I don’t know if anyone has tried it but I would like your thoughts.
If I shaped say a 7 foot board in EPS without a stringer and 3" thick. After shaping what if say a 2" deep slot was cut in the deck say 5.5 to 6 foot long where the stringer would run and a piece of wood, ply or plastic glued into it. Then seal the board and layup with polyester resin 1 x 6oz bottom 2.5 x 6oz on deck. Two smaller strips could be added either side of the main slot for extra strength say 4 foot long.
The EPS is really light and not very strong. Its basically only there because it’s easier than making a hollow board! But all your strength is in the glass (with a little in the stringer if you have one). If you’re doing a stringer at all, its easier & stronger to just cut it in for the whole board. If you hold it back from the last 9" or so on each end, I think you’ll break the nose off or hit something with the fin and break the tail off. Your glassing schedule is fine, but why the poly resin? Cost?
Keep in mind that poly resin is not flexible. But EPS is. EPS will flex & squeeze under your feet and epoxy resin is flexible enough to go with it and then both materials can return to their original shape. Unlike poly foam which, once crushed, stays crushed, and poly resin which does not like to flex - it pressure dings instead. The flex characteristics & materials match of EPS & epoxy seems to be very good. Might be worth the extra $40 for a 7’ board.
“Keep in mind that poly resin is not flexible. But EPS is. EPS will flex & squeeze under your feet and epoxy resin is flexible enough to go with it and then both materials can return to their original shape. Unlike poly foam which, once crushed, stays crushed, and poly resin which does not like to flex - it pressure dings instead”
your glassing depends on the density of the eps… if it a home depot foam i’d add more… also i would never use poly on eps. 1st you would have to have the eps sealed perfectly, second epoxy is stronger, more flexy, bonds better, dosen’t stink, and is all around a better product…
try cutting down to the bottom glass with like a router, end to end, then get some fiberglass thats rolled up and epoxy resin and stuff the “rope” in the crack you made, and seal it up with epoxy. It should be good.
Others have addressed the glass question, let me take a few shots at the stringer idea(s).
If you shape, then inlay a 1/2" x 1/2" stringer top and bottom, you’ll gain very little except weight. The two inlays will bend just like 1/2 by 1/2 sticks = a lot.
If, however, you not only split the board lengthwise, insert a thin vertical sheet of wood and bond the 1/2 by 1/2 to the top and bottom, you’ll have a I-Beam stringer, the best possible with reference to bending strength. That’s why steel beams are made in that shape. P.S. steel beams aren’t called I-beams any more, but “wide flange”.
Structurally, the 1/2 by 1/2 will carry the tension and compression loads, while the web or thin section carries the shear. If these concepts are beyond you, post further and the list members will surely get into them.
The classical analogy to this is bending a deck of cards while trying to keep the ends of the deck square with the top and bottom card. The topmost card on one side wants to stretch (it’s in tension), the bottom-most card wants to shrink (it’s in compression), and the cards in between want to slip a little relative to each other (shear stress). The stringer wants to place material where it will most effectively stiffen the board. It so happens that the most structurally efficient way to do this is to put the materials as far as possible from the card which doesn’t want to either stretch or shrink. Thus, the 1/2 x 1/2 “flanges” at the top and bottom surface, with the thin sheet or “web” between (terms from structural engineering).