I have made a few perimeter stringer boards, and a high percentage of them are buckling. None of them have actually broken, the glass just buckles in between the stringers and the stringers stay intact. All of these boards are 2lb eps with 3x4oz deck and 2x4oz bottoms. Buckles have been on the deck and the bottom. Stringers I have used are wood and PVC.
The boards ride great - they have a dampening affect, giving a smoother ride - it is fun to tune the flex by using different kinds of stringers - the really bendy ones are incredible in the pocket! But it is a shame they are buckling. Same foam with same glass schedule stringerless or center stringer have not had the same fate (although one center stringer did completely break in half…)
I use to have problems with my boards buckling, but the problem has pretty much gone away. For me the key is to make the board stiffer in the area that they are buckling. I’ve done this with horizontal stringers, but I’ve had much better luck controlling the deck shape/rail angle. I only make longboards and I use way less glass than you do so I don’t think throwing more glass at the board will solve your problems.
We’ve had issues with the perimeter stringers, also. Heel dents compress the foam between the stringers and at some point get compressed enough that it causes the fiberglass at the seam of the stringers to either delaminate or rip. As you can imagine a heel dent close to the perimeter stringer - the stringer stays at it’s original height, the foam doesn’t until…the glass rips. Same holds true with different density foam rails - it doesn’t compress like the 2# EPS and so as the heel dents become more pronounced, the glass tears…for us, always along the seam and then inward towards the middle of the board at a right angle from the stringer.
We are trying a few things. One is to move the stringers further out towards the rail, another is a corecell skin on the deck and finally a carbon patch along the stringers overlapping the stringer/foam seam. I have no definitive results yet.
I used to ride PVC (center) stringers and broke and or buckled four boards in a row. Maybe coincidence, maybe I’m too big and put the board in the wrong place. I switched to wood stringers (center and perimeter) and haven’t broken one of those yet. I have also build two compsands with wood (multiple layers) perimeter stringers and have put them through the ringer and haven’t broken those yet.
As Greg and others have mentioned, thickness and glass schedule have a lot to do with it. I only ride eps/epoxy, if that matters.
yep - you got it! well not totally - one board is fairly thick relative to length - a fish. one buckled at the nose but it buckled in the same fashion as the ones under the front foot. i have buckled boards with pvc stringers AND wood stringers - on the deck AND on the bottom! I have not had any issues with splitting at the seams. They have been instant issues in shallow water tuubes - so i think it happens when they get slammed into the sand. I was thinking it had to do with the way the board flexes differently (which is also why they ride differently than center stringer or stringerless) and so it puts a concentrated strain on the foam/glass between the stringers.
it is interesting that others are seeing the same thing…but not as much in the compsands - the answer must be that the 2lb foam and fiberglass is not strong enough to handle these forces - need a higher density ‘skin’! i bet when the compsand fail, they still fail in the same way - anyone buckle or break a perimeter stringer compsand?
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Let me guess!
The buckled boards are thin
They buckle under the front foot dents or just ahead of the front foot.
I’ve never had any of my parabolic stringers buckle, and I’ve done them in PVC and laminate plywood. I have had a few PVC center stringer boards break, but that was because they were glassed lightweight and destin to fail.
But i don’t make parabolics in small boards, and I don’t use any thing besides warp or impact glass with epoxy. Most boards are 2 5/8- 2 7/8 and at least 7’10"-9’6", glassing is 2x 6oz impact top, 6 oz impact bottom.
oh yea - the other weird thing is on a couple of the buckles, the glass seperated from the foam making a ‘delam’ spot but the glass itself was not damaged, but the glass did crack right above the stringers…different flex points…doing funny things…at least the boards are not breaking completely, making them easier to repair…
Interesting. Like you said, perimetered boards flex a bit more and have more compression stresses in the skins under severe loads. Ironically, the middle of the board is one of the worst places for big deck dents. The reason some boards dont break at the dent is because in some cases the foot actually supports the board there, so some boards will break just ahead. However, big deck dents are weak spots.
You can do certain things to reduce the issue. Add more board thickness, stiffen your skins or add foot patches. Thickening the board will add very little weight and reduce flex real fast.
RR epoxy comes in various flex properties. For #2 eps in heavy waves, I would use RR 2000 with 464 or 466 deck. They wont be as light but in heavy waves light boards arent all that necessary. Not only will the skins be stiffened, the board will stiffen as well.
I would suspect that if some of those boards were pupes, some may have broken cleanly instead of just buckled. Buckles are not too hard to fix.
Most of my boards have strategic reinforcements on the front half and under the front foot. The most common failure facing a compsand is delam leading to weak areas prone to over flex and breaking. But I havent broken one so no data to support. Compsands are far stronger IMO, but much harder to make.
oh yea - the other weird thing is on a couple of the buckles, the glass seperated from the foam making a ‘delam’ spot but the glass itself was not damaged, but the glass did crack right above the stringers…different flex points…doing funny things…at least the boards are not breaking completely, making them easier to repair…
HA! We posted at the same time. Are you sure the glass seperated from the foam or the glass pulled the foam away from itself? I bet its the latter. There was a discussion on this recently. Do tell!!
To clarify - the skin (fiberglass plus thin top layer of foam) is pulling away from the core without actually damaging the glass in some instances - looks like a normal delam - not your typical buckle.
It would be interesting to know how a typical pu/pe or a stringerless or center stringer eps surfboard would have reacted to the same situation…
Of course more core thickness and skin strength (more glass or compsand) will make a stronger board…
I was thinking it had more to it than just ‘more’ flex, because my stringerless boards with same glass schedule and foam have not had this problem. It seems to me that the perimeter stringers are focusing or compounding the forces somehow. This is what i really wanted to get to the bottom of…
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Interesting. Like you said, perimetered boards flex a bit more and have more compression stresses in the skins under severe loads. Ironically, the middle of the board is one of the worst places for big deck dents. The reason some boards dont break at the dent is because in some cases the foot actually supports the board there, so some boards will break just ahead. However, big deck dents are weak spots.
You can do certain things to reduce the issue. Add more board thickness, stiffen your skins or add foot patches. Thickening the board will add very little weight and reduce flex real fast.
RR epoxy comes in various flex properties. For #2 eps in heavy waves, I would use RR 2000 with 464 or 466 deck. They wont be as light but in heavy waves light boards arent all that necessary. Not only will the skins be stiffened, the board will stiffen as well.
I would suspect that if some of those boards were pupes, some may have broken cleanly instead of just buckled. Buckles are not too hard to fix.
Most of my boards have strategic reinforcements on the front half and under the front foot. The most common failure facing a compsand is delam leading to weak areas prone to over flex and breaking. But I havent broken one so no data to support. Compsands are far stronger IMO, but much harder to make.
good stuff - the pics from the kayak website show what is happening - the buckles. one of the types shown is buckling outwards taking the bonded layer of the core with it - like a delam - i have never seen this from any of my surfboards before - they always buckle inward like the other photo. BUT what would really be great to get to the bottom of is - why is this happening on perimeter stringers but not stringerless or center stringers?
something much different is going on with these perimeter stringers! they seem to be concentrating the forces in a unique way…
from bow building: only 10% of the wood on the belly is in compression, and the outer 15% on the back is really under any tension, the middle part really doesn’t do much but dampen the flex and add resistance(i could have the %'s flipped). so a 2.5 inch board would only have the top 3/8" in compression, if the deck is domed the area that the force is distributed over a narrower area. all that energy focused in such a small area is going to begin those stress fractures, which are going to run around the board eventually.
This principle(i believe) is why people are able to design these incredibly super strong skinned boards with an incredibly weak and flimsy core. it is also the reason(in my opinion) that Bert was always touting wider, thinner, and flatter boards.
fire back
constructive criticism welcomed and encourage on all of my post EVER
BUT what would really be great to get to the bottom of is - why is this happening on perimeter stringers but not stringerless or center stringers?
Okay then good question! On your perimeter and non perimeter boards what is the core material and the rest of the construction schedule?
(Keep in mind that #2 eps is very different from light clark pu foam. pu foam has far more tensile strength. But that’s not where Im going…‘measuring’ board flex would reveal a lot. Btw, Bert liked thin wide because he felt normal dimensions felt like ‘boats’…also too thick compsand is way too stiff and feels like crap)
Well things can get complicated quickly with increasing info but my safe guess is that schedule results is a really light board with less then stiff skins that are prone to buckling.
The complicated parts are: the amount of resin used, the resin’s modulus, the style of glass, length to thickness ratios, whether the eps foam is good foam (fusion), whether the foam is truly 2 lb density (this varies a lot in mfg), the tensile strength of the foam, etc etc.
IMO, small thin hiperf boards (not compsand) have to have a certain amount of weight in them for realibility in powerfull surf. Boards under 6 pounds (w/o fins) are more prone to break than boards closer to 7lb. That extra 1 - 1.5 pounds is basically two more layers of glass which can make a world of difference. For me it would be 664 deck, 64 bottom.
If your core foam is good, then it would be safe for me to say that with that construction style, there’s simply not enough structure in the skins (esp under the feet) to be worthy of heavy waves. Also some surfers are more prone to breaking boards than others. I have friends that break boards almost yearly, I have only one broken board in 29 years. Ive surfed Mex pipe too, but not big, just a few feet overhead if that. The boards I took over there were heavy duty.
That is all good information BUT it does not explain why the perimeter stringers are buckling but not the stringerless ones.
Both constructions have 5 layers of glass on the rails - which would be much stronger than the deck or bottom so I can see how the flats would buckle but stop at the rails (I have seen many cases of this in normal center stringered boards as well). Maybe the stringers are introducing a weak point or stress concentration, initiating the breakage - that could maybe explain things BUT in all my cases, the stringers are still intact! Maybe they are introducing a stress concentration high enough to buckle the skin but not high enough to actually break the stringers???
What do you think? Could the perimeter stringers introduce higher stresses in the skins as opposed to stringerless? Or are the buckles on certain boards just random chance?
Craftee - thanks for your insight! Anyone else want to take a stab at this? GregLoehr - you seem to recognize what is going on here - any insight?
If you dont mind me jumping in here I have a few thoughts.
A center stringer is straight, so when it bends, it simply flexes, no twisting involved. A perimeter stringer is curved, so when it bends, it wants to twist also (same for higher density foam rails). Non-symmetry makes things want to twist, flexing and twisting are coupled. This twist could be loading the glass in more compression than it would normally be loading with, causing buckling under the same load that would not buckle the glass on a center stringer board. Also could explain the delam as the shear stress between the twisting stringer/foam and the glass would not exist with a center stringer board.
