strange design question
I must say, that you have gone long past ''strange'' on the ''Wierdometer'' with this question. 8^) BUT I will try to answer it for you.
I guess to start with, catamarans get their speed advantage not from being able to plane (most cats really cannot plane in the traditional sense) but from having very low drag, both in terms of wave making and in terms if wetted surface. This low drag comes from a couple sources.
First of all this low drag comes from light weight. In theory, a catamaran is all about form stability rather than ballasted stability. It is the rapid shifting to leeward of the center of buoyancy relative to the center of gravity that gives a cat its stability. Since cats require minimal ballast, they can have smaller hull volumes in the water and so less surface area to create drag.
The other part of this is that the two hulls each carrying a little more than half of the displacement at any given time can be very slender. (Roughly twice as slender as similar displacement monohull) We now know that properly shaped these very slender hull forms produce very small bow and stern waves. In the absense of large bow and stern waves, the hullspeed can creep up from the traditional 1.34 times the sq rt of the waterline length, well up into 2 times (and in the case of big racing cats to 3.5 times) the sq rt of the waterline length without planing.
Now then, when you talk about bolting two Catalina 27''s together, (ignoring that the structure of a Catalina 27 is not robust enough to take the high loadings that occur at the connections between the hulls) you are adding the weight and drag of each hull to the system so that you would end up with a 27 footer with twice the weight and drag of a normal 27 footer. If you think of this as a 17,000 lb catamaran, you would expect that as a 17,000 catamaran the waterline length would normally be 35 to 50 feet. So you have all of the surface area and all of the wave making drag on a very short, wide waterline. You will have more than double the drag of a normal Catalina 27 because of the added weight and drag of the connecting structure and wake interaction of the two hulls.
A 27 foot performance catamaran would weigh in at around 1500 lbs and a 27 foot reasonable performance 27 foot cruising cat might only weigh 3000 lbs.
Then we get to the rig. Except in a very narrow range of wind angles, there would be tremendouse interation between the two rigs. In other words one or both sailplans will have dramatically reduced efficiency due to the proximaty of the other rig. This reduced efficiency sail plan will be trying to overcome higher drag. There might be a slight offset caused by the fact that this boat won''t heel much and so in a horizontal wind condition (actually in practice pretty rare)there will be slightly more wind felt by the sail when reaching. The reality is that you would probably need to add a single sparred sailplan that is more closely commensorate with a 17,000 lb monohull to even achieve the speeds of a Catalina 27
So cutting to the chase, what kind of sailing ability would this monstrosity have? It would be slow as compared to a single Catalina and not terribly manueverable. It would have a very weird motion and would be a bear to engineer. Now if you took to Catalina 27''s, tool off the ballast keels and made smaller rudders.......nah,never mind.
Tell your friend that for less money than he can buy a well maintained Catalina 27 he can probably buy a 27 foot Stilletto and have all of the advantages of a purpose designed 27 foot Cat.
(BTW the fighter plane looking canopy design on the Stilletto 27 was my idea. I came across the guy who started Stilletto when he was working on the design. He was John Holmes Boatyard down in Florida building a hull model and I happened in. He had previously been a Hobie Cat distributer and I had met him years before when I worked for a company commissioning and repairing Hobies. We got into a discussion about the Stilletto and he mentioned that was trying to come up with a jazzy idea for the companionway. John Holmes, the boat yard owner, was building a stunt plane and the yard was filled with abandoned plexiglass canopies which were the result of his failed experiments with building his own canopy. I joked that Stilletto could pick these up a couple of these pretty cheaply and it would make a pretty dramatic companionway hatch. I was joking but the idea stuck. we kicked around posible slide mechanisms but none that made much sense. But that is how Stillettos came to have thier distinctive fighter plane canopy looking companionway hatches.)