: I just got a response from Noah's marine saying polystyrene is lacking
: compression and sheer strength. However, they sell core materials, so I
: was hoping to hear from a more indepentent source about its compression
: and sheer properties.

As noted in my first post - if you plan on doing endos off waterfalls, or standing the kayak on it's end in surf - then polystyrene is insufficient for that purpose. Nomex is one product (of many) used in the marine industry for building. It is still a foam, but of much higher density (and price). I suppose you can also use balsa end grain as a core material ( Baltek ) but in the amounts needed for a kayak, and the quick bends required due to design, this wouldn't be a recommended material - it is much better for flat surfaces like decks, etc. - but unlike foam is very sensitive to moisture migration - especially around mouted fitting and hardware.

Originally, it sounded like the project was for a very inexpensive kayak, thus the foam strip suggestion.