That wood database site is very nice. Yep looks like birch will suck up water. I think Henry Ford was cheap and used birch as he owned the forest GM didnt use it for their woodies. I'll stick with the sapelle I used in the DR. Now what to use that birch for.......

Birch is not rot resistant.

I would think that if birch was used by auto makers to do the exterior trim on woodies, it would be suitable for the transom. I have used okoume plywood laminations for the rear pad and both sapele and ash for the inside with a lamination of ribbon mahogany plywood over the solid stock. They have both held up well. Keeping the boat out of direct sunlight and dry while stored will go a long way toward adding life to the parts. I have some surplus Okoume plywood in 3 and 4 mm if you need a bit.

Merky

Have you gone here... The Wood Database ? I've found this useful to compare various wood species characteristics. I'm thinking that birch is not going to be particularly durable, even if varnished and stored covered/inside. I hang on to wood that I find or am given too, and understand the temptation to use a piece for something because it's in the pile, just waiting to be made into something. There might be a better use for that birch though.

I recently inherited from a car club member some very nice solid birch rough sawn lumber. It was leftovers from a Model A woodie wagon restoration 25 years ago. Been in indoor storage the whole time. Not enough to do interior work but plenty to do a transom. It seems heavy and solid but not sure if the transom application is appropriate. Everytime I search birch and transom I get plywood which this is not. I figure for a trailer boat that rarely spends overnight in the water it should do with some Captains Varnish to seal. Bad idea or go for it? I just dont want to have to redo it in 5 years or waste some nice boards.