Take an egg, empty it's liquid content, and then see how it holds up to compression loads. Hint, it isn't very good now. The egg (really any spherical shape) works well in compression provided that (a) the load is symetrical about the entire body), and (b) it has an absolutely perfect body or a core.

The marine study you found about boat hulls was very good. But I think you missed a key point. He was lambasting not cored hulls per sey, but the elimination of adequate skin strength for impact and abrasion. Thats why he mentions turning the hammer around. That key point is one that you the builder must decide. I know I'm rather hard on things, so I overbuilt my hull to be able to take hard sharp blows. There's pro's and con's to this. My boat is now rigid, and essentially brittle. So if I were to be supported at only the bow and stern, mine would be more likely to break then a flexible skin type kayak. Neither is inherently right or wrong, they simply are different, and are better suited for different conditions. That's why tuperware type kayaks do so well in whitewater conditions, they just go "boing" when they hit a rock, vs going "crack".

Yes, you are right about the improper application of flat plate modeling for strength. There's a whole lot of people that fall into that one, and many of them should darn well know better then to use flat plate modeling. It's completely wrong for determining curved panel loadings. So be it, it is still what is often used. If nothing else, it does at least give one some comparitive numbers to use between various building techniques. Not surprizingly, more glass equals more strength. That's about as shocking as bigger I beams being stronger then smaller ones. But hey.

I do disagree with what you seem to be saying about compound curves though. A kayak has a minimum curvature in a fore and aft direction. So little, that it can be reasonably modeled flat in this direction. So simple curved panel modeling is quite accurate.

You must also accurately model your own loading on the boat. Are you going to be bashing it into pointy rocks, or are you going to be loading it by wave action. The two are completely different in their application of forces, and the requirements of the body. This is why powerboats like Bayliner are perfectly adequate for recreational boaters, and sink if they are worked commercially. A Bayliner can comfortably handle wave action, but holes if it sees ice flows.