Original SeaCraft transom design ???

[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?