I was going to knock the PVC stringer, but on further thought, they are more uniform than wood. And for that reason it is a good call for this experiment.
They have served well through the years, but all things considered, I’d with go wood on my keepers. These are obviously not keepers.
I am glad you have an environmental conscience. I agree that people should avoid making boards with pvc…UNLESS the boards longevity compensates for the toxicity. There was a good surfboard study done at Berkeley that demonstrated this phenomenon:
I don’t believe that an environmental assessment can be made on a new construction until a structural assessment is made. And this study is structural. Sometimes the seemingly more toxic material, is less toxic over it’s lifetime because it lasts forever. I don’t mean to say that it’s so complicated that we shouldn’t even try. We really need to be thinking about these things in their entirety as Schultz did from Berkeley.
Arguing against wood stringers:
Efficient structural designs are done such that all members of the structure have the same lifespan. All members fail roughly at the same time (unless it is a life critical structure like a building. You don’t want the whole thing to collapse at once). It doesn’t make sense to have an irreplaceable part of the structure degrade faster than the rest. Imagine if cars had chassis that failed after 5 years. You’d have to throw the whole car away!
That’s what wood stringers do to surfboards. Their lifespan is probably 1/4 that of the epoxy and 1/10 of glass. They are organic and degrade quickly with sun and water as they are meant to do in nature. I believe performance degradation is primarily caused by wood stringers. What’s worse is that they create stress concentrations on the laminate that reduce the life of the laminate. Essentially wood stringers cut short the lifespan of the other materials, which means we have to consume more of them.
I think wood stringers with wood skins makes sense, because then the whole structure degrades in unison. But I think if you are using polymer skins and polymer foam, you should also use polymer stringers. With that said, there are less toxic polymers foams that I could have used for the stringer. Unfortunately with the added weight of all the sensors, I needed the weight savings of pvc.
I do think about my impact on our environment and I am stoked that you do also.
…So do it without stringer and more fiber in good place. PVC stringer (and wood to) is only weight for nothing if you make good lam, no?
Well personally I agree with you, but so many people love stringers that I must concede.
In argument for stringers:
The top and bottom skins have to be equal and opposite in force. One in tension the other in compression. To balance each other they must be connected, otherwise the skins will freely slide past each other like a deck of cards. Let’s assume that the foam does nothing. Therefore the compressive skin forces must travel through the rails AND stringer to cancel out the tensile skin forces. If there is no stringer, the loads must travel all the way to the rails to achieve equilibrium. This is a roundabout load path. This means more energy is being put into shear. It would require less energy for a load on top of the stringer to be transferred through the stringer. Loads near the rail are more efficiently transferred through the rail. The shortest load path is generally the most efficient. By having a stringer (as opposed to no stringer) you cut the load path distance in half.
I don’t think I explained this to well, but Solid Mechanics textbooks cover it pretty thoroughly, if interested. Key words: shear flow, shear, i-beams, bending stresses
In short, a stringered board would have less damping because less energy is going into shear. Less damping equals livelier…sometimes.
Personally, I think the difference is negligible, not enough to justify using a stringer. My favorite boards have been stringerless.
I think so to now. I try lot of things and first i was disappointed with my stringerless because they bro(a?)ke. I found that they became more en more sloppy then brake, my PVC stringer board were better for durability. Now i make more uniform shell and it’s better. My springer wood deck seem to be durable to.
Now i think that thin strips of UD carbon on rails can give good durable feeling because of the long life dynamic stress resistance of HR carbon. But as carbon is stiffier it take load first so i try to not put it to far of the neutral fiber.
Sorry for my english, it’s to bad to explain what i think !
Getting off topic here, but I don’t use stringers anymore. Just glass and bamboo here and there. Unless it is a longboard then you have to use them because there is a law that says you have to.
Did boards in PU/PE with and without stringers … the boards were too flexible without stringers … lacked drive. When we went to EPS/Epoxy we found we didn’t need stringers … in fact they have little effect. We’ve done production studies on EPS/epoxy breakage strung vs. unstrung and there is little difference. Also did EPS/Epoxy vs. PU/PE breakage. EPS/epoxy had double the lifespan. Also did PU/PE vs. PU/Epoxy. Breakage was about the same on these two. Also did rail channels vs. none. Rail channels had less than 50% of the breakage of the boards without. Interesting results. Rail channels work and stringers don’t even though the perception is that stringers do so much.
Some other observations, higher density foam with less glass has better break strength than lower density foam with more glass (equal weight finished boards). But lighter foam with more glass is much harder to ding. Many broken boards begin to break from a ding. Flat weave is better for breakage than twist but twist dents less. Double bias fabric was always the big winner although it’s difficult to use and kind of heavy for most surfboards unless your bagging. Modern EPS with no seal has better break strength than the older stuff that had to be sealed. Board thickness is huge when it comes to break strength as well. Boards with a 3.5% thickness to length ratio or better tend to be very tough against breakage.
Bit of history … there is a perception that boards need stringers for strength and certainly for performance in PU/PE this is correct. So for years we had to use stringers in EPS/epoxy even though all they did was add weight. In early EPS, the foam leaked a good bit and we ended up with a cosmetic problem … the wood would bleed into the foam when it got wet. So we switched to PVC stringers which eliminated the cosmetic problem. When selling a product perception is something that always needs to be taken into account. Pain in the ass that in business you can’t simply make the best product you can.
So there are a million variables … production studies tend to take in all those variables. Tests like this one are great as indicators but we always found production studies to be the final decision maker. Great to see someone doing this kind of testing … we need better indicators.
Just made a donation, use it to take some pics and post them up! 
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Just made a donation, use it to take some pics and post them up! :)
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as requested by donor, thanks.
All samples were subjected to the same impact.
The left sample and two right samples are in the spreadsheet. Please refer to it for details. The middle left sample is not in the spreadsheet. It has 3 layers of 4oz e-glass with polyester resin. It should be noted that the W and E samples are stiffer than the 3 I 3(45) sample, yet the impact resistance is much worse! Stiffness and durability are not the same thing! The 12oz sample is about 100% stiffer than the 3 I 3(45). W ~ 20% stiffer and and E ~ 5% stiffer if I remember correctly.
Imagine how violent the impact is to penetrate through 3 layers of 4oz. The 3 I 3(45) only exhibits the slightest pressure ding...no cracking!
maybe better resolution?
I am not sure why the 12oz looks so much better than the E. In person, it is only slightly better...pretty much the same. Epoxy handles impacts way better than Polyester. Only the 12oz was made with polyester.
surfteach,
Thank you very much for the generous donation.
Interested in seeing the Surfblanks in the test. Also,
cudos to Marko for stepping up. Todd P is a good guy.
Shapers help other shapers!!!! Karma and all that stuff!
Keep up the good work..........
BKB
Ben
First off, thank you for your all your efforts to date.
You mentioned that the results were posted without units.
I’m assuming that the strength and displacement columns are the raw data for your analysis . Will you tell us what units correspond to these values?
The majority of the spreadsheet (stiffness/strength/toughness etc) is obviously related to the 3 point bend results and analysis .
I wonder if you can provide some additional insight to the failure modes for the various laminates. I realize ‘broke’ is usually a good enough indicator to suggest a legit design limit. I was mostly curious to hear if you filtered the data based on failure mode and/or repeated tests that failed in one mode over another?
In addition to sample geometry, you appear to use foam density and total sandwich weight as a control for most of the test. That said, this batch of data seems to be all about the layup
Was there any attempt to normalize the bend data based on the amount of fiber aligned with the principal bending stresses ?
Others have already mentioned design based on fiber alignment with stress’ of interest.
Can I assume that only the 0 degree fibers (or cos( fiber angle)) contribute to bending strength in this test?
If this is close, can you talk a bit more about how fiber alignment contributed to the impact test results. ?
i.e.
are the ‘isotropic’ laminates the most impact resistant?
did cracks show up in one particular direction relative to the fibers present?
Is there any particular advantage to placement of impact related fibers on top/bottom or middle of laminate?
But mostly, your thought /observations regarding the laminate considered most impact resistant..?
What were the magnitudes of the impacts or how were the tests conducted?
Regards,
-bill
exactly, on the rails is the only place carbon should be used. Keep it close to the neutral axis, unless you like stiff boards. Then you can put carbon anywhere.
Are you putting the carbon strips below the glass or on top?
We did a surfboard-to-kiteboard conversion and added carbon strips… tape, I think, over the lamination, over the stringer on both the top and bottom. The theory was to keep it lively, give it a bit more snap resistance, but take some of the flex out. Does that make sense?
I never rode the board, but the guy who does rides it as his go to waverider. Says it feels great.
Yes, if you want to reduce flex with carbon strips, then the best place to put it is over the stringers top and bottom.
First i lam bottom with cut lap, then i bag wwod skin on deck, then i put carbon strips and i lam deck, both cut lap on edge.
Sorry for my frenglish
Thank you Peter (or Robert?) for the donation!
Ben, I notice on the data sheet that the 2 pound EPS foam differs in stiffness between the 2 boards. One glassed with warp and the other not. Can I safely infer that warp glass will result in a stiffer board? (still thinking about my sail board glass schedule).
