You know your J's.
My transom did have 2 layers of 3/4" and 1 layer of 1/2" as you mentioned. It was a perfect example to see how polyester resin does not adhere to wood very well- especially regular "construction grade" spruce plywood.... Each layer of plywood separated at the thin layer of glass that was supposedly used to bond each layer together. At best I would rate it at about a 70% bond. I would assume they didn't coat the wood beforehand so when you lay up the transom the wood absorbs the resin and there isn't much left to hold the layers together. The wood wasn't really rotten it was just soaking wet and man did it stink. It smelled like cleaning out the outhouse
I could see that each layer of glass trapped the moisture between each layer of wood. In short - it would never dry out like that. IMO the 2 layers of 3/4 marine ply are as strong or stronger than 3/4, 3/4, 1/2 crappy grade plywood.
Yes I am adding some knees to tie in the floor, transom and motor well. I was considering a motorwell delete - but I don't want to throw the wet ski rope and wet jackets and gloves on the floor so it makes sense to keep the motorwell to throw the wet stuff until I get back to the dock.
They used the same engineering thought process with the floor as they did with the transom. They layed down the first layer of 1/2" crap plywood, glassed over it and then layed down another layer of 1/2" crap plywood. I peeled the top layer of plywood right off the 2nd layer and there was water trapped in between the 2 layers. Again, there was nowhere for the water to go. They didn't coat the underside of the plywood so it just kept absorbing water and moisture like a sponge.
Instead of 2 layers of plywood, I am going to install a stringer down the centre to help stiffen up the bottom and just use 1 layer of 1/2" marine grade ply.
Should have some more pics by the weekend.