rotted motor mount and plywood

[jreason]

Hello,
Looking for some advice.
1984 21' Cobalt, Mercruiser 350 inboard V8
Rotted motor mounts, cut one motor mount out, discovered the plywood under the motor mounts that stretched the length of the engine compartment (sandwiched between fiberglass) is also rotted.
I was thinking:
clean out all the bad rotted plywood
Use epoxy resin and install new marine grade plywood
rebuild motor mount box with marine plywood cut and epoxied together
scuff existing fiberglass and fiberglass over plywood, new motor mount, and over existing (original) fiberglass a few inches, to make good structure

Am I right, totally wrong, better ideas?
Because this is motor mounts, and the plywood is obviously some sort of structure, I want to make sure I do this correctly, and not have to do it again!!!

Thanks in advance!!
Jim

[Fuzz]

Your basic plan sounds good but I would do some more inspection first. Have you checked the transom over very close? How are the stringers? If either stringer or transom are rotten you have a whole higher level of work looking at you.
Good luck and welcome to our little group.

[jreason]

Thank you for the reply!

Stringers are hidden pretty well, so I have not checked on them, I will drill a sample and hope for the best!!.
Visually, the transom looks good, but I have not done a sample test on it.

Jim

[fallguy1000]

So, hard to comment without pictures; I can try.

Suppose the mount is made from 2 pieces of 3/4” ply and glassed over. You have 1.5” of rot to hog out. Then you will need to be real careful to bond in the replacement pieces. They will need to be prefit and precoated with epoxy resin and then you will need to peanut butter things and create some vent holes in the bottom of the glass that remains. If you don’t vent; you’ll have a hard time pushing new wood into an epoxy bedding.

[cape man]

I'm with Fuzz on concern that you may have more issues than you know. If that whole deck below the mounts is rotten, I'd check the stringers and transom before deciding what to do. I'm not familiar with that brand of boat, but if they used ply that wasn't sealed well elsewhere you may be looking at a much bigger repair.