A lot of supposition about twist also revolves around the head of the sail opening and closing in respose to wind strength. Good theory, but the problem with it is that it is very hard to see that happening much in any modern windsurfing race/slalom/speed sails.
Over the years I think some significant attempts have been made to get high dynamic twist. Few seem to have survived.
Winglets also in crease lift by stopping the air flowing of the end, air flow at end is higher than by body.
air flow on leeward side is faster and doesn't flow upwards anywhere near as much so battens are align for that not the slow moving air on windward.
Cover your sail in telltales and you'll see lots of them blowing at 90degrees or more to each other.
Select to expand quoteBen1973 said..
Cover your sail in telltales and you'll see lots of them blowing at 90degrees or more to each other.
That would be logical, especially on the windward side. That would be caused by turbulent airflow. How do they look in the Leeward side?
Select to expand quoteBen1973 said..
Cover your sail in telltales and you'll see lots of them blowing at 90degrees or more to each other.
A while since I've seen a telltale. Do they reflect the bulk of the flow or are they well immersed in the boundary layer that will grow out from the surface of the sail? Maybe the streamlines of the diagram are of the flow 1mm or so from the sail surface. A minor pressure pressure gradient generated by the bulk flow can easily bend the air very close to the sail surface if it's going slow enough. But it's the momentum change in the bulk flow that gives lift so why would they present us with the first mm? As Sailquick says it's a diagram that needs explanation. We're all guessing.
Could be analogous to what happens in the planetary boundary layer. The Eckman Spiral. The highs and lows set themselves up with the pressure gradient being matched by the Coriolis force. The wind mostly runs along the isobars. Friction with the ground slows the surface wind but pressure from above permeates to ground level. Pressure thus gets the jump on slower moving air and wind at ground level veers to the right of isobars going around lows and to the left of highs. Scaling from the boundary layer on the planet to that on a sail is stretching the imagination. But maybe that's what the mystery diagram is showing???
So much fun to argue about what we believe (or not), especially if we can bash marketing departments in the process!
For sail boats, there are a number of studies that looked at actual pressure measurements and CFD simulations, for example a master's thesis "Derivation of Forces on a Sail using Pressure and Shape Measurements at Full-Scale" at publications.lib.chalmers.se/records/fulltext/147899.pdf and another one at hal.archives-ouvertes.fr/hal-01071323/document
That's a bit different from the question Andrew was asking, though. I bet there have been some big studies for the America's Cup boats, but those are probably closely guarded.
Part of the problem might be the computational complexity. Good CFD simulations for fins are hard enough, but at least there, the speed is constant over the length of the fin, and the fin shape is (relatively) stable. For sails, the wind speed changes from bottom to top, and one of the most important performance aspects is how the sail reacts to gusts. Part of that reaction is a change in mast bend and therefore sail shape, which in turn changes the flow. Besides having a lot more variable in the simulation, they are closely coupled rather than independent, which creates a bit of a computational nightmare (and strongly limits the usefulness of "simple" CFD simulations). Reminds me a bit of my masters thesis, which was about calculating the result of interactions between coupled complex electronic oscillators in one dimension. A much easier problem 
.
In comparison, gathering empirical data about forces on the sail should be easier. What is the actual distribution of forces in a windsurf sail? How much does the loose top section contribute? How do things change in gusts? All we need is an aerodynamics professor and a windsurfing grad student in search of a thesis theme...
I didn't think one image would create such debate.
Just remember flow will be from high to low pressure and you need to think 3D. The high pressure on the windward side of the sail will flow under the foot of the sail to the lowest pressure on the leeward side which will probably be about a 1/3-1/2 way up the sail. These vortices are even more pronounced on raked sails but I see even AC boats had this effect.
Select to expand quoteHow do things change in gusts? All we need is an aerodynamics professor and a windsurfing grad student in search of a thesis theme...
What's a gust? It's not a uniform increase in windspeed from top to bottom. Google the physical interpretation of the Von Karman constant. The familiar K in the logarithmic wind profile but 100 years on nobody really knows what it is exactly. I remember a local guru at a conference 30 years ago saying " What this field needs is another Newton". He or she still hasn't tuned up. I don't think your average professor will cut it.
Select to expand quoteIan K said..
What's a gust? It's not a uniform increase in windspeed from top to bottom. Google the physical interpretation of the Von Karman constant. The familiar K in the logarithmic wind profile but 100 years on nobody really knows what it is exactly. I remember a local guru at a conference 30 years ago saying " What this field needs is another Newton". He or she still hasn't tuned up. I don't think your average professor will cut it.
A gust: I know one when I feel one. That's good enough for me, I don't need Newton to describe it for me.
What I'd love to know is how exactly the pressure in the sail changes in gusts, at different points in the sail. Measuring things can be done without a complete understanding of the underlying causes. Grad students are good for that 
. Measured numbers might shed some light on whether the loose top of the sail works primarily as a "drag reducer" like a winglet, or adds lots of extra forward momentum, or whatever else your favorite hypothesis is. Although it probably will be more a "how much" rather than a "yes or no" thing.
Thanks for this paper links Peter. 
As you said: not so much about sail twist, but lots of intering stuff anyhow. Might have a read them over again a few or a few dozen times though... 
Select to expand quotesailquik said..
I am gathering old, out of date smoke fares.
Sailquik, are we returning to the ol smoke n mirrors hypothesis again?
Im keenly following this thread. As theres no easy solution to explain the why n how of what we love so much.
Regarding seeing the sail leech OPEN on a gust. Dont think its visible bcoz, its more to do with the mast laying off n the leech tension governing/ aiding this dynamic of the sail in response to a gust.
Daffy, it says its blowing 32 knots ENE at the Prom. Why are you on here like us workers? 
No mirrors. Just smoke and action cams. 
I have done a lot of following close behind other sailors watching the sails and I have to say it's very hard to see any dynamic deflection, either sail or mast, or both from wind variation. Some can be seen when pumping, but very hard to see from wind gusts. Thats not to say it isnt there, just very hard to see.
Select to expand quoteka43 said..
Daffy, it says its blowing 32 knots ENE at the Prom. Why are you on here like us workers?
Its 3-4 knots at Sandy. Wind hasn't got over here yet. Also waiting for the tides. Should be good late this arvo!
Select to expand quotesailquik said..
No mirrors. Just smoke and action cams.
I have done a lot of following close behind other sailors watching the sails and I have to say it's very hard to see any dynamic deflection, either sail or mast, or both from wind variation. Some can be seen when pumping, but very hard to see from wind gusts. Thats not to say it isnt there, just very hard to see.
Easier to see from above.
Maybe we all need a 'Bow Sail'
i would love to be able to get my old 6.8m Rushwind slalom sail back and put it head to head with my 8.6 M2. Bet there's no difference in power.
Select to expand quoteBen1973 said..sailquik said..
No mirrors. Just smoke and action cams.
I have done a lot of following close behind other sailors watching the sails and I have to say it's very hard to see any dynamic deflection, either sail or mast, or both from wind variation. Some can be seen when pumping, but very hard to see from wind gusts. Thats not to say it isnt there, just very hard to see.
Easier to see from above.
Maybe we all need a 'Bow Sail'
i would love to be able to get my old 6.8m Rushwind slalom sail back and put it head to head with my 8.6 M2. Bet there's no difference in power.
Drone duty.
Select to expand quotesailquik said..
I am gathering old, out of date smoke fares.
How many do you need?
Select to expand quoteRAL INN said..sailquik said..
I am gathering old, out of date smoke fares.
How many do you need?
If you can spare a couple, that would be great!
Select to expand quoteka43 said..
Daffy, it says its blowing 32 knots ENE at the Prom. Why are you on here like us workers?
yep. Came through here alright. 20-30+ ENE. Tide was very late in the day and not that low so lots of scary rough water. had a few runs and decided to retire unhurt,
Whoops! From one of those papers linked above. Must have had one of the undergrads do the graphics labeling.
Here my take on things.
It's very simple.
Look at the America's Cup foilboats, then look at a windsurfer sailing next to it.
We need four dimensional twist incorporated in a truly 3D designed rig concept.
The human force, or better lack of it is the limiting factor. A large and heavy rider can take that limit a little up but he won't go much faster.
So the next thing to wonder about is the following: why design for fat old bastards if you could design for young and talented riders possibly not having the pure force of the fat and heavy?
The ideal windsurf sail balances itself through twist, leechcurvature, sidebend and shape. If well balanced no force is needed and glide will be the answer. Glide means effortless speedsailing in a new realm to be created by the next generation sails.
Select to expand quoteIan K said..How do things change in gusts? All we need is an aerodynamics professor and a windsurfing grad student in search of a thesis theme...
What's a gust? It's not a uniform increase in windspeed from top to bottom. Google the physical interpretation of the Von Karman constant. The familiar K in the logarithmic wind profile but 100 years on nobody really knows what it is exactly. I remember a local guru at a conference 30 years ago saying " What this field needs is another Newton". He or she still hasn't tuned up. I don't think your average professor will cut it.
Hey Ian,
It may be easier to define 'a gust' than it is to define 'your average professor'. Wind is an ensemble of gusts that can be quantified with wave number and frequency spectra. A professor is a human being who on average has one breast, one ovary and one testicle.
Select to expand quotesailquik said..
I have done a lot of following close behind other sailors watching the sails and I have to say it's very hard to see any dynamic deflection, either sail or mast, or both from wind variation. Some can be seen when pumping, but very hard to see from wind gusts. Thats not to say it isnt there, just very hard to see.
You could always attach a couple of 10 Hz GPS units, one on top of the mast and one near the boom. Maybe make the top one a phone with an accelerometer app. Then you'll just have to sort out the directions. With a ublox unit, you can look at velN and velE in GPSReader - best to align your runs going straight N-S or E-W.
If you're going SW-NE or similar, you'd have to transform the data to get the components in travel direction and perpendicular to it. Not really hard, but an extra step. On the bright side, once you figured out how to do that, you might be able to use a Motion. I don't think the .oao files have velN and velE, but you could calculated them from speed and doppler heading.
Select to expand quotesailquik said..
Whoops! From one of those papers linked above. Must have had one of the undergrads do the graphics labeling.
Hehe, Now that SIR, explains it ALL. 
Heres me cursing the DH. No twist...
Any wonder i cant gybe!!!
OH well, back to the drawing board....
Select to expand quoteboardsurfr said..
You could always attach a couple of 10 Hz GPS units, one on top of the mast and one near the boom. Maybe make the top one a phone with an accelerometer app. Then you'll just have to sort out the directions. With a ublox unit, you can look at velN and velE in GPSReader - best to align your runs going straight N-S or E-W.
If you're going SW-NE or similar, you'd have to transform the data to get the components in travel direction and perpendicular to it. Not really hard, but an extra step. On the bright side, once you figured out how to do that, you might be able to use a Motion. I don't think the .oao files have velN and velE, but you could calculated them from speed and doppler heading.
An interesting idea for sure.
But the math involved makes my eyes roll back.
One issue with this approach would seem to me to be: How would I isolate the whole rig movements from the twist and side bend? Possibly doable, by measuring movement at the boom, if the resolution of the velocity is extremely precise, but I am not sure that it is sufficiently.
And I would need quite a few of those loggers.
I think the key to sail performance is to reduce the vortex round the head and foot of the sail. The vortex, and hence drag, is caused by the movement of air from the high side (windward) to low side (leeward) of the sail, at the head and foot boundaries of the sail.
This is achieved at the foot of the sail by raking the sail aft and closing the gap between the foot of the sail and the deck of the board.
It is achieved at the head of the sail by reducing the angle and profile, through twist and reduced batten tension, of the sail to the wind, so that the windward and leeward pressure in the sail at the head is close to being the same.
Select to expand quoteBen1973 said..
Easier to see from above.
Maybe we all need a 'Bow Sail'
i would love to be able to get my old 6.8m Rushwind slalom sail back and put it head to head with my 8.6 M2. Bet there's no difference in power.
Have you seen any drone footage from above of a 'Bow Sail'? That would be interesting!
Select to expand quoteJohn340 said..
I think the key to sail performance is to reduce the vortex round the head and foot of the sail. The vortex is caused by the movement of air from the high side (windward) to low side (leeward) of the sail at the head and foot of the sail.
This is achieved at the foot of the sail by raking the sail aft and closing the gap between the foot of the sail and the deck of the board.
It is achieved at the head of the sail by reducing the profile and angle, through twist and reduced batten tension, of the sail to the wind, so that the windward and leeward pressure in the sail at the head is close to being the same.
Yep. That is one of the most widely accepted traditional theories. I dont think anyone has ever quantified it though. For instance, if those mechanisms worked optimally, but how much % would overall drag be reduced?
Or an even bigger question: Is there any evidence that top twist in a windsurfing sail even does that at all? (what we feel may be something completely different)
Just been out Felling some firewood
that does Feel good.
last weekend I tried chasing DAFF down the course my overpowered 4.7wave sail had a plenty loose leach and at speed and locked in was flapping as if there was no pressure on it From any where
