Its clear that the cross beams on a multihull need to be VERY strong. In my case - an ‘Atlantic’ design project, they need to withstand the force of lifting if not ‘flying’ the main hull while burying the ama. My instinct is to keep it simple and light by separating forces, using wires for tension.

I’d appreciate any input from this knowledgeable and experienced community. Pointing out any howling gotchas would be good 😊 ‘Scuse me for giving dimensions in feet and inches, these days I think in that medieval system :(

Also -  I’ve read that the Atlantic /Pacific debate has been well worked over in these pages but I can’t find the relevant threads - please point me. I know its a bit boring to rehash old arguments.

My dimensions are at this point notional - (stitch n glue ply build)
Main hull 34’, 6’ max beam, 54” draw, 24” freeboard.
Ama, 20’, 24” max beam, 24” draw, 24” freeboard, 
Ama may be canted out slightly, or built asymmetric.
Distance between centerline of mainhull and centerline of ama, 8-10’
Crossbeams extend out to windward at least 4ft to support a long ‘cockpit’ and possible a safety Ama - the idea being to put water tanks out there. (And hang the tender under)

Do any of these proportions look silly?

I will post a drawing or two soon 😊 

I’m thinking three main crossbeams. Why? - three low aspect masts (25’?), one stepped on top of each crossbeam along centerline (or maybe toward leeside) of main hull. Each of these units (mast+ cross beam) will be tied together as a triangulated unit, with stays to masthead, and ‘bobstays’ (?) tying ama end of cross beam to mainhull at about waterline. Mainhull has beefy bukheads to take the load of mast and crossbeams.

So essentially, the mast-bulkhead-crossbeam structure is a distorted kite diamond.

I will post a drawing or three soon 😊 

At this point, my crossbeams are 18-24” high under the mast step, tapering to 6-9” over the ama.
(Oh yeah, over the main hull, they form the cross section of a more or less conventional ‘cabin-top’, cute eh? The cabin sides (smooth vertical extension of mainhull, no sidedeck) and cabin top will provide more rigidity between mast/crossbeam units.

The crossbeams are 6-9” wide, forming a tapering box with vertical sides. Internally, ply plates form a conventional triangulation with vertical and diagonals (like an old iron bridge). Under mast, three plates, one vertical, two descending toward gunwales. I’m all about spreading stress. 😊
Crossbeams will be sealed (and foam filled?) for floatation.

I will post a drawing or four soon 😊

Assuming 1/2” or 3/8 build with all joins glassed with 4-6” tape, and, probably, one layer of glass on all exterior surfaces, does this general volume/construction sound like its in the right ballpark strength-wise ?

Its all very rigid, at least in this ‘cross section’ sense. What do I need to think of in terms of diagonal and twisting forces? I’m assuming tension wire diagonal cross bracing between cross beams, between mainhull and ama - two crosses.

Ama is bolted on. Three sets of double bulkheads project above ama deck and sandwich the crossbeams, with 4 or 5 1’2” SS through-bolts at each pair of bulkeads. Crossbeam sides/end panels appropriately thickened/braced over ama. That sounds strong enough 😊

Maybe for transport, the main hull can be tilted on its windward side (with appropriate cradles) with crossbeams pointing diagonally up - it would be quite a sight! 😊 There is also the question of windage, I’m considering some kind of simple sloping plane spoiler at each leading edge.

This my design in a nutshell, more details coming on steering, sailplan, interior layout, etc. Looks like I’ll be posting lot of drawings 😊 Masts *may* be rotating teardrop (iceboat like), _not_ wingsails in the interest of reefability. I’d want the total ‘wing’ area to be roughly equal to minimum storm sail area. (Does anyone have experience heaving-to a wing mast schooner?)

Sorry for the long post, and sorry for all the annoying smileys.

thanks!
Simon