Originally Posted by McNish
Thanks very much for the 8/10 info. The transom is now sandwiched between two pieces of ply making it around 60mm thick. Have since spoken to the seller and apparently the rot was quite localised, so I may strip back to the original transom with a bridging filler repair between two 3mm ally plates. I bought it in cheaply to see how I get on with an air deck.
That will merely camouflage the problem at hand until the base substrate has rotted away, leaving no base material to attach aluminum plate to.
I would recommend removing and replacing the transom with a new piece of marine plywood.