seth, i agree with steve but you need
to dig back the core under the existing fiberglass ( which is what I think steve was saying) so that when filled, it supports the fiberglass deck and interior fiberglass liner. This is if it is a cored area on the deck or transom.You can dig out the core under the deck with a bent nail. If you dig out the core under the fiberglass even 1/4" is enough to support the stress of a new bolt to keep the core from crushing. 3/8th's is better and 1/2" would be the best.You need to clear the core from deck to liner, no old core can remain for this to handle the connection without any compression problems.That was alot, I am sure someone here will explain what I said, if you don't understand, please ask, better safe than ......r.w.landau