Select to expand quoteSchobiHH 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.
How many is there?
Select to expand quoteSchobiHH 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.
Select to expand quoteremery said..Basher said..
Harsh? Yes maybe.
I was just thinking how we test sails now and there is some method to it:
You usually start with the sail you already have and you record speed with a GPS etc.
Then you alter something in the sail, just one thing, and test run again in the same conditions, to see if you getter better or worse speed.
Ultimately the new sails are then tested by two sailors side by side, and then at regattas.
In the modern age, I guess we might also stick small cameras on the sail to record how the leech reacts in gusts etc.
Testing in the real world does produce results and gradual improvements. There are a lot of variables in windsurfing but sailors like Antoine Albeau are famous for their written log books where they record every setting used in their session - from fin size to mast foot position etc.
How do you ensure these "same conditions" in a natural system.
That is difficult, but there are places in the world which do have consistent conditions from day to day.
On a single day you might test stuff like different downhaul and outhaul settings, or you might change battens within a sail.
But changing leech tension is usually a sewing machine job, and it can take a while because to release the leech you often have to take luff round out of the front of the sail or alter the broadseaming.
Another, quicker option, might be to change the mast to one with a different bend. If a softer mast works better, then you might re-cut the sail to achieve the same shape, albeit one that fits on the production mast you already sell.
In practise, sails from bigger brands are tested over time with two sailors often working together, and knowing how fast you both are then sets a suitable benchmark to then experiment from.
When you sail day after day on the same gear, you soon get to notice even the smallest differences, not just with GPS speeds recorded, but with feel.
I remember some older sails having a thin chord running down the leech which you could adjust the tension by wrapping it around a little knobby thing. I presume it was for adjusting the leech tension. It fizzed out.
For the recreational freerider sailor, too much of a loose leech/twist isnt always beneficial.
www.windsurf.co.uk/test/ezzy-lion-3-7-5m-2015-test-review/
We actually went about 1cm beyond the maximum setting on the downhaul, which we believe improved the twist and precision of the power.
I already had these sails on order when I read the review, it was interesting. Soon after receiving the sails I had a session at the same spot they did the test. Its flat water and fairly consistent winds compared to my home spot of Poole Harbour. I sailed for an hour at the max downhaul setting, then another hour at +1cm. The top speeds at the +1cm setting were faster. I thought why hadnt Ezzy realised this. Then I looked at the 1 hour average speed, and both were the same to 0.1 knot. So the faster top speeds were matched by slower speeds in the lulls. The wind was consistent over the session, measured by an anemometer 1 mile away.
Back in choppy gusty Poole Harbour, max +1cm wasnt so good, as speeds were slower and the lulls larger. So I stuck to max downhaul and tweaked the outhaul by 1cm depending upon conditions. Only setting +1cm when the tide was low and decent wind, at the correct angle, with no weed in the water, and I was feeling up for some flat out blasting.
The 2020 replacements had a bit more downhaul tension and twist. They feel faster than the 2016, but as I havent hit the correct set of conditions for flat out sailing, been foiling more, etc, I have yet to go faster on any of the 3 sails 6.5/7.5/8.5m.
I see a few sailors with leeches flogging to death and think, whatever. Guess it depends if you want flat out speed on a few runs or a decent planing most of the time session.
Select to expand quoteImax1 said..
I remember some older sails having a thin chord running down the leech which you could adjust the tension by wrapping it around a little knobby thing. I presume it was for adjusting the leech tension. It fizzed out.
The leech line was a traditional add-on to sails that fluttered along the trailing edge. The idea originated from cotton sails, which of course stretched when wet. Cotton sails disappeared sometime in the 1960s I think.
The idea was continued in early windsurf sails of the triangular Dacron or Polyester type, which sometimes also fluttered.
I still had some sails in the 1980s which were fully battened by then, but which still had a leech line sewn inside the leech cloth tabling. This was not adjustable, but the thickness of the chord gave the sail a more robust feel.
The leech on sails nowadays is often 'scalloped' - with a hollow between batten ends which in turn keeps the line of tension along the sail's trailing edge to effectively stop any fluttering.
Stiffer non-stretch modern sail panel materials have probably solved this problem.
Yes it's a lot of feedback which is great.
Hopefully the AI will sort it out although it might take a lot of computer power for this one 
I've learned the difference between twist and looseness. A perfect example is the super freak whose Dacron doesn't loosen while there's existing twist.
If one tries to hold a flag into the wind the pole will tug and vibrate, that's too much looseness.
If one tries to hold a door into the wind it'll be more controllable but with zero lift (or drive).
The best sailor at the beach today was boosting perfect 35 foot back loops.
His sail when static on the beach had zero looseness between the battens.
It was strong winds
In the last 2 days I have taken a close look at all the different (wave) sails on the beach. Only one had the flappy floppy back edge that was normal only a few years ago. All the others were tighter, and some no looseness at all.
But hey maybe I am imagining it.
Select to expand quoteBasher said..Imax1 said..
I remember some older sails having a thin chord running down the leech which you could adjust the tension by wrapping it around a little knobby thing. I presume it was for adjusting the leech tension. It fizzed out.
The leech line was a traditional add-on to sails that fluttered along the trailing edge. The idea originated from cotton sails, which of course stretched when wet. Cotton sails disappeared sometime in the 1960s I think.
The idea was continued in early windsurf sails of the triangular Dacron or Polyester type, which sometimes also fluttered.
I still had some sails in the 1980s which were fully battened by then, but which still had a leech line sewn inside the leech cloth tabling. This was not adjustable, but the thickness of the chord gave the sail a more robust feel.
Some sails still had them much later. IIRC Tushingham were the last ones I saw with them. I thought it was last century, but google returned this test from 2014.
www.windsurf.co.uk/test/tushingham-rock-5-2m-2014-test-review/
Select to expand quoteBasher said..Imax1 said..
I remember some older sails having a thin chord running down the leech which you could adjust the tension by wrapping it around a little knobby thing. I presume it was for adjusting the leech tension. It fizzed out.
The leech line was a traditional add-on to sails that fluttered along the trailing edge. The idea originated from cotton sails, which of course stretched when wet. Cotton sails disappeared sometime in the 1960s I think.
The idea was continued in early windsurf sails of the triangular Dacron or Polyester type, which sometimes also fluttered.
I still had some sails in the 1980s which were fully battened by then, but which still had a leech line sewn inside the leech cloth tabling. This was not adjustable, but the thickness of the chord gave the sail a more robust feel.
The leech on sails nowadays is often 'scalloped' - with a hollow between batten ends which in turn keeps the line of tension along the sail's trailing edge to effectively stop any fluttering.
Stiffer non-stretch modern sail panel materials have probably solved this problem.
The sail I remember which had a adjustable chord tension was a NP 6.2m. I believe one of the first camms. 2 possibly 3. It was a sail that I pulled out the battons before rolling the sail up over the mast.???
Its going back a while.
Select to expand quoteSchobiHH said..Manuel7 said..
Old sails had a slimmer taller leech and less loose material up top. New sails seem to have more.
What's the point of having so much of it?
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.
yes, insight expected.
other than basher,
roo has posted himself in one of the most iconic photos in windsurfing sail history.
beyond that there are people here who have designed windsurfing and sailboat sails professionally. there's international and national competitors across multiple classes. there are many people who are current sail testers for windsurfing brands. there are a few ex pwa competitors floating around and others with 30+ years experience.
doesn't mean we can't challenge or disagree with what someone says but rest assured there are experts.
Select to expand quotesheddweller said..
The best sailor at the beach today was boosting perfect 35 foot back loops.
His sail when static on the beach had zero looseness between the battens.
It was strong winds
In the last 2 days I have taken a close look at all the different (wave) sails on the beach. Only one had the flappy floppy back edge that was normal only a few years ago. All the others were tighter, and some no looseness at all.
But hey maybe I am imagining it.
agree with you. part of that i'm guessing is to do with static twist v dynamic twist. with new materials dynamic twist may be back on the table. so not that sails have less twist just that they do it more on the water and less on the beach.
i don;t really rig wave sails by head looseness anyways. always use the batten position against the mast and the tension feel at the clew.
i've also noticed sails with less battens have tighter static heads. have always guessed that's because they open up more under load.
Select to expand quoteBasher said..Imax1 said..
I remember some older sails having a thin chord running down the leech which you could adjust the tension by wrapping it around a little knobby thing. I presume it was for adjusting the leech tension. It fizzed out.
The leech line was a traditional add-on to sails that fluttered along the trailing edge. The idea originated from cotton sails, which of course stretched when wet. Cotton sails disappeared sometime in the 1960s I think.
The idea was continued in early windsurf sails of the triangular Dacron or Polyester type, which sometimes also fluttered.
I still had some sails in the 1980s which were fully battened by then, but which still had a leech line sewn inside the leech cloth tabling. This was not adjustable, but the thickness of the chord gave the sail a more robust feel.
The leech on sails nowadays is often 'scalloped' - with a hollow between batten ends which in turn keeps the line of tension along the sail's trailing edge to effectively stop any fluttering.
Stiffer non-stretch modern sail panel materials have probably solved this problem.
Yes, I have had sails that would buzz loudly unless the leech line was really tight. Some Dacron sails were only good for a few hours before they stretched and blew out. Seems now some of the local wingers are turning over their wings fairly regularly for much the same.
Select to expand quoteBasher said..
I think I've already broken down this issue in my earlier itemised post, but there are a lot of comments here about 'wind shear' which seem to be a bit confused.
Let's try and clarify this, because there are two definitions of that term, one which is relevant, and the other which is not. The goal issue here is still why we want twist in our sails.
1) There is a theory where the wind at ground and sea level is streaming at a different angle to the wind flow way up high, like at cloud height. And we can often see that difference when we feel wind on our face, and then the clouds above seem to be flowing in a different direction. This is true 'wind shear', where the wind at higher atmospheres is often travelling faster and in a very different direction than at sea level. There are a number of reasons for that differential, one of which is surface friction acting on the wind at ground and sea level, slowing it down. The wind directions are also affected at different heights in the stratosphere by changes in atmospheric pressure, with wind in low pressure systems heading towards the centre of the low, etc. The atmosphere above us is in fact a complex mix of different winds, sometimes with uneven wind flows - like a badly mixed cake has lumpy bits and sweeter bits.
2) But the other wind shear people talk about is where a simple differential occurs in 'apparent wind', with a different wind angle felt across the deck of the board (or boat) compared to the wind angle seen at the mast tip.
Apparent wind is what the sail sees and that's the wind angle when sailing along. Apparent wind is made up of the 'true' wind of the day and the 'created' wind which is down to the speed of the board. The true wind across the deck of a non-moving board is lighter in strength compared to the true wind higher up, say at mast tip level. You can test that difference in strength by measuring the wind at ground level, and then climbing a ladder to take another reading. But note that those two wind readings may well have the breeze flowing in the same or a similar direction - because the height difference is not that great.
The created wind however is purely down to board speed and that is the same at deck level and at the mast tip, and that wind flow is fixed in direction, parallel to the centre line of the board or flowing opposite to the direction of travel..
It's only when we combine these two, created wind and the true wind, that we get the wind strength and wind direction we call apparent wind. The ratio of created wind to true wind changes at the mast tip purely because the true wind is measured stronger up there. And that is why there is a different wind angle for apparent wind at the mast tip compared to below the boom.
So that, in turn, is why we set the sail with more twist at the head.
Sometimes it takes a while to get your head around this, although it's actually not that complicated.
It's no different for windsurfers and the various dinghies used, except where the board or boat speed varies, and where the craft are used in lighter or stronger true winds.
We won't learn much more about our windsurf rigs by looking at racing dinghies or yachts, as each craft has a different need for full or flat sails and for more, or less, sail twist. Each craft just develops the rig where, over time, competition shows what works best.
Note that the angle the craft sails to the true wind also makes a difference, because the true wind may flow at right angles to the board on a beam reach whereas the created wind still flows with the direction of travel. When we sail upwind the true wind is a bit more in line with the created wind flow, but on planing days we may still want sail twist where that allows us to dump excess power.
When not planing or when sailing in relative light wind, a windsurf or other craft may prefer a tighter leach to maximised power, so in those circumstances sail twist is less of a thing.
The key in all conditions is to be maximising drive and lift from the rig while minimising drag. And sail twist becomes a very useful factor in controlling that relationship.
Note that foiling dinghies (like Moths) that sail fast in light wind will of course use a lot less twist in their sails - simply because created wind plays a bigger part in the apparent wind make up. They can sail faster than the true wind of the day.
And I should know, given my competition past.
Yes, but while there is "wind shear" due to apparent wind, that would affect all rigs of the same height in the same way (assuming speed and angle were similar) - and yet rigs of the same height but on different craft have very different optimum twists even when sailing at the same sort of speed and angle. Therefore, wind shear seems to be only a minor reason for the greater twist in windsurfer rigs.
Yes, each craft has a different need for full or flat sails or leach twist. My point is that we CAN learn from that, because we can see how the different needs and ratios of those craft affect the optimum twist for the different craft.
Moths still use a lot less twist than boards even when at very high speeds, which indicates that the twist in windsurfer rigs is only about wind shear to a limited extent. I know your competition past and I also know and sail against guys like Landy (who won the same worlds, but in a boat he built and with a sail he built, and who then went on to win more worlds and an Olympic medal) and Steve S, Mark T, and Emmett L who all won multiple worlds in the same class, and they all use much less twist than most windsurfer rigs when required, which shows that it isn't all (or mostly) about wind shear. Landy's world champ A Class sails may often sail at the same sort of speed and angles as a board and therefore would experience similar wind shear to a board, but he uses far less twist than a board much of the time and it's a quite different sort of twist.
I'm away from home so don't want to do the calcs, but it would be interesting for you to run the numbers to show how much the apparent twists over the 5m or so of a windsurfer rig.
I agree about most of your reasoning behind the twist we use; I just don't think that wind shear is responsible for the far greater degree of twist we use compared to other sailing craft because they experience similar wind shear and yet find that much less twist works better. Therefore different factors are at play in many respects, and we can learn from them.
Select to expand quotemr love said..
Also Chris, every photo you posted are yachts going upwind, when sailing down wind in planing conditions you are faster if you twist the sail off.
Yes, I know (which is why I have a nice collection of national and state championship trophies in the most popular racing yacht, the most popular racing board and the most popular racing dinghy among others), but the boats don't normally twist the sail as anywhere near much as a normal windsurfer does unless they are depowering when conditions are extreme - and in some of the classes (ie Laser) there's often no twist even downwind when sailed at optimum. I'm not saying either type is wrong, but looking at the different situations to see what we can understand from that.
Plenty of dinghy classes have unstayed rigs and little more righting moment than a windsurfer but they still go slow with windsurfer-style twist even in windspeeds when the dinghy is overpowered and when windsurfers with lots of twist go faster. And lots of the cat and dinghy sailors windsurf so they know how well windsurfer-style twist can work on a windsurfer. The thing is that things like the low induced drag of bell-shape spanload foils and Drela's point about the need to maximum the L/D ratios of the entire craft, not just the L/D ratio of the rig, give us some understanding about WHY these differences occur.
And yes, the A Class rig would be overpowering on a windsurfer but with respect that misses the point - if the amount of twist a windsurfer carries was used just so that it could match wind shear or because of the speed a board sails at or because of "exhaust" or other reasons sometimes given, then the A Class cats (and Moths etc) would run around with lots of twist too because they move at similar speed to boards. They don't, so those factors cannot be as important as some claim and therefore the reason behind the extreme twist most come from somewhere else, something experienced by boards but not by other craft. Those reason can include things like the angles boards mainly sail at, the desire for good gust response, the lift/drag ratios of windsurfers, etc.
If you had an International Canoe, a Formula Board and a Raceboard all sailing around a track in maybe 8 knots they may all be going at similar speeds and angles but they would all use different sail twists for optimum performance despite all encountering similar windspeed, wind shear, boatspeed etc, which underlines that the physical characteristics of the craft have a lot to do with the amount of twist they perform best with. That seems to teach us about why we put so much twist in boards.
Select to expand quoteCarantoc said..
All those boat classes have a fixed sail size.
And they race in fixed bands of wind strengths.
Moth sails are designed to be competative with wind speed from 6 knots to 25 knots. And they race more often towards the bottom of that range than the top.
Pretty sure A2 doesn't get out of bed until it hits 26 knots. Pretty sure a laser would be unsailable in the conditions in the photo.
What would a Moth with a fixed sail size look like if it had to race in 50 knot winds ?
What would A2's sail look like if he had to run down that canal at 6 knots of wind ?
Yep, agree that apart the implication from the second line - boat sails generally have a vastly wider range than windsurfer sails. A Laser sail is used to race from almost zero to a true (ie no BS) 27 knots or so, and an A Class or Moth sail go from about 6 to 22-2 knots. By the way I'd reckon the top Laser sailors could go downwind in those conditions although far slower than AA.
I'm not saying any sail is better or worse, but the point is that as with many types of analysis we can look at different cases and from them we can draw lessons to help us find out what is going on.
Yes, a Moth would use a different sail in 50 knots and AA would use a different sail in 6 knots. That's part of the point, because when we analyse why he would do that we can learn how sails work. We don't learn by ignoring differences.
The differences show us that it's not just wind shear (IMHO) and not "exhausting" or whatever that leads to different sail twists. It's about apparent wind strengths and apparent wind angles, aero and hydro drag and their relative levels, sail sizes and a bunch of other factors like the way a leach twists to give gust response. Aerodynamics are an incredibly complicated area so it's not surprising that questions lead to very complicated answers.
Select to expand quoteRoo said..
@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
Now I have to Google proverse yaw - interesting.
At this point in the thread, i'm beginning to think most of the aerodynamics experts on here skipped physics class.
Im no expert, and flappy leeches and creases in a fully lit sail (like the example of the NP at Ludeitz above) have always bothered me.
Yacht and dinghy sailing we always tryed to eliminate any interruptions to the laminar flow, so a smooth sail is ingrained in my brain!
However there is no doubt that the newer sails are
1 -easier to ride in general
2 -more forgiving in a gust or over powered situation
3 -arguably faster
4 -lighter per sqM
5 -bigger than previous sails that we rode in relation to set wind speeds so have greater latitude
6 -feel way more 'solid' and 'locked in/ridged' (even if the photos don't lie and they are in fact twisting off and flapping)
7 -more powerful than of old per sqM
8 -more dependent on exact settings and correct mast so arguably more twitchy to set well.
and lastly
9 - under way more tension in every way except perhaps out haul than of old
Why they work so well is obviously something that there is much to learn about for most of us, some of it theory that's hard to prove empirically (other than gps results), some R&D driven, some of it just fashion/trends that appear to be working so manufacturers go with it, some of it customer preference - brand x is doing this so why arnt my favourite brand y following suit and getting the results?
There has been plenty of good information here plenty of differing opinions and lots of learning for us all, but too much snippy sniping for me.
I love this forum hay- lets stay nice to each other...
When you say old you mean 80s to early 90s?
Early 2000s is when masts shrunk, right?
Also, mast bases moved closer to the rider.
Select to expand quotesheddweller said..
Which people have designed windsurf equipment used to win windsurf events at elite level?
why dont we ask them what they think? if they answered would anyone listen?
This.
Select to expand quoteChris 249 said..Roo said..
@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
Now I have to Google proverse yaw - interesting.
Try this one Chris 249 : openscholarship.wustl.edu/cgi/viewcontent.cgi?article=1092&context=mems500
Even though some people dismissed the ADTR at the time it did enable the sail design that allowed windsurfers to be the fastest sailing craft in the world and hold the 500m speed record for the first time. It was obvious from the data which sail was the fastest and that was the one that got the record in Sotavento. The reason the program stopped was Gaastra no longer wanted to spend 1/2 a million dollars a year on the project once they had been sold to investors.
Select to expand quotepeterowensbabs 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
Select to expand quoteChris 249 said..
if the amount of twist a windsurfer carries was used just so that it could match wind shear or because of the speed a board sails at or because of "exhaust" or other reasons sometimes given, then the A Class cats (and Moths etc) would run around with lots of twist too because they move at similar speed to boards. They don't, so those factors cannot be as important as some claim
I did the calculations some time ago, a logarithmic wind profile, got the roughness length from a look up table, and the change in apparent wind angle over the height of a windsurfing mast is a lot less than the angle of twist. So yes, not as important as some experts claim.
Select to expand quoteIan K said..Chris 249 said..
if the amount of twist a windsurfer carries was used just so that it could match wind shear or because of the speed a board sails at or because of "exhaust" or other reasons sometimes given, then the A Class cats (and Moths etc) would run around with lots of twist too because they move at similar speed to boards. They don't, so those factors cannot be as important as some claim
I did the calculations some time ago, a logarithmic wind profile, got the roughness length from a look up table, and the change in apparent wind angle over the height of a windsurfing mast is a lot less than the angle of twist. So yes, not as important as some experts claim.
Finally someone with the real approach,i.e. Check the data!. I also did this wind profile look up a while ago and came to the same conclusion. There is too much BlaBla talk without baking things up with a deduction from physical principal's in this kind of discussion. Which in the end leads to nowhere.
So where is the data then?
Here is one example (from www.onemetre.net//Design/Gradient/Gradient.htm):
This is a pretty recent paper, Dec 2023, but they look at IQFoil sails: www.sciencedirect.com/science/article/abs/pii/S0029801823026793
I don't have access to the full paper but they do mention a big factor being how far back the sail is swept, as it contributes to spanwise flow, as well as a reduction in rolling moment (or us having to counterweight the sail forces).
These sails though, as they are foil sails for a lot less true wind (ideally), don't have anywhere near the twist of something like a camberless freeride sail (like a cheetah).
I'm wondering how much the spanwise flow and other unmentioned factors contribute to the high twist and loose leech in sail design. It's been too long since I've really done aerodynamics.
Thanks for the link, looks like they modelled the iQFoil sails with their static (unloaded) profile and twist.
Will be a nice read tonight. :)
Select to expand quoteSurferKris said..
So where is the data then?
Here is one example (from www.onemetre.net//Design/Gradient/Gradient.htm):
you should at least take the effort and read the article before you post. There is a better picture in the EXCEL file which actually shows exactly what I am saying. The gradient has not so much of an effect. One still needs to calculate from the gradient the effect of the apparent wind. Then you know hon much the sail needs to twist.. I leave the calculation up to you. Some vector analysis of 9th grade school.
Surely there are differences between:
- flying at 50 knots in 40 knot winds (sail speed > wind speed)
- sailing along at 20 knots in 25 knot winds (sail speed > wind speed)
