Perry writes (quoting Parker): Full length battens are very useful devices in creating good sail shape & have been known & used for centuries. This is a doubtful statement, since their main purpose is to support the leech of a triangular sail with a very large roach – and with modern sail-making materials they can perform the secondary effect of assisting good sail shape - neither of which has a history which goes back "centuries”.
Anyway, Perry, it is a misleading paradigm in relation to junk rig. It’s a common mis-understanding due to unfortunate terminology. Junk rig “battens” have no relation to what people normally regard as full length battens (they merely look like it to the casual observer). A better terminology for junk rig would have been “boomlets” since their purpose is not at all to create good sail shape, but to divide up and transfer to the hull some of the driving forces of the sail – like multiple booms – via sheetlets - thence to the main sheet. (As well as, in combination with the lifts, to give an incredibly simple reefing mechanism). However, history has given us the terminology “battens”, which we can’t really change now. They are not “battens” in the usual sense of the word, they are spars.
To do their job, they are best as rigid as possible (though on a SJR with its multiple downhauls, being able to induce a very slight downward bend helps to keep the luffs nice and straight).
(We should ignore the case of bendy spars in modern bermudan racing rigs - their purpose, when the sail is sheeted in tight, is to reduce the draft of a cambered sail in moderate to heavy wind, not to increase it).
The problem with bendy junk battens is that they bend more in strong wind and less in light wind, which is the reverse of what you want. Furthermore, they will bend differently depending on which tack.
An exception is “articulated battens” which do bend for the purpose of giving shape to an otherwise flat junk sail, but they are a special case and the articulation is deliberately limited mechanically, to a constant amount. These are the "hinges" referred to by Alastair in a previous post. It is arguably simpler and easier for most people to use rigid battens and create sail shape mainly from the cut of the sail.
Perry, your suggestion of a hollow flexible tube with a semi-rigid and perhaps variably flexible central core is a concept worthy of consideration, but I do not understand your description. If a complex hybrid “batten” capable of being sheeted from the leech is considered a better proposition than simply sewing shape into the sail, then the challenge would be to find a central core which would make it curve most in the first 35% of the chord and be relatively stiff for the remainder – and reversible so it works correctly on both tacks. The forces on a batten are different on different tacks, so how to avoid curve at the leech, and an ”S” curve when on the “bad tack”?
The nearest I have seen to successfully solving this problem is David T’s soft wing junk sail which is cambered by a combination of: articulation in the otherwise rigid batten, and the cut of the very special first 30% of the chord of the sail. It works on both tacks.
Another interesting example is Annie's sail which I saw when Fanshi was first launched. This too (Tyler inspired I believe) was a combination of articulation of otherwise rigid battens, together with shape cut into what looks like the the first 50% of the chord of a conventional cambered sail.
Ancient Chinese junks were not built using modern materials, and a little bit of shape was no doubt gained fortuitously from spars that might have been a little bit bendy, and sail cloth which unavoidably stretched. And there is a theory that the wind 'sees" camber in fanned panels too, which some Chinese junk sails did have. But why wouldn't you just use rigid battens and put the shape you want, the amount you want, where you want it, into the sail - by designing and sewing it in?
Anyway, Perry, we are drifting away a little from the subject of Karl's thread. Perhaps it would be good to start a new thread, as you progress through the evolution of a special sail for your project build.