Not knowing too much about the weights of each type of foam (EPS and clarkfoam)what would be the difference in a 9’0’ longboard made using EPS and Volan cloth and a standard clarkfoam 9’0’ glassed with volan. would the difference be signifigant.
Urethane blanks are about 2.75 puonds per cubic foot. IE 2.75ld density. EPS is readily available in 1 and 2lb densities.
EPS is available in 1#, 1.5# and 2# densities. They are the lightest foams for surfboards. After them follows Clark. Densities are Approx. 2.3 for ultra, 2.5 for super, 2.8 for blue and 3.1 for green. After that there is balsa and then redwood, and bass. Choose your density…
Greg, Are the EPS blanks (both you and JP talk about) stringerless or do you use stringers with them? If yes, are they used for strength or are they simply cosmetic? I have a Beatty Rocket Fish that doesn’t have a stringer and I wonder if it’s because as the blank gets longer (above some certain length) does tensile strength become an issue?
I’ve had boards with and without stringers. I know many don’t agree but I beleive stringers are a cosmetic apendage and Clydes boards are proof of this. As far as tensile strength, IMHO compression strength is the key when your thinking about boards. The tension side of composite structures rarely fails.
I have a lot of respect for Greg and what he’s doing to keep the industry (and this board) up to speed on tech stuff. That said, I refer to actual lab testing, which I know isn’t the same as real surfing, done by the Patagonia guys (Chouinard) several years ago. Their conclusions along with several snapped Surftechs I’ve seen keep me in favor of stringer(s). Patagonia also uses epoxy resin to glass their boards - maybe a belt and suspenders thing.
I agree with Greg, after seeing a number of snapped and buckled boards. The stringer, being less flexible than the foam, usually breaks/fractures and serves as a trigger point for glass failure. That’s when the board fails and breaks or buckles. The stringer’s three functions, far as I can see, are to change the initial stiffness of the blank ( though not the ultimate strength of the board ), to allow different blank rockers from a given molded foam blank and to serve as a centerline for the shaper. For that matter, when you have a very heavy stringer, well- just how strong is the foam-wood bond? That is, when you have the foam and the wood both flexing very differently, what happens? The foam-wood bond fails in shear and then the stringer becomes kinda irrelevant. The thing is, it’s a bitch to sell boards without stringers. The perception that the stringer adds a lot of strength is all but universal. doc…
“The foam-wood bond fails in shear and then the stringer becomes kinda irrelevant.” Not sure what you mean by “foam-wood bond fails”… are you saying that the foam separates from the stringer lengthwise? I’m thinking that a piece of wood as thick as the board (say 2-4" thick) even if it’s only 1/8" wide is going to add a fair amount of breakage resistance. Easy enough to prove - just take a couple of ribs after cutting the outline. Cut to size to make sure they’re the same dimensions. Slice one and add a stringer. Test 'em both by bridging across a couple of blocks and adding equal weights until they break. Chouinard did it with a little pressure dial to quantitate the results but for a simple stringer vs non-stringer test, it’s probably not necessary. Might be a fun Swayloholic’s Anonymous 2004 project. Basically try and replicate Chouinard’s experiment and test for validity? That’s how it’s done in the real world. If it can’t be replicated, it doesn’t count.
Uhmm… lets see. John, are you thinking slices across the width of a board or lengthwise? Glassed or not? To do this…well, and replicate how a board gets slammed out in the water as opposed to replicating the Chouinard/Point Blanks tests, you’d almost have to take a panel made up with a stringer in it, glass it and then drop a large bag of water ( or maybe a sandbag?) on it to simulate how a board gets smacked by the lip, so that as the board deformed under load the pressure would stay in contact with the surface. I don’t know if steadily adding weights, especially adding weights to an unglassed foam/wood structure, would really act like a board out in the impact zone, y’know? I’m sure a foam piece alone would deform more than a similar foam piece with stringer under the same load. Thing is, the foam bends more than the stringer, and the glass goes with the foam and that would tend to trigger delamination and structural failure, when the glass-foam bond fails. You sometimes see, in buckled boards, a kind of channel in the glass lamination along the stringer either side of the buckle where ( I think) the lamination has failed. Possibly, testing to destruction with heavier and heavier sandbags or water bags dropped from a given height until the test piece broke would show something like this before the complete failure of the test piece. Hmmm… the test piece might be a simple shaped item, with a rounded top and a flat bottom but straight sides? Like an extrusion in the shape of a letter 'D’with flat/squared off ends? I oughtta sketch out something… Leastwise… that’s my thinking on it. Dunno, though, it’d be interesting to find out. doc…
Doc - Let me know what ideas you come up with. I was thinking for consistency just cutting basic rectangular blocks. I think Chouinard used 2’ long X 2" thick X 24" wide. Maybe test some Clark and EPS foam. Glue up some with stringer leave stringer out on others. Maybe glass them all with different resins - i.e. epoxy, poly, vinylester. I’d like to try some of that Australian foam Peter Hosking makes too. Obviously, actual wave action is a variable that would be hard to replicate in a “lab.” I agree that a sudden slam by a heavy lip or a surfer landing an aerial maneuver adds stress that might break any board. I still remember an old magazine photo of Peter Cole holding a snapped longboard that had many thick stringers.
The thing you can’t replicate accurately in a lab is impact and that’s where boards break. We’ve done plenty of testing on compression, tensile and sheer but it all goes out the window because it generally comes down to impact on the entire sandwich which includes all of these together along with plenty of unmeasurablables. With all due espect to Yvon, his tests were therefore inconclusive. If they weren’t he wouldn’t be using Dow extruded foam right now. Because of it’s directional cell sructure that foam has the lowest break strength per weight of any.
Hi John, Yeah, designing a test like this is, at best, kinda tough. What’s going to behave like a wave and at the same time be something you can replicate on dry land with conditions you can measure/test. With more or less surfboard shaped ( in cross section ) items, though they’d be harder to make and definitely harder to test, you’d get the effect of the arched deck which would act as a stiffener for the structure. The square sided panels - well, they’d buckle a lot easier, I think. It might be a good idea to make the test pieces, oh, something like 12" wide and scaled down to 1 1/2" thick maximum, so you’d have something that would break with a relatively light impact…don’t want to be having to lift and drop a sandbag that weighs in at 200 lbs to break a panel, y’know? In addition, this would economise on materials and maybe allow the test panel sides to be made or roughed out on a wood shaper; quick and dirty but at the same time fairly consistent. Might be a good idea to make the test panels more like 3’ long, 12" wide and 1 1/2" thick, that would be, I’d imagine, closer to a board on the water with the lip coming down or Heavy Air Dude coming down with it to flat water. I think the 2’ x 2’ x 2" box-type panels Chouinard tested …well, there’s not a helluva lot of similarity to those and a surfboard in use, so I’d have to question how useful the test results are. 4 oz glassing might be ‘scaled down’ enough so that the effects of the glassing wouldn’t be exaggerated. It’d be nice to set up something, maybe just a quick and dirty kind of indicator using friction on a pivoting arm that came up under the thing, to see how much the test panels deflected under a given impact load. How much the thing would deform and return without breaking. If the things were made essentially with a straight bottom, that would be very good for finding out things about permanent deformation - I have a feeling that a stringered structure would be far more prone to permanent deformations than something that was just foam and glass. You mention different foams and resins; excellent idea. I think you’ve got a very good handle on it. I’ve just been bumping out a few ideas off the top of my head, and I know theres a number of working engineers who read all this, so their input would be far more useful than my meanderings… doc
Do you think that floating the test samples would make the test a little more realistic. Since it sounds like only the ends would be supported as opposed to the entire sample.
Yeah, it would be a much more realistic simulation, but it’d make the testing much more complex to do. Maybe that could be simulated to a certain extent by putting a little but fixed amount of ‘give’ in the end supports? Maybe, just tossing an idea in, something like a strip of the elastomer stuff they use for mountain bike fork springing? That stuff has the virues of being pretty consistent and being something you can get in several levels of ‘rebound’. Dunno- there’s a lot of brighter people than me on the forum, what do you think? doc… http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=41592
Re: deck and rail contours… No doubt they contribute but introduce too many variables for a simple experiment. So would glassing technique - i.e. full overlaps vs zippered rails, etc. Greg’s input on the “real” world is well taken but also introduce variables that I can’t see quantitatively or qualitatively applying to testing. I’ve seen 3’ “double up square tube” waves that would impact a board more severely than a 10’ “slow roller” wave. For GENERAL COMPARISON purposes a press with dial gauge should do the trick and foam planks just glassed top and bottom (exposed foam on all sides)would eliminate glassing technique variables. Dimensions wouldn’t be all that important as long as they were consistent. Wasn’t the original idea to test the breakage resistance of a STRINGERED blank vs NON-STRINGERED blank? If deck and rail contours apply, it should apply to either. Same with glassing technique. I’m not all that interested in comparing pros and cons of multiple layers over light weight core vs single layers over heavy core, etc. Just to keep it manageable, I’d be happy to test STRINGERED EPS glassed single 6 with epoxy vs NON-STRINGERED EPS glassed single 6 with epoxy. I’d also be happy to test STRINGERED Clark Foam glassed single 6 with poly vs NON-STRINGERED Clark Foam glassed single 6 with poly. Just for kicks, we could do the Clark foam with epoxy and/or Vinylester too. We could do multiple samples of each and average the results. Sound fair?
Greg, Do you have a website? I would like to order a board from you, are you making any Lis type fishes?
John, Comparing a 6 oz. laminate with a stringer will of course be stronger that a 6 oz. without. Compare a 6 oz. stringerless with a 4 oz. with a stringer. Weight wise that would be equal and fair. But I have already done that test. The stringerless won. Oh yea, laps matter A LOT. John F, No website yet but you can e-mail me @ .
I was just curious as I’ve often given the sales pitch re: double stringers on either side of a single box, multiple stringers in longboards, etc being less likely to break. Comments re: cosmetic appendages or even that stringers make a board more likely to break deserve comment and investigation.