Re: Cockpit drain question
If the cockpit sole is truly 4-5 inches above the (travelling) waterline then thru-transom drains may work well. It's actually quite rare for following seas to climb into the cockpit, but if the stern wave rises enough it's possible that you'll back flood that way (though that would already be happening if that was the case)
In any event you could add little flaps on the outside of the transom drains to discourage that. If you have the space and clear routing for draining out the back it may well be a viable option. Just make there there are no low spots in the system that will sit stagnant and grow weird biological cultures all year long.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)