I just read this entire thread. Please forgive my unsolicited thoughts.
I respect the plans as written and will make my Zip with solid timber frames, but if I had a stack of marine grade 3/4" ply or had a line on a good deal for some, I wouldn't hesitate to build it with ply frames.
Let me see if I can do some theoretical problem solving here.
Strength:If the solid timber is say..., 20% stronger (for the sake of argument), simply make the plywood frame proportionately wider from outer edge to inner edge. That would seem to solve that problem.
Endgrain won't hold fasteners as well:As for end grain and fasteners, there seems to me to be a very simple fix here--looking at the plans for my Zip reveals it. We use a solid piece of plywood for the transom, but we add solid timber as a 'frame' in this area. However, it is not really a 'frame' in that location as the transom is a solid 'bulkhead' if you will. Rather, it is just some solid wood screwed to the plywood for the purpose of holding some screws.
With that in mind, why not just use small scraps of mahogany screwed to the plywood frames next to the notches cut for the longitudinals. This gives you some solid material to screw the chine, sheer, keel, battens, etc into. This would seem to easily address the problem.
Endgrain soaking up moisture over time:If this is a concern, I would borrow a page from the stitch and glue process. I would add a small epoxy fillet to all areas in question. Would this not seal the area adequately?
I mean, we use plywood gussets near the bilge. The book indicates that we can even use a lower grade exterior ply for this. Why aren't we concerned with that ply soaking up water? If encapsulation works in this area, why would it not work on plywood frames?
I'm not advocating the economy of such a pursuit, because I simply don't know. But if a builder wants to do it, I'm convinced it can be done with excellent results. Sometimes, people just want to put their own spin on the process. I imagine that it's rewarding. Maybe it's a creative thing.
On the other hand, I may have no idea what I'm talking about.
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