I used to do some hang gliding, and I remember that rigid wings were way more efficient than the standard "flex" wings, which were already more efficient than sails because they had double surfaces (i:e they had actual internal volume instead of a single aerodynamic surface).
Should this "rule of thumb" not be transferrable to modern sailing?
That would make BMWO sail much more effective.

[edited for typo]

What do you understand of efficiency?
Thickness (e.g. volume) is just one parameter of a profile. The big advantage of thickness is that you can put the supporting structure inside the aerodynamic surface, hence creating less drag. The second advantage is that a wing keeps its profile under load (more or less), hence it can be optimised. Long time the sahpe of sail was a result of mast stiffness, batten stiffness, sail cut and the skill of the sail trimmer. However this changed with time, as modern computer software can predict the deformation of a sail and hence it can be designed to have optimum shape under load.
However you can't look only at aerodynamics. The weight of the sail/wing and it center of gravity is a major contributor to overall efficiency. Clearly a wing is in disadvantage here.

Cheers,

Klaus

Agree that you can predict deformation of soft sails, but as conditions change, the sails will also deform/change with the pressure changes so you will spend more time with the sails at a non-optimal trim. I dont know how this will apply to the monster rigs on the AC boats, but it is pretty interesting.

Same for the wing. It is optimised for one single case. To adapt you have to articulate it, hence the flaps, slots, etc. At the end you may have many more control devices than a conventional sail. Which sounds good in the first moment, but to adjust them, you need also a lot of input information. That's one of the reasons why BMWO has this huge number of sensors on board.
Sometimes simpler is faster. In one interview, the design of Alinghi 5 said, that they decided against a wing, because they lack the time to sort out all the details associated with. And I am pretty sure that's why the BMWO wing is so conventional/simple (some may not agree, but I mean conventinal or simple in a positive sense).

Well, and sometimes predications can be terrible wrong not only for sails...

Cheers,

Klaus

Simple is good. Single slot becouse they can use a headsail downwind I heard. I am at the egde of my seat waiting for the racing to start..

I hope your arms don't get tired hanging on to the edge of your seat cuz the racing won't start until Wed. On the up side if there are 3 races I will be able to go back to bed after the last race. Assuming they run it on the weekend.

This is exactly why my wife hates sailing...

(light air, no air)

I can't say as I blame her.

Hey now, light air is great for:

Stantion mounted BBQ
scuba diving

Efficiency in the case of rigid-wing hang gliders pretty much boils down to less drag and more lift. My gut says those are not just correlated parameters though, there's got to be more about it than just drag. The differences result in higher speeds for the wing at the same glide path (I think this compares to higher VMG) or insanely flatter glide paths (pointing higher) as compared to flexible wings, Part of the equation is definitely having supporting structure inside the wing. The leading edge ultimately "becomes the mast" as it were at a certain point. I seem to recall that having a separate upper and lower surface gets rid of lots of parasitic turbulence, but I don't recall the theory all that well, so I'm not sure.

The thing that gets me comparing wings to sails is that wings on gliders (and rigid-wing hang gliders) are not all that complicated but very efficient all the same. I don't see why sailboats should need so much more complexity to use a wing sail and be competitive, whereas all these gliders don't...

or perhaps its just not needed/reasonable to have dozens of adjustments on a glider... more things to break... and manage...

if the tri breaks a part.. they can get towed home, if the glider breaks a part...

its also a matter of rigging time and what do you do with the wing when you aren't sailing?

Peter Harken went into the practicalities of the issue on his **** hour with the Anarchy crew yesterday.

I was gonna get one, but when i heard it doesn't even fit under the Golden Gate Bridge at high tide, i decided against it

At 223 ft it won't fit under the Skyway either. Buy the way, the wing is 7000 sq. ft. LOL.

Hi Dennis,


I wouldn't say that a wing sail has always more lift. A (wing) sail must work with wind from left and right, which is a significant limitation in profile design. A symetric profile without flaps has a lower maximum lift than a classic soft sail. Flaps increase the lift, but also drag. The ratio between lift and drag is closely related to vmg upwind, as long as there is enough wind. This is comparable with the best glide path of a glider, which is directly related to lift to drag ratio.
However you can significantly reduce drag by increasing span (thats way gliders have a lot of span). Since a soft sail is lighter, you can have a higher mast than a boat with wing sail and reduce aerodynamic drag or have the same mast height and a lighter boat and decreasing hydrodynamic drag.
Downwind you need maximum lift. A good genoa (or gennaker) - mainsail combination can provide a maximum lift close to that of a wing with flaps (however a genoa with wing sail...). Again you could have more sail area for same weight or less weight for same area.
As you see, it is not as simple as on glider planes, where aerodynamics rule everything. For a sail boat, you have to make a careful trade between weight and aerodynamics. As weight is more impoartant in light winds than in strong winds, soft sails are in general better in light winds.
Efficient because of asymmetric profiles, simple because they have no flaps and because you don't store your glider outside on the beach for six months. Last but not least your glider operates at higher speeds and hence higher Reynoldsnumbers, where fixed wings are more efficient.

If you have a free rotating aerodynamically shaped mast, you can minimise the turbulence zones by a fair amount. But on a fixed mast, the turbulence behind the mast is significant. So much that you can say that the rotating must is the key enabler for performance of cats (don't tell the skiff community...)

Remeber A/cat world championship 2008 in Florida: Ben Halls wing sail wasn't the superior in any wind speed. This is certainly related to Reynoldseffects, hence not transferable 1 to 1 to the large AC boats.

Cheers,

Klaus

Nevermind, pleas carry on.............wow

Hey Thanks klaus, are you a professor at Hamburg? If not, you'd make a darn good one IMHO!

The asymmetric profile of the glider never occurred to me... That sure makes my comparison a case of apples and oranges...

And cats being "interface vehicles" (term I borrowed from Richard Woods) makes the hydrodynamics vs aerodynamics trade off a really big deal too.

I'll look into Reynolds theorem again although I have to admit I was never able to "get" it before. I just can't handle that much algebra.

I am just a normal mortal aircraft engineer.
Actually you don't need algebra for the basic understanding of the Reynoldsnumber effect.
I try to attach some pictures to explain. In the first picture the lift coefficient is plotted over the Reynoldsnumber, in this special case for a windmill section (for general aircraft or foil sections the effect is not that strong). The second pictures shows the effect of Reynoldsnumber on drag coefficent.
The Reynoldsnumber itself is the product of the fluids speed passing over the sail/foil/wing times its length divided by the viscosity of the fluid. The smaller the wing the lower the R.number and the slower you move the lower the R.number.

For the lift you can see, that below a certain R.number there is a drop off. In general this drop is off is stronger for thick sections and smaller for thin sections, hence the advantage for a soft sail. Friction drag may raise with low Reynoldsnumber because of the behaviour of the boundary layer, the air which is closest to the body. A common trick to make this boundary layer more stable is to make the surface rough, as done on tennis and golf balls. Again a fabric with sews and overlaping patches might be closer to a tennis ball, than a smooth airfoil.

Since BMWO wing is large and the boat is relative fast, it is less affected, than Ben Halls A-cat or a C-class.
Another intersting point is, that the viscosity of water is different than that of air. The Reynolds number of the smallst rudder are larger than of a sail. That's way aircraft foils work fine for this application.

Cheers,

Klaus

Ok, pardon my what I now understand from all this is that there are three "zones", "high Re" where lift and drag scale well, "low re" where they also do, and "the middle" where drag is much reduced and lift is maximized?
You say the "drop off" is less for a soft sail, but does that not also imply that max Cl is also lower overall?

Another question: is this Reynolds related lift-boost something one could experience on an old cat like my Nacra 5.2? It would account for some experiences I have had when reaching where the boat picks up a gust and after accellerating all of a sudden the boat starts to pitch-up (where it had been rapidly going bow-down) and takes a sprint with apparently less drag and heeling moment. feels like planing in a skiff or dinghy) but I'm sure that is not it.

Ok you talked me into it. can i come by friday (so we can epoxy it to my 5.5)?

Yo, if you have wing questions, now is the time to ask. Today we will have Dimitri Despierres, lead engineer for USA17's wing, Francis Hueber from the wing design team, Fred Eaton who is current C-Class champion to answer any questions you may have.

Go to Sailing Anarchy OTW to check it out. The show starts at 12:30 PM EST.

Oh, and tomorrow we're doing the show from onboard USA17 with some really special guests.