Phillip wrote:Yes, use some 13mm MARINE ply [at least 200 x 200mm in size] seal it in whatever paint you like.
Use stainless steel bolts when you bolt the bracket on.
Make sure you use plenty of silaflex between the ply and the fibreglass at each bolt.
Goodluck.
SP, from experience, 200 x 200 is way too small. The idea of a backing board is to spread the load of the outboard' bracket's four mounting bolts. My backing board is almost the same shape as the transom, but about 200 mm smaller all the way around. It is covered with an epoxy resin and has been glued in place using epoxy glue. Essentially, it's become part of the interior structure, and it's important that it is strong and spreads the load so the force of the outboard surging around doesn't stress the fiberglass, which is thinner on the transom.
When I bought Dulcamara she had four small timber backing plates on the inside of the transom, and when under motor you could see the transom flexing like Rolf Harris' wobble board. It also meant that there is no a few hairline cracks in the gelcoat. With the new improved backing board, there is still a little flex, but over the whole area, not just on each bolt.
On the other hand, it may pay you to do a really good service on your diesel and make SURE it's going to be safe amd reliable, thats what I would do. If you had both engines, you may be the only Top Hat in existence to have two auxiliary power sources and it seems like a lot of work for a "just in case" scenario.