Most areas have constantly changing winds.
Tight leech sails are fastest within their wind range.
Bigger looseleech sail allows for more variance in wind speeds..more control over wider changing conditions like course racing venues.
Bigger heavier sailors can leanback and force any sail to deform.
Smaller sailors can be just as fast if they can compensate for changing wind and water conditions.
Freestylers rig tight leech small sails,
similar to freeride foilers today.
Select to expand quoteChris249 said..
But a cutaway wasn't the first way to make a loose leech, per se. Look at the amount of twist in these 1973 sails;
hum ... I see a real lot of deformation in those sails. Loose everywhere but no twist!!!!
Jeff Magnum admitted in the ADTR article that he was sitting in the truck looking at the data and one of the techs was poking the test rig and he spotted it when the leech was prodded so he asked the tech to keep doing it. He realized something good was going on.
Accidental?? I'd reckon.
Sorry Nubie. Washout is designed in aeroplane wings to ensure that the root of the wing will stall before the tip. This is to make for a soft stall which is predictable, not sudden and enable the ailerons, closer to the wingtips, to still be somewhat effective as the wing begins to stall. I dont think it has much to do with drag reduction.
Also, I think the ADTR days were pre-PC! If they were measuring lift and drag, I very much doubt that it had a computer readout. More likely, spring scales. Roo could tell us. 
Yep, sailors have known about twist for eons, as have wing designers. Select to expand quoteduzzi said..
hum ... I see a real lot of deformation in those sails. Loose everywhere but no twist!!!!
??? There's heaps of twist - look at the vid after 1 minute, for example. For the same sort of thing, see here;
The angle at the top of the sail (near the logo) is clearly very, very different to the angle of the bottom or near the boom. That's twist. Windsurfing started with sails that had TOO MUCH twist, rather than not enough, and it caused speed and handling issues. We knew about it, which is why some people experimented with a line running from the clew to the foot. The idea was that putting the clew down would tighten the leach.
As Basher says, boat sailmakers had known about twist for eons. It was spoken about and written about all the time. But much of what happened was that older materials made it hard to create a leach that stayed flat and would twist the right amount at the right time. Once materials improved, and the rules and style of sailing changed, you could design a flatter sail that would open up effectively.
It wasn't so much a case of anyone learning anything (or shouldn't have been) but of them being able to do something better once materials got better. However, to be honest many windsurfer sailmakers do seem to be pretty ignorant, and a touch arrogant, about boat sails in my experience. It's probably because many of them came from the USA or Europe, where boat designs are more restricted. In the UK, NZ and Australia there had been more design development because we had more classes with few restrictions.
Select to expand quotesailquik said..
Also, I think the ADTR days were pre-PC! If they were measuring lift and drag, I very much doubt that it had a computer readout. More likely, spring scales. Roo could tell us.
Spring scales...you're dreaming Daffy. Electronic load cells and a steady stream data output. Nothing PC about me! We knew all about twist, all of us working on the ADTR were pilots. What we found was the more you offloaded the head of the sail the less the healing moment was, that's what you could see and was recorded in the numbers when the tip of the sail was pushed up and the angle of incidence changed. We had lots of data collected over a few years testing. Twist was a way to make the sail easier to handle rather than reduce drag.
can we have the source of those photos above ??
would love to put em on my blog and credit source(s) !!
Select to expand quotejoe windsurf said..
can we have the source of those photos above ??
would love to put em on my blog and credit source(s) !!
I took those photos, still have the slides I scanned them from. Here's another. Brian Sprout took some a year later that were used for the Gaastra brochure, we painted the truck to match the trailer for those!
Just a bit of a prod there Roo. 
I stand corrected. You were obviously on the bleeding edge.
There must be some VERY interesting data from those tests.
Thanks for a data backed answer to one of my big questions.
Obviously that setup could measure a lot of things. Could it measure drag very well?
Select to expand quotesailquik said..
Just a bit of a prod there Roo. I stand corrected. You were obviously on the bleeding edge.
There must be some VERY interesting data from those tests.
Thanks for a data backed answer to one of my big questions.
Obviously that setup could measure a lot of things. Could it measure drag very well?
It could measure everything well. Lift, drag, pitching moment and heeling moment along with angle of attack and airspeed.
Select to expand quoteRoo said..
What we found was the more you offloaded the head of the sail the less the healing moment was, that's what you could see and was recorded in the numbers when the tip of the sail was pushed up and the angle of incidence changed. We had lots of data collected over a few years testing. Twist was a way to make the sail easier to handle rather than reduce drag.
www.seabreeze.com.au/img/photos/windsurfing/15121379.jpg' />
Does twist on windsurfing sail have something with wind gradient?
(because some sailing books descirbe twist because of wind shear)
Can you explain that?
(if you have sail on car like you did,than sail will not has wind shear in top part,becasue wind speed is eqaul on bottom and top,but if wind shear do not exist when sailing ,then sail on car can be good simulation for sail when sailing in real life.. )
Select to expand quoteNubie said..Roo said..
What we found was the more you offloaded the head of the sail the less the healing moment was, that's what you could see and was recorded in the numbers when the tip of the sail was pushed up and the angle of incidence changed. We had lots of data collected over a few years testing. Twist was a way to make the sail easier to handle rather than reduce drag.
www.seabreeze.com.au/img/photos/windsurfing/15121379.jpg' />
Does twist on windsurfing sail have something with wind gradient?
(because some sailing books descirbe twist because of wind shear)
Can you explain that?
(if you have sail on car like you did,than sail will not has wind shear in top part,becasue wind speed is eqaul on bottom and top,but if wind shear do not exist when sailing ,then sail on car can be good simulation for sail when sailing in real life.. )
I very much doubt it. See my post using YP nad MI as an illustration on the other SB 'GPS and Speed' thread.
Not sure who exactly invented the loose leech but in my mind the masters became the Blaaw+Bringdal combo. The MW5 was revolutionary being the first loose leech wavesail & their race sails had massive range making you feel like you truly flew.
What lead to this? Me thinks... rapid improvement in sail design over a decade from about the mid-80s. Materials, RAF then the cambers on the Gaastra Powerfoil, awesome bottom end but topped out quickly. Next gen was the Gaastra Speedfoil which still had a std. leech but with extra down haul on the first carbon masts (controlled flex rather than assuming a relatively static ali stick!) the top couple of battens would get weird loose sections between them. We thought it looked wrong on the beach but it had better top end on the water. I think around this time Gaastra got their hands on Rushwind who had great race sails & the truck-rig was providing all manner of data. And as they say, the rest is history.
Select to expand quotePacey said..Basher said..
When I was Moth world champion, my light wind sail was quite tight leeched but my windy weather rig had a looser leach and was cut much flatter to twist off at the head. (1970s).
But it's also key here to understand what you can do with a stayed mast on a dinghy or yacht, and what you can do with the un-stayed windsurf rig. In windsurfing our only mechanical controls are downhaul and outhaul, which work in conjunction with mast stiffness and mast bend.
Well I was never a world moth champion, but I did once work for Frank Bethwaite, who told me that the top NS14 dinghys at the time used active batten rigs where the battens were stiff and used high tension, with the result that by down hauling the luff hard the battens would be compressed into the mast, bending the mast and freeing the leech dramatically. This allowed them to depower the rig in strong winds far more effectively than the more conventional rigs in use at the time.
I suspect that is also the source of the bendy 18 footer rigs that shoodbegood mentioned, this is when Julian Bethwaite first designed his own18 and started introducing several radical innovations, some of which culminated in the 49er Olympic class.
That may be the case although I don't know the history of NS14 dinghies as others might. (But I wouldn't describe the rig control that way because batten tension and mast bend tend to be separate things.)
The key point I was making is that racing dinghies and yachts had and have several options for rig control which our unstayed windsurf rigs don't, and the loose leach or 'control of sail twist' has been a thing in sailing for decades before the windsurf rig was invented.
The early windsurf rigs were obviously a very crude triangle of cloth, and seen as very unsophisticated by dinghy racers. It then took a couple of decades of windsurf rig development before we realised we had one major mast bend control option which was downhaul.
Downhaul nowadays controls the mast bend, and we use that in conjunction with a deliberate mis-match between the sail luff round and the mast bend curve.
I'm loving this discussion and some of the retro pics.
Sail design goes back a long way and you can read up about some of it on Wiki with a quick bit of googling.
The original windsurf rigs did indeed twist and, as has already been said, they did so uncontrollably, which usually meant the drive from the head when overpowered would reverse and dump you in to windward.
So our windsurf sail development started from that problem and several wrong turnings were made - such as the idea that a tack strap could act like a boom vang (also known as a kicking strap amongst dinghy sailors) in keeping the boom end down and thereby controlling twist. The tack strap certainly doesn't work that way now - but that's another discussion in itself.
Sail twist and sail fullness are connected issues because we are using our sails as a foil to deflect the wind to create 'lift'. There is more wind at the mast tip than there is nearer water level, and the apparent wind direction is different too, so we often need a different sheeting angle at the head of the sail than at the foot, and that difference also depends on the whether we are sailing broad or tight to the wind. (For example, when sailing off the wind, we can set our sails fuller and with more twist. )
Sail fullness is a bit complicated because a fuller foil section can create more drive but at the risk of increased drag, so there's a pay off between having a powerful sail that helps you plane early, but which then becomes a handful when overpowered.
The there issue to mention with loose leeches is that we often find they look floppy and loose on the beach but they usually tighten up when we are on the water and the sail is under load, with the upper mast bending sideways. So part of the loose leech idea is to anticipate this mast bend.
I see the pictures above showing a sail apparently being tested on top of a car. That's really dumb because it does not replicate any real life situation. a) a driving car just creates a head wind, and b) Our sails operate vertically and not horizontally, and that means an apparent wind differential.
But never mind. A bit of armchair windsurfing is always fun and you can learn stuff that helps you rig your sail better.
Select to expand quoteChris249 said..
Yep, sailors have known about twist for eons, as have wing designers. duzzi said..
hum ... I see a real lot of deformation in those sails. Loose everywhere but no twist!!!!
??? There's heaps of twist - look at the vid after 1 minute, for example. For the same sort of thing, see here;
The angle at the top of the sail (near the logo) is clearly very, very different to the angle of the bottom or near the boom. That's twist. Windsurfing started with sails that had TOO MUCH twist, rather than not enough, and it caused speed and handling issues. We knew about it, which is why some people experimented with a line running from the clew to the foot. The idea was that putting the clew down would tighten the leach.
As Basher says, boat sailmakers had known about twist for eons. It was spoken about and written about all the time. But much of what happened was that older materials made it hard to create a leach that stayed flat and would twist the right amount at the right time. Once materials improved, and the rules and style of sailing changed, you could design a flatter sail that would open up effectively.
It wasn't so much a case of anyone learning anything (or shouldn't have been) but of them being able to do something better once materials got better. However, to be honest many windsurfer sailmakers do seem to be pretty ignorant, and a touch arrogant, about boat sails in my experience. It's probably because many of them came from the USA or Europe, where boat designs are more restricted. In the UK, NZ and Australia there had been more design development because we had more classes with few restrictions.
That is not twist. That is the whole sail deforming. If anything the area around the boom is more stretched out than the top: the opposite of "twisting". The sail is designed like a triangular dinghy sail from the 70s, with most area at the boom a very narrow top, The little twist you see at the top is not intentional and is more than obliterated by the huge distortion everywhere. The angle of the "spinnaker" bulge has an even wider angle. It is just a very badly designed sail (and I used it! I am that old ... my gods!!!!!)
Select to expand quoteBasher said..
That may be the case although I don't know the history of NS14 dinghies as others might. (But I wouldn't describe the rig control that way because batten tension and mast bend tend to be separate things.)
The way it was explained to me was that if you have stiff battens under high tension and lots of tension in the sail, its like a bow and arrow, with the batten being the arrow being pushed into the mast by sail tension. The net result is pressure on the mast which encourages it to bend. Increasing the luff downhaul tension increases this thrust and bends the mast further.
I think modern windsurfing sails use the same effect, the mast is bending not just because of the buckling effect of high luff tension, but also because of the bending force exerted by the battens.
The person who gave me this interpretation was Phil Darby, who also worked for Frank and was the NS14 national champion at the time, so I have no reason to doubt that it was the current thinking in the class at the time.
First 2 photos are sail not powered up.
Last 2 photos are sail powered up.
The top 1/3 of your mast bends sideways when powered up and tightens the leech. If you rig your sail on the beach to look like the last 2 photos you will end up with an extremely tight leech which will stop the exhaust and choke the sail your board will not lift and fly it will stick to the water. YOU DON'T SAIL ON THE BEACH !!
photo by Mari
photo by Mari
photo by Dezza
As ever, it's possible to talk at cross-purposes when discussing rigs.
What the slalom racer wants from a rig is not the same as what the recreational sailor often wants. And the way we rig wave and freestyle sails is different again.
Often, these differences are down to the different requirements of speed versus manoeuvring, and for when acceleration and early planing comes before the need for top speed. Some of our boards need to be flown off the fin, whereas others don't.
Select to expand quotePacey said..Basher said..
That may be the case although I don't know the history of NS14 dinghies as others might. (But I wouldn't describe the rig control that way because batten tension and mast bend tend to be separate things.)
The way it was explained to me was that if you have stiff battens under high tension and lots of tension in the sail, its like a bow and arrow, with the batten being the arrow being pushed into the mast by sail tension. The net result is pressure on the mast which encourages it to bend. Increasing the luff downhaul tension increases this thrust and bends the mast further.
I think modern windsurfing sails use the same effect, the mast is bending not just because of the buckling effect of high luff tension, but also because of the bending force exerted by the battens.
The person who gave me this interpretation was Phil Darby, who also worked for Frank and was the NS14 national champion at the time, so I have no reason to doubt that it was the current thinking in the class at the time.
I'm afraid a lot of that is wrong. Battens simply hold the luff apart from the leech and they tend to take up the shape of the sail but they don't exert much pressure on the mast and have nothing to do with mast bend.
We use tapered battens to encourage a flat exit at the leech and their thinner forward ends can help hold some curve at the luff.
With camber induced sails the batten is used to wedge against the mast to help push out fullness that is already cut in the sail but when sailing the mast bend tends to cause the battens to pull back from the mast, and nowadays our cams are sewn to the luff of the sail and only push against the mast when we add spacers to the batten ends before fitting the cam, and that's really about shaping the leading edge of the sail, or fine-tuning it for locked in drive.
In your dinghy sail above, the sail appeal is cut with radial seams which suggest (visually) a forward force on the batten, but I'm betting there isn't any. A lot of those seaming claims are just marketing bull****.
On most stayed rigs as seen here, sailors control mast bend with a mast ram at deck level, with the kicking strap, and with the setting of the spreaders. On yachts they can also use an adjustable backstay. All those key adjustments can affect sail twist, as well as mast bend.
Dinghy and yachts however don't use downhaul to bend the mast as we do, but they do adjust tension on the mainsail luff panels and those sails may be cut with stretchy cloth, so that increasing luff panel tension can pull fullness forwards in the sail, or else it can be used to flatten the sail entry for upwind sailing. Some dinghies still use a 'Cunningham hole' for this purpose, and you can google that. Mr Cunningham lived quite a life and died at the age of 96.
Select to expand quoteBasher said..
As ever, it's possible to talk at cross-purposes when discussing rigs.
What the slalom racer wants from a rig is not the same as what the recreational sailor often wants. And the way we rig wave and freestyle sails is different again.
Often, these differences are down to the different requirements of speed versus manoeuvring, and for when acceleration and early planing comes before the need for top speed. Some of our boards need to be flown off the fin, whereas others don't.
If that's what they want they buy a freeride or wave sail not a race sail they are designed different with a tighter leech etc.
Select to expand quoteNubie said..
Does twist on windsurfing sail have something with wind gradient?
(because some sailing books descirbe twist because of wind shear)
Can you explain that?
(if you have sail on car like you did,than sail will not has wind shear in top part,becasue wind speed is eqaul on bottom and top,but if wind shear do not exist when sailing ,then sail on car can be good simulation for sail when sailing in real life.. )
Sail twist is a bit complex but I'll try and explain it here.
1) Firstly the idea is that the sail should be sheeted in to the correct angle on the wind at all the points up the sail luff, from the tack to the head. If the sail is cut fuller near the boom and flat at the head then some twist would be needed to get the correct angle of attack at each section because our sails are wider at the bottom. You could achieve the same angle of attack with twist at the head, or else by cutting the sail fuller at the head (but fullness at the head also means a tighter leech at the head. )
2) The sail panel needs to deflect the wind if it is to create lift, but we like our sails to drive from low down, rather than to pull from the head - so it's arguable that we like the sail head to sheet out naturally from the head, to exhaust properly and to dump excess power when overpowered. A sail that is full in the head or which has a tight leech tends to feel top heavy. That can work well in light winds, but gets catapulty when overpowered because the tight leech doesn't release power at the head well. That's why most windsurf sails are cut with the top batten set pretty flat.
3) We are actually sheeting our sails to the 'apparent' wind - which is the combination of the true wind of the day and the 'created' wind that is made by the board moving forwards. The true wind might be, say side-on, but the created wind will run from nose to tail along the board, so the apparent wind will flow at some angle between these two directions. This is further complicated by the created wind being the same strength at deck level as it is at the mast tip, whereas the true wind is usually stronger at the sail head than it is across the sail foot. So that means the apparent wind often flows at a different angle at the mast head than it does across the board, and this is another reason we might want twist in our sails, again to get the optimal sheeting angle.
4) There is another issue, often called wind shear, a term which is often wrongly used and which doesn't affect us here - because our rigs are simply too short for this to be relevant. The wind shear is where the wind direction way up in the air is in a different direction from that same wind flowing along the ground.
We can sometimes see clouds way above flowing at a different direction to the wind we feel on our face. However on open ground, if you measured the wind direction 5m off the ground, it would probably the same as at ground level. You might find a difference in wind strength though. Wind sheer is often mixed up with my point 3), above. You can google the term, to find its usual meteorological meaning. People also talk about wind gradient.
5) If you set a rig on a car as shown in the photos here, positioning the sail to be horizontal takes wind shear and differences in apparent wind out of the equation. So it's not clear what they are measuring, and really they are just showing off, as if they were being scientific in their rig development, when in fact they are not.
All you could hope to do with a rig on a car is to try and measure the lift you got at different sheeting angles at different motoring speeds, and then you might change the sail shape until you found one that that seemed to work better - on the car. But then you'd have to start again with the rig on the water, set in the vertical plane, and with the sailor hanging off it causing new rig loads. And so that would be like starting from scratch, once the usual apparent wind and other sail dynamics issues come into play. So the 'road testing' is a waste of time.
Select to expand quoteduzzi said..Chris249 said..
Yep, sailors have known about twist for eons, as have wing designers. duzzi said..
hum ... I see a real lot of deformation in those sails. Loose everywhere but no twist!!!!
??? There's heaps of twist - look at the vid after 1 minute, for example. For the same sort of thing, see here;
The angle at the top of the sail (near the logo) is clearly very, very different to the angle of the bottom or near the boom. That's twist. Windsurfing started with sails that had TOO MUCH twist, rather than not enough, and it caused speed and handling issues. We knew about it, which is why some people experimented with a line running from the clew to the foot. The idea was that putting the clew down would tighten the leach.
As Basher says, boat sailmakers had known about twist for eons. It was spoken about and written about all the time. But much of what happened was that older materials made it hard to create a leach that stayed flat and would twist the right amount at the right time. Once materials improved, and the rules and style of sailing changed, you could design a flatter sail that would open up effectively.
It wasn't so much a case of anyone learning anything (or shouldn't have been) but of them being able to do something better once materials got better. However, to be honest many windsurfer sailmakers do seem to be pretty ignorant, and a touch arrogant, about boat sails in my experience. It's probably because many of them came from the USA or Europe, where boat designs are more restricted. In the UK, NZ and Australia there had been more design development because we had more classes with few restrictions.
That is not twist. That is the whole sail deforming. If anything the area around the boom is more stretched out than the top: the opposite of "twisting". The sail is designed like a triangular dinghy sail from the 70s, with most area at the boom a very narrow top, The little twist you see at the top is not intentional and is more than obliterated by the huge distortion everywhere. The angle of the "spinnaker" bulge has an even wider angle. It is just a very badly designed sail (and I used it! I am that old ... my gods!!!!!)
That's very definitely twist. Neil Pryde, for example, define twist as "the amount the top third of the sail twists to leeward relative to the bottom". The North Sails Fast Course book defines twist as "the change of each chordline's angle to the centreline of the boat from one height to the next". Quantum Sails define it as "the change in the angle of attack from the bottom of the sail to the top." Marchaj and Bethwaite show it in diagrams that look like the sail in the pic. Every sailmaker I have sailed with, including America's Cup winners, has defined twist in the same way.
Yes, the twist in the pic is not intentional and is caused by the fact that it's a primitive early Windsurfer sail deforming - but that doesn't mean it's not twisting. The point is that just about ALL sails used to twist heavily because of distortion, therefore no one "invented" the loose leach; for many, many years loose leaches were unavoidable because the technology wasn't good enough to create tight leaches most of the time. In his 1964 book Marchaj, for example, says that it was very difficult to reduce twist to as little as 3.5 degrees; the sails, controls and spars weren't good enough to stop the sail from twisting. As sailmakers say, what has happened is that materials have got better and therefore an open leach can be made that has a more controllable shape, with flatter aft sections.
Extra marks for Chris249 for mentioning Marchaj.
I'm sure I've still got that book on a shelf somewhere.
Sadly, it's 1am in the UK, and bedtime for me.
Select to expand quoteBasher said..
Dinghy and yachts don't use downhaul to bend the mast as we do
You are IMHO dead right with your points about earlier sails and about twist and aerodynamics. However, with respect, some dinghies DO use downhaul to bend the mast; for example the Bethwaite designs and Australian International 14 sailors. Phil Darby, who Pacey quotes, used to work with the Bethwaites. The recent world Tasar champions expressly say that they DO use downhaul "to really bend the mast and flatten the sail" for example.
Pacey's pretty much up to speed with aerodynamics, by the way. His source Phil Darbs and I used to be close rivals. I'm not sure how much downhaul-induced bend is created by batten compression and how much is simple mast compression. I tend to the latter but then again, it's a fact that the Tasar battens are certainly under compression; we used to specifically carry zero batten tension on the lower batten in a breeze because otherwise the compression would force it into an inefficient S bend.
Cats also definitely use downhaul to bend the mast; it's one of the most critical controls and is often explicitly referred to as a depth control. It very definitely works; you can play with it in action and see and feel the effects just as you can with an IMCO (to mention one windsurfer with an adjustable downhaul), Tasar dinghy or Bethwaite 18 Foot Skiff. The operation of all of them appears to be very similar, with the possible exception that the cat downhaul tensions seem even more frightening than that of an old Formula sail! (EDIT - and the fact that the Bethwaite skiffs used to concentrate downhaul-induced bend in the top sections).
So rigs for very differing craft have, in some ways, moved towards similar shapes and control methods as materials technology has allowed sailmakers to do things they once just dreamed of. Where twist and shape were once hard to control, they can now be adjusted in most dimensions. Loose leaches were not invented by any one person or even any one part of sailing (although it sounds like Roo etc made a leap in application in windsurfing) but have just been refined and made more useful by modern technology.
I think the more pertinent question is who developed the square top (i should say squareish top)?
That (to my mind) is where/when they really started to take the loose leech as a beneficial shape to to windsurfing, and work an intended looseness into the design.
Select to expand quoteChris249 said..Loose leaches were not invented by any one person or even any one part of sailing (although it sounds like Roo etc made a leap in application in windsurfing) but have just been refined and made more useful by modern technology.
Exactly. 
Thread question answered!
