It’s problem though is its flatness. Flat sails have no power, especially to windward because what a sail needs to go, just like an aircraft wing or a woman, is curves.
This is the sailmaker’s art.
If you look at the picture below right, what you see is a curve. Not only that but the top is twisted away from the wind more than the foot and that is not a quirk – it has to be for efficiency, for flow over the whole sail.
In the Little America’s cup, they had great wingsails but what was lacking in a rigid wing was twist. An aeroplane has it designed into the tips but sails are more dynamic.
All sails hoping to go to windward need camber of a certain amount. This is the curvature of the sail in its base position.
If you lay a sail on the floor, it should be a bit baggy, not lying flat. This is because of the clever way the panels have been cut and “bunched up” a little in the sewing, a principle not unknown to dressmakers too.
It’s very precise, as you’ll see in this clip from the series: How to Draw the Camber of a Sail — powered by eHow.com.
Basically, the NACA tabulated precisely what sailmakers had known for centuries and came out with a seried of curves which are the most efficient in certain conditions and for certain foils. Here is an interesting article on foils, specifically those for keels and rudders but the principle is the same.
They discovered certain things about the position of maximum camber.
Mast and sail combined
If you have a flat[tish] wing mast of 20% of the length of sail and mast, it is less efficient than a 10% wing mast [and 90% sail]. It only gets more efficient at 35-40%. Therefore, one either goes the whole hog with a wing or else sticks to the traditional mast sail. That’s why the old sails will never die because, within a small angle of attack [the angle at which the wind makes the sail fill and operate], they are almost as efficient as a solid wing at its best.
The advantage of the solid wing is a wider angle of attack, meaning it will be efficient either let out or pulled in and is more forgiving that way. On the other hand, it won’t de-power and every sailor knows there are moments every few seconds when one wishes to de-power to keep the boat upright.
So, it’s all a compromise.
Vertical is not horizontal
The greater the camber, the more the power and here another factor comes in. A sailboat is always in the wing-effect zone close to the water whereas a horizontal aircraft wing is there for a second or two. The top of the sail is meeting different wind patterns to the foot but that is not so with an aircraft.
Therefore, in the sail, the top must be cut flatter for less power and it must allow twist.
There have been attempts with the junk sail to design camber into it and it works well although adding to the cost and defeating much of the appeal of the traditional junk.
Again, it’s all a compromise.