if you’re layering 3/8"-1/4" thick balsa, bamboo, cedar or paulownia with glass under and multiples layers of glass over with 3/4" to 1" thick solid wood rails I don’t think you’ll or should get get too much pressure compression issues with the core.
The key with a good compsand is that the primary strength is in the shell and the solid rails and all the core should be doing is providing filler. If the thing was engineered properly in the first place like Eva said the core of a modern sandwhich surfboard would just be air(maybe aerogel instead) …
I think that’s why Bert’s been such a strong proponent of the super lightweight cores cause his focus is in making bullet proof prefab shells and tailoring the flex with shape which includes core/rail and skin thickness. And if built correctly what ever that process is, I don’t think the cores in bert’s designs ever feel any of the types of stresses that would normally dent or fracture a core.
The question is how can you flex a xps core without stressing it to either outgas or break. For me based on a little experience bending the stuff for rail bands, it seems that if you glue up thin sheets inorder to to create the needed core volume and still allow for some slippage between them with a flexible adhesive then the thin sheets will bend very easy without too much stress. Also if the core it small enough and concentrated along the center of the flex axis then it really won’t be bending all that much anyway which is what I think Surflight is doing by wrapping that thick polypropylene shell around the narrow xps core.
For home made boards where you could careless if your board breaks or bubbles up then why not play with it since its waterporoof. But I’m with GL in that there’s just too many great foam choices out there to be monkeying around with the stuff for any type of production work. That is unless you have real good process to prevent pissed off clients.
The guys I know who are buying the XTRs are not buying them to last forever they’re buying them for the ride and the fact that they’ll last a little longer than a PUPE. These guys seem more interested in board performance than how long a board lasts… If they did they would gravitate to a Surftech but they don’t like the ride of those boards. In fact one of my shortboard lineup budz initially made the jump to surftech and after a couple of months switched to a JS XTR and now a Lost SD2 XTR.
I’m old enough to know to listen to the guys with the most experience and with this compsand stuff from what I understand that’s Gary Young and if he tells us that he’s changed his compsand wood board technique to use XPS inplace of wood rails and he’s at a point where he doesn’t even have to glass them at this point, I’ve got to respect that. The stuff everyone’s pretty much showing and doing with wood compsands today he was doing back in the 70’s and that 's a long time to figure what works and what doesn’t…
Your solid corecel or a solid wood kite board is going to be strong.
But for pure strength there are other options like Nidacore, Foaming Aluminum. Nomex Honeycomb, Ceramics, Carbon and Basalt fabric all expensive options but a stronger end product than whats being used in the surfboard construction industry. Just depends on how much you want to spend and whether you can get the stuff. For a kiteboard project if you want super tough and long lasting then you can resin infuse nida-core over with balsa/paulownia with carbon or basalt cloth with this new nanoparticle-epoxy resin. Should be bullet proof…
I wonder how soon someone will figure out how to cast a blank mold out of foaming aluminum to use as a core.