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Who is inventor of loose leech?

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Created by Nubie > 9 months ago, 8 Apr 2019
sailquik
VIC, 6066 posts
13 Apr 2019 12:56PM
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Basher said..
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.



Well explained.
This is what I meant when I referred to the Maquarie Innovation wing and the testing that was done to design it. Lindsay Cunningham measured the wind strength at various heights above the sand spit, to, presumably, the height of his wing. He was aiming for a boat speed of 50 Knots and they designed the wing twist to work to those perameters. The difference in wind speed he measured could not have been very large, as the wind has quite a subtle amount of twist. In those days we had very clean wind off the ocean and accross the very low, smooth sand spit. I dont think there was much boundary layer turbulance at all, or much increase in wind strength at the height of our rigs. Therefore, the twist we needed to account for that was minimal. Certainly far less than we see in modern speed and slalom sails. So, really, wind velocity increase at the top of the sail is not the reason why we have twist in speed sails at venues like this.

Sandy Point 50 Knots day 27-09-09:
Tony Wynhoven: 49.11 - 2 sec
Spotty on his way to 50 peak.:
me - slower.

The Luderitz canal however, may be a very different matter. The wind there is VERY turbulent. Add to that my great frustration that in my time there the water was very low in the canal and therfore it was like sailing behind a half meter wall. There was very little wind strength getting into the lower sail.


Is it any wonder that the sails there work much better with massive twist? Although it has to be said that when this pic was taken, and when all the WR were set, the canal was right up to ground level. The wind was still extremely turbulent though, off the rough low hills upwind. (Like those in the background)


















sailquik
VIC, 6066 posts
13 Apr 2019 1:03PM
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The use of the 'Cunningham hole' is interesting because the explanation says increasing downhaul tension moves the draft forward in the sail. In my experience, this is opposite to what it does in a windsurfing sail, at least in cambered race sails.

Wikipedia quote: By either hauling or easing the line, the tension in the luff can be changed, thereby shifting the point of maximum draft of the sail forward or aft respectively, optimizing sail shape and thus performance.

Pacey
WA, 525 posts
13 Apr 2019 11:19AM
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sailquik said..
The use of the 'Cunningham hole' is interesting because the explanation says increasing downhaul tension moves the draft forward in the sail. In my experience, this is opposite to what it does in a windsurfing sail, at least in cambered races sails.

Wikipedia quote: By either hauling or easing the line, the tension in the luff can be changed, thereby shifting the point of maximum draft of the sail forward or aft respectively, optimizing sail shape and thus performance.


On a soft sail and/or stiff mast, it will pull the draft forward and free the leech to some extent. On a windsurfing sail it has secondary effects such as bending the mast more, which may flatten the front of the sail.

olskool
QLD, 2444 posts
13 Apr 2019 1:24PM
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Sailquik, pulling more DH on windsurf sail effectively does the same thing as pulling more cunningham? More DH loosens leech so depowers the sail.
Cunningham also depowers the upper sail by moving the draft forward. Therefore depowering the leech. Just does it a slightly different way, due to much stiffer mast??

sailquik
VIC, 6066 posts
13 Apr 2019 2:51PM
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olskool said..
Sailquik, pulling more DH on windsurf sail effectively does the same thing as pulling more cunningham? More DH loosens leech so depowers the sail.
Cunningham also depowers the upper sail by moving the draft forward. Therefore depowering the leech. Just does it a slightly different way, due to much stiffer mast??




Well of course, up in the top!
But I have a whole lot of measurements of speed sails that show the Draft moving back, and the entry getting finer in the lower sail as more downhaul is applied.. It is due to the mast bending more and taking out some of the mismatch between the luff curve and the mast.

The finer entry is more pronounced that the Draft moving back.

Interestingly, if you bend the mast sideways, and you have enough slack in the outhaul to allow it, the Draft moves forward

sailquik
VIC, 6066 posts
13 Apr 2019 2:57PM
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A bit off topic, but an interesting read for those who missed it.

www.clubmarine.com.au/exploreboating/articles/24-3-50-knot-out

olskool
QLD, 2444 posts
13 Apr 2019 7:47PM
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Sailquik, "the finer entry is more pronounced than the draft moving back." Hmmm A desirable outcome for us windsurfers. Draft shifting greatly is PITA. As the sail becomes flatter im guessing the draft must shift aft somewhat. Otherwise the sail would have a blunt entry n very flat turbulent rear section. Hardly creating any lift.We have only 2 stings to pull to create the desired shape. Quite a varied shape really. Really highlights having to use the correct mast the sail was designed for. Otherwise draft n leech tension n twist will be way out of whack. Making the sail hard to control n produce no power.

sailquik
VIC, 6066 posts
13 Apr 2019 8:03PM
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yes, all true. But the draft definitely moves back. That does not mean the sail becomes unstable. When sailing the draft is still locked.

Draft moving slightly back with finer entry is great for upwind, and also for reaching in lighter winds (high apparent wind compared with true wind speed). Maybe not so good for deep downwind speed sailing in strong winds. Sometimes it's better that have less DH and more OH for upwind. Keeps the leech tighter. But during speed sailing sessions, I never change the DH to sail upwind. Too time consuming and hard to do on the water. I DO adjust the outhaul for upwind though. That is easy and makes a big difference..

Basher
522 posts
13 Apr 2019 8:19PM
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Subsonic said..
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.


Those of us who are old enough to have lived through that era know that the wider head sails in windsurfing were simply an attempt to replicate what was already happening in other sailing craft.
Indeed, dinghies and catamarans had had wide head sails for decades. Although initially, some of those wide head sails were developed out of a need to maximise drive from rigs which had class rules which limited maximum sail area.
So for non-planing days they wanted to have a tall and narrow sail that would catch the best of the wind at mast tip level. The challenge was then to make the same sail work in stronger wind. My International Moths had high aspect sails with broad heads from the early 1970s - when windsurfing still had crude triangular rigs. Catamaran rigs had broad heads in the 1960s.

The windsurfing rig development was interesting though if I remember correctly, with one early Neil Pryde rig requiring a cantilevered 'gaff' extension to the mast that was to support the leach at the head.

Basher
522 posts
13 Apr 2019 8:31PM
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sailquik said..
The use of the 'Cunningham hole' is interesting because the explanation says increasing downhaul tension moves the draft forward in the sail. In my experience, this is opposite to what it does in a windsurfing sail, at least in cambered race sails.

Wikipedia quote: By either hauling or easing the line, the tension in the luff can be changed, thereby shifting the point of maximum draft of the sail forward or aft respectively, optimizing sail shape and thus performance.







Well I think they are simply different controls. But let's give a bit more detail on how they work.

A Cunningham hole is or was traditionally used in conjunction with a stretchy luff panel and this is meant to flatten the luff entry rather than to bend the mast as we windsurfers do. In the days of cotton* and then dacron sails, the luff panel cloth was positioned so the cloth bias would be under tension - that being stretchier than if you line up load with warp or weft.
The tension is applied to the boat mainsail for upwind work so that the boat will point higher - or to allow the jib to be sheeted in tighter, also for upwind work.
Another aspect of the luff panel tension is that it can stabilise the sail entry shape, compared to a rig with no luff panel tension - so in that sense it can hold the sail draft forwards. Laser dinghies are obvious examples of this, although their adjustable luff tensioner can't strictly be called a Cunningham hole. The Laser sailor bends his (unstayed) mast by sheeting in hard which in turn adds pressure to the mast via the boom, or by cranking on the kicking strap for upwind work. Those are two control options we windsurfers don't have.

(*The original Cunningham hole was invented to deal with the sag you got with cotton sails when they got wet!)

Our windsurf rigs are different from this concept, so downhaul and Cunningham hole tension are not the same.
Our windsurf racing sails are cut with non-stretch luff panels so that downhaul tension bends the mast. And so increasing downhaul tension flattens the sail at the luff - which might be good for upwind work or because it also releases the leech when we are overpowered. Where the leach is released with extra mast bend, the sail drive should stay where it was, even though the luff is flattened. But when we add downhaul on the beach we usually add outhaul too.

The RS:X boys will let their adjustable downhaul off for heading downwind, because the reduced mast bend gives them more belly in the sail to go broad. Obviously they slacken the outhaul too, and that's good for twist which you want more of when heading downwind.

Speedsailing has also been mentioned on this page, and let's not forget that twist is very important for them - they need their sails to depower and to exhaust well, but they also need a lot of twist when heading so broad off the wind.

duzzi
991 posts
13 Apr 2019 10:44PM
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Chris249 said..




duzzi 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.











Whatever ... we are just playing with words. Sure it is "twisting"/"deforming", similar to the way a beach blanket would "twists" if "rigged" on a stick! But that triangular sail has nothing to do with how sails started to be designed from the early 90s onward.

Basher
522 posts
13 Apr 2019 11:05PM
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Those early windsurf rigs were simply different beasts.
They started as simple triangles of dacron cloth, sleeved over a crude mast and held in a triangular shape by a sloppy floppy wishbone boom.
Any twist in the sail leach was only held back by the weight of the boom, or by the sailor hanging off the boom.
But the sails would still twist open at the head as the mast bent sideways under load. The lack of battens and lack of sailcloth panel tension meant the shape of the sail could easily distort - and flattening off with downhaul and tightening the downhaul knot and perhaps adding a tack strap tensioner was all you could do to keep control in windy weather. The bendy boom meant outhaul would slacken under load.

The sails then developed of course and boom attachment got better (slowly) and battens were added at the head first to flatten off the foil shape there. Eventually we had a pulley and cleat arrangement at the mast foot which could be used to add tension to the luff panel. But these were the windsurfing equivalent of Topper dinghy rigs. A bit basic.
What year did we stop rolling our sails around the mast?

It might be interesting to document the changes in our rigs with photographs from each year, if someone can be bothered to do the research.
On topic, loose or open leaches always existed, as did sail twist, but the concept of the 'loose leech' came to mean something more special for a while in windsurfing, once the concept of using downhaul to control mast bend came into play.

Chris249
357 posts
14 Apr 2019 3:03PM
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duzzi said..













Whatever ... we are just playing with words. Sure it is "twisting"/"deforming", similar to the way a beach blanket would "twists" if "rigged" on a stick! But that triangular sail has nothing to do with how sails started to be designed from the early 90s onward.


It's relevant because it shows that there have always been loose leeches in windsurfers and in boats. Every decent sailmaker has always known about loose leeches, leach twist, and trying to optimise design of the aft sections of their sails. For example, some 470 dinghy sailmakers like Steve Benjamin used to use so much mast bend that their sail exceeded the luff curve and inverted, which opens up the leech to depower and drop the centre of effort.

Sure, due to things like advances in technology, around the late '80s/early '90s windsurfer sailmakers were able to improve leech twist. The testing that Roo etc did probably highlighted its value, perhaps because technology allowed them to get away from the old problem where cloth stretch etc would make it almost impossible to control the way the leech opened up well enough. But it was certainly not some great new concept.

choco
SA, 3994 posts
14 Apr 2019 7:34PM
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Basher said..
Those early windsurf rigs were simply different beasts.
They started as simple triangles of dacron cloth, sleeved over a crude mast and held in a triangular shape by a sloppy floppy wishbone boom.
Any twist in the sail leach was only held back by the weight of the boom, or by the sailor hanging off the boom.
But the sails would still twist open at the head as the mast bent sideways under load. The lack of battens and lack of sailcloth panel tension meant the shape of the sail could easily distort - and flattening off with downhaul and tightening the downhaul knot and perhaps adding a tack strap tensioner was all you could do to keep control in windy weather. The bendy boom meant outhaul would slacken under load.

The sails then developed of course and boom attachment got better (slowly) and battens were added at the head first to flatten off the foil shape there. Eventually we had a pulley and cleat arrangement at the mast foot which could be used to add tension to the luff panel. But these were the windsurfing equivalent of Topper dinghy rigs. A bit basic.
What year did we stop rolling our sails around the mast?

It might be interesting to document the changes in our rigs with photographs from each year, if someone can be bothered to do the research.
On topic, loose or open leaches always existed, as did sail twist, but the concept of the 'loose leech' came to mean something more special for a while in windsurfing, once the concept of using downhaul to control mast bend came into play.


They also had a leech tightening line from memory

Basher
522 posts
14 Apr 2019 7:39PM
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choco said..

Basher said..
Those early windsurf rigs were simply different beasts.
They started as simple triangles of dacron cloth, sleeved over a crude mast and held in a triangular shape by a sloppy floppy wishbone boom.
Any twist in the sail leach was only held back by the weight of the boom, or by the sailor hanging off the boom.
But the sails would still twist open at the head as the mast bent sideways under load. The lack of battens and lack of sailcloth panel tension meant the shape of the sail could easily distort - and flattening off with downhaul and tightening the downhaul knot and perhaps adding a tack strap tensioner was all you could do to keep control in windy weather. The bendy boom meant outhaul would slacken under load.

The sails then developed of course and boom attachment got better (slowly) and battens were added at the head first to flatten off the foil shape there. Eventually we had a pulley and cleat arrangement at the mast foot which could be used to add tension to the luff panel. But these were the windsurfing equivalent of Topper dinghy rigs. A bit basic.
What year did we stop rolling our sails around the mast?

It might be interesting to document the changes in our rigs with photographs from each year, if someone can be bothered to do the research.
On topic, loose or open leaches always existed, as did sail twist, but the concept of the 'loose leech' came to mean something more special for a while in windsurfing, once the concept of using downhaul to control mast bend came into play.



They also had a leech tightening line from memory


Leach lines date back to cotton sails and were added to deal with leech flutter. Even on windsurf rigs of the 90s you'd still see leech lines to stop flutter between batten ends.
The flutter issue was then dealt with by using better non-stretch materials in the sail panels and for tabling the leech.
But the other trick to stopping leach flutter is to have a slight hollow in the straight line between batten ends. That hollow maintains the panel tension along the sail's trailing edge.



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"Who is inventor of loose leech?" started by Nubie