Hey I was watching at Kurnell last weekend and couldnt help noticing that a lot of guys were using largish sails and that the top third was just flapping around with no sail shape at all.

I had a good view from the shore and could see that as guys were fully sheeted in and on the plane and the top third of the sail was just flapped around.

I havnt sailboarded in a long time and can remember my pryde 5.5 4WD cambered sails were similar but no where near as pronounced.

What is the general feeling about this? Sure the top third layed off when a gust hit but it didnt really seem that it was doing much in the first place.

Grumps is on the money, its the area where the wind "exhaust" out of the sail.
Setting it tight causes the air to turbulent (sort of) and not flow across and out of the sail on the windward side. Good smooth air flow across the low pressure on this side, as to apposed to air flow across the outside of the sail and thus the drive.
Sport sailors may give a better explanation, but roughly that’s how it works.


Mineral

It's a weird thing, watching just how much a windsurfing sail twists when there's a good wind blowing. Other sails (like the ones that 18 footers use) don't seem to twist nearly as much.

One thing to note is that the area of the sail near the boom normally has a lot more curvature than the battens near the top of the sail. It's also worth noting that a flat airfoil produces a lot less lift than a curved one. Also, changing the angle of attack of an airfoil (angle to the wind) changes how much lift is produced. So if you want absolute max power from a certain size of sail, you would want the same curvature all the way up the sail, and no twist at all. (8 cams, anyone?)

Flat airfoils (like the top batten) can't be sheeted in as much as curved ones (like the one near the boom) before they stall, so if you've got a flat top batten and a curved bottom batten, then for max efficiency I guess you have to sheet out the top batten a bit, which is where the twist comes in.

I've got an 80's era Bombora speed 6.2 sail that has almost no sail twist, and in a good wind is very prone to wanting to luff up into the wind. This is known as "the centre of effort moving backwards". It really has a strong twisting moment when the wind's blowing, and can be difficult to push back when it's determined to luff up!
My early 00's RX1 has a lot more sail twist, and doesn't want to luff up nearly as much. I haven't rigged the Bombora sail for a while, so don't know which one gives more push.

I think the answer is control -- with sail twist the center of effort doesn't move backwards as much in high wind. The sail also has a much wider sweet spot with regard to how much you're sheeting in, because there will always be at least one part of the sail that's at the right angle of attack. It also turns itself from a sail with a large area into a sail with small area in higher winds, meaning that you don't have to come in and re-rig every time the wind changes a few knots.

Fluid dynamics is a strange beast, many times the explanation the makes the most sense is completely wrong, and can be proven so with the right experiment... so muck around with your downhaul and see what happens

My 2cents worth- 30 years ago I was sailing Moths, 18ft skiffs, keelboats and windsurfer one designs, had a best friend who was a sailmaker.I was sailing in a world Moth Championship and asked this same question because large roached (leech area outside sail triangle) sails were developing. The answer I was given was... The windspeed increases the greater the height above the weter surface (eg 1m= 15kts, 2m= 18kts, 3m=21kts etc) so salils are designed so that bottom is full & powerful for the lower speed wind, middle is flatter and slightly twisted-off to accomodate the different 'apparent wind' angle caused by the greater windspeed caused by the greater height above sea level, and the top is flattest and more twisted for the same reason.This should cause a gradual change in the angle the airfoil and in the 'chord'(depth) of the airfoil. A sailboards mast has lots of 2 dimentional bend and the sail has to be designed so that it will also depower itself automatically when a gust hits (twist off) and power up after the gust(twist back) by using this bend. The loose leach is therefore a design feature to give the 'non-flapping'part of the sail an optomim shape- 90% of the sail working, 10% no drive bus assisting shape. I have noticed that NASH sails have less roach area and nore ccrve cut into the top of their slalom sails, while NORTH sails seem to have large roaches, less luff-curve in the tops of theirs. NASH sails therefore appear to have lots less 'flapping' sailarea to achieve their designers required shape. Lay sails of different brands( but same app area) over top of each other and see how differently some luff curves are cut.
Longwinded explaination, but a complex question. Then again, I could be speakin through my ass. Any sailmakers want to give their explination!

The original design came from nature. Take a look at the twist at the wingtips of an eagle, especially when its soaring in gusty conditions. Awesome sight. The tips just twist off for a beautifully controlled flight.

eightfootplus,

That was a very gusty day, and I guess a lot of guys had a lot of downhaul which increases the amount of loose leach and lets the sail twists of easier in a gust. If the wind had been lighter and/or steadier, then you would probably have seen a lot of sails rigged with less downhaul and therefore a lot less loose leach and twist.

It is this ability to adjust downhaul and therefore adjust loose leach and twist that means a modern sail has a much larger useful wind range.

(Did I get that right?)

Regards,
Harrow.

Hi Guys

Just remember that all these loose leech sails are also much larger that previous tight leech sails. The twisted head is a more efficient treatment of drag and the power is contained down low but other boats like skiffs and racing dinghies can't use mega twisted sails because they are restricted in sail area. If the sail area was restricted to the old race board size of 7SqM then you would see twisted sails disappear overnight.

i'm on board with jethrow,

anything above 7.0m and you need loads of downhaul. otherwise when the gust hits your arms will be ripped off. modern sails are also a lot more rigid, so they need the flex in the head to soften things up.

the downside with big sails is that there is less lift because of a flatter draft due to downhaul. so you either go bigger for the wind (more lift) or pump like crazy to get on the plane.

the upside is they can sail on all points of sail through the lulls better.

i have used big sails with min downhaul and they suck.. they are slow, to powerfull and not manouverable. once you set med downhaul and pump onto the plane they are a joy to use. very fast, and very light in your hands.

i think this is why some people don't like big sails. they just don't rig them with enough downhaul so they feel heavy, slow, and a handfull.

Hi Mikey,

The main trouble with things like this is that the people who really know what they're doing with regard to design, know that laypeople don't understand words like angle of attack, chord, wind shear, stall angle, reynolds number and the like.

So they say things like "Sail twist reduces tip turbulence". (This could be just another way to say that it decreases the angle of attack to stop the tip stalling, possibly because of the flatter airfoil or higher windspeed at the tip.)

The trouble is that other people who have half a clue take those words as gospel, without understanding fully what it is that the designer was paraphrasing for the masses...

OK just for grits and shiggles, you posted:

quote:

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generating lift towards the tip of the foil can lead to instability

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Another way to stop lift being generated at the tip is to cut the tip off altogether and make your wings shorter! It also moves the 'center of effort' closer to the root of the wing/sail. This is clearly a bit of a silly solution, but it has the same effect (in terms of tip lift and COE) as washout. It even reduces drag! So why don't we just use a smaller sail rigged fuller if we want to move the COE down? Clearly, something else is going on.

It's a thorny issue, there are many explanations, like good shoes one size doesn't fit all, so find one that works for you and stick with it in your own head. And the name of the game is to experiment, experiment and experiment some more. It doesn't matter how technical your explanation is if it can't be transferred to the real world

this page has some interesting stuff. may explain the differences everyone has with closing the gap versus sheet angle versus downhaul tension versus outhaul versus mast versus twist versus drag versus handling etc etc etc.

can't say i understand it fully but it looks like fun reading..

www.onemetre.net/

ok, nice photo, had to post that link... here is the better stuff...
www.onemetre.net/Design/Twist/twist.htm

quote:

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nebbian who is Kenn Batt???

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NFI, ex-sailor now employed by the BOM? Apparently he's the guy who briefs the Sydney-Hobart yacht crew. Not sure if that means he knows what he's talking about or is full of sh!te Some of the things he said made sense. They seemed to tally with what you said about wind shear as well, but go into more depth... ie turbulent air doesn't have any wind shear, but laminar air does (which makes sense I guess).

Has anyone had the situation happen where:
You're slogging along, wanting to get onto the plane, you feel a gust on your face, and you think "OK this is plenty strong enough to plane in", but can't get planing.
Then another gust comes which feels about the same strength on your face but just about rips the boom out of your hands?? Perhaps this is to do with higher wind further up the sail, or a change in direction further up the sail?

Maybe we could stick waterproof wind meters all the way up the mast to finally get a definitive answer

Sail development seems to be about 10% theory and 90% empirical. I think thats how those really floppy leaches came about - theory of twist, less power in the top etc then why not try a bit more to see what happens. Modern sails have less power than old ones size for size but they are more rangy and conrtolable.It seems the top of the sail is just a valve but over downhauled its just a flappy draggy flag.

can anyone tell me what wind speeds you can expect to find wind shear develop in?? is there any rule to it or is it something that can happen on any given day in any given wind strength or is is something that is restricted to certain wind speed and certain tepmreture differences??? or something????