Hi Chaps.
First time poster from Cape Town, South Africa here.
Have a 14ft prone epoxy paddleboard where the knee well area has cracked through, because the epoxy laminate is simply too thin in this high stress area of the board.
In addition to an extra layer of fibreglass cloth and epoxy underneath the new foam knee pads, i would like to install 3 bamboo stringers running (parallel) vertically down the length of the board. The centreline stringer will be +-800mm in length, with the 2 shorter parallel equidistant stringers (L and R) being 500mm in length each - situated directly underneath the brand new 2 foam replacemebt kneepads once they're glued down in the final step.
Now for the question/advice part:
Will the following bamboo pictured below make a suitable stringer once it is routed in? The 20mm surface will run flush with the board once bonded and epoxied in.
Think this will work, though as newbie stringer installer just would like to get an additional experienced opinion before going ahead.
Many, many thanks in advance. >>
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..
Some excellent advice and suggestions here indeed, thank you so much for taking the time and trouble to explain here to the level and detail that you have.
Okay, so the stringer idea has been tossed neatly straight out of the window.
I do have some leftover biaxial carbon cloth sufficient to use as a template directly underneath the 2 new foam kneepads. Beforehand, will 'pick off' the existing paint off board using a stanley blade (the board has not been glassed as such) leaving a textured polyester cloth to be scuffed a little before applying the epoxy base coat, biax cloth, and epoxy pigment top coats.
The pads use up the vast majority (95%) of the top-deck width, so a longer central carbon tape 'stringer stripe' will nicely transition the stress load linearally and directionally, as per the well explained 3-piece iron centred fishing rod analogy.
In addition, will place epoxied carbon tape on the high-points of the boxy angular rails, to re-inforce them from going soft over time as these are frequent contact points, taking weight/pressure frequently when 'popping-up' transitioning from prone to knee paddling and vice-versa. Here will also remove the existing paint to get a structurally integrated and low(er) profile once they are complete.
Thanks a ton again, am definately on the right track here again now and truly appreciate all of the above invaluable advice.
Exactly. I do enjoy when somebody gets the idea and runs with it, makes my efforts worthwhile.
Don't hesitate if you have some ideas you want to bounce off us. That's what we're here for.
doc...
You might try adding a cork skin over a repaired deck and put a layer of FG "over" the cork skin. Cork has much higher compressive strength than foam, is lighter than FG with epoxy resin, is impact absorbing, does not soak up water and will increase deck skin thickness.
Just a "thought" to consider/explore for a potential composite sandwich deck skin...
My next experimental build tech for this summer is a cork deck skin over an XPS foam core, then cover with FG. May get ambitious and do the bottom with cork under glass too...
Swaylocks Surfboard Design Forum: thoughts & theories ... practical & theoretical
RAIL PROFILE http://bgboard.blogspot.com/2014/03/march-82014-afterr-seeing-recent.html
Interesting. Would you consider sandwiching the cork between a couple of layers of cloth laminate? You'd probably want to vacuum bag it but it would be wonderfully stiff for the weight.
doc...
Doc,
Yes. That is one of the experimental options...
Bill
Swaylocks Surfboard Design Forum: thoughts & theories ... practical & theoretical
RAIL PROFILE http://bgboard.blogspot.com/2014/03/march-82014-afterr-seeing-recent.html
Hi Bill,
Nice. I would guess you're thinking about a 'standard section' and measuring deflection?
doc...
My orginal interest in cork is/was its impact absorbing properties (compression and rebound) combined with decent compressive strength. And it' seems to be fairly cost effective.
The project is evolving -- haven't really thought much about testing.
Swaylocks Surfboard Design Forum: thoughts & theories ... practical & theoretical
RAIL PROFILE http://bgboard.blogspot.com/2014/03/march-82014-afterr-seeing-recent.html
Will you believe it, have +- 3 running square meters of cork lying around at home.
As there is no core foam or any other structural damage to the paddleboard, will save this for a different rainy day.
Get some high density foam 1/8" to 1/4" thick. I have 4' x 8' sheets in stock, Cut into deck side with long as possible pencil router bit to match the width of you high density foam. router some length wise cuts however long necessary. Place appropriate depth high density foam stringer in the routed out cuts with epoxy resin. When cured. sand flush to deck. for the foot traffic areas cut rail to rail direction half the depth of the nose to tail cuts. insert hight density foam as mention before and sand to deck shape again. Glass over repair. Cuts maybe 3-4 inches apart both directions. Board will fall apart around repair. Don't be shy to preemptively place with next build. I vac bag channel crossing pro boards for one time race use in hawaii. Super light. No breaks yet.
Vacuum bagging is the key for optimal strength when using any type glassing recipies.
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