Roger Long wrote:

6 x 6 backing block x 638 = 22,968 pounds. That would lift this
particular boat, almost two of them, in fact.

My real question was whether the stuff appears to grab with close to
its potential strength on old fiberglass.

I'd certainly lay up a nice epoxy and glass backing panel in this area
if I could reach it. If 5200 holds even 60% of its book number on the
ground fiberglass, it's probably plenty strong though.

Roger,
If you can sand the fiberglass surface then clean with mineral spirits,
5200 will stick. I have used it several places to bond things to the
hull during my boat refurbishing.
krj