Ummm, interesting -
Now, i’m assuming you are trying to stiffen this up a bit. Not surprising, those boards are built really, really light. Foam and glass are absolutely the lightest thing they can get away with. Thin glass, just enough to keep the water out, foam that will dent if you give it an unkind look - it’s about as dense and as strong as the foam on a glass of beer. They are competition boards, most of them, not meanr to last or be used for a lot more than races, you practice on something else…
Using them a lot, knee paddling, as you have found the construction is so light that you squash the glass and foam, reinforcing them is a really good idea. Adding a layer or two of cloth, excellent, one layer stepped back from the other so you don’t gety any abrupt transitions in how stiff the board is, it will tend to break at those transitions, as if you had taken a three piece fishing rod, say, and replaced the middle section with a steel pipe.
So far, so good. And you want to stiffen up the board as a whole? Good. But stringers, bamboo or redwood or carbon or whatever in an already shaped, already glassed board? Not so good.
See, in order to put that stringer in, you have to cut through the glass on the deck, all the way through the foam and out the glass on the bottom. Then rout or otherwise cut the foam such that the stringer fits in perfectly, then use some sort of glue, then shape it to the contours of deck and bottom perfectly, then glass it well , perfectly,so everything is tied in nicely and contributes to the overall strength and stiffness.
And first, cutting through the glass. You just compromised the strength of the skin, which is the only real strength in this thing. When the board flexes, well, when that happens the skin will likely buckle and you’ll get what is called a sudden catastrophic failure. The board breaks like a bread stick.
Next, cutting through the foam and cutting it to fit the stringer and doing it perfectly. Good trick. In truth, doing it with hand tools, hand power tools and improvised jigs and fixtures would be hard. You wind up with something oversized, so you have to use filler, adding weight and weakness and worse.
Shaping it - you wind up with the wood even with the existing glass. And then you need to put several bands of cloth on, top and bottom, so it’s tied in to the existing skin as best you can do it. And again, in this very light board you’re either accomplishing nothing structural 'cos the foam has no real strength or if you get real lucky (if you want to call it luck) you made a really stiff spot like I mentioned above with the three piece fishing rod with the same breaking at the ends.
Or, most likely, you have something stiff surrounded by something flexy, not bonded all that well, so it breaks free and it’s sort of floating in there, making new and exciting problems. Not good.
Okay - what can you do? Well, to stiffen the board, you have to stiffen the skin. That beer-foam-weak foam core inside you can and should ignore. Don’t cut anything, in other words. You need to reinforce the skin. How you gonna do that?
Me, I’d try reinforcing with carbon fiber, carbon fiber tape with more strength longitudinally than side-side. Not terribly narrow, as with something real narrow you’ll find in flexing the stuff could compress and buckle the skin/existing glass with bad effects.Top and bottom, a little more on the bottom. One long band in the middle, shorter bands further towards the sides. Well beyond the knee wells, I’d go most of the length of the thing. Sand where they will go lightly so you get a good bond, , saturate the carbon well with the resin,squeegee well for your strongest lamination without excess weight, coat again afterwards to fill the weave, sand and polish carefully ( you don’t want to heat it too much in sanding and polishing) and you wind up with as good and as stiff a paddleboard as you can get without (really) starting over from scratch.
And it’s comparatively easy. Never a bad thing.
hope that’s of use
doc…