I'll throw, another effect in here just to confuse things.
After varies experiences at places with glassy water due to oil from weed, It's apparent that the amount of chop on the water affects wind gradient. At Lake George and other places I've sailed, the water can be pure glass at 20kts.
Counter to inexperienced LG sailors, you go slower when you hit what looks like a gust (small ruffles on the water) and faster when you come back into the glass.
A kiter mate said it was very strange launching a kite there, the usual increase in wind force on the kite as it went up, wasn't there. He said the wind must be laminar, there's no wind gradient.
And yet we aren't changing the amount of twist we tune our sails for to go fast in the glass. (maybe we should?)
I have a mate that says it's got to be faster on ball bearing chop, because of reduced board drag, but I haven't experienced that. for me it's faster in the glass because of laminar wind, creating more force at the bottom of the sail.
Select to expand quotedecrepit said..
I'll throw, another effect in here just to confuse things.
After varies experiences at places with glassy water due to oil from weed, It's apparent that the amount of chop on the water affects wind gradient. At Lake George and other places I've sailed, the water can be pure glass at 20kts.
Counter to inexperienced LG sailors, you go slower when you hit what looks like a gust (small ruffles on the water) and faster when you come back into the glass.
A kiter mate said it was very strange launching a kite there, the usual increase in wind force on the kite as it went up, wasn't there. He said the wind must be laminar, there's no wind gradient.
And yet we aren't changing the amount of twist we tune our sails for to go fast in the glass. (maybe we should?)
I have a mate that says it's got to be faster on ball bearing chop, because of reduced board drag, but I haven't experienced that. for me it's faster in the glass because of laminar wind, creating more force at the bottom of the sail.
In the video looks like most sails have a nice even twist except @1.28 in Phil's LG Video, the guy on the Dutone sail, over downhauled?
Select to expand quotechoco said.. In the video looks like most sails have a nice even twist except @1.28 in Phil's LG Video, the guy on the Dutone sail, over downhauled?
I guess that depends which theory is correct.
Some say that floppy stuff is just drag, some say it's a barrier to stop tip vortex.
Both cases have been presented here, but I see no data, indicating the validity of either argument.
Personally I think the tip vortex is highly likely, but obviously can be overdone.
We know from aviation design, that tip vortex barriers save airlines money. but how effective is the floppy tip at doing that, is the question.
Just doing a thought experiment, if the twist is even, there's still wind pressure at the head, if it's fluttering, there is 0 angle of attack and the wind is symmetric both sides of the sail, so that at least means there's no lift where it's fluttering, so there will be no high and low pressure side, and no flow over the top of the sail
Select to expand quotechoco said..decrepit said..
I'll throw, another effect in here just to confuse things.
After varies experiences at places with glassy water due to oil from weed, It's apparent that the amount of chop on the water affects wind gradient. At Lake George and other places I've sailed, the water can be pure glass at 20kts.
Counter to inexperienced LG sailors, you go slower when you hit what looks like a gust (small ruffles on the water) and faster when you come back into the glass.
A kiter mate said it was very strange launching a kite there, the usual increase in wind force on the kite as it went up, wasn't there. He said the wind must be laminar, there's no wind gradient.
And yet we aren't changing the amount of twist we tune our sails for to go fast in the glass. (maybe we should?)
I have a mate that says it's got to be faster on ball bearing chop, because of reduced board drag, but I haven't experienced that. for me it's faster in the glass because of laminar wind, creating more force at the bottom of the sail.
In the video looks like most sails have a nice even twist except @1.28 in Phil's LG Video, the guy on the Dutone sail, over downhauled?
?si=qXfGI9rd9N8448uh
Choco, that's me @ 1.28. Definitely not over down hauled. I clocked a 36.8 2sec on 7.7 and 115 during this session and held my own against some very good sailors on the golden lake that day.
Select to expand quoteJohn340 said.. Choco, that's me @ 1.28. Definitely not over down hauled. I clocked a 36.8 2sec on 7.7 and 115 during this session and held my own against some very good sailors on the golden lake that day.
Which adds credence to the floppy tip reducing tip vortex theory.
Select to expand quoteBasher said..SchobiHH said..
OMG, what did you start.... I didn't know how many sail makers we have on this forum. Do you really expect any insight from all of these experts.
Actually, there are some good comments in this thread with some racing sailors contributing.
I'm certainly well qualified to talk about this stuff as someone who worked for a windsurf sailmaker for many years.
With a strong sailing background I also worked as a professional yacht rigger, and am a former world champion in a dinghy class pictured on this page. But these are contentious issues where there is often a lot of disagreement.
I remember doing an Olympic campaign one year and we'd developed some fuller sails for our 470 dinghy, looking for better lightwind performance. The new sails seemed very powerful, and our main concern was how manageable they'd be in strong winds. ... And then a French pair turned up at one light wind European regatta, and they won all the races with sails that were much flatter than ours, and with a more open leech.
And that was a lesson that sail efficiency often trumps sail power.
Discussions about sail twist will probably go on forever.
I'm surprised that no one has yet mentioned the differences between our windsurf foiling rigs, compared to our slalom sails.
High aspect rigs have made a comeback in foiling, simply because of apparent wind issues when you sail in light airs on a craft that can move faster than the true wind of the day. The relatively stronger 'created' wind means the sails can have a tight leech.
Foiling sails can also have a shorter boom because the sailor is no longer driving the board off the fin, and foiling stance is very different because of upward lift from the foils.
Are there really any good comments in this thread? For me that is clearly a No. To many things mixed up.
The whole thing in my judgement is not so complicated. (Having a Physics and Math Masters btw shall qualify to add to the discussion)
There is a foil, i.e. the sail. The foil is mainly defined be the aspect ratio and the camber depth.
Aspect ratio is about efficiency of the foil. The higher the more efficient. Camber depth is about more lift with more camber. If you would have steady wind. We could stop the whole discussion here.
Many talk about the wind gradient is done here. But assuming a 90? course in 20knts of wind and assuming that you have 1-1.5 times the windspeed as boatspeed. The change of apparent wind angle is about 5? . So not really a deal when it comes to "twist".
But what is true is that the wind is stronger the higher it is, but also the wind is of higher quality i.e. less turbulence. That is why high aspect ratio sails are actually wanted. Get the wind from higher. Look the americas cup. These sails are crazy high (35m) now.
So next, what do you want as a sail designer. You want to have a sail that can "depower". Because we do not have steady wind.
Depower means 2 things: Angle of Attack change (i.e. twist ) as less AoA means less lift, and 2nd chamber depth change. As less chamber depth means less lift.
So the current approach in windsurfing foils, i.e. sails. is to have so much sail area outside the triangle of the sail to achieve that. That is all to discussion. We could stop here again.
This whole sail area allows for excessive AoA change and chamber depth reduction. And practice shows that you can achieve what you want. But if comes with a price. That you actually have much bigger sails than you actually need. The art of sail design is that you balance this properly for racing sails you make it much bigger as this allows to get the maximum effect. And with wave sails, you try to minimize it as size is always an issue.
So actually the whole thing is really boring. Because there is not so much physics behind all this. And that is also why sails haven'T really improved in the last 15 years. (The rdm Mast has made a change in the sail design, because it allows for higher bending curves) The magic comes from the fine tuning. And I hate if a sail designer changes a sail (just because marketing dept needs something new) . There are some really good years with some sails and some really bad ones. Just because of "little" changes,
Am I really missing something with that argumentation?
Select to expand quoteRoo said..mr love said..
A big problem with comparing windsurf rigs with yachts is a simple fact...yacht rigs are held up with stays and weight is used to counter the healing moment. On a windsurfer we are holding the rig up unassisted. If you used a sail like the A class on a windsurfer it would be like wresting a crocodile, you would be pulled over. Having a twisted sail means the top of the sail which has the most leverage over us runs at a low angle of attack or even zero angle of attack so produces low lift and less healing moment. Most of the drive is coming from the middle and bottom of the rig which has less leverage. This allows large sails with very power profiles in the bottom half that don't pull us over. You do need some twist in a sail on high speed sailing craft as it lowers drag, if you didn't have it the top of the sail would stall, this is more pronounced downwind but yes we use very large amounts...due to our unique ergonomics.
@mr love is right, it's all about being able to control and handle the rig by reducing the heeling moment. I remember when we first discovered it on a dry lake bed in California back in the late 1980s with a device called the ADTR. It was one of those aha moments that dictated the future design trends for windsurfing sails. Other designers had been looking at twist but for the first time we had real data that showed the improvement. The extra material at the top of the sail defines the twist characteristics that results in a bell curve lift distribution profile very similar to what Prandtl discovered back in the 1920s. The twist/washout in the sail also improves the stability of our sails due to a phenomena called proverse yaw.
Myself, Pascal Maka and Jeff Magnan
Was there a video made about this? I remember reading about it in the magazines
Select to expand quotesaid..Basher said..SchobiHH said..
OMG, what did you start.... I didn't know how many sail makers we have on this forum. Do you really expect any insight from all of these experts.
Actually, there are some good comments in this thread with some racing sailors contributing.
I'm certainly well qualified to talk about this stuff as someone who worked for a windsurf sailmaker for many years.
With a strong sailing background I also worked as a professional yacht rigger, and am a former world champion in a dinghy class pictured on this page. But these are contentious issues where there is often a lot of disagreement.
I remember doing an Olympic campaign one year and we'd developed some fuller sails for our 470 dinghy, looking for better lightwind performance. The new sails seemed very powerful, and our main concern was how manageable they'd be in strong winds. ... And then a French pair turned up at one light wind European regatta, and they won all the races with sails that were much flatter than ours, and with a more open leech.
And that was a lesson that sail efficiency often trumps sail power.
Discussions about sail twist will probably go on forever.
I'm surprised that no one has yet mentioned the differences between our windsurf foiling rigs, compared to our slalom sails.
High aspect rigs have made a comeback in foiling, simply because of apparent wind issues when you sail in light airs on a craft that can move faster than the true wind of the day. The relatively stronger 'created' wind means the sails can have a tight leech.
Foiling sails can also have a shorter boom because the sailor is no longer driving the board off the fin, and foiling stance is very different because of upward lift from the foils.
Are there really any good comments in this thread? For me that is clearly a No. To many things mixed up.
The whole thing in my judgement is not so complicated. (Having a Physics and Math Masters btw shall qualify to add to the discussion)
There is a foil, i.e. the sail. The foil is mainly defined be the aspect ratio and the camber depth.
Aspect ratio is about efficiency of the foil. The higher the more efficient. Camber depth is about more lift with more camber. If you would have steady wind. We could stop the whole discussion here.
Many talk about the wind gradient is done here. But assuming a 90? course in 20knts of wind and assuming that you have 1-1.5 times the windspeed as boatspeed. The change of apparent wind angle is about 5? . So not really a deal when it comes to "twist".
But what is true is that the wind is stronger the higher it is, but also the wind is of higher quality i.e. less turbulence. That is why high aspect ratio sails are actually wanted. Get the wind from higher. Look the americas cup. These sails are crazy high (35m) now.
So next, what do you want as a sail designer. You want to have a sail that can "depower". Because we do not have steady wind.
Depower means 2 things: Angle of Attack change (i.e. twist ) as less AoA means less lift, and 2nd chamber depth change. As less chamber depth means less lift.
So the current approach in windsurfing foils, i.e. sails. is to have so much sail area outside the triangle of the sail to achieve that. That is all to discussion. We could stop here again.
This whole sail area allows for excessive AoA change and chamber depth reduction. And practice shows that you can achieve what you want. But if comes with a price. That you actually have much bigger sails than you actually need. The art of sail design is that you balance this properly for racing sails you make it much bigger as this allows to get the maximum effect. And with wave sails, you try to minimize it as size is always an issue.
So actually the whole thing is really boring. Because there is not so much physics behind all this. And that is also why sails haven'T really improved in the last 15 years. (The rdm Mast has made a change in the sail design, because it allows for higher bending curves) The magic comes from the fine tuning. And I hate if a sail designer changes a sail (just because marketing dept needs something new) . There are some really good years with some sails and some really bad ones. Just because of "little" changes,
Am I really missing something with that argumentation?
There's some good comments there, but I wouldn't say there's not much physics involved. There's actually a lot of physics involved, but the required physics involved are that well locked in, that sails don't really change drastically year to year because I guess, sail designers don't want to get too far away from what they know works, which I guess is what makes it "boring". The physics of the matter is the elephant in the room (of this thread) as far as I'm concerned. A few have tried to bring it to light, but have been cast aside (mr love, Roo etc) by others.
What's lost in this thread to all the discussion on aerodynamics and related numbers/formulas for sail design, is the reality that a windsurf sail has the sailor directly tethered to it, trying to deal with all the wind variables and keep the sail in a relative static position, as well as the sail being attached to a board that is directly interacting with chop and water state (yes, others have stated that too). So whilst there maybe an Uber efficient aerodynamic shape for a sail to be from an aerodynamic standpoint, leech twist or not, other factors have to be applied when the sailor is a major part of the link to transfer the sails power to forward movement. Other well respected sailors/designers have discussed this over the years on seabreeze, and have stated that leech twist (in a windsurfing sail) has largely to do with the benefit of lowering the centre of effort in a sail, which makes it possible for a sailor to control and benefit fully from the power a given sail size might provide.
The tight leeched windsurf sails of yester year developed to a workable point, but they were nowhere near as user friendly as modern windsurf sails are.
note: I'm not at all a qualified physicist, or "aerodynamics expert" just a keen windsurfer and sailboat sailor, who takes an interest in what makes the things I like to do work.
thanks to SurferKris and aeroengr for the articles, interesting reading
@AusMoz can't ever remember anyone coming out to film it, a few photographers as we were near Windsurf magazine's offices.
As @Subsonic said it has a lot to do with control and being able to handle the power. Today's sails are a lot more powerful than the ones from the late 1980s yet easier to handle so we go faster....pretty simple.
Nice thing was the sails we tested on the ADTR I got to use on the water. This was the sweetest of them, the 4.3.
Roo, Sotavento
Drag racing for Surf Magazin/Windsurf/Planche Mag test. Roo & Benoit Treguilly
Select to expand quotePaducah said..peterowensbabs said..
Yacht and dinghy sailing we always tryed to eliminate any interruptions to the laminar flow...
Laminar flow, in reality, occurs much less frequently than we assume. Mostly, when people say "laminar flow", they actually refer to keeping the boundary layer attached. Keeping the flow laminar is extremely difficult and not necessarily a desired characteristic as laminar flow detaches quite easily and becomes turbulent. That's why vortex generators can be very effective is that they energize the air flow making it less likely to turn turbulent.
The much vaunted P-51 Mustang and it's laminar flow wing, in practice, was more often than not, non-laminar The Germans didn't pursue laminar flow wings because, in practice, they didn't feel comfortable that they could mass produce a wing smooth enough. The turbulence and handling penalties of detached flow outweighed the possible benefit of a laminar flow wing.
I'm not suggesting that a rough surface is ideal nor that efforts shouldn't be made to smooth flow. After all, even non-laminar flow wing designs greatly benefited from flush rivets, etc. But, laminar flow is a difficult thing to achieve in the real world and requires modern manufacturing techniques and absolutely clean and dry wings.
For a deep dive (5-8 min in) below. If you want the 30 second version: ww2aircraft.net/forum/threads/laminar-flow-airfoil.36417/?post=996485#post-996485
As I said...no expert, but it was always referred to as laminar flow you may be correct and its miss named (nit picking?) but the concept is what's relevant here, smoother airflow over both sides of the sail no interruptions no sudden changes of direction of air flow, we used 'tell tails' to visualise the flow over the sails, sometimes far too many sets and any turbulence was adjusted for or at least attempted to be. Not a sail maker nor a scientist nor an engineer just an observer.
We are not alone trying to work out all this stuff-forums.sailinganarchy.com/threads/why-windward-middle-and-top-tell-tales-stream-up-at-jib.238735/page-4
www.wumtia.soton.ac.uk/services/wind-tunnel-testing/sailing-yacht-aerodynamics#:~:text=Wind%20tunnel%20tests%20can%20be,rig%20strength%20calculations%20and%20stability.
Even multi million dollar big boats get it wrong, one 100footer at the yard I'm at has a multi million dollar mainsail that is useless it's going to be got rid of.
Subsonic...If you scroll back in the thread this is exactly why I said twist works on Windsurf sails. We physically hold the sail up, no stays, no mast mounted into a hull, if we let go the rig falls over. For us to be able to do that the sail needs to have the power low so we dont get pulled around or pulled over or exhausted after a couple of minutes. So high aspect rigs as efficient as they are just dont work in our situation.
To test this out go for a sail with less dowhaul than recomended, a tight leech and see how your rig feels when a gust hits. I will guarantee it is heavy , too physical and pulls you around.
The twist allows us to have a sail that is managable upwind but still has sufficient power to sail deep off the wind. If we used a smaller tight leeched sail upwind would be OK but we would be very underpowerd downwind. So the twist is effectively letting us go upwind with a sail that is effectively too big but perfect when we bear off and sail accross or downwind.
So regardless of any arguments about the twist and aerodynamics, a twisted sail works on a windsurfer because of the ergonomics.
Select to expand quotemr love said..
Subsonic...If you scroll back in the thread this is exactly why I said twist works on Windsurf sails. We physically hold the sail up, no stays, no mast mounted into a hull, if we let go the rig falls over. For us to be able to do that the sail needs to have the power low so we dont get pulled around or pulled over or exhausted after a couple of minutes. So high aspect rigs as efficient as they are just dont work in our situation.
To test this out go for a sail with less dowhaul than recomended, a tight leech and see how your rig feels when a gust hits. I will guarantee it is heavy , too physical and pulls you around.
The twist allows us to have a sail that is managable upwind but still has sufficient power to sail deep off the wind. If we used a smaller tight leeched sail upwind would be OK but we would be very underpowerd downwind. So the twist is effectively letting us go upwind with a sail that is effectively too big but perfect when we bear off and sail accross or downwind.
So regardless of any arguments about the twist and aerodynamics, a twisted sail works on a windsurfer because of the ergonomics.
Makes complete sense Martin, which is unsurprising from a sail designer.
Recently I've been exploring wind foiling with small sails, using less downhaul to tighten the leach extend the low wind capability of a sail. It works until it doesn't. As soon as the wind reaches a threshold the sail becomes unmanageable. Add downhaul and the sail transforms.
Raceboard, Formula and RSX sailors have been using this trick years, adjusting downhaul on the fly. I suspect IQ foilers do the same.
Select to expand quoteJohn340 said..mr love said..
Subsonic...If you scroll back in the thread this is exactly why I said twist works on Windsurf sails. We physically hold the sail up, no stays, no mast mounted into a hull, if we let go the rig falls over. For us to be able to do that the sail needs to have the power low so we dont get pulled around or pulled over or exhausted after a couple of minutes. So high aspect rigs as efficient as they are just dont work in our situation.
To test this out go for a sail with less dowhaul than recomended, a tight leech and see how your rig feels when a gust hits. I will guarantee it is heavy , too physical and pulls you around.
The twist allows us to have a sail that is managable upwind but still has sufficient power to sail deep off the wind. If we used a smaller tight leeched sail upwind would be OK but we would be very underpowerd downwind. So the twist is effectively letting us go upwind with a sail that is effectively too big but perfect when we bear off and sail accross or downwind.
So regardless of any arguments about the twist and aerodynamics, a twisted sail works on a windsurfer because of the ergonomics.
Makes complete sense Martin, which is unsurprising from a sail designer.
Recently I've been exploring wind foiling with small sails, using less downhaul to tighten the leach extend the low wind capability of a sail. It works until it doesn't. As soon as the wind reaches a threshold the sail becomes unmanageable. Add downhaul and the sail transforms.
Raceboard, Formula and RSX sailors have been using this trick years, adjusting downhaul on the fly. I suspect IQ foilers do the same.
Some of the top ones use adjustable downhaul for that reason (iqfoil). I asked one to show me his setup and asked him when he changed it. He said he would between races or heats if the wind changes. I guess that's a plausible good reason for an electronic downhaul setup.
Tweaking that big 9.0 on the water, mostly outhaul but downhaul sometimes, really helped my feel for when a sail needs something. I'm surprised sometimes by how much dropping the clew eyelet (more twist), dropping the boom, and adding some downhaul can make something scary much more manageable.
Yep and in certain circumstances a tighter leech is more efficient, race board in displacement mode punching through chop upwind for example. However if we are talking about what most people do, blasting fully planing, then twist is your friend. It can be taken too far though and there are definitely some sails which IMO have too much.
I am in awe of how the iQ foil guys hang onto those massive sails in 20 knots.....try that without lots of twist. I was out foiling a few weeks ago in 20 plus knots fully powered on a 4.7 wave sail and they were flying around on 9s
Chris...If you want an example of a yacht class that does something similar to a modern windsurf rig look at the Star. Massively overpowered when the wind is up they use the backstay to bend the mast and twist the sail off upwind but have lots of power downhill without a spinnaker.
Select to expand quotemr love said..
Subsonic...If you scroll back in the thread this is exactly why I said twist works on Windsurf sails.
Yep, that you did. Select to expand quoteSubsonic said..
A few have tried to bring it to light, but have been cast aside (mr love, Roo etc) by others.
Sorry if i buried that acknowledgment too deep in my wall of text.
Select to expand quotemr love said..
I am in awe of how the iQ foil guys hang onto those massive sails in 20 knots.....try that without lots of twist. I was out foiling a few weeks ago in 20 plus knots fully powered on a 4.7 wave sail and they were flying around on 9s
Absolutely true. Flappy floppy is a good de-powering device.
The iq is also speed limited by its foils, so there isn't any advantage in a sail that can go any quicker than the foils.
Now take the same sailor, put them on a faster set of foils and a smaller taughter sail and they will go a lot quicker.
There isn't any argument about this is there?
Select to expand quotemr love said..
Chris...If you want an example of a yacht class that does something similar to a modern windsurf rig look at the Star. Massively overpowered when the wind is up they use the backstay to bend the mast and twist the sail off upwind but have lots of power downhill without a spinnaker.
So the twist is to deal with being massively overpowered. That isn't the same as being efficient or even desirable in a wider context.
Personally I prefer to sail comfortably powered up and can and do change down sail sizes. Normally smaller sails are lighter and more fun to use, so it makes sense to use a smaller taughter sail than a bigger flappy floppy no? Unless you have a specific goal, such as upwind downwind racing where there is no substitute for area to go deeper and faster?
Select to expand quotemr love said..
Yep and in certain circumstances a tighter leech is more efficient, race board in displacement mode punching through chop upwind for example. However if we are talking about what most people do, blasting fully planing, then twist is your friend. It can be taken too far though and there are definitely some sails which IMO have too much.
Amen
Select to expand quotemr love said..
.
To test this out go for a sail with less dowhaul than recomended, a tight leech and see how your rig feels when a gust hits. I will guarantee it is heavy , too physical and pulls you around.
.
Well yes, of course. You are using a sail designed to be used in a certain way, in a different way, it's pretty obvious that the risk of failure is high.
Now design the sail to be used with that firmer back edge, optimise it like the other one has been, adjust to the different handling and let's talk about it again afterwards. ( Like has been happening with wave sails)
Select to expand quotesheddweller said..mr love said..
.
To test this out go for a sail with less dowhaul than recomended, a tight leech and see how your rig feels when a gust hits. I will guarantee it is heavy , too physical and pulls you around.
.
Well yes, of course. You are using a sail designed to be used in a certain way, in a different way, it's pretty obvious that the risk of failure is high.
Now design the sail to be used with that firmer back edge, optimise it like the other one has been, adjust to the different handling and let's talk about it again afterwards. ( Like has been happening with wave sails)
I've used a well designed tight leach sail in good condition from 2011 and a modern well designed loose leach sail from 2022. Both 4m wave sails (North Voodoo & Naish Force 5). I know which sail I'd rather use.
Select to expand quoteJohn340 said..sheddweller said..mr love said..
.
To test this out go for a sail with less dowhaul than recomended, a tight leech and see how your rig feels when a gust hits. I will guarantee it is heavy , too physical and pulls you around.
.
Well yes, of course. You are using a sail designed to be used in a certain way, in a different way, it's pretty obvious that the risk of failure is high.
Now design the sail to be used with that firmer back edge, optimise it like the other one has been, adjust to the different handling and let's talk about it again afterwards. ( Like has been happening with wave sails)
I've used a well designed tight leach sail in good condition from 2011 and a modern well designed loose leach sail from 2022. Both 4m wave sails (North Voodoo & Naish Force 5). I know which sail I'd rather use.
I used a 5.6 sail recently that 3 separate people told me was excellent. I have to say it was probably the worst sail i have ever used. I almost never believe other people anymore, with regards to MY personal experience. However, I cannot argue with them and their experience, It just demonstrates that there is no one true answer, we like what we like. There are trends and fashions though- and the collective like changes over time. It is not necessarily superior its just where the emphasis lays at any time in the cycle.
The guy at the local beach boosting 35 back loops the other day was on a 2024 wave sail with no visible looseness when static on the beach, It was however twisting dynamically under load.
Wave sails are a different situation to Slalom , blasting sails, which I am talking about. Thats why there are sails designed for different applications, no sail can do everything just like no board can do everything.
Select to expand quotedecrepit said..
I'll throw, another effect in here just to confuse things.
After varies experiences at places with glassy water due to oil from weed, It's apparent that the amount of chop on the water affects wind gradient. At Lake George and other places I've sailed, the water can be pure glass at 20kts.
Counter to inexperienced LG sailors, you go slower when you hit what looks like a gust (small ruffles on the water) and faster when you come back into the glass.
A kiter mate said it was very strange launching a kite there, the usual increase in wind force on the kite as it went up, wasn't there. He said the wind must be laminar, there's no wind gradient.
And yet we aren't changing the amount of twist we tune our sails for to go fast in the glass. (maybe we should?)
I have a mate that says it's got to be faster on ball bearing chop, because of reduced board drag, but I haven't experienced that. for me it's faster in the glass because of laminar wind, creating more force at the bottom of the sail.
I concur Mike. It is quite obvious at LG that it is faster in the 'Glass' than in the 'Riffle'. The 'speed talk' in my ear also reinforces this. I think it points to the drag resistance on the board being less in the 'Glass', contrary to the theory that the 'Ball bearing' water is faster. I think that idea came before we had a chance to sail in actual 'Glass'. It certainly was faster to sail in the 'ball bearing' water close the the bank at Sandy Point, than further out in the larger chop. But it seems that practical experience shows us that the 'glass' is even faster, and causes even less resistance. Perhaps it is true that the less air bubbles under the board, the less drag there is?
And the wind gradient seems much less on the Lee end of the lake. Much more turbulence at the windward end off the shore, even when there is 'Glass'.
But as Chris so well points out. We can clearly discount the old explanation of massive twist in windsurfing sails being due to wind shear. It just is not. Even in rougher water. The evidence is clear.
And I am not convinced a large amount of twist in a windsurfing sail is getting us anywhere near the benefits of the 'Prandtl' 'bell shaped span load' aerodynamics either. The AR and span length is just far too short.
Explanation short summery: Prandtl calculated that the Elliptical shape wing was the best L/D design when wing span was limited. (it still is)
But then a few years later, he calculated that if wingspan was not the limitation, a different design ( referred to as the 'Bell shaped span load') that eliminated wing tip vortex was far more efficient, and also explained why birds don't need a tailplane to counter Adverse Yaw. (The Bell shaped span load produces 'Proverse Yaw'.)
Now it may be conceivable that the higher aspect sails on fast foiling sailcraft could take advantage of Prantdl's Bell shaped span load design, but I don't see any evidence that that is happening yet? Perhaps this is because the AR or wing (sail) area is limited?
ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160003578.pdf
Select to expand quotedecrepit said..
Just doing a thought experiment, if the twist is even, there's still wind pressure at the head, if it's fluttering, there is 0 angle of attack and the wind is symmetric both sides of the sail, so that at least means there's no lift where it's fluttering, so there will be no high and low pressure side, and no flow over the top of the sail
Agree with the first part but not sure about the last part I highlighted in bold.
There could still be spanwise flow at the tip even in zero angle of attack - it comes from the rest of the sail. ??
Select to expand quotemr love said..
Subsonic...If you scroll back in the thread this is exactly why I said twist works on Windsurf sails. We physically hold the sail up, no stays, no mast mounted into a hull, if we let go the rig falls over. For us to be able to do that the sail needs to have the power low so we dont get pulled around or pulled over or exhausted after a couple of minutes. So high aspect rigs as efficient as they are just dont work in our situation.
To test this out go for a sail with less dowhaul than recomended, a tight leech and see how your rig feels when a gust hits. I will guarantee it is heavy , too physical and pulls you around.
The twist allows us to have a sail that is managable upwind but still has sufficient power to sail deep off the wind. If we used a smaller tight leeched sail upwind would be OK but we would be very underpowerd downwind. So the twist is effectively letting us go upwind with a sail that is effectively too big but perfect when we bear off and sail accross or downwind.
So regardless of any arguments about the twist and aerodynamics, a twisted sail works on a windsurfer because of the ergonomics.
Exactly this!!!
I would add - and I am shocked that no-one has even mentioned this directly, that what we need most is Dynamic Twist.
In my experience, static twist is very much less use than ideal! It really doesn't do what Martin Describes so well.
I have sailed some sails with very loose leech and almost NO dynamic twist, and they felt awful. No power to get going and then like and ON-OFF switch in the gusts and lulls.
The best sails I have used and seen have a twist that responds and adjusts to load. (and that is much more difficult to engineer!)
It's all about ergonomics, and bugger-all about aerodynamics. 
Oh, Ok, maybe a little about aerodynamics. 
Daffy the Bell Curve lift distribution also works on lower aspect spans. The Horton brothers used it to good effect with the Ho-229 flying wing. Three of their planes are on display at the Smithsonian Air and Space museum at Dulles airport in Virginia. I go to look at them every couple of weeks for inspiration, their original blueprints are also in the archive there.
"If you are not prepared to be wrong, you will never come up with anything original" - Sir Ken Robertson.
Proverse Yaw (and also nice explanation of modern aircraft wingtips):
Select to expand quoteRoo said..
Daffy the Bell Curve lift distribution also works on lower aspect spans. The Horton brothers used it to good effect with the Ho-229 flying wing. Three of their planes are on display at the Smithsonian Air and Space museum at Dulles airport in Virginia. I go to look at them every couple of weeks for inspiration, their original blueprints are also in the archive there.
I would very much like to be convinced Roo. 
