EPS Design Theories

XPS delams, isn’t it?

I believe XPS retains a lot of blowing agent, and is well known for delamination. Not sure if you’be heard of Greg Loehr but I believe he recommends EPS because of this.

hi soul, can you provide a link or a company name for this super light xps ?

this is what i’m looking for. for the moment i’ll go with eps but would be nice to have a watertight core even with a sandcomp.

the blowing agents, in europe are air or co2 b/c of enviremantel laws. ?? will this bubble ??

wasn’t there somebody who sold this dow stuff, maybe you can jump in.



It was the LD 80 -04 I thought I wanted the extra thickness, but ended up taking it out. Let’s keep the big board hi-performance thing too ourselves. I don’t want anyone to find out about how to catch more waves and still go straight up.

I like the PVC stringer because it has more memory, it won’t break. You can load up the board, so the epoxy is bending, the stringer is bending, and the foam is sort of bending. Using a wood stringer probably gives you quicker return, but used with epoxy which has great flex is going to fails catastrophicly when overloaded.

The issues I have isn’t with a board breaking in 2, it’s usually a buckle. It’s the foam that’s failing. The glass & stringer are in perfect shape, it’s just that I get a big ol crease in the bottom of the board, then that crease turns into a rail cut, then it’s time to fix.


You can load up the board, so the epoxy is bending, the stringer is bending, and the foam is sort of bending. Using a wood stringer probably gives you quicker return, but used with epoxy which has great flex is going to fails catastrophicly when overloaded.

Do you think it’s the case that a relatively stiff strip of wood with foam on either side trying to hold it in place and all encapsulated in resin is going to focus stress more owing to its grained structure, where PVC or especially a stringerless would be flexing throughout? That occurred to me after reading your post.

Not 100 years but close to 20. I used bondo because I could add it to an exsisting shape and get accurate results. Building a new board changes too many things all at once, even if you try to make two exactly the same. Plus you have to make an entire board to test one little thing. If you ever read Greenough’s stuff he always was altering exsisting shapes to get results. It’s the best way to work.

I was with Ed Angulo in Aruba and he and Rush Randall were talking about dimple design and Rush wanted Ed to shape one without some of the dimples to test. I said, “just fill em in with Bondo.” You should have seen their faces. One of those, “Oh f#@k, why didn’t I think of that long ago.”

What I found with tail rocker is that there are different ways to get different release off the tail and different ways to create drive. Everyone knows that straight lines create speed but generally everyone uses that only in overall rocker profiles not in confined areas of the rocker so much. I tried a number of alterations that confined changes to certain areas and got new and better results. Well, I thought so.

the purpose of a stringer is to distribute pressure applied at a point (or small area) along a greater linear distance. in the case of wood, a longer grain will tend to be more effective in achieving this.

I think resinhead is saying that if the stringer’s stiffness is not supported by the rest of the materials (e.g., they need to bend more before providing stiffness), then the use of a stringer could cause a board break more easily than the same board without the stringer. This sounds logical in theory, but I’m not saying I agree with it in practice. I’d think flex response of the materials would need to be really far off for this to apply, but what do I know? It wouldn’t be too hard to test - just expensive.


you outta try perimeter stringers…

hi greg, surfline has got a foto doku about j. carper. in one of those pictures you can see a board with parabolic wood/pvc stringer, but the interesting thing for me is that the pvc-stringer ends about 5-6" and the wood about 2 or 3 inches from the tail. this should give you a lot of flex and you’re able to keep a flat rocker for speed is this what you’re talking about?

one more question the board i’m going to build will be a fish with a deep swallow tail the idea is to use the thin out swallow like a cut away fin. what do you think about this approach?

i’m pretty new to this and appreciate every input



Yes, it can delam.

When I started laminating really airy and light I haven’t seen any delam’s. The pinholes let the air from the busted bubbles out.

When the bubbles start to burst it is like when EPS starts to pressure ding. A coarse sanding before glassing also lets a lot of air out.

There is no super light XPS, lowest weight I found is 1.75 lb/sqf or 28 kg/m3.



These two foams I work with.

Everybody should weigh the advantages and disadvantages for their self.


I’ve been thinking of how I can get a board surface that I can continually alter, to see the affects of small changes. I was thinking of a 1/2" thick sheet of plastic, but using Bondo is a great idea!

Greg, what about the weight? Adding a layer of bondo will also add weight, did you find that a problem?

Well, I’ve broken and buckled a few of Greg’s EPS w/PVC stringer short and long boards (and Sam’s). I switched to EPS w/wood stringers 'cause I just couldn’t continue to buy new ones, and besides, Greg isn’t around here much anymore. I haven’t buckled or broken the wood stringer boards yet. I think that the PVC has much more elasticity than the epoxy bound fiberglass cloth, and that is why I kept buckling and breaking them. Sam said it was just coincidence, but I couldn’t afford to find out. So, that glassing schedule for strength vs. flex for PVC seems key to me.


I like the PVC stringer because it has more memory, it won’t break. You can load up the board, so the epoxy is bending, the stringer is bending, and the foam is sort of bending.

I recall it was Hartley as your test pilot in those days. The tail I remember was the one where you added a little bondo right at the tail edge to form a cup, almost a concave right at the tail. If I remember correctly. I was thinking tail lift and gettiing a quicker launch as you caught the wave and the tail was lifted a bit quicker. Fond memories. sorry to digress.

I checked the file and it’s 6 megs. Sways won’t let me upload such a large file, sorry.

I’ve ridden EPS with laminated wood stringers (two different thicknesses), fiberglass stringers, and PVC foam stringers. The only one that ever buckled was one with a fiberglass stringer. Coincidentally, that board felt the stiffest of all. The only explanations for the failure I can think of is that either the fiberglass stringer is the weakest material, or the fiberglass stringer is the stiffest material, failing rather than flexing. Putting together the two observations (stiffest board, AND the only failure) I’d have to say that fiberglass stringers have the least amount of flex (which translates into weakness) and that with an already stiffer foam, the better materials would be those that flex more - up to a point.


not to muddy the waters here but as they say there’s many ways to skin a cat.

Bert and Greg may be gods to S-Lockers here when it comes to composite and EPS boards, but there are good folk out there doing some incredible things that just don’t see the need for the limelight eventhough their solution maybe light years ahead of what the popular phenomena has latched on too.

This humble guy on the northshore has been doing this special thing behind the scenes since the 90’s and knows a thing or two about composite structures, chemicals, and most importantly sourcing the right materials for what he’s trying to do…

I was thinking about visiting him again regarding another board possibly a custom when I realized he’s been talking about all this stuff on his website for over half a decade already. I guess nobody noticed or hois solution was just too out there to be considered serious…

My only comment, is that if you are open minded enough you will discover many treasures that most pass by…

The core

The skin

The story



Surflight Hawaii was formed in 1996 by Jim Richardson to develop and improve a technology for making an explosive and fast flexing surfboard. What we do is put controlled and engineered flex and spring into our boards. By controlled and engineered, we mean that our boards are specifically designed to flex just the right amount in the right place, spring quickly back, and resist twisting. The flexing qualities are matched to the weight of the rider and the size and type of board.

So what is the deal with flex?

Why is so it important to surfing really well, better than you ever have before?

Think of flex as adjustable rocker.

Most shapers agree that rocker is crucial, and can make or break that magic board. But most would also agree that different wave conditions call for different rocker. So we should have several boards in our quiver, possibly the same size, but with different rocker for different breaks. In fact, each wave, each turn and cutback, would benefit from a slightly different rocker; it would be better to have less rocker for driving through a section or getting speed to launch into the air. So the shaper must compromise, settling on the best overall rocker for the waves and style of the surfer.

Fortunately, water is pretty squishy. The surfer can compensate for lack of rocker by pushing harder to make those tight turns. By contrast, on snow and ice, snow boarders and skiers must have a flexible rocker to make different turns and adjust to changes in the slope. Rigid snow boards restrict turning and would be dangerous.


Do surfboards benefit from more flex?

We have learned working with some of the best surfers and shapers on the North Shore, that the answer is Yes.

But the flex and spring have to be just right, or the performance is worse, not better.

Remember water is squishy, and surfers need to generate speed in quick bursts to do vertical maneuvers. If the board has too rubbery a flex, bends in the wrong place, or worst of all, twists too much, it will slow down and feel dead. But if the board has just enough flex in the right place with a very quick reflex, you will be blown away by the springy and lively feel, the explosive speed you can now generate, and the way your board remains glued to the wave and under control even in the most extreme situations. The larger and more powerful the wave, the greater the degree of flex that is beneficial.

We are not the first to work with flex in surfboards. We have built on the insights and innovations of others before us. Flexible surfboards have been around for decades. Many of the late 1960’s long boards had stringer arrangements and scoops or bumps in the thickness flow to increase flex. George Greenough, Mike Tinkler, and Tom Morey worked with innovative flexible constructions in the 1960’s and '70’s. Since that time, others have continued to experiment with flex. Jim Richardson, inventor and founder of Surflight, began experimenting with alternative materials in the 1970’s, trying to put more flex into boards. The big problem was the materials. Today’s thin, lightly glassed conventional boards have some flex which quickly deteriorates with use and age.

Many surfers are aware of the importance of that little bit of flex and understand that it is the lack of flex that makes thicker, more heavily glassed boards feel dead. But these lightly glassed boards tend to snap in half and the bond between the glass and the foam rapidly breaks down, causing the boards to lose their springy feel. Putting even a reasonable amount of flex in these boards stretches the conventional technology beyond its limits.

Jim’s initial experiments convinced him that if you want the board to flex and not break or fall apart, you have to eliminate the brittle eggshell fiberglass skin on conventional boards.

Tom Morey’s flexible foam surfboards were inspirational, but lacked performance because of too much floppy bend, slow reflex, and way too much twist. They needed something inside to control the twist and give quicker reflex.


Around 1992, we made some major improvements and decided we had a viable technology.

What we did was make a “core” of composite materials that looks something like a large snow board; we then surrounded it with flexible foam, similar to body board foam but lighter and firmer. Think of the core as a large snow board or small surfboard inside of a custom shaped, flexible foam surfboard.

Surflight’s composite cores are combinations of carbon graphite and specially woven glass fibers that are laminated with epoxy resin under pressure and cured with heat so that the materials meld, performing like they are one thing rather than a collection of separate materials, similar to the way snow boards, skis, tennis rackets and jet fighter airplane wings are made.

The composite carbon/glass fiber core gives the board a very snappy flex and is difficult to break. The “torsion box” shape of the core recommended by our aerospace engineer consultant reduces twist and allows the board to flex the right amount in the right places.


Once the blank is complete with the core inside, the boards are custom hand shaped and finished, similar to conventional surfboards, but without the outer fiberglassing step. After shaping, our boards are coated with a tough, satin smooth urethane finish. FCS fin plugs or other compatible fin system boxes are sunk into the core. The result is a very smooth but resilient outer surface that resists dings and bounces off people and other boards. The boards are lighter than conventional fiberglass boards of similar strength.


What’s the bottom line?

Their quickness, powerful bursts of speed and incredible control open up a universe of new possibilities.

Just a few examples: the boards explode out of turns; the flex and spring make it easier to launch and land airs; if pulling into sick pits is your thing, especially Waimea shore break or the Wedge, these boards will hold in to the drop like radial tires and when you do eat it and get smacked by the board, you’ll paddle out for more. Flex also endows our boards with the control to ride a smaller board in larger waves; overall, flex can give a board a broader range of sizes and conditions.

oneula, i saw the web some weeks back during my “research” and it sound very exciting. bert’s metod of vacuum bagging is pretty simple if you figure out the small hidden traps, so even “joe or uzzi” can do it.

how about a foto thread how to build it without a aerospace ing.?


NJ, here is what I said about stringers about a year ago. Still makes sense to me. Probably.

"You can build a board that is short enough (low bending moment, low strain) and thick enough ( so that it doesn’t require a stringer to keep it from snapping. Generally, in sandwich construction, stiffness increases exponetentially with increasing thickness (7-fold when thickness is doubled, 37 fold when doubled again). How about a 5 inch thick board?

I think if you are talking 6 feet plus or minus and not less than 2.5 inches, stringers are just added weight, just lines for shapers, just decoration for the glasser.

Actually, someone here turned me on to a great little book: “Understanding Airfcraft Composite Construction”. there is an antecdote about hanging a 1000 pound weight on a chain and a bungee, each capable of carrying 600 pounds. They don’t hold 1200 total. First you have to connect the bungee and let it carry say 550 pounds, then connect the chain and continue to lower the weight. the bungee has to be given a lot of stretch to carry its share of the load , but the chain does not.

Apply this to composites. Some aircraft wings are made mostly with epoxy and E glass, but with carbon on the spar caps. Carbon is much stiffer that the glass, so the carbon willl carry nearly the entire spar cap load. If any glass is used in parallel with the stiffer carbon, the glass componet is essentially wasted because it will never experience enough strain ( stretch) to carry a useful load. the carbon gets nearly all the load. Same with stringers in many circumstances. Short boards don’t put enough strain on the stringer. Stringers are therefore just heavy decoration. Longer (How Long?) boards do strain the stringer by virture of more flex.

Flex is another issue. Some like more , some less, "

so… are you saying is that, depending on the glass schedule, and with thickness held constant, a foam and fiberglass shortboard’s flex characteristics won’t change by removing a stringer, if the glass schedule is heavy enough to flex less than the stringer? Or are you talking about stressing to the point of failure only?

If you’re talking about the former, an “eggshell” of fiberglass, with no foam or stringer, would have the same flex characteristics as a foam and fiberglass board with a stringer. If you’re talking about the latter, they would both fail at the same point.

I dont’ know what I’m saying. Ha.

No, actually, my point is not that stringers don’t have an effect on the flex characteristics, but that the glass and stringer, unless designed to work together (“how” is an imponderable for me), don’t add up to a stronger board. In fact, from my experience they seem to fail sequencially or at least separately and therefore don’t help each other and therefore, maybe you don’t need a stringer. At least in a SB. It would be a good thing if the glass and stringer worked together and failed at the same point. But, like the chain and bungee cord illustration, having two strong products in the same composite doesn’t necessarily add to a stronger composite.

I think LB’s are different because they flex more and engage both elements of strength. Probably.

I’ve sort of given up on stringers. At least stringers down the middle. Other threads tell the story.

The problem with all of this is it is just speculation. Some informed. Some not so much. I dunno. Several conversations going on at the same time. Whoever said “…just build it, surf it and report your results back here” said it best.

I need a beer.