What's the purpose for so much loose material in the leech?

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mathew said..

sheddweller said..
Loose material is not the same as twist.
lots of the time sails have a load of uselessly flapping material doing absolutely sweet **** all When loaded up. This ain't slippery or exhausting or anything other than a load of flap.

Says you. Based on what reasoning ?

It is clear that sails have gotten faster over the years - as verified by gps measurements - more people are able to go fast in more locations. It is also clear that sails have more flapping material vs previous designed.

In other words, the flappy designs are indeed faster.

Unless you have some kind of reasoning to back your statement, that you can support with actual numbers.

Says me indeed and what the **** do I know eh? Just another faceless bod on an internet forum

Take it or leave it, it matters not

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sheddweller said..

mathew said..

sheddweller said..
Loose material is not the same as twist.
lots of the time sails have a load of uselessly flapping material doing absolutely sweet **** all When loaded up. This ain't slippery or exhausting or anything other than a load of flap.

Says you. Based on what reasoning ?

It is clear that sails have gotten faster over the years - as verified by gps measurements - more people are able to go fast in more locations. It is also clear that sails have more flapping material vs previous designed.

In other words, the flappy designs are indeed faster.

Unless you have some kind of reasoning to back your statement, that you can support with actual numbers.

Says me indeed and what the **** do I know eh? Just another faceless bod on an internet forum

Take it or leave it, it matters not

I explained sails are easier to handle in stronger wind and gotten faster - we have the evidence through gps-measurement. You didn't explain any reasoning, just made an unfounded claim.

Clearly it matters to you or you wouldn't have responded. Or you're just someone trying to have the last-word.

Thanks Kris for this other video. I had watched it but not with this topic in mind. The obvious question is why don't manufacturers shape their panels as such to begin with?

Does Antoine do it?
Do speed people do it?

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decrepit said..
We must allow that all that loose stuff flapping around doesn't look functional.
It's very easy to assume that it's only drag.
However as Mathew says, experience says otherwise.
To me the theory that the loose stuff acts as a barrier to flow around the sail, decreasing tip vortex and reducing the resultant drag. Makes sense.
That's what they are doing with aircraft wings now, and it's what the end feathers of eagles do.

Getting the optimum amount of flap, is another question, I assume too much would indeed be draggy, as would not enough.

This down to experimenting with downhaul, to find the sweet spot.

It may not be that the loose material acts like a physical barrier that smooths down the tip vortexes, as some have said. If it did, boat sails (which get shedloads of cash thrown at them) and aircraft wings (even more) would have similar skinny, flappy or twisted sections on them to get the drag benefits. Aircraft due sometimes use washout for vortice reduction but it's mainly for control.

Given that for the maximum lift (drive)/drag ratio for a given heeling moment we want what's called a "bell shaped span loading", the top of a sail should be developing a certain amount of lift compared to the sections lower down. The amount of lift and induced drag (also known as "drag due to lift", which shows how inescapable it is - no drag = no lift) created depends on the width of the sail at each point (the chord), the amount of depth at that point (camber) and the angle of attack (ie how much twist there is when sheeted in). So span loading (or power and drag) isn't the same as the physical amount of sailcloth in that section of the sail. A big flat section of sail twisted off can produce the same power (and drag) as a smaller deeper section of sail with no twist.

If we plot the optimum span loading it looks like the outline of a 1980s sail. But we want to make the leach twistier to get better gust response. However, if we just make the head wider so it can twist off, the sail in that section will be developing too much lift, which inherently creates more induced drag at the top where we don't want it, because that lift will create a bigger vortex coming off the top of the sail just next to it. A sail loaded at the top like that also wants to fall to leeward more.

By making the head of the sail wide but making it flatter and more twisted we can reduce the span loading; there will be plenty of area but it won't be creating much lift OR drag for its size because it's flat and twisted. That way we still have enough area to give good gust response with twist, but we don't have so much area that we create lots of tip vortex drag and induced drag up high.

The downside is that there's a lot of area not developing much drive, so the sail gets bigger and heavier for the same amount of drive. It also may

All of this is just from stuff I've picked up from reading and internet chats with real aero experts and sailmakers so I'm not saying it's the way it really is, but it does seem to be the way all the theory and practise fit together.

Basher hit the nail on the head. End thread.

Thanks all for the nice feedback. I guess maybe this conversation could have been split between categories.

Even in waves, there are sub categories, side on jumps and side off riding have different requirements don't they?

I guess extra leech is nice for support while narrower leech makes it more reactive and light feeling. A shorter compact sail logically should have a bit more than a taller one.

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SurferKris said..
It seems to me like some people are looking for problems that do not exist. A twisting sail is a good thing, it compensate for the lower windspeed near the water surface and it also act as "wash-out" on airplane wings, i.e. ensures that over-sheeting is not done at the top of the sail and that a stall would start at the lower part of the sail.

A sail that flops a little at the top on an upwind course, will be just right when going downwind:

And some people have actually tried to learn from the world experts like Professor Mark Drela. who designed the world-record human powered aircraft and designed and pedalled the world record human powered boat as well as designing America's Cup wingsails and being the guru in aerodynamics at MIT, one of the world's great unis. Some have also sailed with guys like John Bertrand of Australia II (he did his masters thesis on small craft sails) and Grant Simmer (who has won about seven America's Cups as a sailmaker, sailor or design leader). These guys aren't morons so it would be wrong to ignore what they say.

With respect, wind shear is not the whole answer. Wind shear affects all craft and yet extreme twist only makes a few craft go faster. If wind shear was the answer then Laser dinghies, which have a mast about as high as a windsurfer, would go faster with the same amount of twist as a windsurfer - in fact they go dog slow with the same amount of twist as a shortboard. So does a longboard. So does a high performance cat. So wind shear is not the answer, and we can tell that from getting out on the water and seeing that the wind does not shear as much as a high-wind windsurfer sail twists. We can do that very easily by looking at sail tufts, which often show that there is not actually much wind shear at all, and that wind shear is almost never as large as the twist in a fast windsurfer sail. On a big boat we can be looking at tufts that show us wind shear from about three metres above the waterline to about 30m above the waterline, and it's basically never as big as a windsurfer's twist is. The same applies in smaller scale on things like Moth or Laser rigs.

The Moth guys may well be going faster than the windfoilers a lot of the time and they are going to great trouble to REDUCE twist. Look at this pic of top Mothies going upwind - where's the huge twist like shortboards carry? If windsurfer twist was all about wind shear then why don't the top Moths, which are a similar height off the water, also twist off to suit the wind shear?


Here's the Olympic Laser gold medallist going upwind. Like the Moths, he is using massive amounts of vang tension to REDUCE twist, yet his mast height is similar to that of a board and so the wind shear would also be the same - so why does he not ease mainsheet and vang to twist off the sail if twist was faster? Answer - because windshear isn't a big factor.

Here's a taller mast but on a very efficient boat - the A Class cats. On these tall sticks there should be lots of wind shear but the fast guys don't carry anything like as much twist as a windsurfer sail. The world foiling A Class champ, by the way, is an America's Cup champ/sailmaker who probably earns more than all the windsurfer sail designers do put together and he has access to F1 style wind measurements and tech. The non-foiling division world champ is a sailmaker and Olympic medallist, so these guys are not morons who are ignoring wind shear.



Wind shear is very well known to big boat sailors because when you are sailing in light winds and ocean swells, the swells mess up the windspeed low down and cause wind shear to be absolutely critical to correct trim a lot of the time - and yet no fast big boats carry twist like windsurfers and if you give them that much twist they go slow. So it's a lot more than wind shear.

There's no particular reason why rigs need washout to control stall like an aircraft wing does as far as I know. There's plenty of boats where you can adjust the rig to stall at the head first and it's not disastrous; in fact in some conditions (flat water and light/medium breeze in medium-speed boats) it's actually very fast.

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.

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.

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Roo said..

mr love said..
A big problem with comparing windsurf rigs with yachts is a simple fact...yacht rigs are held up with stays and weight is used to counter the healing moment. On a windsurfer we are holding the rig up unassisted. If you used a sail like the A class on a windsurfer it would be like wresting a crocodile, you would be pulled over. Having a twisted sail means the top of the sail which has the most leverage over us runs at a low angle of attack or even zero angle of attack so produces low lift and less healing moment. Most of the drive is coming from the middle and bottom of the rig which has less leverage. This allows large sails with very power profiles in the bottom half that don't pull us over. You do need some twist in a sail on high speed sailing craft as it lowers drag, if you didn't have it the top of the sail would stall, this is more pronounced downwind but yes we use very large amounts...due to our unique ergonomics.

@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

This is all so wrong.
What windsurf rig can be tested horizontally, and on a car? Think it through, please.
(This photo was just a stupid marketing gimmick.)
You take away true wind and therefore apparent wind - and so you only test for created wind, as the car drives along.
What does that tell you? Nothing, because that is not the real world wind we windsurf in.

Without true wind as an essential component of apparent wind, you certainly can't get the sail twist right. No sails head straight into the wind, so any 'lift' measured by this rig, would not replicate a real world situation. .

And 'healing moment' in windsurf rigs, Mr Love? To be fair, I think he means heeling moment.

But heeling moment in yachts and dinghies is nothing like leverage issues in windsurfing - because of the way we counteract that rig load and any leverage.

The lack of rigging stays on windsurf rigs simply means the mast bends in a different way, and we set up sail rigidity and structure using downhaul tension. In yachts and dinghies - with rigging stays and spreader bars and kicking straps etc - you have other tools to control mast bend, so downhaul is not used in the same way.
But most yachts and dinghies aren't focussed on planing speeds anyway, so there's very little comparison.
Note that the old Olympic Finn class and the Laser dinghy class, do not have rigging stays either, but they do not use downhaul to bend the mast as we windsurfers do.

Not sure where this thread is going.

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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?

Now look at what you done! You lit a fuse!

I just realised what all the loose material is in the head.

its the foreskin

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Gestalt said..
I just realised what all the loose material is in the head.

its the foreskin

I think you mean the bell curve end.

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.

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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.