On Sat, 11 Jun 2005 21:10:47 -0700, Evan Gatehouse wrote:

Mac wrote:
I don't think the joints in the core NEED much strength. I mean, you can
use Styrofoam as a core, and that can be broken easily by hand. Also,
balsa core is not very strong laterally, but it is an excellent core
material (unless it gets wet).

You can use Styrofoam but you shouldn't.

With cores, the important thing is that it adhere well to the skin, and
that it have good compression strength. Butted plywood with, say, epoxy
filler at the joint, should make a good core.

With cores, the most important property is the shear
strength. Obviously if it doesn't stick to the skins it
isn't core... Transoms should have a higher compressive
strength - balsa is good, plywood is o.k., high density foam
(= 120 kg/m3 is ideal)

As long as it is not polystyrene? ;-)

But then I am not a mechanical or structural engineer or naval architect,
so take my opinion for what it is worth.

--Mac

I am a m.eng. and naval architect

Evan Gatehouse

Thanks for chiming in! It's always nice to have people who actually know
what they are talking about.

So, to come back to the OP's question, if a transom is originally
constructed with fiberglass skins and a plywood core, and some of the
plywood is rotten, would it be OK to replace the rotten parts with several
pieces of plywood butted together? Or would it be better to scarf in one
piece of plywood to replace the rotten area?

I guess another way of asking the question is, would butt joints in the
plywood core significantly weaken the transom?

I am thinking that they would not weaken the transom.

--Mac