In my opinion:
Compressive strength: yes the plywood helps more than foam.
Bending: Not likely. The extreme outer fibers take the brunt of the load. Outer fibers being the skins. The math gets complicated because of the different stiffness of the fiberglass versus the ply, the fiberglass might be quasi isotropic- it doesn't care which way the strain goes. But the plywood definitely cares and has different stiffness. And extreme outer fibers of ply aren't necessarily oriented correctly. (But it helps that the shear stiffness is higher than foams, generally.)
The higher (but variable with orientation and moisture and condition) shear stiffness of the the plywood generally is also what probably allows the lightweight inner skin, as the load is transferred to the inner skin better, when the plywood is dry.
So the math on a dry transom might work out fine as is. A foam core probably will need more laminate on the inner skin. But the foam should never loose its shear stiffness. Plywood will as it gets moist.
I can't say on pure tension. I think the tensions loads are usually a load that goes with bending.
Executive summary: More glass on the inner skin with a foam core. And no weight penalty and no rot.
Or: More glass on the inner skin with plywood. It will be overbuilt and heavier until the plywood gets damp. Then it will be less underbuilt. And even heavier.
In either case, I would be interested in a solid laminate around the I/O penetration.
Quote:
Originally Posted by FLexpat
Didn’t want to derail anyone’s rebuild thread but I want to make sure I understand the engineering behind the original SeaCraft transom as I design a layup to replace it…
When I disassembled the transom on my 23 Sceptre I/O, I found that the outer glass was ~1/4”, the plywood core was 2 layers of ¾” stapled together and apparently wrapped with thin cloth (8-10 oz?) along the bottom and sides (tape?), and the plywood assembly was glassed into the hull with woven roving – something like 24 or 36 oz. This created an inner skin of about 1/16” or .0625”. I didn’t dissect the exterior skin but it appears to be a combination of mat and roving. It was definitely a hand layup.
When I attempt to reverse engineer the design loads, it appears to me that the strength comes from the 2 layers of plywood rather than a sandwich or cored construction. The inner skin does an ok job of attaching the plywood to the hull along the edges but does not seem to do very much structurally other than form a weak horizontal tension member. The inner skin is marginally effective at sealing the plywood transom from water in the bilges, but mine was open at the top (so not a vertical tension member other than from adhesion to the plywood). If it didn’t have a zillion holes letting saltwater in, it might have still been a little dry.
The inner skin of .0625” is MUCH less than what you would expect from Gerr’s scantlings for cored construction; he indicates an inner skin of ~75% (or more) of the outer skin thickness. Following that, the inner skin should have been at least 3x thicker or about 3/16”.
So am I right in thinking that the original transom strength came primarily from the compressive, tensile, and flexural characteristics of the plywood rather than the ‘cored’ construction?
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