[optiguy]

After having done 6 Seacraft 23 transoms in the past 2 years for customers, I think that plywood is still the best all round solution. The only thing we do different than most people is to mark the motor bolt holes before the exterior glass goes on and put in 4" diameter cores of Divinycell H100. If it was my own boat and cost was not an issue,I would use the H100 or H120 for the whole transom. As far as Nida core goes, it will work for a transom. But the glass skins will have to be thicker and there will be no advantage to using it. Also on the subject of saving weight. You always have to remember, if you lighten a boat from it's design weight, the ride changes. We have build boats very light compared to design weight for customes looking for speed, but to it always backfires when it comes to ride quality and handling. One Seacraft we modified by removing near 800 lbs rode like crap and pounded in 2-3 ft seas. It would also layover onto a hull flat if you did not use the tabs going straight. Now when it was loaded to go tuna fishing with full fuel and 1000lbs of ice. It behaved like a Seacraft should.
Just my 2 Cents.
Bill