Getting to the Core of the matter
Observation: Flat areas are cored to reduce flexing. This would be like your cockpit sole and mine. Side decks can flex a little, too, as well as the foredeck.
Note that this is not specific to Ericson.
The problem the designer and builder had to overcome with flat surfaces is "having enough 'web' in the I-Beam". If you consider the top and the bottom of the cored section to be like the flat parts of an I-Beam, and the core (whether vertical grain balsa in the decks or the plywood I found under my pedistal) to be the "web" or the part that separates the top and bottom, it is that separation distance and firm attachment to top and bottom that provides the stiffness. Same idea, on a realy large scale, in those truss sections you put under the flooring in your house so it will not bounce up and down when you walk on it.
I suspect that, as per normal, Guy has it right that there may a lack of adhesion between the coring and top laminate (or bottom laminate). Sometimes a section of coring will be a bit starved for resin during layup.
Sidebar: If you are imagining this as happening on boats depending on cost, don't. The cheap boats may have starved areas from labor and material cost cutting; the expensive boats can have the problem from trying TOO hard to limit the amount of resin to what is needed for strength and wasting none on creating excess panel weight. My last boat was from respected builder, Hinterhoeller Yachts, and I found a small dry area near one side of the foredeck and several other small ones in the layup of the roving in the hull. All fixable and I did so.
Since our Ericson built 1988-layed up boat has no flex in the cockpit when I clump around with my 210#, I could speculate that your problem indeed might be cured with some epoxy injected in some sort of one inch square grid pattern. Fact is, that while I know someone that did this, I have not.
Can you perhaps provide a photo with the suspect area outlineed in blue tape?
Trouble shooting at long distance is tricky, even on the best day...
Advice given by strangers may be worth less than 2 cents, and YMMV.
Good luck,
Loren in PDX
'88 Olson 34
