How do those new Double Mainsail's on the AC72's work?

[Hunter216]

During the single race yesterday that was called on lack of wind - timed out, the commentators had a lot of time to ramble on

One of them spoke to a theory that the change in sail shape - draft was created by having the windward layer fairly flat while allowing the leeward layer to become more “curved”. When the boat starts to foil and pick up speed the leeward layer is pulled flatter. Hard for me to describe

I managed to grab one screen of the NZ main. LOTS of widgets in there!!!

 

[DArcy]

I haven't been able to follow this thread closely and maybe don't have that much to contribute. But I am interested in the difference between a sail - which is not shaped like a wing - and a sail that is shaped like a wing. A lot of questions arise: Is the wing shape more efficient? How much does the speed of the wing thru the atmosphere affect efficiency? What about drag? I've read that the most efficient shape for a wing is the Spitfire airplane wing. Most lift for the least drag. But speed still matters - I think.

Cost was the real driving force behind getting away from rigid wing sails. The wing shape is certainly more efficient than our simple sails and wind speed also has a big impact on efficiency. I'd guess that, with enough analysis and testing, the double layer soft sails could be made more efficient over a wider range of environments just because you can play with the shape more than a rigid sail.
It is also much more likely for this technology to trickle down to us plebes since it was really driven by cost.
The Spitfire wing may have been the most efficient at the time for that specific type of aircraft but airfoil shape really needs to be tuned to the application. That's why the shape of an F/A-18 wing is so much different than a glider wing. But even a highly dynamic Spitfire has much different requirements than a Dash-8 even though their speeds and operating altitudes are not that much different.
Computational fluid dynamics software has really come a long way in the last decade, making modeling air flow much more accurate. This allows the engineers to make design iterations fairly rapidly, but also to model the performance over a wide range of wind and boat speeds with different sail configurations. They are probably using multiphysics solvers to predict boat speed and optimize foil design at the same time as sail design.
Efficiency is a very complex problem for these foilers, much more complex than our pedestrian, low riding displacement hull boats.

 

[walt]

A little .02 for what its worth... You can also get a little insight into the sails required on these very fast boats by looking at the equations for lift and drag Lift and Drag

Apparent wind speed is the vector of true wind and boat speed. With a displacement hull, boat speed is always limited to hull speed +/-. But on these fast boats, for the same true wind, they will see significantly higher apparent wind speed than a displacement hull.

Lift is proportional to angle of attack and wind velocity square (plus some other constants). Emphasis on the square for wind velocity - ie, if you double the wind velocity, this term goes up by a factor of four. On the fast boat, the magnitude of wind velocity square becomes a large number quickly and since lift has to be limited to not capsizing the boat, the sail angle of attack with respect to apparent wind generally operates at a smaller angle compared to a slow boat.

And possibly more important is that drag also goes up with the square of apparent wind speed. On the hull speed limited boat where apparent wind is generally significantly lower, drag is of course important but on the fast boat, the drag is going up with the square of wind velocity so this factor in the equation can be much larger. So dealing with the drag of the fast boat sail is more significant and important.