David:
I wonder realistically how much more a junk rig weighs than a Bermuda rig with all the standing rigging, winches, heavier sails, etc. The taller mast of a typical Bermuda rig will put at least part of the sail into a better wind gradient, but it all adds up to considerable weight if you look beyond the mast itself. There is no doubt that a biplane rig with two shorter masts will be heavier for the amount of drive it is able to achieve, though shorter masts can be lighter than taller ones. It looks to me like the mast weight itself is the main consideration, and the fact that is further forward.... which itself is not insignificant.
Several articles I've encountered suggest that the further aft the rig is on a catamaran the better. This suggests that a cat designed specifically for a junk rig from the keel up is what is really needed, but almost no boats are designed ... North Atlantic 29 is the only one that comes to mind at the moment.
logically a standard catamaran with a bridge deck cabin could be built to have the masts set at the forward end of the main hull cabins or the aft end of the cabin forward of that. The mast could be offset a foot or so inboard rather than hull stepped to allow access forward, and utilize the bridge deck cabin top as part of it's supporting structure. The keel or daggerboard would be changed to move the CLR aft. The rig would be designed for the maximum practical forward balance, which it would seem would limit your choices to a split rig or maybe a soft wingsail... I don't know what the balance limits are. If the boat were designed as a fractional rig, the masts could be aligned according to the original location, otherwise underwater "compensation" would be needed.
That compensation in CLR could take the form of a different location for the daggerboard, a change in the keel, or even the addition of a board. I'm thinking of what Bernd Kohler calls "luff boards", which are foils.. like a daggerboard mounted to the inboard side of the hulls on a pivot that fold back like a centerboard. Drawings below from his web site. These were designed with the KD 860 in mind.
The huge problem with multihulls is weight...... light weight is desirable, and it's a balancing act between bridge deck clearance and windage.... it's all driven by the obsession people have with speed. You want to carry load, you need displacement which equals drag, which means a larger rig and a larger boat. When you look at multihull designs, they tend in two directions. One is light weight minimalist speedsters, and the other is heavy condomaran pigs. The design that most appeals to me has inadequate payload due to the desire for performance. Payload is sufficient for typical use, but not for serious voyaging. People never seem to include everything they should in payload... they "cheat" a lot. The numbers sound "generous", but when you start counting galley equipment, ground tackle, safety equipment, spares, tools, food, fuel, water, clothing bedding, electronics, dinghy, watermaker, batteries, charging system, scuba bottles propane bottles, etc.... that .9 ton gets gobbled up very rapidly.......... I said most people "cheat".... really they conveniently overlook things. A ton sounds like a lot... but I'm here to tell you it isn't!
When I was looking at building.... and I'm not anymore because of time and cost, and the simple fact that buying something someone else has built makes far more economic sense and time sense... though you have to settle for something less than "ideal", I was looking at the lightest possible construction methods... infused foam sandwich. Money spent up front to build light pays dividends for the life of the boat.... It means a smaller, less expensive boat to own and maintain. A win win situation. "pay forward" they say.
H.W.