Eric, the battens I used were very bendy. In fact I used them because they were marketed as being able to bend almost to a circle without breaking. The various sheeting methods stemmed from the fact that the battens were so bendy.
I wanted to keep everything as straight and square as possible and that required a lot of tension otherwise the bendy battens would bend. I sheeted the sail from the boom with all of my sails because this applied a downward pull that flattened out the leech. At first I used the same fiberglass rod for every batten and the yard but not the boom. On square top sails the yard was very difficult to keep straight and horizontal. It had to be supported by a series of lines attached along the yard with adjustable knots leading to a ring or soft shackle. I used midshipman's knots.
This created a structure of support that I tuned to be as straight as possible when hoisted. One problem was that if the halyard is too close to the flimsy rod it could not support the entire yard. I had the most issue with the luff and leech drooping. This required the point that all the supporting lines lead to be placed at some distance above the yard. Otherwise, the shallow angles created were not strong enough and that meant the yard could not be hoisted to the top of the mast because room was needed for the supporting lines. The distance from the yard that the halyard needed to be depended on how the material flexed.
It was difficult to adjust the tensions of the supporting lines leading to the yard from the halyard as when sheeted, the tensions change due to stresses from load. Halyard tension became extremely important. Too tight would cause the yard and entire sail to deform. Too loose meant a reduction in efficiency to windward. Once the yards supporting lines were tuned to stay straight when the main sheet was brought in tightly, it worked well on all points of sail. I did not reef the sail due to my lack of ability to re tension whichever batten was acting as the boom. Tying the battens together at the leech and luff worked for me but I did not experiment much with reefing, and I did not build my sails with reefing in mind. I may experiment with that in the future
I eventually decided to use 1/2” wood dowels as yards on my square top sails. The weight difference was minimal, and it was easier to keep straight. The supporting lines remained important since the wood dowel would snap with too much bend. On the fan top style sails I used the fiberglass rods for the yard and bending or deforming was never as much of an issue.
Paul, the reason I said a fan top could be a better choice in high winds is the angled top panels seem like they would make a good stay or storm sail while reefed down to top panels alone. I have never experimented with sailing under the top panels alone in any design. I have only tied the first panel or two down to see if it would work and I did not sail like that for long. I do not have much to say about reefed junk sails.
The outline of the sail itself is not as important to me as the shape inside it.
That said, it did seem like a square sail was more powerful when compared to a fan top, but both sail outlines performed much better when the top and bottom panels were flat with shape induced by cambered panels in between. I even disassembled a fan top sail to add a square top. I removed the top angled panels and replaced them with a single rectangular panel with an extra batten. It seemed to sail better, even though the sail was shorter and had less sail area overall. The performance difference was very noticeable in light winds.
Here is that sail. Square Sail In Light Wind It was a fan top sail.
My theory is it may have something to do with the twist in the sail among other things. The angled panels seemed to have less effect as far as twisting the sail while a rectangular one seemed to act like a lever.
Slieve, here is a video of one of my square top sails. You can see tell tails occasionally. https://youtu.be/7u5r-Du4mkg?si=omKiGNzyyfk8Ngc6
-Colin Clayton