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?
I think the answer is control.
The majority of power is generated in the bottom half of the sail. The top part is mostly for control and by having so much loose leach the sail can twist/breathe to release excess power in gusts.
Maybe see it as a suspension, the more loose leach the softer the sail might feel in powered to overpowered conditions?
Try giving 4 cms less downhaul next time and you'll notice how twisty the sail feels and it will be challenging to sail in powered up conditions.
I thought it aligned the air from both sides as it exited the sail, hence is less vortexes and less drag.
I do see it as some sort of dampening feature yes especially on wave sails.
Ezzy's are a little bit different and feel different, superfreaks too probably because their leech work differently.
How about for wave riding when there's lots of sail movement?
How about freestyling?
It makes the sail much more efficient. The air needs to be exhausted from the sail to work properly and the loose leach is where this happens. Those old sails with a tight head will feel awful compared to the new sails. A tight leach might give you the feeling of more power but it's not translated into speed, it will tug like hell but not go anywhere.
www.northsails.com/de-dach/blogs/north-sails-blog/north-u-understanding-twist-by-bill-gladstone
"More on Twist
Twist is necessary because due to less surface friction, the wind is stronger aloft than at the surface; this phenomenon is called wind gradient. True wind and boat speed combine to create apparent wind. The stronger true wind up high creates both a stronger apparent wind and a wider apparent wind angle aloft. The upper part of the sail must be twisted out relative to the lower part of the sail to match the more open apparent wind angle."
Pretty much explains why we need a bit of twist in sails.
Select to expand quoteManuel7 said..
How about for wave riding when there's lots of sail movement?
How about freestyling?
You'll notice that in freestyle the sail is generally trimmed much flatter with barely any loose leech. I guess that might work better in tricks and to some extent the same applies to wave sailing depending on what you do on the waves.
Select to expand quoteJC1994 said..
It makes the sail much more efficient. The air needs to be exhausted from the sail to work properly and the loose leach is where this happens. Those old sails with a tight head will feel awful compared to the new sails. A tight leach might give you the feeling of more power but it's not translated into speed, it will tug like hell but not go anywhere.
Define "Exhaust' 
Or better still, explain the concept.
Show me where any aerodynamics expert has ever used the term in describing airflow over a wing? This is a BS term made up by ignorant sales people.
Utterly meaningless. 
Always fascinated me this one : Does a 7m sail with a very loose leach (ie some cloth not capturing much wind) work effectively the same as say a 6m with a tight leach (where all the cloth captures wind)?
which one should I rig when conditions are between the two sizes?
Select to expand quoteJasonwave said..
Always fascinated me this one : Does a 7m sail with a very loose leach (ie some cloth not capturing much wind) work effectively the same as say a 6m with a tight leach (where all the cloth captures wind)?
which one should I rig when conditions are between the two sizes?
A bigger nicely rigged sail will always:
- be faster
- point upwind better
- be more stable
- feel heavier to move around
- be slower to toss around
- slower to waterstart
A smaller minimally downhauled sail will:
- feel lighter
- faster to transition
- be slower
- less stable
- faster to waterstart
Less downhaul generally equals more initial lift. More downhaul means better control and faster top end speed.
It depends on what you like to do. Some boards need more lift than others. I prefer smaller sails always. I privilege reactivity over planing and speed. One can always adjust outhaul on the water too.
Select to expand quoteJC1994 said..
It makes the sail much more efficient. The air needs to be exhausted from the sail to work properly and the loose leach is where this happens. Those old sails with a tight head will feel awful compared to the new sails. A tight leach might give you the feeling of more power but it's not translated into speed, it will tug like hell but not go anywhere.
A sail's exhaust is where you put the false waste gate to make your sail sound like it has a turbo, you silly. :-)
As you say, there's a lot more to creating a low-drag shape than just the leach.
Select to expand quoteJasonwave said..
Always fascinated me this one : Does a 7m sail with a very loose leach (ie some cloth not capturing much wind) work effectively the same as say a 6m with a tight leach (where all the cloth captures wind)?
which one should I rig when conditions are between the two sizes?
As I understand it from what aero guru Prof. Mark Drela says, what matters is essentially the combined (ie both air and water drag) drag and lift of the entire unit - ie both the sail and board. That complicates matters because the board's drag varies dramatically as the wind strength changes, while the sail's drag and power rise more steadily.*
In light winds, a shortboard may be schlogging or pushing water when it tries to get onto the plane - that creates a lot of hydro drag at a time when the light wind means there's not much power available. In strong winds, the shortboard is planing so it creates dramatically less drag, compared to the tons of power available from the strong wind.
Generally speaking, a deep tight-leach sail will create more lift (ie power) but at the expense of increasing aerodynamic drag. So if you are trying to obtain maximum power (ie schlogging or trying to get onto the plane in light winds) a deep sail can work very well. You have a high-drag board and a high-drag sail, but lots of power to push all that drag.
However, once you pick up speed and get on the plane, the windspeed increases so aerodynamic drag starts to hold you back more. At the same time, the drag of the board decreases (or decreases compared to the amount of wind power). If you had a deep, draggy 6 metre you now have a lot of aerodynamic drag that you don't really need, because your board is now easier to push. You'll feel a bit of a speed limit as aerodynamic drag continues to rise.*.
Conversely if you put a flat loose-leach 7m on when you are trying to get planing, you don't have as much grunt to get the board moving as it schlogs along. The sail may be producing little aerodynamic drag but that's not very helpful because the board is producing lots of hydrodynamic drag.
Once the board gets on the plane and the hydro drag drops* then the flat sail works well, because you don't need much power to drive the low-drag planing board. You now get the benefit of low aero drag to give you a lower-drag low-power unit.
For the same sort of reasons, the flat and open-leach windsurfer style sail isn't actually better per se on longboards or boats. A longboard or a dinghy has higher drag (in planing winds) than a shortboard does. If you put a shortboard sail on a longboard or a dinghy, you have a high-drag unit with a low-drag low-power driving force and you go slow.
* drag doesn't actually reduce as boards and sails go faster, but it reduces as a proportion of the speed and as a proportion of the power available from the wind.
Select to expand quoteManuel7 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?
Here's what the pro-designers have to say about it:
Select to expand quotesailquik said..JC1994 said..
It makes the sail much more efficient. The air needs to be exhausted.
Define "Exhaust' Or better still, explain the concept.
Show me where any aerodynamics expert has ever used the term in describing airflow over a wing? This is a BS term made up by ignorant sales people.
Utterly meaningless.
I guess exhaust means release, escape, bleed, etc. Monty Spindler used that word once in that video above :).
When they say "breathe" isn't it the flex of the sail causing it to shed a gust?
Select to expand quoteWillyWind said..sailquik said..JC1994 said..
It makes the sail much more efficient. The air needs to be exhausted.
Define "Exhaust' Or better still, explain the concept.
Show me where any aerodynamics expert has ever used the term in describing airflow over a wing? This is a BS term made up by ignorant sales people.
Utterly meaningless.
I guess exhaust means release, escape, bleed, etc. Monty Spindler used that word once in that video above :).
You can make an umbrella shaped sail that would be incredibly powerful but have no release/exhaust. That's essentially what I mean by exhaust. The air has to flow through the sail otherwise it'll feel awful and not create any useable power.
Freestyle sails have a tight leech for low end power and stability in ducking manoeuvres.
Select to expand quoteJasonwave said..
Always fascinated me this one : Does a 7m sail with a very loose leach (ie some cloth not capturing much wind) work effectively the same as say a 6m with a tight leach (where all the cloth captures wind)?
which one should I rig when conditions are between the two sizes?
Always better to rig your Sail properly given the choice. An under downhauled sail will tug you around like crazy. Much better to go for a larger size that's stable and releases properly.
Most windsurfers are under downhauling their sails to great detriment.
Select to expand quotesailquik said..JC1994 said..
It makes the sail much more efficient. The air needs to be exhausted from the sail to work properly and the loose leach is where this happens. Those old sails with a tight head will feel awful compared to the new sails. A tight leach might give you the feeling of more power but it's not translated into speed, it will tug like hell but not go anywhere.
Define "Exhaust' Or better still, explain the concept.
Show me where any aerodynamics expert has ever used the term in describing airflow over a wing? This is a BS term made up by ignorant sales people.
Utterly meaningless.
The movement of air over a sail has three phases. In front, over and behind the sail as the sail moves through the wind. Presumably "exhaust" is a marketing term that describes the airflow behind the sail. Laminar flow through all three phases is desired for efficient design of the sail. A loose leach promotes twist which reduces tip vortex at the top of the sail and improves laminar flow behind the sail. A similar shape occurs in bird wings.
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.
What it is is a reduction in effective sail area, so you dont have to change down- just crank on the downhaul.
If when fully loaded your sail has loose flapping cloth, then you could happily be on a smaller sail( without flappy material), with less drag less weight and more fun.
Surely if the sail is rigid it won't twist so the sail needs to be both rigid and loose and the art is knowing where those 2 elements should occur and how they interact to control the twist
At risk of getting into another heated debate over baloney, I'm just going to say here's the quite similar discussion from a few months back.
www.seabreeze.com.au/forums/Windsurfing/General/Wtf-5
Select to expand quotesheddweller 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.
What it is is a reduction in effective sail area, so you dont have to change down- just crank on the downhaul.
If when fully loaded your sail has loose flapping cloth, then you could happily be on a smaller sail( without flappy material), with less drag less weight and more fun.
The problem with these discussions is we can't agree what we're talking about. Loose material vs twist vs exhaust vs release. Everyone wants to feel like they're right 
Windsurfing is about fun ! Any science behind our sport is pretty dodgy. The feeling and performance on the water is what counts.
If you want to rig your sail with a tight leech, go for it!
Select to expand quoteJC1994 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.
What it is is a reduction in effective sail area, so you dont have to change down- just crank on the downhaul.
If when fully loaded your sail has loose flapping cloth, then you could happily be on a smaller sail( without flappy material), with less drag less weight and more fun.
The problem with these discussions is we can't agree what we're talking about. Loose material vs twist vs exhaust vs release. Everyone wants to feel like they're right
Windsurfing is about fun ! Any science behind our sport is pretty dodgy. The feeling and performance on the water is what counts.
If you want to rig your sail with a tight leech, go for it!
That around about sums up how these conversations nearly always go, arguments over semantics.
im not sure I'd say the science behind our sport is dodgy though. Windsurfing is quite unique in many ways and sometimes requires thinking outside the box. There are plenty of designers in windsurfing that have come up with unique ideas to solve problems unique to windsurfing. Not all of them have had a degree, or even a background in aerodynamics, or hydro dynamics outside of windsurfing, but it doesn't make their ideas less scientific. A formally trained aerodynamics expert might not ever be inclined to used the term exhaust when speaking about a wing, but to a layman windsurfer who has even a slight thought of what it feels like to be attached directly to a sail with a tight leech, the term makes sense.
Ok cool, so yesterday I felt the leech flapping slightly right above the boom.
Interesting to hear that it's a feature for sails on a downwind course!
I'll add that a tighter leech adds more mast foot pressure pushing the nose of the board down, reducing speed.
Select to expand quoteKen767 said..
I'll add that a tighter leech adds more mast foot pressure pushing the nose of the board down, reducing speed.
I guess you are referring to straight line speed?
When turning on a wave, frontside, a high forward pull from the sail actually helps to keep the rail down and maintain the speed through the turn.
First one has to find out on what application the OP is asking about, wave, longboard (non-planning), slalom or speed?
Here is what Wojtek has to say about slalom sails:
Select to expand quotesheddweller 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.
Select to expand quotemathew 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.
I haven't got a clue but it just struck me that maybe sailors are going faster because the race sails are more controllable and stable so they can hold a larger sail in the same winstrength ?
