What I can tell so far is that the O/B versions seemed to have a thicker inside skin than the I/Os - maybe to make up for the notch and lack of a full width cap. As long as the plywood was dry it is really stiff and strong. When the plywood gets wet both the stiffness and strength seem to crater. Adding rot, termites, carpenter ants makes it sooo much better.
Dry plywood is better than Coosa on every number. Wet plywood seems to be worse on stiffness. Wet plywood appears to fail more gracefully - incrementally. To get the Coosa to perform comparably to the plywood it needs a much thicker inner skin and all the through bolts need to be overbored and filled - just like what you would do to protect plywood. This gives the Coosa an advantage on the weight side but dry plywood will always be better in performance.
One of the big differences to me is that if the glass on a wood core ever lets water in - from screw holes or damage of whatever sort - it will need to be ripped out and replaced. Plywood wins on cost big time - unless you have to replace it again. I never want to have to do this again on this hull.
I also want to make it strong enough to handle a bracket if I change power a couple of years from now.
So here is my current plan for going to Coosa:
Pirate FishStrecher's idea of a garolite section around the drive.
Overbore and backfill all the through transom fittings/bolts with epoxy/glass fiber.
Add at least 2 layers of 10 oz cloth on the inside of the outer skin.
The inner skin is to be alternating layers of 1808 (0/90) and 1708 (+/-45) biax for a total of ~0.3 inches. According to Gerrs scantlings this works out to an equivalent solid layup of about .8" but the cored construction (ignoring Coosa properties) makes it much stiffer. Layup is in vacuum bag and targets 40% glass to epoxy resin. This still gets the transom thickness less than the 2.25" limit for installing the drive.
Terry - a 25 Bert w outboards sounds really nice - In a lot of ways I do miss my old Bertram 20 - the curved transom just looked nice to me too. Sceptre rides lots better though.
I heard that the guy that bought my Bertram lost it in Eluthera when Andrew hit.