> If we decide to double the core thickness and leave the layup the same,
> then we could expect the panel strength to double. Ignoring things such as
> the modular ratio and assuming that the layup due to its higher modulus of
> elasticity takes most of the tension and compression forces in a stressed
> panel and that the core (the wood) transfers only shear stresses then the
> layups have equal and opposite forces and together form a couple (a pair
> of point loads separated by the core thickness plus a layup thickness).
> Thus if one doubles the core thickness the couple forces also nearly
> doubled and so for a given load layup stresses are cut in half and if
> layup stresses remain the same then the load can double.

I have been thinking about this post for a couple of days because, although it seems okay in theory, it doesn't match up with practical evidence.

What I think is this. If you double the thickness of the core you will add very little to the strength of the composite. You will probably double the strength of the "core" but cedar strips add little or no strength to the composite because they have very little structural strength to begin with.

In stripper construction the strips serve two purposes; one is a form over which we can lay our structural material (glass/epoxy), and two as a form of artistic expression for the builder.

If we laid all of our epoxy/glass on the outside of the form the boat would be just as strong! But it wouldn't be waterproof. If we laid up all of the epoxy/glass on the inside and then removed the strips the boat would be just as strong! But it wouldn't be pretty. So we put half inside and half outside we get everything we need from this method. However, George using 1/8 strips and claiming is boat is just as strong is "TRUE". The fact that George claims his boats are actually stronger means that he has figured out another property of composites that the rest of us overlooked or felt wasn't important.

The cedar strips are not a component of this composite. The composite is the glass/epoxy only, just as in a commercial fiberglass boat. Fiberglass boats are heavier for many reasons; gel coat, hardware (most homebuilders talk weight before rudders, foot pegs, rigging , etc., are installed.)and liability considerations.

So now the mystery is how to get more strength for the same weight in our layups.I think that George has found that multiple lightweight layups are stronger than single heavyweight layups.

Ian Johnston