed about getting good gust response on my shortboards and personally I'd rather a tighter leach since I like a sail that generates more power for its size. That's purely personal, but it means I've never really looked into the reasons for extreme twist/washout in any detail. I know at least one world-class aerodynamics professor has indicated that sailors tend to be stronger on practice than theory so the theory that windsurfer sailmakers talk about may not be 100% correct.

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).

Any minute - wing sails.

3 - 2 - 1 ...........

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Wing 11 said..

tryharder said..
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.

If velocity is same up to 4m,why then in all text book wright that twist on wsurf sail is because of wind gradient(increase velocity with height) ?
In my opinion twist main job is to depower upper sail ..

The wind speed at the front of the mast will be exactly the same at the top or the bottom of the sail.

However, and this is where its different, the wind speed does increase across the sail , its just very hard to read.

As the sail twists, it also acts like a ramp for the wind to then run off.....like a bike down hill.....well, not like that at all but its using twist to accelerate.

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Madge said..

The wind speed at the front of the mast will be exactly the same at the top or the bottom of the sail.

However, and this is where its different, the wind speed does increase across the sail , its just very hard to read.

As the sail twists, it also acts like a ramp for the wind to then run off.....like a bike down hill.....well, not like that at all but its using twist to accelerate.

Can I ask for evidence that the windspeed is the same at the top as the bottom, and that the windspeed increases across the sail?

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Chris 249 said..

Madge said..

The wind speed at the front of the mast will be exactly the same at the top or the bottom of the sail.

However, and this is where its different, the wind speed does increase across the sail , its just very hard to read.

As the sail twists, it also acts like a ramp for the wind to then run off.....like a bike down hill.....well, not like that at all but its using twist to accelerate.

Can I ask for evidence that the windspeed is the same at the top as the bottom, and that the windspeed increases across the sail?

not really got any to hand. Years of sail making experience and using pressure and speed sensors in wind tunnels and rig testing on Sail Rocket is what I am using.

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?