Rocker differences with epoxy vs. polyester

Okay, I understand I’m likely opening a big debate after looking at my own results and then reading about mechanical properties of epoxy versus polyester resin.  I started off with asking myself if the differences between the flex characteristics of the two resin types would change the rocker profile on two identically shaped boards (with the same glass) if you wanted them to surf with similar dynamics.  I originally thought that epoxy was more flexible given the fact that I let a little epoxy dry in the bottom of a plastic cup alongside the same cup with about the same amount of dried polyester, and the epoxy had much more of a flexible tendency when popping and peeling the dried resin out of the cup.  The polyster seemed to be much more brittle when popping the dried resin out.  This led me to believe that the epoxy was more flexible, thus making a board more flexible.  Then I read in other posts that when applied over fiberglass, the epoxy model is about twice as stiff as polyester.  So that’s part of my confusion.

So I guess my question is two part. 1) Is an epoxy board typically stiffer than a poly board, and 2) does anyone change the rocker profile given the differences?

I’m still very new to all this, having built 17 boards to date.  Four of them were poly, the rest epoxy. I’ve not yet shaped two identical boards.  I’ve done similar boards in both epoxy and poly, but they were for two different people, and those two people are totally different in the way they surf and like their boards.

So, back to my original thought. If the two resins have different flex characteristics, could I take the more flexible of the two and shape a little less rocker into it making it paddle faster?  If it’s more flexible, the theory is that loaded up in a turn, the more flexible board would have increased rocker over the stiffer board, acheiving a scenario where you could build a board that paddles faster, but still turns well. I know there’s way more to the comparison than just flex characteristics, but if we could keep the discussion on rocker, that would be great.

Thanks in advance for the replies.

I’ve been down that road with exactly the same thoughts and curiosities.  I’d say that from my limited experiments that in theory what your are trying acomplish is understandable but by simply altering your rocker you probably won’t be able to reach your goal.

I think that by using variations of materials, placement of various things as carbon, and by using rail channels you can alter a boards flex characteristics to improve a boards ride quality but ultimately too little rocker or too much rocker will be just that.  The rate of flex rebound is what seems to be the dynamic factor that people are trying to quantify and control.  

Throw in the mix stringer material, rail shape, foam density, cloth types, lay-up shedule, concaves, lap widths, overall board length, rider weight, and on and on it is such a cosmopolitan mix of what a board is and how it rides.  

So to stick to your request the answer is no.  A slower rebound flex of an eps/epoxy board with less rocker than a comparable pu/pe with more rocker won’t yield a better riding board just a different riding board.  Now alter said rebound flex pattern with materials designed to work with epoxy and then you might be getting somewhere.  Now take one exact board shape and do 35 variations with materials you might get lucky and find a magic mix/formula.

Also minor tweaks of your rocker profile won’t be as important as the overall displacement of a given material for paddling.  You also need to consider inertia.  Some of my eps board float and paddle like nothing else.  Throw in a stong offshore wind with chop and they become a nightmare on take offs. 

The problem I had was that the gains vs expense formula wasn’t working out very well.

I firmly believe shape will always trump materials. 

Thanks for the reply. The one thing I forgot to mention is that I’ve been using epoxy over PU. I love shaping PU, and I love the durability of epoxy and S glass. I’m not a good enough surfer to notice the subtleties of what I’m asking, nor have I built boards for anyone so picky. The questions came for ideas rattling around in my head.

So is an PU/epoxy board stiffer or softer than a PU/PE board?