olskool said..
Mathew, sails n fins are only trying to mimic what is found in nature. Sail designers mustve looked at a birds wing for inspiration. Twist n looseness of leech is required bcoz we need to control the power produced. The sail is not part of the human body. We cannot control its shape or power as a bird infinately controls its wing in flight. We are stuck with a lifeless man made substitute.
Sure lets do the tests n refine the design. But until we have say, 97 sensors n servos which can infinately change the shape n power at will for performance, we are stuck with what we have. A design which has come a long way in the past 40years. But still has a LONG way to go to be equal with what is found in nature.

Just no. No bird has infinite power and shape. And the SR-71 Blackbird is proof alone that humans understand aerodynamics.

When downhalled properly, the twist is already in the sail. It doesn't need a gust to open up.

The streamlines on the sail below are unlikely be right, but illustrate the general point that the flow is 3D. Without twist the flow on the upper sail is horrible - streamlines are way too long for the small camber on this part of the sail, very draggy. Twist allows more exhaust from just above the boom and corrects the airflow. The designed-in tension in the sail from the clew to the Neil Pryde "N" allows this twist-off of the upper sail, avoiding the old-school tight leach. Speed is a bit both about thrust but more importantly about reducing drag, especially at higher speeds. Shame about the overly floppy bits of leach, a bit hard to have twist mid-sail and a not prevent over-twist up the top.

mathew said.. >>>> BTW: The whole tall-poppy / anti-intelligence movement is just tiring. If windsurfing is about simplicity, do feel free to *not* comment on a thread that is discussing the performance merits of sail twist.

Yep!!!!!!!!!!!!!

It's great to enjoy the simplicity of just blasting around.
But it's rather stupid to think that you'd enjoy it just as much, without people doing the extremely complicated stuff of trying to understand, how to do it.

OK, as we're getting personal and you choose to set-up a pissing contest yourself if someone like oldskool or yourstruly mention something that doesn't fall into the equation you are trying to set up, while failing miserably yourself on trying to find an explanation -and I explain later why- first the following.

I will beat most of you guys sailing with my head in my arse when it comes to a competition. And no, I won't keep it there, I stick it out to discuss, as I thought this was meant as an open discussion, not as a self centered science club that is basing themselves purely on physics.

Again...where does inspiration come from? There is something before physics. Physics responds, physics doesn't create, at least not in an organic form. For that you need cell bioligy. Are you starting to catch my drift?

Call me a tall puppy or anti intelligent creature whenever you want -just like you do with hyper intelligent people who dare to step out of the box again and...my dear God...dare to use metaphores as our language simply doesn't have the right words for that which cannot be seen...hey?...my whole point-, but yesterday I was standing in only a slight breeze. My back against the wind, shirt put of, arms spread to notice directions and fluctuations in direction, like a bird can do. I tried to feel as good as humanly possible and noticed my back was getting a massage from the wind and there were lots of pressure points. It was as if tenthousand fingers were pushing my back, nearly all at a different force.

Now, the point oldskool tries to make is that a bird can adjust the tiniest parts of it's Wings and we as humans cannot.

We are forced to seek answers in the mechanical world and have to do so by applying the laws of physics on the models with which we are creating for sailing.

What I tried to point out is the following: can we as humans trust our own calculations for being right and what do we base our theory on? Hence my suggestion to see the human, board, fin and sail like one organism within the equation to calculate the optimum amount of twist in relation to all parametres involved. The human being holding the set to the water being the most important, and this is very important...LIMITING...factor.

And I am not even going to say sorry for the fact that I got the refugees and bombs in as I knew this was coming. Before even starting out to try and understand where these tall puppies are coming from that in the now are rising in vast numbers spread over the earth, you have to start to at least try and understand where physics got us in the here and now. This is why I showed this very simple lecture of Bruce Lipton who does sound weird and like an old hippie to most, but does that make him wrong and doesn't it have to do with twist? If you don't understand what I am trying to say, I won't say the same like you are saying so I won't request you to leave the discussion, but I WILL keep on adding my comments as this is an open forum.

Twist cannot be seen apart from the response needed within the whole set consisting of the weight, length, strength, response time etc. etc. of the rider, the board, the fin and the sail.

First of all the industry is putting us up with stuff that only can be sailed effectively by giants and proves simply because of this fact, it's moving the wrong way. If I, as light rider, follow in the footsteps of the giants, I will loose upfront whenever I would want to enter a competition.

So the first challenge is to rethink and realize we're all bound by our size, but not by the size of our brain, which is sometimes better organized than some of you think as ridiculing seems to be the accepted norm by at least two people.

choco said..
Did this thing have any twist?

I dont think so - the sail is largely a semi-solid wing. It does appear to have something going on with the top of the wing which allows for the tip-air-flow to be different from the rest of the foil. I''m not privy to the design, so no sure what it does.

It also had a rudder and vertical stabiliser.

Models like this are instructive for sure, but the reason twist works for windsurfers might be tied up in the fact that real wind isn't laminar (as well as the leverage issue).

All the models presented in this thread use laminar inputs. We all accept the logarithmic profile but forget what is behind it. The K in it equals 0.4 and can be loosely interpreted as gusts from 0.4 of a metres continuously mixing up in to the layer 1.0 metres above ground. Gusts from 0.8 metres jumping up to 2 metres and vice versa. It's a mess.

It's not quite so messy if you're travelling a fair bit faster than wind speed. Although the gusts scale up fairly well with windspeed doing 17 knots on a wave board in 30 knots of wind is not smooth going. Foil boards are often doing better with windspeed ratios but suffer from similar righting limitations. Do foil-specific sails have less twist?

(I'm only guessing.)

Laminar modelling and wind tunnelling is more valid for airplanes because they are self-propelled through comparatively still air.

TGale said..
^^^ and Sailrocket's sail doesn't rake backwards - our windsurfer rig rake is one of the reasons we need twist to be efficient. They also don't need twist to quickly reduce the height of the center of effort on sheeting out so as to reduce overturning moment when powered up and hit by a gust - their thrust line goes through their hook foil so they can basically just absorb a gust and just go faster .

I like this post and especially the following tiny part, metaphorically and physically speaking.

..their thrust line goes through their hook foil..

We are bound by our size. We need as much power per sqmtr to keep the Centre of Effort down and use our thrust line -which goes through the fin- in the most optimum way.

We also need to cope with the tiniest of gusts and the sail needs to absorb those gusts within a gust. That's why twist, or better a total breathing rig is needed.

Power per sqmtr. and gust absorbtion.

Follow that route and you get close to making (much) better sails than now available to us mortals riding those silly vessels.

Effectively we need to make masts softer and adjust the total foil in a way that it pulls us forward, not sideways as that's what I feel when holding 90% of the sails in my hands. ****loads of sideways pressure and I can FEEL.....this is not good as it's simply energy lost.

That's why the largest are fastest. Combine this with the way the PWA is set-up, like gybing in the windhole (what you never do when freesailing, or GPS racing going for the alpha) and you've got your modern day **** for brains sails that we now have to cope with.

We can go much, much faster if we rethink the whole concept, not just the twist alone. Twist is dependent on way more variables than all physicists within this discussion seem to think. You are all forgetting the bigger equation and that's feeling. A top rider feels the wind better than a mediocre rider. That's why a top rider could give worthful comments from a deeper insight seen through the eyes and shared based on the feeling this rider has. I dig it that sailmakers listen to top riders. The main problem being that most top riders think they don't suffer from a lack of strength as they are mostly larger than your average rider, when in reality even they do suffer from their very limited amount of force and leverage

Kwibai said..

We also need to cope with the tiniest of gusts and the sail needs to absorb those gusts within a gust.

Exactly. Kwibai is on to it.

The sail is not working from a clean slate.
If it was it'd be this simple.




Richardson's adaptation of a poem about fleas sums up all you need to know about what stumps the largest computers.

"Big whorls have little whorls Which feed on their velocity, And little whorls have lesser whorls And so on to viscosity."

en.wikipedia.org/wiki/Siphonaptera(poem)

You lot need to go for a sail
BTW I CAN beat all of you

olskool said..
So what do the actual sail designers use to support their designs n ideas? Surely they have the $$$ to investigate n test their designs n actually find out what works n why? Is it a SECRET SOCIETY SCIENCE that they dont want us to know?
Or are you telling me they just whip up a sketch n go 'wow that sure looks great, lets make it n market it for all to use'??? Tell em its the best yet n theyll believe it!
Surely someone on planet earth has done these tests before n found out why twist is needed to control the power n thrust of a windsurf sail???
Please excuse my naievity.. but,
40 years of windsurf sail design n NO ONE has done testing??

It's called trial and error. Lots of testing of ideas and seeing which ones work for them. They don't really know why it works either, just that what they test works in the certain situations. That is why development has been so slow. Lots of dead ends and contradictory ideas.

TGale said..
The streamlines on the sail below are unlikely be right, but illustrate the general point that the flow is 3D. Without twist the flow on the upper sail is horrible - streamlines are way too long for the small camber on this part of the sail, very draggy. Twist allows more exhaust from just above the boom and corrects the airflow. The designed-in tension in the sail from the clew to the Neil Pryde "N" allows this twist-off of the upper sail, avoiding the old-school tight leach. Speed is a bit both about thrust but more importantly about reducing drag, especially at higher speeds. Shame about the overly floppy bits of leach, a bit hard to have twist mid-sail and a not prevent over-twist up the top.

Thanks for you theories TGale, but this just sound likepure supposition on your part.
Define: 'the flow on the upper sail is horrible'
Explain why'streamlines are way too long for the small camber on this part of the sail',and why this is 'very draggy'
Explian why the twist would line the streamlines up more acrosss the chord of the sail?
Explain what is'exhaust'and why it'corrects the airflow'
Explain why tight leech is 'old school' when it is designed into many modern sails - even windsurfing sails which win course races.
So is the twist up top correct, or is it 'over twist'. If it is 'over twist', what is it doing and why is it correct?

And then how about some eveidence to back your theories as you explain them?

I am temped to say your spiel sounds like 'Marketing Crap' but I'll give you the benefit of the doubt awaiting you dissertation.

Sailquik Dictionary definition: Marketing Crap - What the sales people tell you when they have no freaking idea, but they think it sounds plausible enough for you the believe it and be impressed

My point is, all these meaningless, or out of context words and phrases have been thrown around at us for so long that we start to actually believe it all without any real evidence. We put whatever interpretation we have in our mind on what it means, and we think that they, and us, understand it.

My theory is that the reason why none of the top sailmakers will come out and explain their designs with physics, is that they just don't know the 'WHY'. Sure, they know that what they are doing is working, and it is taking them forward, albeit, slowly. Sure, they have their theories, just like many of us, but they also know that they have very, very little to really explain how and why it works, so they do not want to expose themselves to embarrasment. I can't blame them for that. I would be the same in their shoes.

so we disagree on the wisdom shared within the video. By the way, I don't support all of his views but find his view refreshing, not in the least because he dares to speak out in a way we don't feel comfortable about as lots of our knowledge seemingly contradicts with what he says.

As for the picture....I am waiting for the right sailmaker who understands and translate. As said, physics is for the physicists, and I know my limitations. I am looking at this discussion from the other side of the room, remember :-). I am in awe of what's expressed here and really think this is a very valuable topic, so thanks for that!!!

The description to the picture is pretty vivid though and could be implemented pretty easily in a software program by someone skillful enough to translate the needs as expressed by me. I know I would want one if it could be made :-).

Sorry, I just saw your additions. I didn't need to quote just yet. It seems we're both on fire :-). I couldn't agree more on your judgement on many sailmakers. What could be more important though, is building the right 3D software program, calculate all variables (so far we can't as forces on/from...man, board, fin, waterstate, windflow, rotal rig can't be measured properly) and as a follow up the implementation of the right variables.

And no...we don't need floppy twist. We need controlled twist, functioning as a whole. But so far there seems to be no better when windspeeds are catching up with boardspeeds. Proof is with the pudding, in this case raceresults.

What I do know is that we need to go lower aspect and we need to get self balancing rigs that can be steered by the fingertips. I've got my preferences in the now and I simply feel who is on track for me (my personal sailingstyle, weight and length) and who is not. No marketingtalk in the world could convince me otherwise.