Hello
As man interested in aeronautics I have two easy question for sail designers when look at windsurfing sail at first:

1)Why sail has rake if upright sail has better lift over drag ratio?
*why dont move mast track back and design upright sail with larger foot,you can gain more speed at Luderitz channel.
*swept wing at subsonic speed ,significantly reduce wing efficiency


2)Why battens are not parallel to the water/airflow when sail is raked during planning?
*battens gives sail ideal airfoil profile so air must follow batten path,look at any hang glider,they all have battens parallel to the airflow/parallel to longitudinal axis,why this is not case at windsurfing sail?

Noah

Just to clarify:

Sails need to be upright at first as the board is not moving fast. As the board speed increases so the centre of lateral resistance moves back and as the centre of effort in the sail needs to stay in line with this point therefore the rig moves back too.

The batten point is good and if anything they should point slightly higher as when the wind goes over the sail it rises and the battens should then align to make air flow easier.

This used to effect sails more but as a lot of sails now have battens that are not a seperate pocket, rather now they are constructed within the shaped seams of the sail, they are not so important to be in line with air flow. Again it helps when they are not sailing fast as their position is then more in line. Sometimes its aesthetics too rather than function.

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Madge said..
Just to clarify:

Sails need to be upright at first as the board is not moving fast. As the board speed increases so the centre of lateral resistance moves back and as the centre of effort in the sail needs to stay in line with this point therefore the rig moves back too.

The batten point is good and if anything they should point slightly higher as when the wind goes over the sail it rises and the battens should then align to make air flow easier.

This used to effect sails more but as a lot of sails now have battens that are not a seperate pocket, rather now they are constructed within the shaped seams of the sail, they are not so important to be in line with air flow. Again it helps when they are not sailing fast as their position is then more in line. Sometimes its aesthetics too rather than function.

We can design sail which will be upright when planning and when going slow we can move sail forward with arms,so we can adjust technique to use "upright sails" . That will be good for speed records where aerodynamics efficiency is important.
As you can see every year foil sails become more and more upright and has higher aspect ratio,these two things are key for sail efficiency=speed..

I dont talk about battens angle because of skin friciton ,I am talk that batten represent ideal airfoil profile which gives to sail, so it is best for air to follow this path..
When battens are angled so much down when sail is raked (as in all race sails ,I think excpet new EVOs) then sail behaves like it has more sweep angle then sail with "horizontal battens"

Foil sails and windsurfing sails are two different breads.

Firstly foil sails give lift, lots more than a windsurfing sail, yes you can use a windsurfing sail on a foil and vice versa but they are both different. Try using a foil sail when totally over powered and I tell you, they are terrible, they are more efficient by far at lower speeds.
Gliders are really efficient too and they are really high aspect but you'd never be able to rig one as the mast would be like 8m long.

Foil sails have much higher leech tension to get the foil up out of the water. Where as windsurfing sails give more forward drive.
Windsurfing sails can be used much more over powered too so the leech twists more. The luff curve shape is totally different on foil sails too, almost the opposite of a windsurf sail.

" Upright sails " as you call them also wouldn't work as at speed and when over powered you cannot then level against them with your body weight.

Also twist in the sail not only exhausts wind, it also flattens and changes the angle of attack of the luff angle when they twist which flattens the entry of the sail.

I know where you are coming from but when in actual use its a different thing all together.

Boat sails are different again as they can change so many settings when sailing compared to a windsurfer.

Pryde have in the past had battens that were horizontal to when the sail is raked but they didn't look very nice and when not fully powered were terrible.

If still unsure check out " Sail Rocket " its both raked back and at an angle and is the fastest sail craft by a long stretch.




I also wouldn't really call them battens are they are part of the foil but they are parallel.

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Noah Prandtl said..

*why dont move mast track back and design upright sail with larger foot,you can gain more speed at Luderitz channel.

Damn, why didn't we think of that? It's so obvious, we could have been doing 60 knots years ago. Thanks for your expert advice.

Battens and mast angles are the least of you're problems when you have PEBAB

My turbo gts like to be kept more upright than my M3s. Feels weird Like your about go over the front at any point

You bring up some interesting thoughts, Noah. Here are a few things to mull over:

There's a lot going on having to do with apparent wind and twist which makes this more complex than you might think. Very rigid, upright windsurfing sails with almost no twist were tried in the 80s and 90s and were just unmanageable as speeds increased.

The lifting portion of our sails has a fairly upright entry anyhow, because of the mast bend. It's the top section which is severely raked. The upper section of the sail, once we're moving, is doing almost nothing, and as this article points out, is most effective, once we are at planing speeds, if it has zero angle of attack altogether.pdfs.semanticscholar.org/c575/74df4eea997fefe4b48bea067e08449ef6bb.pdf?_ga=2.148697397.3216351.1598280006-197449037.1598280006. Most of the actual lift at planing speeds, which is when the sail is raked most, is centered at or right above the boom, which is a pretty vertical section. Using something like a foot-operated mast track to change the location of the center of lift has never been practical except with big raceboards.

Many sails for both foil and fin (Phantom, Avanti ) DO in fact align battens with flow. It's efficient but a severe pain in the butt when we come to roll them up. Some sails from the late '80s from Gaastra, Hood and others, needed to have the battens removed before the could be rolled. Also, when the angle between mast and batten gets too acute you can get some really funky rotation issues.

There are all kinds of ways in which sails could be made more efficient - a tapering entry ahead of the mast is one, a flat windward surface as with Stan Mostoviy's designs is another, but they mostly come with a cost in price, weight or convenience or we'd be doing them already.

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elmo said..
Battens and mast angles are the least of you're problems when you have PEBAB

Whats a PEBAB...????

Me,
Problem Exists Between Boom And Board

Noah. Pretty much everything you mention has been tried over the last 40+ years.

There has been very intense competition at a professional level in windsurfing for most of that time.

The current designs are the pinacle of all that development. And guess what? They all look very similar.

Why dont you try to explain why they DO work. I and many others would find that very interesting.

Also, pretty much all of the current top slalom and speed sails are relatively low aspect. This flys in the face of casual aero theory. And it proves beyond doubt that the theoretical aerodynamic advantages are well and truly outweighed by practical considerations like centre of effort, leverage, stability considerations and control.

Same goes to some extent for many of the currently fastest speed fins.

Memories of MW Sails thread/s

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Mark _australia said..
Memories of MW Sails thread/s

LOL

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elmo said..
Me,
Problem Exists Between Boom And Board

Or
problem exists between board and brain

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Noah Prandtl said..

2)Why battens are not parallel to the water/airflow when sail is raked during planning?
*battens gives sail ideal airfoil profile so air must follow batten path,look at any hang glider,they all have battens parallel to the airflow/parallel to longitudinal axis,why this is not case at windsurfing sail?

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Noah Prandtl said..
I dont talk about battens angle because of skin friciton ,I am talk that batten represent ideal airfoil profile which gives to sail, so it is best for air to follow this path..

They've already covered this pretty well here:
aviation.stackexchange.com/questions/80507/how-ribs-angle-change-wing-overall-shape

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sailquik said..
Also, pretty much all of the current top slalom and speed sails are relatively low aspect.

Some sail brands, notably Neil Pryde are moving to a higher aspec design for their slalom race sails, similar to their foil sails. For example the previous NP Evo XI 7.8 sail is now offered as 7.4 in the Evo XII. The luff remains the same but the boom is shorter. See the following for details.

www.neilpryde.com/collections/sails-2020/products/rs-racing-evoxii

It will be interesting how they compare when the PWA starts up again.

Is Noah Prandtl a Neil Pryde sales stooge? First posts ever and very specific. Sound very suspicious to me, especillay in the light of the timing of the above "news"

Interesting handle though, for those who know who Noah Prandtl was.

And no replies!!

A quick google found a few instances of a 'Noah prandtl' that recently joined a number of aerodynamics forums and Car Engineering forums and started trolling.

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sailquik said..
A quick google found a few instances of a 'Noah prandtl' that recently joined a number of aerodynamics forums and Car Engineering forums and started trolling.

I think these days theres not a lot of difference between trolling and being an idiot. He seemed to know a bit about rig shape etc just not the concept of actually handling a sail or whats been used in the past and why it's not used now.

Anyway each to their own I guess.

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Mark _australia said..
Memories of MW Sails thread/s

What ever happened to those sails, thought they were better than anything else on the planet, apparently not.

I don't think a troll but eitiher an engineering student look for homework answers or someone curious but not enough knowledge or experience to understand the answers given. He had trouble understanding how a rib not in the direction of the airflow could support the proper shape.

On the NP, Nico Prien has mentioned in the videos that while training this past (his) winter, he would use his foil sail on slalom gear and was surprised by how well it did. I think that with a bunch of A/B comparisons that can be easily made on PWA gear, it will be interesting to see if booms shorten up.

MW seems to still be around although he's moderated his aspect ratio after telling everyone here how wrong the windsurfing industry had gotten things. Good times...

Replay to what?
Replay to am I idiot,troll,student,Neilpryde seller or "expert"?

I do not participate in forum with that type of conversation.

P.S.
listen at 45:50 - 47:10 why/how Goyard hold his sail vertical as much as possible.


Follow next few years in which direction foil/slalom sails are going.Indeed,even now, if you watch carefully you can see in which direction they are going.

Mr Noah,

I have been sail making and into windsurfing for 35 years now, during that time I have seen lots of changes, some great, some good, some bad, some absolutely terrible.

A lot of what goes on in the windsurfing world is simply aesthetics, some is engineering, some is design.

A lot of what goes into production is not what the pro's use and some is.

Looking at windsurfing as an engineering point of view is great but can be very misleading if that is not put into practice and then used for a while by a professional sailor and then proven to be a good concept.

By all means do ask why things change, thats good but don't assume that because they don't make sense either on paper or in some aeronautic book or even within a different class of windsurfing, don't assume that this is correct for everything and every sail, and don't assume that everyone sails in the same way with the same style. Sail shapes are all different because some sailors want a flatter entry or more shape further aft or more twist and the list goes on.

Basically a windsurfing sail is held by someone and therefore can feel great to one person and work exceptionally well and terrible to another person.

I can tell you now that slalom sails will never be that high aspect, light wind course sails and foil sails yes but not high wind slalom.

Probably going in the direction of where they once were

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Gestalt said..
Probably going in the direction of where they once were

Nah, the old sails with no twist at all, they were great for getting up and going but terrible in any gusts. New sails are soo much better.

They aren't so powerful but more controllable, have a wider wind range and are lighter in the hands.

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Ben1973 said..
My turbo gts like to be kept more upright than my M3s. Feels weird Like your about go over the front at any point

That's what I noticed going to free ride sails from cammed..

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sboardcrazy said..

Ben1973 said..
My turbo gts like to be kept more upright than my M3s. Feels weird Like your about go over the front at any point

That's what I noticed going to free ride sails from cammed..

yes I think you went from ods to turbos as well.
I don't think I mind it to much with the 9.2 As that's my light wind only sail but I find the ODs so much easier to sail when powered up. I've messed around a lot with mast foot position etc to get comfy with the turbo but just can't in anything but lighter end of its range, it either loads up the back foot or is trying to launch me over the front.

as soon as it hits 11knots plus I'm on the 8.6od going faster with much less effort.

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Noah Prandtl said..


I do not participate in forum with that type of conversation.

I don't blame you. They're prone to playing the man when they don't know the answers.

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Noah Prandtl said..
Replay to what?
Replay to am I idiot,troll,student,Neilpryde seller or "expert"?

I do not participate in forum with that type of conversation.

P.S.
listen at 45:50 - 47:10 why/how Goyard hold his sail vertical as much as possible.

Follow next few years in which direction foil/slalom sails are going.Indeed,even now, if you watch carefully you can see in which direction they are going.

LOL! Look at it like this:

First post in these forums. We have no idea who you are or what your background is or what you already know. If you were a windsurfer and you have an interest in aerodynamics, you would most likely know the answers to your questions and be able to ask more interesting and relevant questions. Forums are a two way street when it come to technical questions. it is nice to provide background and some concrete contributions as well, especially when you post under a anonymous screen name with not even your location listed and nothing in your profile. Usually, it is better to get to know the forums by starting a converstion at the basic level before you dive into the PhD levels.

So that leaves the field wide open for us to interpret your motivations and the point of your questions.

So you say you 'dont participate in forums with that kind of conversation' but then you go ahead and participate anyhow??

As already said. Foiling and Windsurfing have very different requirements. You didn't bring that into your original post at all.

How about you give your theories first with all the experience and data you have to back them up, and then, maybe, a dicussion might follow.

It's one thing to know all the classic formulas and theories of aerodynamics, the books and papers are there for all to study. It's another thing entirely to work out how all that applies to the windsurfing situation. In that space you will find far less, and IMHO, most of what you find is based on an abundance of assumptions and generalisations. It seems to have proven to be very difficult to make meaningful progress on windsurfing sail design through the classic pure application of science. The sail, sailor, board and water interface seems to have proven far too complex. The way progress has been made is more from intuitive trial and error, (with an eye on basic physics and aerodynamics) and the progress through competitive comparison. You can come up with all the theory you like, but if the thing you build does not win races and feel good to use, what is your theory worth?

That is why I say, don't ask why 'this idea' or 'that idea' won't work. But try to explain why what we are using now works as it does, and that might then give you some insight and ideas about what can be done to make it work better.

Or you could go ahead and completely disregard, and disrespect everything that now exists and try to reinvent the wheel.

Hope this helps.

PS. You could also do a forum search here and read up on what has been discussed before.

^ Or he could just do what he did in the reasonable expectation of getting a civil response.