Hey Kurt, excellent!
Did I miss what wood you will use for the compression beam?
Depends on where the struts, the verticals are going. All the way to the cabin side (as 338) or are you keeping the forward stateroom, and will have a doorway?
Could guess that if you were going with the doorway 2 X 8 would be OK. In white oak. Wider apart and deflection could be a factor. I would go to 2 1/2" And I might think to lag in a piece of 5/16" steel on edge - cut exactly to the arc of the cabin roof - as insurance. There is a problem with this if you are laminating: your lags are going into the lams which spoils their integrity.
There is good reason why beams are usually deeper rather than flat. You know you'd have a superior beam if you put the 8" on edge. I think 338's is like 2 3/4 X 5. With smaller section plain whiteoak struts on the sides, at the ends of the beam.
Maybe you could weld up a 2 X 8 s.s. arc as a 'C' channel. That might work good. No wood, just steel. Very good, if you through bolted across the cabin top the whole length.
Don't think I'd do away with some kind of verticals going down to the berths.
Theoretically any arch can't deflect if its ends cannot move. On the Triton site at least one guy assured his cabin arc by fabricating a curved athwartship mast base - thrubolted with the beam, no doubt.
At the end of the cabinsole where the V-berth deck rises -right there at the doorway in the bulkhead - I've glued in at 2'' thick mahogany 'floor'* (crosspiece) and I will be doubling that (and maybe a bit more) thinking that I may have to put a compression pole in because the beam is not adequate. They naturally end where the original settees rise. With a pad on them and the pieces fitting well andthe remaining pieces of the old bulkhead tabbed in solid they may spread the load of the mast over the keel well enough. I've heard that poles are great to grab onto below. Imagine a well rehearsed two handed 180 swing from a crouch to a 360 landing on the Airhead...
White oak is not a wood for gluing. Tannins mean the resorcinol won't work. Can't remember about brown glue (the powdered stuff} but white oak? don't think so. There are enough complaints about Gorilla urethane glue to keep me off it. No 'soft' glue will hold stress lamination. And epoxy requires a glue line. Might talk with Smith & Co. about their epoxy AllWoodGlue.
If the beam appeared to be molded into the cabin, not featured, but was disappeared, nicely rounded, into the cabin painting scheme, I bet you could go deeper on the beam scantling and be positive (almost) that the mast would stay out of the accommodation. Incorporating a number of fastenings thru the lamination might insure the mast won't crack it and delaminate. Be cool if you could carve the beam out of a curved branch. Indeed, if you rounded the bottom, curved it like a branch you'd gain more strength without it being obvious.
Interesting problem.
Epoxy has to have a glue line. In curved lams prebending by steaming would allow gluing without force. If your lams are thin and easy to bend (1/8"), and if you have the time: gluing up a few at a time, letting them get hard, and continuing with a few more, etc could work. Gluing a bundle of wood together under stress is not correct anyway. The piece will want to straighten itself out (flatten) which is not what we want to happen - so why build it in?
Wide lams may be difficult to set up a jig and to clamp. But the beam is pretty small. Gluing the beam to the boat would add strength and solidity.
Screws could be used to hold lams together while gluing and backed out after set. I have a bunch of 1/4" luan 1 1/2" squares with a hole in the middle. Drive them in with a grabber to tighten up pieces, get a bulge out.
I think you have to give attention to supporting the full width of the 8" beam with 8" wide verticals. Gotta support the beam!
Go for it!
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*floors connect ribs together over the keel in traditional wood boats. Each pair of ribs, port and starboard, would have a floor, each floor fastened to the keel. Snobby using a term like that in a lit'l ole plastic Ariel. oh well...
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