Most areas have constantly changing winds.
Tight leech sails are fastest within their wind range.
Bigger looseleech sail allows for more variance in wind speeds..more control over wider changing conditions like course racing venues.
Bigger heavier sailors can leanback and force any sail to deform.
Smaller sailors can be just as fast if they can compensate for changing wind and water conditions.
Freestylers rig tight leech small sails,
similar to freeride foilers today.

Jeff Magnum admitted in the ADTR article that he was sitting in the truck looking at the data and one of the techs was poking the test rig and he spotted it when the leech was prodded so he asked the tech to keep doing it. He realized something good was going on.
Accidental?? I'd reckon.

Sorry Nubie. Washout is designed in aeroplane wings to ensure that the root of the wing will stall before the tip. This is to make for a soft stall which is predictable, not sudden and enable the ailerons, closer to the wingtips, to still be somewhat effective as the wing begins to stall. I dont think it has much to do with drag reduction.

Also, I think the ADTR days were pre-PC! If they were measuring lift and drag, I very much doubt that it had a computer readout. More likely, spring scales. Roo could tell us.

sailquik said..

Also, I think the ADTR days were pre-PC! If they were measuring lift and drag, I very much doubt that it had a computer readout. More likely, spring scales. Roo could tell us.

Spring scales...you're dreaming Daffy. Electronic load cells and a steady stream data output. Nothing PC about me! We knew all about twist, all of us working on the ADTR were pilots. What we found was the more you offloaded the head of the sail the less the healing moment was, that's what you could see and was recorded in the numbers when the tip of the sail was pushed up and the angle of incidence changed. We had lots of data collected over a few years testing. Twist was a way to make the sail easier to handle rather than reduce drag.

joe windsurf said..
can we have the source of those photos above ??
would love to put em on my blog and credit source(s) !!

I took those photos, still have the slides I scanned them from. Here's another. Brian Sprout took some a year later that were used for the Gaastra brochure, we painted the truck to match the trailer for those!

sailquik said..
Just a bit of a prod there Roo. I stand corrected. You were obviously on the bleeding edge.

There must be some VERY interesting data from those tests.

Thanks for a data backed answer to one of my big questions.

Obviously that setup could measure a lot of things. Could it measure drag very well?

It could measure everything well. Lift, drag, pitching moment and heeling moment along with angle of attack and airspeed.

sailquik said..
Sorry Nubie. Washout is designed in aeroplane wings to ensure that the root of the wing will stall before the tip. This is to make for a soft stall which is predictable, not sudden and enable the ailerons, closer to the wingtips, to still be somewhat effective as the wing begins to stall. I dont think it has much to do with drag reduction.

Yes washout use for ensure root stall before wingtip but also reduce induce drag.

en.wikipedia.org/wiki/Washout(aeronautics)

quote:
"Washout is a characteristic of aircraft wing design which deliberately reduces the lift distribution across the span of an aircraft's wing. The wing is designed so that the angle of incidence is greater at the wing roots and decreases across the span, becoming lowest at the wing tip. This is usually to ensure that at stall speed the wing root stalls before the wing tips, providing the aircraft with continued aileron control and some resistance to spinning. Washout may also be used to modify the spanwise lift distribution to reduce lift-induced drag."

But Roo just confirm that they looking for reduce heeling moment,that make sense becasue if you depower upper part of sail,you have sail with lower COE,thus heeling lever arm and sail resultant force are smaller.

Nubie said..

Roo said..
What we found was the more you offloaded the head of the sail the less the healing moment was, that's what you could see and was recorded in the numbers when the tip of the sail was pushed up and the angle of incidence changed. We had lots of data collected over a few years testing. Twist was a way to make the sail easier to handle rather than reduce drag.

www.seabreeze.com.au/img/photos/windsurfing/15121379.jpg' />

Does twist on windsurfing sail have something with wind gradient?
(because some sailing books descirbe twist because of wind shear)

Can you explain that?

(if you have sail on car like you did,than sail will not has wind shear in top part,becasue wind speed is eqaul on bottom and top,but if wind shear do not exist when sailing ,then sail on car can be good simulation for sail when sailing in real life.. )

I very much doubt it. See my post using YP nad MI as an illustration on the other SB 'GPS and Speed' thread.

Pacey said..

Basher said..
When I was Moth world champion, my light wind sail was quite tight leeched but my windy weather rig had a looser leach and was cut much flatter to twist off at the head. (1970s).
But it's also key here to understand what you can do with a stayed mast on a dinghy or yacht, and what you can do with the un-stayed windsurf rig. In windsurfing our only mechanical controls are downhaul and outhaul, which work in conjunction with mast stiffness and mast bend.

Well I was never a world moth champion, but I did once work for Frank Bethwaite, who told me that the top NS14 dinghys at the time used active batten rigs where the battens were stiff and used high tension, with the result that by down hauling the luff hard the battens would be compressed into the mast, bending the mast and freeing the leech dramatically. This allowed them to depower the rig in strong winds far more effectively than the more conventional rigs in use at the time.

I suspect that is also the source of the bendy 18 footer rigs that shoodbegood mentioned, this is when Julian Bethwaite first designed his own18 and started introducing several radical innovations, some of which culminated in the 49er Olympic class.

That may be the case although I don't know the history of NS14 dinghies as others might. (But I wouldn't describe the rig control that way because batten tension and mast bend tend to be separate things.)

The key point I was making is that racing dinghies and yachts had and have several options for rig control which our unstayed windsurf rigs don't, and the loose leach or 'control of sail twist' has been a thing in sailing for decades before the windsurf rig was invented.

The early windsurf rigs were obviously a very crude triangle of cloth, and seen as very unsophisticated by dinghy racers. It then took a couple of decades of windsurf rig development before we realised we had one major mast bend control option which was downhaul.

Downhaul nowadays controls the mast bend, and we use that in conjunction with a deliberate mis-match between the sail luff round and the mast bend curve.

Chris249 said..
Yep, sailors have known about twist for eons, as have wing designers.

duzzi said..

hum ... I see a real lot of deformation in those sails. Loose everywhere but no twist!!!!

??? There's heaps of twist - look at the vid after 1 minute, for example. For the same sort of thing, see here;


The angle at the top of the sail (near the logo) is clearly very, very different to the angle of the bottom or near the boom. That's twist. Windsurfing started with sails that had TOO MUCH twist, rather than not enough, and it caused speed and handling issues. We knew about it, which is why some people experimented with a line running from the clew to the foot. The idea was that putting the clew down would tighten the leach.

As Basher says, boat sailmakers had known about twist for eons. It was spoken about and written about all the time. But much of what happened was that older materials made it hard to create a leach that stayed flat and would twist the right amount at the right time. Once materials improved, and the rules and style of sailing changed, you could design a flatter sail that would open up effectively.

It wasn't so much a case of anyone learning anything (or shouldn't have been) but of them being able to do something better once materials got better. However, to be honest many windsurfer sailmakers do seem to be pretty ignorant, and a touch arrogant, about boat sails in my experience. It's probably because many of them came from the USA or Europe, where boat designs are more restricted. In the UK, NZ and Australia there had been more design development because we had more classes with few restrictions.

That is not twist. That is the whole sail deforming. If anything the area around the boom is more stretched out than the top: the opposite of "twisting". The sail is designed like a triangular dinghy sail from the 70s, with most area at the boom a very narrow top, The little twist you see at the top is not intentional and is more than obliterated by the huge distortion everywhere. The angle of the "spinnaker" bulge has an even wider angle. It is just a very badly designed sail (and I used it! I am that old ... my gods!!!!!)

First 2 photos are sail not powered up.
Last 2 photos are sail powered up.
The top 1/3 of your mast bends sideways when powered up and tightens the leech. If you rig your sail on the beach to look like the last 2 photos you will end up with an extremely tight leech which will stop the exhaust and choke the sail your board will not lift and fly it will stick to the water. YOU DON'T SAIL ON THE BEACH !!


photo by Mari

photo by Mari

photo by Dezza

Pacey said..

Basher said..
That may be the case although I don't know the history of NS14 dinghies as others might. (But I wouldn't describe the rig control that way because batten tension and mast bend tend to be separate things.)

The way it was explained to me was that if you have stiff battens under high tension and lots of tension in the sail, its like a bow and arrow, with the batten being the arrow being pushed into the mast by sail tension. The net result is pressure on the mast which encourages it to bend. Increasing the luff downhaul tension increases this thrust and bends the mast further.


I think modern windsurfing sails use the same effect, the mast is bending not just because of the buckling effect of high luff tension, but also because of the bending force exerted by the battens.

The person who gave me this interpretation was Phil Darby, who also worked for Frank and was the NS14 national champion at the time, so I have no reason to doubt that it was the current thinking in the class at the time.

I'm afraid a lot of that is wrong. Battens simply hold the luff apart from the leech and they tend to take up the shape of the sail but they don't exert much pressure on the mast and have nothing to do with mast bend.
We use tapered battens to encourage a flat exit at the leech and their thinner forward ends can help hold some curve at the luff.
With camber induced sails the batten is used to wedge against the mast to help push out fullness that is already cut in the sail but when sailing the mast bend tends to cause the battens to pull back from the mast, and nowadays our cams are sewn to the luff of the sail and only push against the mast when we add spacers to the batten ends before fitting the cam, and that's really about shaping the leading edge of the sail, or fine-tuning it for locked in drive.

In your dinghy sail above, the sail appeal is cut with radial seams which suggest (visually) a forward force on the batten, but I'm betting there isn't any. A lot of those seaming claims are just marketing bull****.
On most stayed rigs as seen here, sailors control mast bend with a mast ram at deck level, with the kicking strap, and with the setting of the spreaders. On yachts they can also use an adjustable backstay. All those key adjustments can affect sail twist, as well as mast bend.

Dinghy and yachts however don't use downhaul to bend the mast as we do, but they do adjust tension on the mainsail luff panels and those sails may be cut with stretchy cloth, so that increasing luff panel tension can pull fullness forwards in the sail, or else it can be used to flatten the sail entry for upwind sailing. Some dinghies still use a 'Cunningham hole' for this purpose, and you can google that. Mr Cunningham lived quite a life and died at the age of 96.

Nubie said..

Does twist on windsurfing sail have something with wind gradient?
(because some sailing books descirbe twist because of wind shear)

Can you explain that?

(if you have sail on car like you did,than sail will not has wind shear in top part,becasue wind speed is eqaul on bottom and top,but if wind shear do not exist when sailing ,then sail on car can be good simulation for sail when sailing in real life.. )

Sail twist is a bit complex but I'll try and explain it here.
1) Firstly the idea is that the sail should be sheeted in to the correct angle on the wind at all the points up the sail luff, from the tack to the head. If the sail is cut fuller near the boom and flat at the head then some twist would be needed to get the correct angle of attack at each section because our sails are wider at the bottom. You could achieve the same angle of attack with twist at the head, or else by cutting the sail fuller at the head (but fullness at the head also means a tighter leech at the head. )
2) The sail panel needs to deflect the wind if it is to create lift, but we like our sails to drive from low down, rather than to pull from the head - so it's arguable that we like the sail head to sheet out naturally from the head, to exhaust properly and to dump excess power when overpowered. A sail that is full in the head or which has a tight leech tends to feel top heavy. That can work well in light winds, but gets catapulty when overpowered because the tight leech doesn't release power at the head well. That's why most windsurf sails are cut with the top batten set pretty flat.
3) We are actually sheeting our sails to the 'apparent' wind - which is the combination of the true wind of the day and the 'created' wind that is made by the board moving forwards. The true wind might be, say side-on, but the created wind will run from nose to tail along the board, so the apparent wind will flow at some angle between these two directions. This is further complicated by the created wind being the same strength at deck level as it is at the mast tip, whereas the true wind is usually stronger at the sail head than it is across the sail foot. So that means the apparent wind often flows at a different angle at the mast head than it does across the board, and this is another reason we might want twist in our sails, again to get the optimal sheeting angle.
4) There is another issue, often called wind shear, a term which is often wrongly used and which doesn't affect us here - because our rigs are simply too short for this to be relevant. The wind shear is where the wind direction way up in the air is in a different direction from that same wind flowing along the ground.
We can sometimes see clouds way above flowing at a different direction to the wind we feel on our face. However on open ground, if you measured the wind direction 5m off the ground, it would probably the same as at ground level. You might find a difference in wind strength though. Wind sheer is often mixed up with my point 3), above. You can google the term, to find its usual meteorological meaning. People also talk about wind gradient.

5) If you set a rig on a car as shown in the photos here, positioning the sail to be horizontal takes wind shear and differences in apparent wind out of the equation. So it's not clear what they are measuring, and really they are just showing off, as if they were being scientific in their rig development, when in fact they are not.
All you could hope to do with a rig on a car is to try and measure the lift you got at different sheeting angles at different motoring speeds, and then you might change the sail shape until you found one that that seemed to work better - on the car. But then you'd have to start again with the rig on the water, set in the vertical plane, and with the sailor hanging off it causing new rig loads. And so that would be like starting from scratch, once the usual apparent wind and other sail dynamics issues come into play. So the 'road testing' is a waste of time.

Extra marks for Chris249 for mentioning Marchaj.
I'm sure I've still got that book on a shelf somewhere.

Sadly, it's 1am in the UK, and bedtime for me.

I think the more pertinent question is who developed the square top (i should say squareish top)?

That (to my mind) is where/when they really started to take the loose leech as a beneficial shape to to windsurfing, and work an intended looseness into the design.