Plywood Catamaran – Frames

APP,
your cat 8-9m is small, unless you plan to beach it, or ride in rocky tidal environments, if you dont want a turtle , you should consider a as thin as possible hull weight is critical in multis. one test that we did years ago when building wooden boats was to have it sit on a round rock at low tide dead center, no waves at low tides there, we put carpets on the rocks. it always passed the test new, we did destroy a few older boats with that test. but better there than in the shipping seaway hitting floating logs. it was common to see scars on hulls , imagine hitting a 20 cm diam. log at 25 knots, it pierced thru the skin. we did add foams of all types, no one wants to sink in front of a cargo ship ... but we enjoyed jumping those wakes ... now we use seadoos at ... knots , not as many logs if no rain, but those damn algeas pack keep getting bigger and like to hide a few smaller logs ...
sit and list what you really want to do , that will establish your program, then ask for a specific design. My program is to be in shallows, so at times, it will be hitting coral heads and rocks, the question is not if, but when. there are tough skin products on the market that can be added instead of thickening the wood.
keep informing us on your discoveries, time to get back to dust and smells, i am restoring an old sail boat, the epoxy should be ok, it was cold last night ...

 

OK. Let me stay with Richard Woods data for the time being.

Example: The 29ft SATURN [http://sailingcatamarans.com/saturn.htm]
LOA 8.8m 28ft 10in
LWL 8.35m 27ft 5in
BOA 5.6m 18ft 5in
Draft (Hull) .42m 1ft 4in (Daggerboard down)
Headroom (Hulls) 1.56m 5ft 2in (Cuddy) 1.54m 5ft 1in
Sail Area (Main) 27 sqm 290 sqft (Genoa) 14.7 sqm 158 sqft
Disp (Empty) 1800kgs (Loaded) 2400kgs
Engine 8 - 9.9hp 4 stroke outboard Material List: [http://sailingcatamarans.com/satmatlist.htm]

Saturn Materials List
(Provisional)

Bulkheads 5 sheets 9mm ply 30m 3" x 1", 10m 2" x 1"
Hull 20 sheets 9mm ply 35m 2" x 1", 55m 1 ½" x 1" 4m 2" plank (Width varies and can be laminated from several narrower strips) 20m 1" plank (Width varies and can be laminated from several narrower strips)
Interior 2 sheets 6mm ply 6 sheets 9mm ply 40m 2" x1"
Decking 6 sheets 6mm ply 5 sheets 9mm ply 60m 2" x 1"
Beams 3 sheets 9mm ply 8 off 5.5m 3" x 2"
Cockpit 5 sheets 9mm ply 40m 2" x 1"
Cuddy 8 sheets 9mm ply 2 sheets 6mm ply 40m 2" x 1", 3m 6" x 1"
Totals: 10 sheets 6mm ply 52 sheets 9mm ply
55m 1 ½" x 1" 240m 2" x 1" 30m 3" x 1" 45m 3" x 2"
Glass/epoxy 20kgs sheathing cloth 200g/sqm (min, 300g/sqm max) 15kgs 600g/sqm +/-45deg biaxial, 70kgs epoxy (option: 5kgs polyurethane glue can replace epoxy for above water glue joints, except beams must be epoxy)
NOTE: All plywood to be best quality marine grade, (gaboon (okume) preferred) All timber to be best quality Douglas fir, yellow cedar, sitka spruce or similar.
Timber sizes are nominal PAR (planed all round). Thus 2" x 1" has a finished, actual size approx 45mm x 20mm

Another material list from Woods is: http://sailingcatamarans.com/studyplans website/Surfsong/matlist.htm
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Any other similar info is welcome. Thanks again.
Regards
APP

 

APP, Gonzo has pretty much nailed it on the head. Scantlings are application specific and aren't "typical" for any general size vessel. This said there are assumptions that can be made about scantlings, once you've got a handle on the engineering side of things. For example you can increase frame spacing with appropriate changes in hull shell scantlings. The same could be said about the hull shell, whereas you could employ a 9 mm plywood skin with 2 layers of 400 G2M 45/45 biax applied on the inside, providing the same strength as a 13 mm plywood shell, but the modified 9 mm would have slightly less stiffness and slightly more penetration resistance for the same weight. A slight increase in number or molded dimension of the longitudinal stringers would compensate for stiffness loses, in this hull shell scantling change, though honestly, you need to look at scantlings rationally, to justify changes or modifications. I'm quite familiar with dimensional timber sizes, though a 1x2 is generally 20x40 mm depending on how aggressive the planning was. I've seen them as small as 18x37 and as large as 22x44, but most often 20x40.

Simply put, there aren't any "specifications/dimensions etc. using plywood for small boats" each is sized for the anticipated loads. Though some boats will seems similar at first blush, once you have a good look, it will approach engineering situations differently, addressing loading situations uniquely in a lot of cases, so there's no hard and fast rules that can apply. Again this in hand, there are assumptions that can be made, but not with your current level of understanding (no offense intended). An example might be that most 18' day sailors will employ 9 mm plywood sides on hard chine designs. Of course some might use 6 mm or 13 mm, but as a rule you can expect 9 mm for most wholesome, none performance dinghy designs. This grand rule doesn't really help much, but can be applied for comparative purposes.