Thanks for chiming in! It's always nice to have people who actually know
what they are talking about.

So, to come back to the OP's question, if a transom is originally
constructed with fiberglass skins and a plywood core, and some of the
plywood is rotten, would it be OK to replace the rotten parts with several
pieces of plywood butted together? Or would it be better to scarf in one
piece of plywood to replace the rotten area?

I guess another way of asking the question is, would butt joints in the
plywood core significantly weaken the transom?

I am thinking that they would not weaken the transom.

--Mac

If they are relying on the core for stiffness and strength,
then butt joints would be weaker. I wouldn't do it on my
boat for example.

I would cut off the outer skin near the transom corners,
chisel and then grind out all the plywood and rebuild the
transom

Evan Gatehouse