Anonymous wrote:
One question I have that people might have an answer to is what is the "ideal" airfoil shape for a sail/batten? Is the upper surface of a symmetric NACA foil just about it? That's two questions in one!
There isn't one. The ideal airfoil shape is dependent on the speed of the fluid the airfoil is moving through. So in an airplane, the airfoil is chosen for the speed the plane will spend most of it's time at and even then flaps and nose movement are used to change the airfoil shape for lower speed use at take off and landing.
With a sail boat one does not have that speed the air foil will spend most of it's time at. So being able to change the airfoil shape depending on wind speed would make a lot of sense. So far as I can tell, nobody does (aside from sheeting tighter or not) this except multi-million dollar racing boats with pretty much solid wings rather than sails. Sails are very flexible and tend to bend more in strong wind than in light winds which is just the opposite to what is desired. To add to this, trying to flatten a sail in strong winds also means using stronger gear all the way from lines to battens to sail cloth.
So what most people seem to have chosen to do, is to change the camber of the sail so that the bottom panel(s) have a greater camber than the top panels because in stronger wind the system will be reefed. The top panels often have no camber at all. This is certainly not a perfect solution because in light winds the top half of the sail is less than ideal. On the other hand, many people want to use a junk rig in the first place for simplicity and the hull has a speed limit which means once that speed limit is approached, efficiency becomes less of an issue.
Anyway, enough background. In your case, changing the foil shape might be done manually (as in depending on weather forecast before setting out), making each batten with a different camber up the mast or some how allowing the flexible part of the batten to be tightened on the fly. While there are some exotic materials that can be stiffened just by passing electrical current through them, that is probably beyond where you wish to go. I think the best you could do is to leave the battens straight cut at the joins and tighten the middle membrane in some fashion.
Getting back to the best airfoil shape. Assuming you want a single shape for simplicity, aim for light wind performance with the theory that strong winds will provide enough power even with poor efficiency. Remember that it is not something to loose too much sleep over because cloth is involved and no matter how close you get the batten to your chosen shape, the cloth will choose it's own shape. you can compensate for that to some extent by choosing a wrong airfoil shape (read everything you can find on both camber cut cloth and SJR for some good info on airfoil shape) but even then, wind speed will still change things... probably negatively.
As a last thought. I find it amazing how a simple cloth with both ends held tight makes a reasonable foil all on it's own...