Ian K said..

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

Yes, that pattern seriously troubles me too. That is either just BS pretty marketing, deliberately deceptive, or someting completely unexplained. Why someone would publish such an image without any explanation all, is even more troubling. Lip service without any substance?

Ben1973 said..
Cover your sail in telltales and you'll see lots of them blowing at 90degrees or more to each other.

That would be logical, especially on the windward side. That would be caused by turbulent airflow. How do they look in the Leeward side?

So much fun to argue about what we believe (or not), especially if we can bash marketing departments in the process!

For sail boats, there are a number of studies that looked at actual pressure measurements and CFD simulations, for example a master's thesis "Derivation of Forces on a Sail using Pressure and Shape Measurements at Full-Scale" at publications.lib.chalmers.se/records/fulltext/147899.pdf and another one at hal.archives-ouvertes.fr/hal-01071323/document

That's a bit different from the question Andrew was asking, though. I bet there have been some big studies for the America's Cup boats, but those are probably closely guarded.

Part of the problem might be the computational complexity. Good CFD simulations for fins are hard enough, but at least there, the speed is constant over the length of the fin, and the fin shape is (relatively) stable. For sails, the wind speed changes from bottom to top, and one of the most important performance aspects is how the sail reacts to gusts. Part of that reaction is a change in mast bend and therefore sail shape, which in turn changes the flow. Besides having a lot more variable in the simulation, they are closely coupled rather than independent, which creates a bit of a computational nightmare (and strongly limits the usefulness of "simple" CFD simulations). Reminds me a bit of my masters thesis, which was about calculating the result of interactions between coupled complex electronic oscillators in one dimension. A much easier problem .

In comparison, gathering empirical data about forces on the sail should be easier. What is the actual distribution of forces in a windsurf sail? How much does the loose top section contribute? How do things change in gusts? All we need is an aerodynamics professor and a windsurfing grad student in search of a thesis theme...

How do things change in gusts? All we need is an aerodynamics professor and a windsurfing grad student in search of a thesis theme...

What's a gust? It's not a uniform increase in windspeed from top to bottom. Google the physical interpretation of the Von Karman constant. The familiar K in the logarithmic wind profile but 100 years on nobody really knows what it is exactly. I remember a local guru at a conference 30 years ago saying " What this field needs is another Newton". He or she still hasn't tuned up. I don't think your average professor will cut it.

I am gathering old, out of date smoke fares.

sailquik said..
I am gathering old, out of date smoke fares.

Sailquik, are we returning to the ol smoke n mirrors hypothesis again?
Im keenly following this thread. As theres no easy solution to explain the why n how of what we love so much.
Regarding seeing the sail leech OPEN on a gust. Dont think its visible bcoz, its more to do with the mast laying off n the leech tension governing/ aiding this dynamic of the sail in response to a gust.

No mirrors. Just smoke and action cams.

I have done a lot of following close behind other sailors watching the sails and I have to say it's very hard to see any dynamic deflection, either sail or mast, or both from wind variation. Some can be seen when pumping, but very hard to see from wind gusts. Thats not to say it isnt there, just very hard to see.

sailquik said..
No mirrors. Just smoke and action cams.

I have done a lot of following close behind other sailors watching the sails and I have to say it's very hard to see any dynamic deflection, either sail or mast, or both from wind variation. Some can be seen when pumping, but very hard to see from wind gusts. Thats not to say it isnt there, just very hard to see.

Easier to see from above.
Maybe we all need a 'Bow Sail'
i would love to be able to get my old 6.8m Rushwind slalom sail back and put it head to head with my 8.6 M2. Bet there's no difference in power.

sailquik said..
I am gathering old, out of date smoke fares.

How many do you need?

ka43 said..
Daffy, it says its blowing 32 knots ENE at the Prom. Why are you on here like us workers?

yep. Came through here alright. 20-30+ ENE. Tide was very late in the day and not that low so lots of scary rough water. had a few runs and decided to retire unhurt,

Here my take on things.

It's very simple.

Look at the America's Cup foilboats, then look at a windsurfer sailing next to it.

We need four dimensional twist incorporated in a truly 3D designed rig concept.

The human force, or better lack of it is the limiting factor. A large and heavy rider can take that limit a little up but he won't go much faster.

So the next thing to wonder about is the following: why design for fat old bastards if you could design for young and talented riders possibly not having the pure force of the fat and heavy?

The ideal windsurf sail balances itself through twist, leechcurvature, sidebend and shape. If well balanced no force is needed and glide will be the answer. Glide means effortless speedsailing in a new realm to be created by the next generation sails.

sailquik said..
I have done a lot of following close behind other sailors watching the sails and I have to say it's very hard to see any dynamic deflection, either sail or mast, or both from wind variation. Some can be seen when pumping, but very hard to see from wind gusts. Thats not to say it isnt there, just very hard to see.

You could always attach a couple of 10 Hz GPS units, one on top of the mast and one near the boom. Maybe make the top one a phone with an accelerometer app. Then you'll just have to sort out the directions. With a ublox unit, you can look at velN and velE in GPSReader - best to align your runs going straight N-S or E-W.

If you're going SW-NE or similar, you'd have to transform the data to get the components in travel direction and perpendicular to it. Not really hard, but an extra step. On the bright side, once you figured out how to do that, you might be able to use a Motion. I don't think the .oao files have velN and velE, but you could calculated them from speed and doppler heading.

boardsurfr said..
You could always attach a couple of 10 Hz GPS units, one on top of the mast and one near the boom. Maybe make the top one a phone with an accelerometer app. Then you'll just have to sort out the directions. With a ublox unit, you can look at velN and velE in GPSReader - best to align your runs going straight N-S or E-W.

If you're going SW-NE or similar, you'd have to transform the data to get the components in travel direction and perpendicular to it. Not really hard, but an extra step. On the bright side, once you figured out how to do that, you might be able to use a Motion. I don't think the .oao files have velN and velE, but you could calculated them from speed and doppler heading.

An interesting idea for sure.

But the math involved makes my eyes roll back.

One issue with this approach would seem to me to be: How would I isolate the whole rig movements from the twist and side bend? Possibly doable, by measuring movement at the boom, if the resolution of the velocity is extremely precise, but I am not sure that it is sufficiently.

And I would need quite a few of those loggers.

Ben1973 said..
Easier to see from above.
Maybe we all need a 'Bow Sail'
i would love to be able to get my old 6.8m Rushwind slalom sail back and put it head to head with my 8.6 M2. Bet there's no difference in power.

Have you seen any drone footage from above of a 'Bow Sail'? That would be interesting!

Just been out Felling some firewood
that does Feel good.

last weekend I tried chasing DAFF down the course my overpowered 4.7wave sail had a plenty loose leach and at speed and locked in was flapping as if there was no pressure on it From any where