I am going to be making a board out of eps foam. I was thinking insted of having to cut the foam in half to put in a wood stringer. I could shape the board then cut a vee shaped grove (deck side) about 1/8 to 5/8" deep down the center(length wise) then when I glass it pour epoxy into the grove making a stringer…has anyone ever done this.
Make sure it has glass in it and do the same on the bottom. The part of the conventional stringer in the middle does nothing. Only the outer edges do anything, and then only in tension.
thanks Greg. so then I could make the groove(s) out near the rails then? maybe in about 3-1/2 from the outside of the rails and the same on the bottom? this board is going to be a 6’-8" egg tri-fin. is a stringer of any kind really needed with a epoxy board this size?. I will be using 6x6x4oz s cloth on the deck and 6 oz on the bottom…
Epoxy weights 2-3 times more than most medium to light woods. Adding glass adds even more weight. just my $.02 regards, Håvard 
Havard has a good point. But wood isn’t as strong as fiberglass in tension. Just don’t over-do with fiberglass to hold the weight down. In my earlier post, I was talking about the center of the stringer when placed in the conventional position. However, by placing them near the rails, you are describing something like “power rods”. Greg Loehr is an expert on those and there are probably others too who have used them successfully.
ive been building boards with stringers in the rails since the late eighties …i could never ride a conventional stringered board again ,stiffening your rail line gives way more drive out of turns after riding rail stringered boards it makes a normal board feel like a rubbery lifeless boogie board …and yes the previous comments on wood and glass are correct combining them in the right way will give good compression and tensile strength…i dont know if i interpreted the comment about not to much glass right???but basically you dont want to much resin but you do want glass …theres no strength in resin by itself the glass binds it together like steel mesh in concrete thus giving compression and tensile strength in one package …
I don’t see how you can say the middle of the stringer does nothing?? It connects the top and the bottom, and thus, the glass layer on the deck and the bottom… creating an I-beam. If you just cut V’s, that didn’t touch, you could get movement between the layers. I don’t know much about EPS and epoxy, and I know that they’re often made stringerless… regardless, I would have to guess that this design would result in more flex.
Personally I think the stringer being an I-beam is slightly off because if the bond to the stringer fails(the weakest point), it’s not an I-beam anymore. The main effect is stiffening up the board which contributes to less chance of breaking the board. All in all, you have to view the construction as a whole because if the glass or stringer or glass/stringer bond fails the board breaks. regards, Håvard
Bert has it on this one. Boards always start to break from the rail first. I snapped one at Sebastian Inlet, it had a second snap about 4" away that went only half way to the stringer. Greg Loehr has shown and talked about this for 20 years
so if I cut a veed groove near the rails do I need to do both the bottom and deck side? do I really need a stringer in the first. the board is going to be some where between 6’-5" and 7’-0". I have read here that some people are not using a stringer in their epoxy boards. for the deck I was thinking of using two 6oz s cloth and 4oz deck patch. the bottom either two 4oz or a single 6oz glass.
Havard, have you noticed that MOST broken boards have the stringer at the break deflected off to one side. During the actual instant of breakage, the foam is compressed to one side or the other and the stringer breaks sideways as much or more than vertically
9 times out of ten a board breaks in compression tending to crease and fold slowly ,when a board breaks in tension its like its been blown up just shattered with glass torn right off it ,if you want your eps core not to break work on better compression strength.as stated earlier fiberglass has way better tensile strength than compression …timber has better compression strength than fiberglass but not as good in tensile …when they do over hanging concrete floors is the steel mesh in the top or the bottom of the concrete its put in the top, taking advantage of both materials,there strongest assets used in combination… regards BERT…
To shape the rail channel use a 1" diameter wooden dowel (about 2" long) and glue 60 grit sand paper to it. Mark a rail line on the deck, both sides, as a guide for your rail channel tool. Depth of the channel should be about 3/8". When glassing push the glass in the channel and the excess resin out. Simple. Rob Olliges
Shipman… Better figure out someway to jig that EPS blank if you plan on shaping it without the stringer in it… The only boards that I have shaped without a stringer out of EPS were paddle boards… and it was a major pain in the ass dealing with blank flex, and getting the rocker to be correct in the finished board. I find that EPS likes to be a real pain in the ass, pretty much all of the time, so figure out some way to hold that blank still, and keep it from flexing like crazy. -Carl
Rail channels work as do power rods. IMHO the primary aspect in board breakage is the compression strength of the foam itself. Stringers, as presently done, mostly just add weight. Bert is right in stating that conventional stringers are placed in the wrong area of the board to effectively reduce breakage. Jim is basicly backing up this argument in stating that the board breaks from the rail first and that a center stringer colapses to the side under compression. Havard is right that often times the bond with the wood stringer is the weakest bond in the board and fails early under compression. Shipman is right that many are successfully building EPS boards without stringers without breakage issues, Clyde Beatty. Bert’s statement about rail stringers (which could/should be called parabolic stringers) is very forward in it’s vision. It also makes use of perimeter weighting which improves rail to rail transition and drive, as he stated. Standard stringer araingment is OLD technology. What is being suggested on this thread is well thought out and visionary. And I don’t see implementation being at all difficult. I’m pretty sure Jim could pull this off, no problem.
Greg (Loehr) - When you say parabolic stringers do you mean curved like the board outline only moved in from the rail a few inches?
Yes. A parabola is shaped like a parentheses. Hyperbola is curved reverse of that. Why skis and snowboards are called parabolic, I have no idea. They are hyperbolic.
hey thanks everyone for all the heplfull knowledge. very interesting stuff. I think I will do two boards(same shape)using EPS foam and do differant stringer setups and see what happens.
The parabolic sidecut of skies/snowboards is in reference to the piece of curve they are cut out of. In the old days this was just a piece of a circle(circular sidecut), nowadays the sidecut is creating using a ‘more complex’ curve(parabolic sidecut) since some genius came to the conclusions that the carves didn’t describe perfectly linked halfcircles… You will see the reference to circles when the describe the sidecut as having a radius(or equvalent radius for a parabolic sidecut). regards, Håvard
When glassing channels do you add any extra cloth to fill the channel or do you just let the resin fill in after pushing the deck or bottom cloth into the channel?