Using Seacast or similar with 5in" Tarnsom Increase

[FLexpat]

Please excuse me while I geek out here, just adding confusion of engineered structures without all the equations and tensile diagrams…
Rebar in concrete is for strength in tension because concrete is weak in tension but strong in compression. When it is in the middle of a concrete slab it really doesn’t help for flex. When a piece of concrete is designed for flex the rebar will be on the outer part, maybe an inch inside the concrete to protect it from the weather etc. That is why you see ‘cages’ of rebar in poured concrete columns.

A similar concept applies to reinforcing a pourable transom. The glass is there for tensile strength and should be on the outer edges; in the middle it just flexes. This is why a slab of Coosa just has a thin layer of glass embedded in both outer edges, not the middle (you can see it if you cut a piece).

A modern transom is cored construction with very high compressive strength (to keep bolts from crushing the core). The Elements of Boat Strength by Gerr has a very good discussion of cored construction – it is worth the read.

You can do what you want, but if I was doing this project I would lay up strong skins (scarfed to the old skins and finished smooth) around a temporary form of a core, pull the temp core and then pour Seacast in. A couple of additional layers of 1808 or 1708 could be added to the inside of the skins (covering the joint by about 3-4 inches) after the form was pulled. That way you would get a transom with construction similar to Coosa without having to completely remove one skin. Use peel ply on the layups. Since Seacrafts like the motor to run a bit high, I would make it either 26 or 31 inches, depending on what motor I was using.

Like others have said, practice on another part first; you don't want to realize you made a mistake by looking back at where the engine used to be. Good luck.