Forgot to add, the amas and mast would be detachable and only used for sailing on ocean or lakes - the boat would be as per regular Chuckanut 15 for the river. I would want the leeboards to attach to the rear ama beam.

You mentioned the cockpit two-up and solo - I've been thinking about whether to have the rear ama beam bisect the cockpit space into two or have it attach to the gunwales with no centre support to keep the cockpit open (obviously it would make no difference once the ama beam is removed for river or fishing use). The latter would be preferable I'm sure but obviously would need significant reinforcing of the cross sections at that point. I keep flipping in my mind as to which would be best. As a possible reinforcement I have in my mind two cross-sections in 6mm ply with a third 6mm ply piece steam-curved to join them on the insides to effectively form a C-section beam curved around the from gunwale to gunwale. I would then glass this. Not sure I can explain that any better without a drawing!

I'm leaning toward having the beam divide the cockpit as it would be removed for fishing and river paddling and I doubt I'd sail one-up very often, so a bisected cockpit might never annoy. Also this will be a short fat trimaran spreading a fair amount of sail on a mast well forward on the hull, so there's going to be a fairly strong bow-down turning-moment when running away from the wind with the sail full out to the side and the center of effort well forward, particularly through choppy seas. This makes me wonder if having weight well back could be a good thing anyway. Finally the rudder will be of barn-door proportions to cope with the amas.