I’d like to understand better what imparts to the sail that vaunted « forward draft » many manufacturers spin in their advertising..

The literal meaning is clear : the sail is so designed as to optimize the forward motion of the board under the wind, while minimizing the lateral motion component… But how so?

I remember having read that it has to do with the sail progressively becoming softer towards luff end. This is achieved, according to some, by the battens becoming stiff or stiffer towards the leech than towards the luff.

Stiffer battens towards the leech constrict the shape of the sail there, with the result that, since the wind energy must be discharged somewhere across the sail , it gets discharged( it shifts) towards the front, where the sail is softer and can thus take a fuller shape than at leech-end.

Some even suggest to insert a segment of a 2nd batten in the pocket(s), ¾ across the sail, to increase the stiffness towards leech-end.
I tend to think that there is some truth in this, because in many sails the battens are tubular ( visibly thicker and stiffer) at the leech-end and they become flatter, thinner and softer towards the mast.

I wonder if anybody can comment on this

Thanks

Francone

You are focusing totally on the sail, none of what you somewhat correctly explained can happen without considering the mast as well.
Without the bend in the mast, there can be no forward draft in the sail.
That's all I have to say about that.

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Mastbender said..
You are focusing totally on the sail, none of what you somewhat correctly explained can happen without considering the mast as well.
Without the bend in the mast, there can be no forward draft in the sail.
That's all I have to say about that.

You are right. The bend in the mast obtained by downhauling loosens the sail at luff-end . In fact, the more you downahaul, the looser the sail there( more shape) and then you have to compensate by outhauling to make the sail tauter, if necessary.

Interesting to know, though, the interplay between downhaul and the tautness of the leech. They are inversely proportional: true, the stronger the downhaul ( i.e. the curvier the mast) , possibly the stronger the forward draft, but at the same time the leech becomes looser the more you d/haul and this reduces the wind power by spilling the excess wind at the top of the sail: the power you gain on one hand is then lost, on the other hand through a floppy sail top.However important this may be in stronger winds and planing, this is not what one may want in light winds.

I had rigged an old N.P. Garda 6.2 m2 from the 90’s on my Bic WindSup yesterday. The winds were light, around 15 knts , sometimes less, sometimes gusting more…
To have enough power to sail comfortably in those light winds, I gave the sail a tight leech by keeping the d/haul to a minimum, this time without a winch,( just by slightly pulling the d/haul twine by hand, until the top of the mast started bending) . On top of this, to facilitate the sail rotation with such a tight leech I also moved the central battens a few inches away from the luff: the result was a fuller shape in the sail and, therefore, a strong draft all along ( I can’t say whether ..forward or other), but sure the board was moving forward quite nicely, at some point getting closer to plane…

This is to say that, to me, in light winds, the degree of bending of the mast is perhaps less important in producing a forward draft, than a tight leech and the battens slightly away from the mast , in addition, of course, to being softer around the luff, as discussed in my post. In the end, more shape in the sail, especially at luff-end .

Francone

I thought that forward draft meant that the centre of effort of the sail was closer to the mast

I am not even sure I understand this question but if it is....How do you position draft in a sail? ... then it can be done by a combination of methods depending on how the designer decides to add shape to the sail design.

Put very simply you control the designed shape of the sail by...........

Luff curve......the amount of curve on the luff that exceeds the curve of the mast
Vertical seam shape...adding shape to vertical panels in the sail.
Horizontal seam shape...adding shape to horizontal panels in the sail.
Batten tapers and stiffness.

Where you place the seam shape and the stiffness and position of taper in the battens impacts the draft position.

Not sure if that is what you were after??

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John340 said..
I thought that forward draft meant that the centre of effort of the sail was closer to the mast

By tilting the mast backward or forward, as you do when tacking or gibing, the board turns upwind or downwind because the center of effort ( C.O.E.) shifts accordingly backward or forward in relation to the vertical position of the mast .

However,when speaking of forward draft of a sail we are not referring to the change of the C.O.E backward or forward, but to the fact that the " focus" of the wind, because of the design of the sail, is concentrated on its forward part, near the mast, where the sail has by design a fuller shape .
Without a forward draft (or a less efficient one) , the wind would be evenly spread backward across the sail as well, from luff to leech. This way the wind force would be dispersed across a wider surface, enhancing the lateral draft to the detriment of a stronger forward thrust.
I may be wrong, but this is how I understand it.
I am open to be corrected, if wrong

Francone

'Forward draft' is a traditional sailing term, meaning fullness cut in the luff panels of the sail.

So when sailmakers talk about forward draft in a windsurf sail it usually means there is some shape cut in there, rather than a simpler, flatter, entry.


What this shape does is to approximate to an aerofoil section that produces forward 'lift' – rather than sideways pull. So in wave sails, as an example, forward draft tends to give you a forward-pulling sail that goes upwind well and which remains stable in strong winds. In contrast, where the luff is cut flatter, then the sail is often more sideways pulling – which some would like for waveriding as it loads the backhand better during manoeuvres.

In bigger rigs, forward draft might be about a sail belly 'induced' into the lower panels with the help of tapered battens and cams – so that's maybe in addition to shape already cut there.


So this is not about tilting the rig.

It can however be fine-tuned with batten stiffness and via mast bend. In the latter case, the downhaul you apply to your sail may affect the fullness cut in the luff panels as an increased mast bend will tend to flatten luff panels.
But forward draft tends to be achieved at the sail design stages via broad seaming (shaping), rather than by luff round.

Applying outhaul also flattens the rear sections of any sails, effectively leaving more relative forward-pulling shape in the luff – which is why we often add outhaul when a sail starts to feel back handed.

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Francone said..

John340 said..
I thought that forward draft meant that the centre of effort of the sail was closer to the mast

However,when speaking of forward draft of a sail we are not referring to the change of the C.O.E backward or forward, but to the fact that the " focus" of the wind, because of the design of the sail, is concentrated on its forward part, near the mast, where the sail has by design a fuller shape .

That's exactly what I said. You achieve it by a combination of:
- cut of the sail
- bend in the mast
- variable flexibility of the battons (soft towards the luff and stiffer towards the leech)
- camber inducers and batton tension to lock the draft into position

There's another factor here in most modern sails, there's a boom cut out with a batten running through the boom area ending up just above the clew.

Depending how much this batten extends past the boom, will provide a balancing load to the wind pressure trying to shape the back of the sail. This is aided by the batten resting on the boom, shifting the pivot point from the clew to further forward in the sail. This all helps to keep the shape forward no matter how great the load. Without this the tendency is for the shape to move backwards as load increases, wind pressure overcoming batten tension.

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Mastbender said..
You are focusing totally on the sail, none of what you somewhat correctly explained can happen without considering the mast as well.
Without the bend in the mast, there can be no forward draft in the sail.
That's all I have to say about that.

Uhh seen a hang glider lately?
Rigid straight tube at the front and same foil shape as a sail. There is no bend in that tube to make a foil shape in the fabric.

The froward draft is simply achieved by seam shaping (curves sewn together) so if you laid the material down it would not be flat. Like if you sewed panels together to make a ball. Lots of flat stuff cut n sewn such that it is now a 3D object.
So no, the curved mast is not necessary to induce a foil to the sail.... and certainly not needed to induce a forward draft.

This shape is then stabilised by battens that are thinner where we want them to bend more (where the max draft sits).

This was all in sails in the mid 1980's.


They just do it way better now.


Mr love and Basher are onto it.

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Mark _australia said..

Mastbender said..
You are focusing totally on the sail, none of what you somewhat correctly explained can happen without considering the mast as well.
Without the bend in the mast, there can be no forward draft in the sail.
That's all I have to say about that.

Uhh seen a hang glider lately?
Rigid straight tube at the front and same foil shape as a sail. There is no bend in that tube to make a foil shape in the fabric.

The froward draft is simply achieved by seam shaping (curves sewn together) so if you laid the material down it would not be flat. Like if you sewed panels together to make a ball. Lots of flat stuff cut n sewn such that it is now a 3D object.
So no, the curved mast is not necessary to induce a foil to the sail.... and certainly not needed to induce a forward draft.

This shape is then stabilised by battens that are thinner where we want them to bend more (where the max draft sits).

This was all in sails in the mid 1980's.


They just do it way better now.


Mr love and Basher are onto it.

Thank you

Your post vindicates my original idea that the battens, along with the way the panels are shaped and sewn together ( which I hadn’t realized at first) is what induces forward draft and not the mast bend.

This leads me to a comment by Basher earlier in this thread. He said :

quote..an increased mast bend will tend to flatten luff panels unquote.

This doesn’t square with the fact that, to the contrary, the more you downhaul, the looser ( or the shapier) becomes the sail along the luff.

In fact, if you already gave some tension to the outhaul, you will notice that as you keep downhauling, the sail becomes looser again and then you have to further tighten up the outhaul in order to make the sail flatter. Then the shape at luff- end caused by the downhaul will disappear..

This is why I don’t understand why the downhaul ( or the bend in the mast) should make the sail flatter..

Is there anything I'm missing?

Francone

Increasing the down haul while maintaining out haul tension will flatten the sail. On a camber sail the entry, or the initial curve of the sail will be noticeably flatter. On regular sail the same will occur once it inflates, the battens will be further back as there is less loose material at that part of the sail to make the curve. Towards the clew of the sail the shape is more influenced by the out haul.

So if you flatten the sail with too much down haul, and then try and get some draft back in the sail by the reducing out haul the front on the sail will be flat (curve with bigger radius,) the clew also be flat, with the draft sort of in the middle, kind like the letter C, or a bed sheet on a clothes line and feel similar. :-)

Hope that makes sense

Francone, Racer X is correct, applying more downhaul bends the mast more and this actually flattens the entry of the sail. If you want the sail to have a bit more knuckle at the front you need less downhaul.
The draft position in the sail is established by the design of the sail...the way the seams are shaped and by the batten tapers. You can change it to some extent by playing with batten tapers and stiffness's but you really need to know what you are doing so you don't screw up the performance and ergonomic of the sail.

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Mark _australia said..

Mastbender said..
You are focusing totally on the sail, none of what you somewhat correctly explained can happen without considering the mast as well.
Without the bend in the mast, there can be no forward draft in the sail.
That's all I have to say about that.

Uhh seen a hang glider lately?
Rigid straight tube at the front and same foil shape as a sail. There is no bend in that tube to make a foil shape in the fabric.

The froward draft is simply achieved by seam shaping (curves sewn together) so if you laid the material down it would not be flat. Like if you sewed panels together to make a ball. Lots of flat stuff cut n sewn such that it is now a 3D object.
So no, the curved mast is not necessary to induce a foil to the sail.... and certainly not needed to induce a forward draft.

This shape is then stabilised by battens that are thinner where we want them to bend more (where the max draft sits).

This was all in sails in the mid 1980's.


They just do it way better now.


Mr love and Basher are onto it.

Yeah, I know all about bias stitching, maybe I should have refined what I meant, but that doesn't apply to all sails even today.
Back when I used to nothing but flat water, I had cambered sails with lots of bias stitching, and they wouldn't lay flat unrigged. But all I use anymore is wave sails, 5.7 to 3.7 (Ezzy Elites), and those are different, as are most wave sails. Those sails I can lay flat (unrigged, w/o batten tension) and they lay flat as flat can be, no bias stitching anywhere, but they'll rig with a nice deep pocket right where it's supposed to be, thanks to the mast, and batten tension I should add.

So we're both right.

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Francone said..

Mark _australia said..

blah blh

Thank you

Your post vindicates my original idea that the battens, along with the way the panels are shaped and sewn together ( which I hadn’t realized at first) is what induces forward draft and not the mast bend.

This leads me to a comment by Basher earlier in this thread. He said :

quote..an increased mast bend will tend to flatten luff panels unquote.

This doesn’t square with the fact that, to the contrary, the more you downhaul, the looser ( or the shapier) becomes the sail along the luff.

In fact, if you already gave some tension to the outhaul, you will notice that as you keep downhauling, the sail becomes looser again and then you have to further tighten up the outhaul in order to make the sail flatter. Then the shape at luff- end caused by the downhaul will disappear..

This is why I don’t understand why the downhaul ( or the bend in the mast) should make the sail flatter..

Is there anything I'm missing?

Francone

I think you are talking about two different things. How a sail is designed to have shape is a few different things. You do not have to have a bent mast to have a sail with shape. Look at yachts, hangliders.

But Basher may have been talking about using a wrong mast, the curve will affect luff panels and draft etc.
He may have been talking about more downhaul induces more mast bend and flattens the luff due to tension (at some points in the rigging process). As the mast bends due to downhaul or wind deflection of course it changes the shape of the sail.
But that is a different topic to "why does my sail have shape in it / how did they design shape into it?"

Agree with mastbender it is one of those topics where everybody is right, its hard to explain without pointing at one....

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racerX said..
Increasing the down haul while maintaining out haul tension will flatten the sail. On a camber sail the entry, or the initial curve of the sail will be noticeably flatter. On regular sail the same will occur once it inflates, the battens will be further back as there is less loose material at that part of the sail to make the curve. Towards the clew of the sail the shape is more influenced by the out haul.

So if you flatten the sail with too much down haul, and then try and get some draft back in the sail by the reducing out haul the front on the sail will be flat (curve with bigger radius,) the clew also be flat, with the draft sort of in the middle, kind like the letter C, or a bed sheet on a clothes line and feel similar. :-)

Hope that makes sense

I rejoin you and mr love on this issue.

I am bit puzzled at all this, because my contention that the d/haul loosens the sail is not based on theoretical grounds, but on my experience, unless I ..live on a parallel universe or.. something is missing from the equation..

I’ll give a brief, step-by-step description of the d/haul-outhaul sequence as I do it.

Usually I do it in increments ( 2 or more, depending on the situation):

first, I connect the clew-end of the sail to the boom and I slightly tension the outhaul twine, then I start d/hauling.

As I d/haul, I notice that the outhaul twine loosens up and the sail hangs loose at the clew end.. Then I re-tension the outhaul and give a 2nd crank of d/hauling and VOILA, the outhaul becomes loose again along with the sail and... on and on with d/hauling, if necessary, until I get the appropriate level of sail tensioning!

The only way out of this quandary would be if the d/haul indeed flattens the sail, as you say, but only at the luff-end, while at the same time loosening it at leech-end.
Is this what happens?

This leads me to a related issue: usually I sail in light winds and I keep the d/haul to a minimum( compatible with the rotation) in order to ensure the tight leech which is necessary in light winds. If the d/haul affects the shape of the sail, as you say,( supposedly at luff-end) and I keep it to a minimum, then a tight leech would correspond to a looser sail (more shape ) at luff-end. Is it so and is it consistent with light-winds sailing?

Thanks for the clarifications

Francone

Ok Francone... I will try to strip this back to basics.

A sail is made out of flat pieces of material so the only way to add 3 dimensional shape is to force it in. You do this by adding curvature to the edges of the panels. Grab two sheets of A4 paper and place them on a bench next to each other, they sit flat, right?? Now cut a convex curve along one edge and tape the 2 sheets together, curved edge to the other sheets straight edge...magic..you have a 3D shape.

So this is what your sail is..a series of flat panels with curvature on some edges sewn together. One of the main ways you add shape is with luff round. What this means is that the luff of the sail has more curvature than the mast it is rigged on, so this difference in curvature forces shape into the sail exactly like the 2 pieces of paper.
So when you have less downhaul the mast is straighter relative to the amount of curvature on the luff of the sail so it forces more shape in. As you downhaul the mast bends so it starts to remove that forced in shape, the more it bends the closer the curvature of the mast matches the curvature of the luff and the flatter the sail rigs.

Now what you are observing with the sail getting what you call " looser" at the clew end is pretty simple to explain. The dimension of the sail from the boom cutout to the clew is a constant. When you have the sail rigged with very minimal downhaul and attach the boom at the recommended length you will see that the clew is well short of the boom end. This is because with very little downaul the sail has a lot of shape forced into it. The arc length (distance from the cutout to the clew around the sail) is a constant so the chord length (distance from the cutout to the clew in a straight line) is shorter the more curvature the sail profile has. Then as you downhaul and bend the mast the forced in curvature reduces so the chord length has to get longer and the clew gets closer to the boom. The more downhaul you apply, the flatter the sail gets and the longer this chord length dimension gets..

I think that explains It???

Sounds very good to me Mr Love, I agree Francone seems to be confusing looseness at the clew with shape in the sail. When in fact a loosening clew means the increasing downhaul tension is bending the mast and moving the bulk of the sail back towards the clew.

How's this for simple ? Forward draft is the feeling that the power pulls near the mast. As apposed to middle of the sail kind of pull. No one wants reaward draft because this is the feeling of being overpowered and out of control. Forward draft sounds and feels safe.

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mr love said..
Ok Francone... I will try to strip this back to basics.

A sail is made out of flat pieces of material so the only way to add 3 dimensional shape is to force it in. You do this by adding curvature to the edges of the panels. Grab two sheets of A4 paper and place them on a bench next to each other, they sit flat, right?? Now cut a convex curve along one edge and tape the 2 sheets together, curved edge to the other sheets straight edge...magic..you have a 3D shape.

So this is what your sail is..a series of flat panels with curvature on some edges sewn together. One of the main ways you add shape is with luff round. What this means is that the luff of the sail has more curvature than the mast it is rigged on, so this difference in curvature forces shape into the sail exactly like the 2 pieces of paper.
So when you have less downhaul the mast is straighter relative to the amount of curvature on the luff of the sail so it forces more shape in. As you downhaul the mast bends so it starts to remove that forced in shape, the more it bends the closer the curvature of the mast matches the curvature of the luff and the flatter the sail rigs.

Now what you are observing with the sail getting what you call " looser" at the clew end is pretty simple to explain. The dimension of the sail from the boom cutout to the clew is a constant. When you have the sail rigged with very minimal downhaul and attach the boom at the recommended length you will see that the clew is well short of the boom end. This is because with very little downaul the sail has a lot of shape forced into it. The arc length (distance from the cutout to the clew around the sail) is a constant so the chord length (distance from the cutout to the clew in a straight line) is shorter the more curvature the sail profile has. Then as you downhaul and bend the mast the forced in curvature reduces so the chord length has to get longer and the clew gets closer to the boom. The more downhaul you apply, the flatter the sail gets and the longer this chord length dimension gets..

I think that explains It???

Excellent explanation. I think I now understand: in reality, this slackening of the outhaul I attributed to the d/haul and which I interpreted as a loosening ( shaping) of the whole sail, is due to the fact that, as the sail flattens( by d/hauling) the chord of the foil becomes LONGER, so that the clew-end of the sail now approaches the boom end and the outhaul twine slackens..

In the end, can’t we say that when the sail flattens under increased d/haul it is because the bending mast becomes somewhat like an arc, at mid-point pulling ( the sail) away from the chord, i.e. the line corresponding to the original straight mast before d/hauling?

Also, as I see it, the loosening up of the outhaul, is due not only to the chord of the foil becoming longer as the sail flattens under d/haul, but also to the fact that the distance between the lowering tip of the curved mast and the clew at boom-end becomes shorter, causing a loose in the sail fabric ( a loose leech).


Francone

Yep, think you got it. Best thing is just rig a sail then play around with the downhaul and watch what happens.

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mr love said..
Yep, think you got it. Best thing is just rig a sail then play around with the downhaul and watch what happens.

I thought that the flattening of the sail under d/haul was explained by the " arc" example I brought in ( as the mast bends, like an arc, it recedes at mid-point from its initial straight position, the chord, pulling the sail back and away, which flattens it.
I thought I could catch this tightening ( or flattening) by observing the d/haul process in... slow motion, step-by-step, as I d/haul.

At first, the outhaul tension would slacken ( as I indeed observed) because, according to your explanation, the distance between luff-end and clew-end in the sail would increase, but I also expected at some point, as I continue d/hauling, to observe CONCRETELY a flattening of the sail and a corresponding increase the out-haul tension.

Unfortunately, I could observe neither : no visible flattening of the sail and the outhaul tension stays the same. If anything, as the d/haul increases, the central battens move away from the mast and this results, to the contrary, in more LOOSENESS at luff-end, which does not seem ( to me) to be an indication that the sail is becoming flatter.

So, in a way I am back to square one : the theory makes sense, but it is not borne out by observation, as far as I am concerned .

I hope to sort this out !

Francone

Ok well you have lost me Francone....Every sail I have ever rigged gets a flatter entry the more downhaul I apply. Yes the outhaul loosens and the sail starts to touch the boom as you downhaul, I guess you could interpret this as the sail getting fuller if you wish.
But going back to your original question...you will get a more forward draft ( more rounded entry into the sail) with less downhaul and as you downhaul the entry will flatten out as the mast bends.
Enough from me on this subject.

Luff curve is the answer you're looking for Francone. Draft is where the maximum chord is in the fore/aft camber of the sail

If you lay the sail flat on the ground it sits flat, and looking up the luff of the sail, you can see that it's curved. When you put a straight mast into the luff pocket all the battens start to bend into shape because the curved luff is being pulled straight (in an ugly fashion) by the mast. This is in turn pulling tension onto the leech and then tension onto the battens.

The more tension you apply to the downhaul the more the mast bends and conforms to the luff curve of the sail, which allows the leech and battens etc, to start flattening off again. That's why your sail gets flatter with more downhaul tension.

Seam shaping is old sail tech. You'll only really see it on old one design classes of boat like 470s' and cruisers where there's no battens.

There are a lot of good explanations here already – but I'll put this in my own way. (If you are new to windsurf rigs, you may have to read this several times to follow)

As sailing rigs go, the windsurf sail is very simple with few controls, but behind the scenes there are more complex things going on – not obvious until you understand them.

You may already know about matching a mast to your sail, but there also has to be a 'relationship' between the sail and the mast it is set on.
That relationship is set at the sail cutting stage, and the mast bend too is set in the factory. We bring the sail and mast together as a matched pair, with the controls then downhaul and outhaul. (In cambered sails, you have a third control, which relates to the induced battens.)

When you roll out a sail on the ground it seems pretty flat, but every sail will have some shape or belly cut in via the panels. In sailmaking we call this shaping 'broadseaming'.

Broad seaming usually means tapered darts are cut in the seams that join horizontal sail panels but you can also add shape via vertical seams. The technique is simply the same as a shirtmaker uses to get a flat material to fit our human shape.

The sail is also cut with luff round – obviously this is the curve of the luff when the sail is laid flat. This luff curve is a deliberate mis-match compared to the mast bend curve. The 'mis-match' in curves is what allows more rotation at the boom. That rotation is partly what adds belly to the sail in use, in conjunction with the board seaming.

When you rig the 'flat' sail on a straight mast it looks a mess until you apply downhaul which bends the mast. With the mast bent, with moderate downhaul, the sail sets with some batten rotation and the belly or broadseam-shaping is revealed – with the sail taking up more of a wing or aerofoil profile.
We say the sail has a 'relationship' with the mast, because the two have to work together as you sail along and as the wind gusts and lulls. A gust will tend to make the mast bend more.


All that is basic stuff, but now we get to the good bit.

The mismatch I mentioned now shows itself on the beach, with more batten rotation in the middle and lower sections of the rig, and less rotation at the head.
As you add more downhaul you simply bend the mast more, and so more downhaul tends to reduce batten rotation near the boom but there is already no rotation at the head, so any increased mast bend there pulls fullness out of the head panels.
It is the mast bend at the head that allows the leech of the sail to open, because that bend has taken fullness out of the luff panels at the head. The leach goes floppy at the head because it has lost its support at the luff.
With some sails, you control this head looseness with just small adjustments in the downhaul setting.

The sail will still have 'fullness' in the boom and foot area due to continuing rotation AND because there is more fullness cut into the panels here, via broadseaming.
On topic, the shape or 'draft' at the luff will depend on how much broadseaming there is – AND on how much luffround is left when the mast is fully bent under downhaul. The mismatch in luffround compared to the mast bend ensures there is plenty of fullness by the boom compared to higher up, but it is the broadseaming that pinpoints where that shape sits.

So the 'forward-pulling' nature of the sail – or the 'forward draft' in this question – will depend on the shape left when the sail is properly downhauled. How forward-pulling a sail is depends on the sailmaker's broadseaming more than on luffround or mast match.

With good/successful broadseaming, a sail should keep some belly to remain forward-pulling even at full downhaul, and, coincidently, that's partly what gives a sail good stability.

We also expect as sail to have a good wind range and so we use downhaul and outhaul to adjust basic power for the conditions on offer. Adding downhaul bends the mast further and that should release the head panels by allowing them to twist open more. (A good sail retains a flatter lower profile when downhaul is added but still has a forward pulling shape near the boom.)

The outhaul control is a lot simpler. As the mast is bent under downhaul load, the sail is flattened and so a rig simply needs more outhaul to match that extra mast bend. So we set outhaul in tandem with the dominant tuning device, downhaul. (Top tip: never add downhaul to the point where it bends the mast more, as that makes the luff panels go slack and prevents the sail from working well).

For any mast bend however, you can also vary outhaul tension – and loosening outhaul adds belly to the lower sections of the sail for more power. Conversely, adding outhaul flattens the sail. Outhaul also control lower leech tension, which can help you sail upwind (i.e. add outhaul for upwind work).

In normal use, your life can be more simple with a modern rig. Set the sail with recommended downhaul, and then change outhaul tension to increase or reduce power.
When that doesn't work, change sail size.

Subsonic.....I think you will find most 'modern" windsurfing sails have seam shaping.

Been sailing for half my life, and I'm old, thought I'd read this thread again, for the hell of it, and it gave me a headache.
Dry feet makes people think too much.

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Francone said..
>>>>>>>
I thought I could catch this tightening ( or flattening) by observing the d/haul process in... slow motion, step-by-step, as I d/haul.

At first, the outhaul tension would slacken ( as I indeed observed) because, according to your explanation, the distance between luff-end and clew-end in the sail would increase, but I also expected at some point, as I continue d/hauling, to observe CONCRETELY a flattening of the sail and a corresponding increase the out-haul tension.

Unfortunately, I could observe neither : no visible flattening of the sail and the outhaul tension stays the same. If anything, as the d/haul increases, the central battens move away from the mast and this results, to the contrary, in more LOOSENESS at luff-end, which does not seem ( to me) to be an indication that the sail is becoming flatter.

So, in a way I am back to square one : the theory makes sense, but it is not borne out by observation, as far as I am concerned .

>>>

Francone

Francone, I think I pointed this out before, you're confusing what the loose clew means, fair enough if you leave the clew loose you'll have excess sail cloth that is free to move around, but this shape will stay at the back of the sail, nobody wants to use a sail like that so you pull more downhaul on!

When you have little downhaul and some outhaul tension, sure the luff is tight, but there's oodles of spare cloth up front, if you push down on the sail at the harness line position, you'll probably be able to wrap the sail around the boom, so although the clew is tight, there's too much shape in the sail. If you increase downhaul, and also increase out haul to keep a slight tension there, you'll see that the amount you can depress the sail is reducing.
So the trick is a constant outhaul tension, not a constant position. This is necessary, because increasing downhaul is moving the sail backwards at the same time it's flattening it (reducing the extra cloth at the luff)

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mr love said...
Subsonic.....I think you will find most 'modern" windsurfing sails have seam shaping.

If there is, it's not much. Certainly no where near as much as there used to be.

With the advent of full length battens and camber inducers, there's less requirement for it. I'm sure a sailmaker will be able to tell us for sure if there's any.

I am telling you for sure that there is seam taper, both horizontal and vertical if the design has vertical panels. Not sure how much you think is much, but it is there.

I had a quiver of 2010 sails that were completely seam shaped, the luff fitted the bent mast exactly, made it a bit hard to achieve full downhaul without wetting the luff sleeve first as the mast/sleeve friction became quite high towards the end. Gybed effortlessly though the 3 cambers had zero mast pressure, they didn't have any shaping roll, just held the sail in place against the mast.