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?

choco 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?

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.

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?

said..

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

Paducah 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.

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.

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.

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.

mr 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?

mr 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

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.

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.

decrepit 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. ??

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.

Roo 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.