I had concerns about binding as well. The batten would be 1.5 inches thick, and the thought was that if the hole was cut to a tight enough tolerance then the batten wouldn't have the opportunity to droop enough to bind. I had also considered using a short section of tubing epoxied into the hole of the batten that fits precisely around the mast.
The engineers here will correct me if I'm wrong: I don't think the tolerance is what matters. Say wind pressure lifts the end of the batten, with a torque of 1 Nm (Newton metre; substitute lbsft at will). Your 1.5 inches are about 4 cm or 0.04 m of vertical separation between the upper and lower edge. To counter a torque of 1 Nm with an 0.04m separation there needs to be a force of 25N on each edge, no matter whether you get that 0.04 m separation from a tight or a loose tolerance. The section of tubing looks to be a better solution, but would interfere reefing, unless, perhaps, you let the tubes be cone-shaped, so that the tubes can slide into each other. Or you allow for movement.
The rods were planned to reach almost all the way up to the next batten, slightly veed out. I was hoping to not have to use lazy jacks because of the increased complexity and potential for failure and would be another maintenance item to worry about.
With only three panels in the sail, that is nearly a third of the height of the mast above deck. That is a lot of weight and windage. And unless you sheet all the way out before you drop the sail, it is likely to still fall outside the rods.
The sail shape in this instance will come from the fact that luff and leech will attached to a rigid batten, so when the sail fills it will take the shape determined by how loosely the fabric is layed between those two points
You are thinking of the shape along the chord. I meant the shape along the span, between one batten and the next.
I have poked around on the JRA website, and all over the place, and haven't found anything quite like what I'm thinking. I was hoping somebody already went through the trial and error! I found your article very interesting.
Bolger's Chinese Gaffer is the closest I know:
Note that a junk sail may have as few as two pieces of running rigging, the halyard and the sheet, along with two kinds of standing rigging, the lazy jacks and Hong Kong parrels that hold the battens to the mast. If you want more control, you could use double sheets, either one or two luff hauling parrels that let you adjust how much of the sail is in front of the mast, and perhaps a yard hauling parrel. So up to six lines, if you want to get fancy.
Bolger eliminates the luff and yard hauling parrels, but then needs both a throat and a peak halyard, he has
three sheets, and he also has two slab reefing lines, presumably because once he reefs, first sheet 2 and then sheet 3 start pulling battens
up. And there is a vang. Bolger ends up with eight lines. I haven't read "103 Small Boat Rigs" or the essays in which Bolger explains the rationale for his choices, so I can't comment.
Still, sticking with conventional junk sheeting should let you do much the same job without the vang or reefing lines, so you may be fine with three pieces of running rigging. I am pretty sure you would need both throat and peak halyard, though. Both the lug and the junk sail align the halyard with the mast, so that the angle at which it pulls doesn't change much as you raise or reef the sail, and they use the proportion of sail before the halyard and mast to determine what proportion of the vertical load is carried by the luff compared to the leach. You propose to do away with the part before the mast, so you can't use the same method, and I think you will need two halyards.
You haven't said why you don't want any part of the sail in front of the mast. The most common motive is to avoid having the mast to disrupting the flow on the lee side of the sail on one tack. If that is it, you don't need your wishbone battens to go around the mast. Let each terminate in a fork or jaw, like this:
Tie a rope from one tine of the fork around the mast to the other tine. You could even let the luff of the sail slide on a little rod so that it is always blown to be in line with the lee side of the mast. Otherwise, use Bolger's planform with conventional junk sheeting. Try it at model size, and if that works, at dinghy size.
The wishbone booms will let you have camber, but unless you let the sail lie on the leeward part of the boom, I don't think you can control how that camber is distributed along the chord. Conventional sails do that by sail cut and tension, but the point of the junk rig is to reduce loads, and so you don't have that much tension to play with. There may be an alternative.
You may have noticed that I proposed, in the article, curved, rigid battens, basically making each batten a Bierig CamberSpar. It looked very good on the model sail. When I tried it at dinghy size, the battens didn't flip as they had in the model, and I never worked out why (I didn't have the boat for long, and was focused on other experimental bits). Nils Myklebust built a c. 25 sqm sail for his motorsailer, chose to flip the battens by 180 degrees instead of about 90, and because they were never going to do that on their own, he rigged a line tied to each batten at the point of maximum camber, and pulled on that to flip the battens. That did work, and he got a good sail shape.
I think the curved, rigid battens could be applied to the gaff/junk hybrid. Take the above picture of the jaw of a wooden gaff. Give each batten some vertical curvature relative to the orientation shown in the picture. Shape the batten pocket to allow the batten to flip around 45 degrees to either side. I am not aware of anyone ever having tried this, so I make no promises, but I think there would be less risk of binding, and better control over the distribution of camber along the chord. And you could make the jaws wide enough that the sail's luff is just behind the lee side of the mast.
In either case, the higher the aspect ratio of each panel, the less the battens will influence sail shape away from the battens. The goals of control over sail shape and simplicity by having few battens are in conflict, and you will need to decide on a trade-off.
Have you had opportunity to sail with either a gaff or a junk?
