What is it? How many degrees? How does one measure it? Has anyone actually tried to measure it?

These were the qustions burning in my mind!

So I made an attempt:





Now this worked out a lot more degrees than I had anticipated, which begs the questions:

How much is required for specific conditions?

And the big one: How much is too much?

And an even bigger one: exactly why is a certain amount, the best amount?

I have heard a lot of theories, i have a few myself,

But I have not seen many science based arguments, or stuff backed up by experimental evidence apart from 'trial and error' - "this feels best/good". And I doubt the experiments on Yacht sails have much relevance to windsurfing sails, or do they?

Any PhD's on it? Any wind tunnel test reports. Even CFD projects?

So many questions and so few good answers.

OK, have a slash at it!

Patrick Deltheim who has done over 52kts at Luderitz was telling a local WA sailor that when rigging for speed he recommended adding 3 to 4cm extra downhaul than recommended, saying that the top of the sail's looser leech came into play when going deep down wind?

I look deep into the past and I see a salt lake. I see a 4wd vehicle with a strange roof mount for a sail, attached horizontally. I see a truckload of cameras, strain gauges, smoke trails. I think I can even see the letters G A A S ........

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MikeyS said..
I look deep into the past and I see a salt lake. I see a 4wd vehicle with a strange roof mount for a sail, attached horizontally. I see a truckload of cameras, strain gauges, smoke trails. I think I can even see the letters G A A S ........

Where's Roo when you need him?

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MikeyS said..
I look deep into the past and I see a salt lake. I see a 4wd vehicle with a strange roof mount for a sail, attached horizontally. I see a truckload of cameras, strain gauges, smoke trails. I think I can even see the letters G A A S ........

oh doesn't that bring back memories .....cutting edge technology circa 1985/86 ish?

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hardie said..
Patrick Deltheim who has done over 52kts at Luderitz was telling a local WA sailor that when rigging for speed he recommended adding 3 to 4cm extra downhaul than recommended, saying that the top of the sail's looser leech came into play when going deep down wind?

Faster you go less twist you need so that doesn't really make much sense.

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hardie said..
Patrick Deltheim who has done over 52kts at Luderitz was telling a local WA sailor that when rigging for speed he recommended adding 3 to 4cm extra downhaul than recommended, saying that the top of the sail's looser leech came into play when going deep down wind?

Patrick is a big guy who no doubt wore weight at luderitz.
I suspect the extra downhaul was to compensate for the extreme mast bend to leeward which would tighten the leach.

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Ben1973 said..

hardie said..
Patrick Deltheim who has done over 52kts at Luderitz was telling a local WA sailor that when rigging for speed he recommended adding 3 to 4cm extra downhaul than recommended, saying that the top of the sail's looser leech came into play when going deep down wind?

Faster you go less twist you need so that doesn't really make much sense.

...............But the less drag you need also. Sail twist affords cleaner release (airflow) and is very similar in principle to tips of bird wings. If it didn't work, airline manufacturers wouldn't be (even retro) fitting winglets on the tips. The fuel savings generated from reducing drag alone are significant to their bottom line.

Sorry, no scientific measurements on this, but in my head pictures it something along these lines....
- the sail is interrupting the movement of the air
- the direction of air movement is changed due to the sail shape and orientation
- effects including skin friction on the sail are providing resistance to the airflow, and in a big-picture sense this slows down the air
- the overall slowing mass of air flows backwards and upwards across the sail (remember the sail is angled backwards, and the airflow is also somewhat upwards relative to the water due to the foot of the sail blocking it escaping downwards and the upper sail twisting off)
- this overall slowing of the airflow leads to what we could call windward side "air stockpiling" as it progresses backwards and upwards across the sail. i.e. the air that can not easily escape is pushing on the top half of the sail!
- with no sail twist this slowed and increasingly "voluminous" air mass will cause seriously airflow blockage and a huge sideways push on the top half of the sail, which will be all wrong for efficient generation of forward thrust
- better to twist off the top half of the sail and efficiently discharge this growing voluminous mass of air, and at the same time get some really nice forward thrust due to increased airflow at a much more efficient angle on the upper sail, together with having the upper sail itself being "sheeted" out and hence producing a lot of forward drive due to its better orientation.

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TGale said..
Sorry, no scientific measurements on this, but in my head pictures it something along these lines....
- the sail is interrupting the movement of the air
- the direction of air movement is changed due to the sail shape and orientation
- effects including skin friction on the sail are providing resistance to the airflow, and in a big-picture sense this slows down the air
- the overall slowing mass of air flows backwards and upwards across the sail (remember the sail is angled backwards, and the airflow is also somewhat upwards relative to the water due to the foot of the sail blocking it escaping downwards and the upper sail twisting off)
- this overall slowing of the airflow leads to what we could call windward side "air stockpiling" as it progresses backwards and upwards across the sail. i.e. the air that can not easily escape is pushing on the top half of the sail!
- with no sail twist this slowed and increasingly "voluminous" air mass will cause seriously airflow blockage and a huge sideways push on the top half of the sail, which will be all wrong for efficient generation of forward thrust
- better to twist off the top half of the sail and efficiently discharge this growing voluminous mass of air, and at the same time get some really nice forward thrust due to increased airflow at a much more efficient angle on the upper sail, together with having the upper sail itself being "sheeted" out and hence producing a lot of forward drive due to its better orientation.

Well explained there Tim, never thought of it all that way. Must be the professor-ness coming out in you.

Iris sails has some nice graphics and seems to have investigated this a bit more than the usual marketing speel.

www.phantom-international.com/iris-sails-overview-and-technologies

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sailquik said..
How much is required for specific conditions?

And the big one: How much is too much?

And an even bigger one: exactly why is a certain amount, the best amount?

I have heard a lot of theories, i have a few myself,

But I have not seen many science based arguments, or stuff backed up by experimental evidence apart from 'trial and error' - "this feels best/good". And I doubt the experiments on Yacht sails have much relevance to windsurfing sails, or do they?

Any PhD's on it? Any wind tunnel test reports. Even CFD projects?

So many questions and so few good answers.

OK, have a slash at it!

The way I look at it is that Windsurfers are righting moment constrained, that's why taller and heavier sailors do better at speed, and why we wear weight, use long harness lines and keep our arms straight to generate as much righting moment as possible.

The flip side of this is that for any particular point of sail we want to generate as much lift, as little drag and as little heeling moment from the sail as possible. Turns out that using a tall sail with a shortish boom and not much twist in the rig, like we used in the 80s, is not an optimal solution to the problem for downwind sailing. Using a shorter mast with a longer boom, a lot more twist and alower third to the sail generates lots of lift, reasonably low dragand low centre of effort. The twist in the sail also makes the rig much more forgiving - get a gust and sheet out and the total lift decreases rapidly, but more importantly, the centre of effort drops lower making it easier to stay in control. Sail into a lull and sheeting on will bring the upper area of the sail back to a reasonable lift coefficient, but the centre of effort also moves up, giving you something to pull against to stop you dropping in the water.

Nice to see a recap of some of the main 'thoeries', but YoYo is top of the class so far with his Iris post. At least they seem to be paying lip service to science.

Closely followed by Pacey, whose theories most closely match my own.

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Ben1973 said..
Faster you go less twist you need so that doesn't really make much sense.

As I see it, that is true to some extent. As the apparent wind comes around closer to the nose, excessive twist would mean the top of the sail is completely luffed, if not just flapping in the wind. i cant see much point in that. But in the pic above, that does not seem to be the case, probably because the true wind strength is still close to his board speed. That Luderitz course is VERY broad, and this requires very strong winds to make it work. The vast majority of speed sailing is done is less wind and less broad angles. I have a feeling that one may be better off just cutting the top panels of the sail off completely, if they are over twisted and just flapping in the wind, but possibly, they are needed just to support the rest of the sail that is working.

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yoyo said..
Patrick is a big guy who no doubt wore weight at luderitz.

Yes. Rumour has it that Patrik wore a massive amount of weight! That is definitely not a beer and pies physique under that white jersey.

Patrik is definitely not as big as many of the other very fast sailors. That makes him very, very good!

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sailquik said..
I have a feeling that one may be better off just cutting the top panels of the sail off completely, if they are over twisted and just flapping in the wind, but possibly, they are needed just to support the rest of the sail that is working.

I reckon this would actually be true on a course with constant wind and a very long approach enabling sailors to gain momentum and enter the starting line at full speed. I don't think Antoine hit the starting line at 53 knots.
From all I know about L?deritz, this isn't the case, making me assume that the top is relevant for propulsion in the "lulls", if you can call a 40 knot spell in 60 knots a lull and during the remaining acceleration phase at the beginning of the course.

MV should know

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sailquik said..
Nice to see a recap of some of the main 'thoeries', but YoYo is top of the class so far with his Iris post. At least they seem to be paying lip service to science.

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yoyo said..
Iris sails has some nice graphics and seems to have investigated this a bit more than the usual marketing speel.

www.phantom-international.com/iris-sails-overview-and-technologies

I have been troubled by this image for some time actually, it doesn't make sense. The streamlines at the bottom and top of the sail look reasonable, but the idea that an air molecule could hit the sail below the boom, head across the sail and then turn 90 degrees and travel upwards (and even forwards slightly) and then turn horizontal again before streaming off the leech just doesn't seem plausible to me. It's also completely testable just by putting telltales on the sail in the appropriate places.

3D CFD codes can be very tricky to set up, and it's easy to get results that bear no relation to reality. To me this looks like marketing fluff rather than rigorous research.

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Pacey said..

yoyo said..
Iris sails has some nice graphics and seems to have investigated this a bit more than the usual marketing speel.

www.phantom-international.com/iris-sails-overview-and-technologies

I have been troubled by this image for some time actually, it doesn't make sense. The streamlines

You've assumed they're streamlines. Could be something as obscure as the contours of the difference in the w components of vorticity between model A and model B. Multiplied by 1000 it gave the most interesting of the hundreds of graphs that CFD models can spit out . Who knows? don't worry about it

Patrik said he was about 95kg at Luderitz but wore 32kg of lead!!

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Jonski said..
Patrik said he was about 95kg at Luderitz but wore 32kg of lead!!

I heard him mention that at Cervantes. Lots of lead. The sails are rigged to the extreme for Luderitz.

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Jonski said..
Patrik said he was about 95kg at Luderitz but wore 32kg of lead!!

I heard him mention that at Cervantes. Lots of lead. The sails are rigged to the extreme for Luderitz.

Luderitz certainly causes sails "rigged to the extreme", and lots of lead. But Patrick seemed to be giving the same advice of "a lot more downhaul" in general, even for the slalom races (more moderate angles), and to windsurfers who did not use any lead.

I find it interesting that TGale's description of air moving up seems to match the Iris graphs. Flatter (higher) sections of the sail will have lower pressure gradients. It seem logical that air would flow from high pressure to low pressure, and therefore upwards. At first glance, using looseness higher on the sail as an "escape valve" makes more sense to me than the common explanation that twist mirrors the gradient in wind speed. Cool to hear a new idea about a topic that has been discussed so many times.

The top 2 panels of the sail should have micro holes set in a pattern perpendicular to the mast, don't know what this would do but sounds good lol

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Pacey said..

sailquik said..
Nice to see a recap of some of the main 'thoeries', but YoYo is top of the class so far with his Iris post. At least they seem to be paying lip service to science.

yoyo said..
Iris sails has some nice graphics and seems to have investigated this a bit more than the usual marketing speel.

www.phantom-international.com/iris-sails-overview-and-technologies

I have been troubled by this image for some time actually, it doesn't make sense. The streamlines at the bottom and top of the sail look reasonable, but the idea that an air molecule could hit the sail below the boom, head across the sail and then turn 90 degrees and travel upwards (and even forwards slightly) and then turn horizontal again before streaming off the leech just doesn't seem plausible to me. It's also completely testable just by putting telltales on the sail in the appropriate places.

3D CFD codes can be very tricky to set up, and it's easy to get results that bear no relation to reality. To me this looks like marketing fluff rather than rigorous research.

Wind speed at the top of the sail is faster than at the bottom due to fiction between it and the land/water. The affect of this is air gets pulled upwards as well as across the surface of the sail .
More so at lower wind speeds and even at high winds it does similar. The diagram is correct, telltales would show the same.

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Ben1973 said..

Wind speed at the top of the sail is faster than at the bottom due to fiction between it and the land/water.

A good question. Are they attempting to input a turbulent boundary layer? The wind speed at the top is faster than lower down. But not by much. And you'd have to leave several anemometers on a pole for a couple of minutes for the averages to reveal this profile. Mostly the wind at the top half is stronger but for just a bit less than half the time the wind hitting the lower half of the sail is stronger .

And streamlines can't do sharp corners without a tremendous local pressure gradient. Maybe this "streamline" is from well in front of the sail bending into the page so the smooth curve is lost from our viewpoint? 3d lofted into 2D is tricky.

(Still thinking it's something other than a streamline. Or at least they've gone thru some sort of harsh filter. Look how the advertisers misinterpreted a highly filtered CFD output for the Quabba fins!)

I did read some time ago that the winglets on aircraft give the same results as if the wing were that much longer. ie: lay the winglet out flat.
so the main reason for the winglet is about saving room at the airport.

As for the windsurfing sails. It has to be looked more at the fact that Air flows upwards across sail relative to say the battens than horizontally ( in line with battens.)

i wonder why speed sails don't align the battens with the Air flow?

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RAL INN said... >>> i wonder why speed sails don't align the battens with the Air flow?

Possibly a structural thing, They'll be longer so heavier and maybe decreased stiffness, there's also their angle to the mast, getting hard to accommodate camber inducers, when the battens are almost parallel to the mast.

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RAL INN said..
I. It has to be looked more at the fact that Air flows upwards across sail relative to say the battens than horizontally ( in line with battens.)

One graphic on the internet and now it's a fact!
Don't let the foilers see it they'll never fly again.

Just remember the big picture, for every bit of momentum sent off in the vertical direction there's an equal and opposite reaction downwards.
F = d/dt(mv) , rocket science 101 but works for everything.

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RAL INN said..
I did read some time ago that the winglets on aircraft give the same results as if the wing were that much longer. ie: lay the winglet out flat.
so the main reason for the winglet is about saving room at the airport.

I don't think this is correct. Winglets reduce the tip vortex that occurs at the end of the wing and hence significantly reduces drag and fuel usage.

I wonder if twist does the same thing on windsurfing sails

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John340 said..
I don't think this is correct. Winglets reduce the tip vortex that occurs at the end of the wing and hence significantly reduces drag and fuel usage.
I wonder if twist does the same thing on windsurfing sails

yep, that is a big question, but there does not seem to be any factual info to back up the theory.