I need to stiffen the flybridge deck/cabin ceiling

I'm a bit confused, what do you mean by perimeter beams? Outline the decking with laminate beams? My original idea was to do 1 beam at each end of the decking, and one in the middle tying it all together at the header strips at the top of the windows. I did do some under deck work in the engine compartment on 90+ days, and yes that was not fun at all. It's the main reason for my inquiries, As I would certainly appreciate any alternatives. Time is on my side so a very good approach, as opposed to a slap dash, is preferable.

 

Yes, a perimeter frame, possibly laminated, with athwart beams would be the way to control the movement of the crown. A simple demonstration might be to bend some cardboard into a simple curve. Mark the two edges that tough the table and push on the center of the curve, deflecting it down. The edges will spread out, as the curve flattens. Now, tape these same edges to the table, freezing them in position and see how much stiffer the cardboard will feel when you try to depress the curve again.

You can physically prevent the roof crown from deflecting, with the perimeter frame and beams tieing each side together or by improving the stiffness of the whole roof, with a core and new internal skin.

 

I think with materials on hand, I will make a up some lam beams, bonded and glassed to the deck, and tying into the header boards over the windows. A 2 birds with one stone cure.