> Okay everybody, This might sound a little heretical, I have a theory that
> questions the very basis of our faith in wood/composite small boat
> construction but here goes:

> Is wood sandwiched between two layers of glass really the optimal method
> of construction for a mostly convex craft?

> Before you answer this, ponder these points:

> -Most tests of this type of construction are done with flat panels
> (usually 12" x 12"). There are very few places on the average
> woodstrip kayak that are that flat, except maybe the bottom and some areas
> near the bow and stern.

> -These panels work because the fiberglass on the back face is put in
> tension when a point load is directed at the front face.

> -On a convex surface, there has to be a great deal of compressive failure
> (and buckling) in the core and outer face before the inner surface sees
> much tensile stress.

> -Think about an egg. An eggshell's shape provides it with one of the
> greatest compressive strength-to-weight ratios of any object known. An egg
> is not made with sandwich construction; it has one structural layer.

> -Think of an egg with three layers: its original shell; inside that, a
> lightweight sandwich layer; and inside that, a layer that is very strong
> in tension (same tensile strength as the outside shell's compressive
> strength).

> If you put a big point load on the egg, the outer shell and sandwich layer
> will fail long before the inner layer. Now, wouldn't the inner layer have
> been more useful as an extra layer on the outside?

> Here's a marine surveyor, David Pascoe, and what he has to say about cored
> hulls on larger boats: http://www.yachtsurvey.com/HiTech.htm Obviously,
> we get a bit better adhesion between epoxied fiberglass and our softwood
> cores than the foam cores shown in the "horror photos", but...
> Look at the diagram showing the effects of stress and bending on cored
> panels.

> Here's another article by Pascoe:
> http://www.yachtsurvey.com/core_materials.htm In it, he says, "The
> other problem with coring a bottom has to do with the inability to
> calculate and estimate stress on complex shapes. It's easy enough to
> calculate stress on a flat panel, but change the contours of that panel,
> introduce the factors imposed by human error, and any benefit that might
> have been obtained by coring the bottom is long lost."

> I'm not trying to knock the accepted method of construction. Heck, I love
> wooden boats; I just got to thinking last night and wondering....

> I won't mind if anyone disproves my theory; I'll then be able to sleep
> better at night! :)

> Shawn

> P.S. I know what Nick's book says, and I know what Moore's book says. I
> just can't reconcile that information with my mind's eye view of a convex
> curved surface.

Shawn,

I read the articles and it seemed that he thought balsa was far superior to foam, I would think that cedar, redwood and pine are superior strength-wise to balsa. I think that okkoume is even stronger. The main thing to watch out for with wood cores is not to penetrate it by attaching things to it, if you do make sure the the wood is sealed with epoxy.

Pascoe also thought that "all wood" (not all wooden boats) boats tended to be very well built. I think that most of our construction methods are quite adequate, but they could always be improved upon.

Mike