Is this a valid theory?

[boatnut]

Recently we had some glass work done on our 18 as we found the typical rot in the lower part of the transom and transom to stringer attachment areas when we removed the engine and drive. We had a long-time glass guy working with us who has built and restored everything from high-performance offshore boats to sailboats over many years (starting in New Zeland). Our repairs consisted of replacing plywood coring in the transom and repairing (filling with epoxy) a few stringer areas mainly where they attached to the transom. We also added some epoxy to fill and redrill the engine mount locations to prevent any compression. Generally speaking our coring in the stringer and gusset/bulkhead areas seemed to be solid and in good shape. However our glass guru had an interesting and I think accurate assessment of the construction of our 1969 Donzi. And he said it was typical of some of the older offshore performance hulls he has worked on. He said they used a relatively thin coring for the stringers, side gussets, bulkheads and constructed this grid as sort of a mold to cover in glass and attach to the hull. It seemed simple pine or some low cost solid wood was used which would likely have less structural strength than a good quality plywood would have. However the glass that covers all of this is fairly thick resulting in the structural strength being more in the glass grid that results by glassing this wood grid. The wood kind of acts like a big mold for the glass work. His theory is that if all the wood suddenly disappeared, the glass grid that remains would be structurally capable of tying the hull, engine, etc together. He said he has seen examples of this kind of construction where the original core wood was pretty much gone and the remaining glass grid showed no sign of structural failure. Makes sense to me, any thoughts?

[gcarter]

Oh boy!
Should I step into this?
In theory, what you say is right. But Donzi didn't put enough glass into the skin of the stringers to benefit, so Donzi was using wood stringers for strength and covering the wood w/one course of roving. The combination of the two is barely adequate, but if either fails (the wood or the glass) the stringer is seriously weakened.
A stringer CAN be hollow, like a Carver motoryacht. But it has to be engineered to supply the strength needed all by itself.
Old Classics usually used 1" X 12" pine boards. If you're building in the same manner as Donzi, then that's the avenue to take. Here's the reason why, if you have the choice of a 1" X 12" solid, clear board, all the grain runs in the same longitudinal direction. In other words, it becomes a beam when edge bonded to the inner hull and covered w/glass. Because the grain is all running fore and aft, you benefit in strength.
If, instead, you use plywood cut into a plank the same size as the 1" X 12" piece of dimensional lumber, only 1/2 of the wood fibers are running in the fore and aft (longitudinal) direction and therefore the wood component of the composite stringer is only half as strong as the solid board.
If a stringer is engineered to be cored w/plywood, foam, Coosaboard, marshmallows, or whatever, then all (or nearly all) the stringers strength as a beam must come from the amount of glass, type of glass, and combinations of lay of glass in t he stringer design. If coring, or plywood is used, and laid the same as Donzi did it, the stringer will be significantly weakened.
Then there's also a difference in the resistance to rot between plywood and dimensional lumber. A board can only absorb moisture from the ends of the board, or holes drilled into the board. In fact the end grain of a board and holes are sealed w/penetrating epoxy or similar, the board won't rot, probably never.
But if plywood is used, there is end grain around the complete circumference of the proposed stringer, and all edges must be sealed.
We can talk about this all we want, but conclusions need to be based on legitimate arguments.

[gcarter]

The AMH boats (starting in '94) were considerably different from the earlier Classics. One of the biggest changes was to use plywood stringers. The stringer spacing was also wider to lower larger engines in the hull.
I have some pictures of an AMH boat undergoing repairs to the stringers. It appears the plywood is 2" thick, which supports my above argument. Also, the work was brought on by the fact the stringers were rotted.
I suppose if I were replacing existing older Classic stringers, I'd use a sandwich of two pieces of 3/4" thick Coosaboard bonded together, followed by a layup of at least triple the original layup schedule using +/- 45* bi-directional stitchmat.

[Scott Pearson]

And this is what happens when coring goes bad...

[MDonziM]

George is spot on based on my experience. That stringer cut in post #3, I have not seen that kind of "Layup" before on the boats I have redone. That skin of glass is not even enough for tabbing imo. Also, I think the stringer strength in terms of bowing/twisting becomes increasingly important with the length of the boat.

[boatnut]

Oh boy!
Should I step into this?

Glad you did George, lots of knowledge in this short thread. I accept and appreciate your explanation. The glass on our stringers etc. seem a little thicker than you describe but I am sure no two Donzis were built exactly the same back then. Fortunately from what we can tell by inspection and sounding most of our wood survived. It is unfortunate to realize that just a few more hours of careful work in sealing all the holes etc. that penetrated these transoms and stringers would have probably eliminated most if not all of the rot issues we all are repairing. I have removed deck hardware on Donzis where there was no apparent effort to seal anything properly. On my 22' Boston Whaler the opposite is true, they seem to be a true quality builder. Anyway considering the classic Donzi is approaching 50 years old and still being sold says it all for the quality of the design. Of course you can tell that by just looking at one. Ed