Originally Posted by mitiempo
the only reason builders use plywood is that it is cheaper and is also faster than multiple layers of glass
Respectfully Brian, I think you're forgetting about about the intent of laminate construction being to provide two skins held at a fixed distance apart to provide MORE panel stiffness than the same amount of material as a single skin. The key to that stiffness lies in the compression strength of the core.
Also, it's not faster...it adds time because it has to be added as it's own step which disrupts the layup timing. Then it has to be flood-coated before the inner lams start going on.
Another take on gwp's suggestion: remember here that the OP wants to use West System because that's what he's got (or what he's most familiar with). Ignoring the cost of filling 1/2" void left by the core, the exotherm will at least cause the glass to print through the gel and will at worst cook the gel, permanently discoloring it.
With that in mind, the exotherm will also make a solid casting a tricky job for someone who is fairly new to it. While they add a ton of strength, fibers don't do much to dissipate the heat. Remember that the heat vs cure curve for epoxy is much different that poly based resins.
Also, given the temps he's likely to see over the next while, he will NOT get a sufficient cure to re-step the mast, re-rig, and sail the boat anytime soon. Personally, I wouldn't feel comfortable restepping the mast on a solid epoxy layup that hadn't progressed very far beyond initial cure. I think it's important to point out as well that the exotherm enables the initial cure. Sure, it may feel hard, but only time can fully cure epoxy.
I can't really think of any issues with a solid layup of epoxy and cloth, again as long as the exotherm is dealt with. A continuous layup (always adding the next layer while the previous is cool but still green) will give all the units a primary bond but should allow the heat to dissipate. Once you compare the added cost to the properties gained, I'd still go with plywood core.
Just different strokes I guess.
Plywood has more compression strength than solid wood in this application unless he wants to cut short pieces and use the endgrain similar to the way a butcher block is made (and the way the balsa is laid).
Plywood has more panel strength than solid wood, so will do a much better job of spreading the loading evenly all around the step. THAT is the primary reason it's used in decks and steps, where balsa is used in cabin sides and coamings. Core materials are an insignificant proportional cost in any well-found production boat. Balsa is easier (read:quicker) to work and lighter than ply.
Again, this is just what my experience has shown me, for right or wrong.