# Wind gradient - twist on windsurfing sail

Discussion in 'Hydrodynamics and Aerodynamics' started by Grunf, Mar 8, 2020.

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### GrunfJunior Member

Aloha !

We can see from graph that wind gradient is higher near the surface 0-1m, so sail must have biggest (relative) twist in that area.But that is area below the boom where there is no twist at all,that sail area is fixed to the boom so boom not allow sail to twist at all!
So how this part of sail don't stall because of flow separation on leeward side??

Does someone try to put telltalles on wsurf sail below boom area and see if lines stay parallel on both side of sail?

Or maybe this wind gradient graph is not case in real life?

Last edited: Mar 8, 2020
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### philSweetSenior Member

You need to transform the graph into the apparent wind speed and apparent wind direction seen by the sail. This transformation depends of the speed and relative direction of the windsurfer. That will explain a lot since when the wind speed is very low near the surface, the wind angle is nearly on the nose as the boat is moving.

When you get into all the hairy details, heel and sweep back also come into play. Sweep angle has a big impact on the sail pressures near the bottom of the sail and is easy to vary while sailing. Even with no gradient at all, a swept airplane wing might need ten degrees of wing twist.

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### GrunfJunior Member

View attachment 154543
Here is vector diagram where you can see that there is huge shift in apparent wind angle in this sail area 0-1m.But problem is that this part of sail dont have twist to adjust this big shift in apparent wind..

Last edited: Mar 8, 2020
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### gonzoSenior Member

Before getting deeper into the discussion, how did you determine the lower part of the sail is not stalled?

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### GrunfJunior Member

As I said ,from this wind gradient graph and lower part of sail without twist, this sail area must be stalled..
So there is just two answer:
1) this wind gradient graph is not correct
or
2)wsurf sail lower part is allways stalled = that dont seem logical

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### gonzoSenior Member

Must be stalled is an assumption. Unless you determine that it is, speculating about it is not logical.

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### GrunfJunior Member

I think you dont understand situation..
If you have part of sail which dont have twist and shift in apparent wind angle of 60 degress on it,it is very clear that this part of sail must be stalled...

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### philSweetSenior Member

Okay, you show a very deep downwind condition and a not-stellar speed ratio (My IOR leadmine can perform like that). Sails aren't optimized for this because you don't have to be any good to sail down wind. The sails are optimized for a more windward condition. Not that this really addresses your question, but you have diagrammed a worst-case scenario.

Running with the conditions depicted, if you had the boom sheeted, say, 20 degrees inside the 1 meter AWA (had the 1 m cordline at a 20 degree AOA), and the sail is untwisted below 1 m, then the sail would unload down to an AWS of about 2.5 m/s, which corresponds to a height of about 0.3 m. Below that, the sail would be inverted. There is only a tiny area of sail down there. In fact, if bow up at all, the sail base is at least 0.3 m high.

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### gonzoSenior Member

The assumption of the wind direction is wrong. Look at telltales and the wind direction is not horizontal. It marconi sails it goes some degrees upwards depending on the angle of heel of the boat. Windsurfers tilt the mast to windward, so there is downwash; the wind direction is downwards and provides a fair amount of lift.

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### GrunfJunior Member

Sail must close the gap between foot and board to stop air leaking from high to low pressure which cause drag.So it has lots of sail area down there...

Deep course is normal in slalom/speed windsurifng and slalom sails is optimize for dowinnd course..

Luderitz speed ,course 140 degress, curent record average speed 53knots over 500m,max speed over 100km/h...So for sail dowind you must have balls and skills,not easy especialy on chop..

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### philSweetSenior Member

You need to pick one operating state and stay there long enough to get somewhere. You have shown pictures of light air close hauled and heavy air down wind and wave jumping utility rig. They are all different. If you go look at the downwind speed run, there isn't a lot of sail down there. For light air upwind, there is a lot of sail down there. And this makes sense in terms of how the sail twist is constrained.

Gonzo, the assumption isn't really wrong, we just haven't added that factor in yet. The next transform is a bit more complex to diagram in the case of twisted wind. It adds the heel factor into the equation, and is usually called the effective wind. Although effective wind may also include other additional factors as well.

I recommend everyone read through the following PDF— OPTIMIZATION OF SPAN-WISE LIFT DISTRIBUTIONS FOR UPWIND SAILS by Peter Richards. Equation 2 on the forth page is the usual effective wind equation. There are some very fine graphics showing how to define the goal of maximum thrust once you have a lift/drag scattergram for that case. Defining the reference AWA to fix the lift and drag vectors is somewhat arbitrary, but usually taken as the 10 m height conditions of AWA. This probably isn't the most convenient one to use for wind surfers, though.

But getting back to one of the original questions. The bottom isn't at much risk of stall, it is unloaded, with a lower AOA than at the boom, and possibly inverted. This washout is known reduce the induced drag. Sealing the bottom gap has not really ever produced the expected benefits in a general purpose monorig. It is very effective on jibs, where it not only improves drive and aspect ratio, it can greatly improve the extra drive off the hull (which isn't measured in the rating rule). It is also worth the effort in very high performance boats that have a very narrow range of AWA that they need to contend with. In the recent AC boats, the difference between the AWA upwind and AWA downwind was about 3 degrees. That's the sort of range where you can really exploit an optimized rig. The rest of us gain more by adding versatility.

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### gonzoSenior Member

I think that a few telltales on the sail and going out in the water is the best way of proving or disproving if the assumption is right.

Doug Halsey likes this.

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### ErwanSenior Member

VR=Vent Réel = TRUE WIND = 7 m/s
TWS= True Wind Speed
AWA = Apparent Wind angle
BS= Boat Speed

From A to F it's the true wind gradient with 3 air stability conditions à 10 meters height and 7 m/s speed
From I to K it is the Apparent wind on an A-Cat sailing windward @45° and 10m/s boat speed
From N to P it is the Apparent wind ...............................................@ 135° and 14m/s boat speed

I cannot find it anymore, but I remember comparing the AWS between the above calculations and the values mesured on the water by Frank Bethwaite in his book High Performance Sailing, I could find almost a perfect fit above 3 feet height.
For the first 3 feet, AFAIR the Log law (used here) overstates the true wind speed compared to F Betwaithe. To be checked

Hope it could help

#### Attached Files:

• ###### A WIND.xlsx
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