How is forward draft designed in a sail?

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

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??

'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.

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.

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.

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???

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.

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

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.

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.

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.

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.