I am an engineer and worked in aircraft design for a number of years. You are mixing two very different problems.

1. Stiffness and strength: If you calculate the strength of the Cedar with fiberglass, you will see that the cedar has only a small influence on the strength. All bending loads are carried in tension and compression by the outer skin (fiberglass). Once you get close to the failure the bond to the cedar becomes important. If the fiberglass is able to seperate from the cedar core in compression, it will buckle and break, thus the laminate fails. If the bond is excellent you may see the layer in tension fail (depending on lay-up).

2. Impact strength is a completely different story and it is very difficult to predict. Your assumption is that your skin construction is superior. It will really depend on the bond between the core and fiberglass. If you achieve a better bond to the hardwood than to the cedar you may have a superior product. Set up a small experiment where you drop a hammer in a predictable fashion on you hardwood/fiberglass layup and then compare it to a cedar/ fiberglass layup. I expect that you see delamination between the fiberglass and wood before you see compression damage in either wood. The impact characteristics are also influenced by the overall stiffness of the sample (if you support the samples on the outside and drop the hammer in the middle, i.e. our kayak hits the rocks - "oh shit" story). To compare apples to apples you should use the same stiffness of the samples, i.e. the hardwood/fiberglass sample will be slightly thinner than the cedar/fiberglass sandwich. Let me know what you find.

Andreas

: Hey, David-
: Me too. I get really scary when I'm varnishing.
: My gut feel is that the likely cause of separation at the point of contact is
: more or less influenced by the compressive strength of the core material.
: If the core material is better able to resist compressive failure, either
: by avoidance (flex) or simply by being stronger in compression, then
: separtion won't happen. I seem to remember that Vaclav's site had a
: comparison of epoxies, and that as a rule slower curing epoxies retain
: greater flexability than faster curing ones. I think you are right on this
: point, though, and it is my area of least understanding. I do know that on
: the new boats, the deck (with the 4 oz glass) deflects noticably when
: pushed hard, but the 6 oz hull doesn't move much.
: So is the dacron weakened chemically, or is the skin weakened due to it's
: lessened ability to "give"? This is kind of what I'm trying to
: get to with the cubic function of the deflection in the half-thickness
: core.
: The I beam comparision is questionable on at least one point, IMHO. A typical
: I beam of steel is homogenous, and has the same strength in tension as in
: compression. Wood, on the other hand, is certainly not homogeneous, and is
: substantially stronger in tension than compression. I don't think the core
: of the sandwich (wood) is acting like the web of a steel beam to a great
: degree. Your point of the bonded epoxy is important, and I think
: complicates the analysis of what is going on there.
: Mike