Custom Bamboo Stringer

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.  

 

 

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…

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…

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

Hi Bill,

Nice. I would guess you’re thinking about a ‘standard section’ and measuring deflection?

doc… 

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.

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.

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.

 

In september '97, I had a 10 foot blank I intended to make a noserider from.

 

Then A hurricane swell came up and the longshore current was so strong I could not get to the peak where these 12 foot face peelers started peeling for hundreds of yards before closing out.

 

if only I had a longboard which could handle the bigger stuff.

 

That night I too that 10’+ blank and carved out a 9’3" x 22 x 3" pintail that a friend coined ‘The Guppy’

It worked well in the waves intended, but also worked pretty well in just about anything.  Since I have carved it from such a big thick clark blank, and it was my first Glass job, it was soft and weak and soon developed stress cracks, and super deep foot dents and the rails dinged easily with a triple 6 glass job.

by 2007 or so after periods of heavy use and then lots of rafter sitting, it  became a lender board, and I did not care if it broke.  One time I took it out as my favorite LB as in drydock, and I forgot the 9.5 inch fin I usually used in it, and I tried it with the 8 inch  fin, and the board felt amazing.  I felt like superman on it.

 

I decided I could not let the board break.

I had some 10 foot Western Red cedar planks,  not quite1/4 inch thick. One was 3.5" the other 5.5" wide.  I routed a 5.5 inch wide strip atop the stringer on teh hull, and along each side of the finbox, laid in another stringer, unequal length, tapering to zero and epoxied those in place.  I laid a 6 inch wide piece of cfiberglass cloth tape into the groove, laid the Saturated cedar plank over it and weighted it in place making sure the board rocker was not changing wth the weight.

 I routed a 3.5 wide strip over the stringer on the deck, laid in some 4 inch wide  fiberglass cloth tape  and laid the 3.5" wide plank on the deck and weighted it.

 

The deck heel and toe prints bothered me. After sanding the cedar mostly flush with the remaining deck,  I clamped my square to the board and routed shallow grooves though one layer of the 6 oz cloth, rail to rail and into these I pulled prewetted carbon fiber  unidirectional strips.  These pulled tight were raised above the depressions from my feet and knees.  I then laid 4 oz over the whole deck but not wrapping the rails.

 

The cedar plank on the hull just got levelled and glassed over with 4 oz, 2 inches either side of where ceda r ended and a new hotcoat.

The board gained about 5.5 lbs.

It was obviously going to be much stiffer

I feared it was no longer going to make me feel like superman.

 

My first wave on it after rebuilding it, I kicked out with total confidence,  laughing like a Maniac.

 

Many thousand waves ridden on it since. 

 

The areas where the stress cracks were on the hull, are turning brown, but I doubt with such a strong I-beam, with the sandwiched planks over/under the stringer, will allow it to break in half.  I’ve done many an underwater cartwheel on it,bearhugging it as I did not want to swim, or have the board hit me underwater.  I can feel it flex in such situations and if it were not reinforced in such a manner I bet most boards would have snapped in half while I was clamped to it doing underwater cartwheels. 

 

Lots of rail dings and deck patches have been added.  Where the rail to rail unidirectional carbon ended the fiberglass would get soft and split.   

 

The rail to rail carbon was intended at the time, two keep port and starboard together, just as much as it was to fill the heel and toe dents/ depressions in the soft and beginning to delam deck.  If I had to do it over the unidirectional carbon would be thinner and not routed through the top layer of glass and the hotcoat, but  honestly its held up amazingly well considering how poorly it was originally glassed, and how well it surfed/surfs considering my inexperience shaping at that time and the multitude of glaring shaping errors so obvious to me now.

 

Good luck keeping your board out of the landfill. 

I expect The Guppy to remain rideable long after I’m gone.

 


WR--  I really like the looks of that board.  Can you post a few more pics?  Different angle?

Lot’s of good valuable advice and info gained from this post and very pleased to have queried. Progress has been bit slow, though has finally reached good momentum.

Might post some pictures of progress (none taken as yet) as i continue and finish the prone board’s repair.

Lowel,

Too many of my Photos have been moved to an external hard drive, but I found these: