The twist loads from a 20' hull are not huge, and can be calculated and built for. A free to pitch hull is a great idea and I used it on U (25' proa) and on W (40' cat). It gets complex though as there has to be some means of stopping the ww hull from flipping. A loose stay to each end of the hull is easiest, but there are other ways. The engineering for a rotating beam is more complex than for a wide beam, plus the wide beam gives you something to walk on. As Rob says, the motion is more lively.
regards,
rob
The single crossbeam idea seems like it has to deal with torsional forces when the ww hull tries to move independently of the lw hull. With only a 2' width on the crossbeam, one would think that dealing with these stresses might be difficult.
I am sure it is doable, but it does make me wonder about just having a circular crossbeam (at least where it joins the ww hull) and a sleeve, like on the mast, and allow the ww hull to pivot independently of the lw hull. I know rob had a hand in building a boat like that once, but I have been unable to find a write up of what the advantages/disadvantages were. Can someone point me to an article? I guess my main concern would be if the ww hull buried itself in a steep wave and pivoted right over, end over end, and the ww hull ended up capsized, even though the lw hull didn't. Is that a realistic concern?- Gardner