multihull frame

[nukem419]

I've been screwing around with auto cad today building a 40' multi hull. Each hull is a flat bottom U hull with a 72" beam. Each rib can be bent out of 2 12' boards but the problem I am having is that I'm not realy sure how thick each rib should be. Could they be bent out of 4x4's then cut in half? (leaving 2 2x4 ribs) I have never realy worked with wood so I am uneasy about it's strength, I only have experience working with metals. I spaced each rib at 3 feet in my design, I want to take out rib spacing further in order too decrease wieght. How far can I space the ribs with out running into problems? What would you sugest building the frame out of? I'm sure I will never build this boat, it's nice too dream though 8).

[john olsson]

Book tip boat strenght by Dave Gerr, very readable book for dreaming boatbuilders, actually cheaper than building....

Im not a profesionally boatbuilder, but I read alot, dream alot and sketch alot.

Framspaceing at 3' (900 mm) seems resonable, however dimensions depend on several things.
Hullthickness, stripplank or plywood hull, stripplankhull can take some more structural forces than plywood, needing lesser framing.
Hullshape, flatpanels do flex alot hereby needing innersupport of framing, roundhull(structural half cylinder) can be rigid by it self.

typical striplank cathull, cross section shape eliptic(half ellipse)
hullthickness about 20mm(4/5"),
frames 45*70mm (laminated or bent) spaced 600-900mm (2-3'),
stringers 20*70mm every 300-450mm(might be overkill)
bulkheads minimum 6pcs, 12-20 mm plywood glued to frames placed were they fit the interior and at structural points such as crossbeams etc.
Hull is suposed to be glassed in and out, aprox 800g/m2 rowing or biax.

/john