As I read through this thread, a couple things jumped out at me. First of all, the thread starts out as a general theoretical design question comparing sloop vs cat rigs. Then it migrates to a very specific question about converting a specific design from a sloop to a cat rig. And then it does a non-sequitor and becomes as a discussion of catamarrans vs mono-hulls and ultimately ends up as a discussion of the egos of felines, and multi-hull and feline lovers. Cheez Louise.......You just have to wonder about this place sometimes.
Anyway, back to the original question, in steady state, moderate to low speed wind conditions a single sail is generally more efficient than multiple sails. But we don't sail in steady state conditions. we sail at a variety of angles to the winds, with gradient winds that require twist to the sails, in ambient winds that increase and decrease in speed, which is then complicated by the impact of apparent wind and velocity shifting, and so on. And in these variable conditions, the multiple sails of a sloop allow us to adapt to the changing boat and wind speed and direction not just for performance, but to balance the helm, reduce heeling, deal with waves or current and so on. This can be done on a catboat as well, but not as efficiently.
Its important to understand that the primary drive in a sail comes from its leading edge and not from its sail area. Increasing sail area without increasing leading edge will increase speed on a beam reach to a run, but does comparatively little for the boat upwind. If we compare a sloop to a catboat of equal mast height, the added luff length of the jib can mean that the sloop will have an extra 60% to 100% more leading edge over the cat. That is significant when close reaching to going upwind.
Of course you can increase the length of the mast on the cat rig in order to equal that of the sloop, but when you do you raise the vertical center of gravity making the cat rigged boat a lot more prone to heeling for an equal sail area and luff length.
Some catboats try to equal the sail area of the sloop while maintaining an equal mast height to the sloop by going to quadralateral sails (gaff or lug rigs). This helps a lot on reaches and runs but adds drag and so hurts windward performance.
The other advantage of the sloop over cat is the ability to maintain luff length while reducing sail area in higher windspeeds. What happens with a performance oriented sloop is that it will have several headsails that it can employ. These headsails are typically of different overlap and flatness, but they mostly all have similar luff lengths. Because of the ability to control headstay sag over a range of conditions, the fullness of any individual jib can be varied with the conditions as well. Because you can pick the right jib for the conditions, the sloop also offers practical advantage over a catboat which can only alter mainsail shape, and reef to deal with changes in windspeed.
More than any other single reason, this flexibility in adapting to changes in windspeed is the reason that larger racing sailboats are sloops and not catboats. The massive inherent upwind inefficiencies of multi-mast rigs is the reason that modern race boats are typically not ketches, yawls or schooners.
Which gets us to the second part of the question, What is the best rig for a puffer, which is the one that I suspect that the original poster had in mind when he started this. For their day, Puffers were a nice design all around. They came with a very efficient sloop rig and I would suspect that converting one to a catboat, even with a taller mast, and greater sail area, one would give up a certain amount of performance particularly at the upper and lower ends of the wind range. Its not because of any theoretical advantage that a sloop may have, but more about the physical properties of the puffer itself.
As to the felines, and multi-hull and feline lovers, go find your own thread.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies