How feasible is this?
Derek Kelsall uses single rudders on his smaller cruising cats, I understand, but I assume this is a rudder on the centerline of the boat.
What would be the downside to putting one big rudder on one hull instead of two smaller rudders on each hull?
I've wondered about this for various reasons, and though my intuitive reaction is that there would be a "good tack" and a "bad tack" intuitive reactions are not always correct. For example, intuitively most of us would want a daggerboard in each hull, but John Shuttleworth demonstrated using tank testing that a single big daggerboard in one hull was more efficient. That's the way I went with Slider, my little beachcruising cat, and it worked very well indeed.
Of course, a single offset rudder would only be feasible for a cruising cat that was intended to never fly a hull.
This would be for a cat that has a rig on centerline-- in other words, not a tacking proa.
I can think of a couple of advantages to such an offcenter rudder. For one, it would probably have a better lift/drag ratio than the sum of two smaller rudders-- I think this is probably why one daggerboard is better than two. (An analogy would be .the superiority of a sloop rig to a ketch rig, efficiency-wise.) Another advantage is that you could hang an outboard off the unused transom, and it would be protected from spray and already in the entrained water from the hull. It would be easier to build and cost less.
So what are the potential drawbacks? It occurs to me that beach cats and other racing cats flying a hull are already using an offset rudder. What problems are associated with steering while flying a hull?