EPS Design Theories


I have a few burning eps design questions.

It seems to me that eps theory is really starting to have a life of its own (instead of just pu theory applied to eps), and I haven’t seen a thread that attacks this directly. I would love to hear your questions, as well as any input on the following:

If you hotwire your rocker flatter than desired and then bend the remaining rocker back into the eps blank when gluing up a stringer, would the “pre-stressed” blank translate to a slightly stiffer – dare I say more pu-esque response in turns?

Can a parabolic stringer effect be achieved with heavier glassing on the rails?

Does a flat eps tail respond like pu vee, since the extra flex in the tail essentially creates vee when you put it on rail?

Does an extra flexi eps tail lend itself to less tail rocker if additional tail rocker is “created” when you dig in with your back foot?



I would tend to think that the heavier glassing schedules commonly used with EPS (at least with the custom board builders around here) limits any potential flex differences. As for tail flex with the axis parallel to the stringer giving more Vee during a turn, you’re talking about a very small surface with a considerable amount of structural strength, so I doubt there’s any more flex going on in that respect than pu/pe.

All I know is the golden rule of EPS - go thinner, glass heavier.

Just my opinions here but …

Rocker in EPS needs to be cut not glued. The EPS blank is somewhat stiffer than PU and the stringer required to hold a rocker has to be more substantial. This makes the board even stiffer. As a result, IMHO stringers are not even a requirment in EPS blanks and if used should be minimal with cut, not glued, rockers.

Glassing heavier on the rail would indeed give you some of that effect.

All my shapes for many years have been concaves. EPS foam, again, is stiffer than PU.

For EPS I use a faster entry rocker because the buoyancy makes the board ride a bit higher. For tail rocker I use a very flat release behind a rocker break placed about 12 - 15 inches from the tail. My total rocker is similar to that of a standard PU just done a bit differently.

Also, because of buoyancy the board may be ridden smaller. One of the things people think their feeling when riding an EPS board for the first time is a difference in stiffness when mostly their feeling a difference in buoyancy. Makes the board feel harder to control. Reducing the buoyancy makes the board feel solid through turns.

Bookmarking this thread right NOW

Thanks GL

gday greg

although the extra glasssing on the rails feels similar wrt torsional stiffness.

flex patterns are entirely different imo.

high density rail material allows for a better distribution of sheer forces

everthing else i agreee with you

Hey, good to see Greg, it has been a while. 2lbEPS foam is a “snappier” foam than PU thats for sure, I had two identical scraps and while each bent the same when flexed, the EPS returned to it’s original while the PU just went limp like a wet noodle all the foam cells had been crushed on the compressed side.

mahalo for starting this

its the best thread we;'ve had around here for a long long long long time

This is the kind of stuff I wish we we’re talking about all the time.

So the question is once it moves from PU to EPS to Compsand then does the design paradigm need to change?

Bert seemed to think so

that the outline, foil and rocker needed to be adjusted to the benefits that the wood sandwich brought to the table.

Greg seems to think the differences are minor when it’s just a variance between EPS and PU but I tend to agree with Bert that maybe an outline variance and the use of thinned concaved decks with flat bottoms seem to benefit the EPS wood railed/sandwich type of builds. I still remember those concave bottomed super flip nose single fins Greg was making way back that I guess started the whole webber banana board concave bottom phenomena that eventually lead to Kelly’s super narrow thruster designs from Al. It’s funny cause BK was riding 16"-17" wide super mini guns with narrow tails back in the 70’s at sunset. It was a statement of managing more power with less board(tow-in?). We always seem to go the extremes and then come back to a more neutral position…

This is actually more of a question than a statement but I’m sure Mike(sabs), Paul(silly) and Dave(meecrafty) have alot of experience about what the benefits of the manipulation of flex along with the reduction of complex bottoms/rockers and perhaps a slight offset in the rail outline and deck profile can do to improve overall performance.

I really like this thread though

Thanks again

HI bernie

my opinion is entirely subjective and based on only 2 years of experiments

and about 20 boards built and developed.

but adding that to the collective opinions of other developers

we can have some pretty solid design ideas established

i dont really have a clear understanding on bottom concaves

or there purpose

but have found that the flat bottoms that flex seem alot faster than

flat bottoms that are stiff(or at least provide a consistently more enjoyable surf)

i feel compsands can carry more width/volume through the nose without any adverse effects

ive only built to 2#eps boards

and the lighter one that pressure dents

feels a lot more flexi and alive then the heavily glassed one

This is what is so cool about the EPS, people complain because of the beads and because it sucks water, but the advantages to a foam that has better memory are huge! Once glassed, PU will obviously spring back better than without glass, but it will wear out faster because of the cell type. I was thinking that EPS is better than XPS or XTR because of this, even though they may have “Better” flex properties, they will have the same issues as PU as they are both the same foam type (Correct me if this is wrong, but is the beaded foam closed cell and the other open cell, or is beaded different). Anyway, it seems that the construction used in EPS leans itself more towards a foam that won’t lose it’s shape as fast. (I have foams for my airplane that are either pu or xps type, and they flex, but as soon as it creases, it’s done). Anyway, do any of you get what I’m saying, or am I way off the mark?



Hey, good to see Greg, it has been a while. 2lbEPS foam is a “snappier” foam than PU thats for sure, I had two identical scraps and while each bent the same when flexed, the EPS returned to it’s original while the PU just went limp like a wet noodle all the foam cells had been crushed on the compressed side.

Brian Hinde gave me a short lesson on cloth and resin a few years ago. I’ve always tried to use his theories simply because it made a lot of sense to me. Bottom of the board was to control and provide flex. He recommended to use E or Warp on the bottoms for their better flex characteristics. The deck was primarily for compression resistance just so that you’d prevent delamination and breakage. Rail wrapping was to create the I-beam, which again was to control flex.

I always loved double bias biax for its flex; they just don’t make it any lighter than 9 oz. and it’s very hard to get in short pieces. I’m already thinking that a .58 oz. unidirectional Kevlar under 4 or 6 oz. warp might be the ticket for my next bottom lay-up to help promote the rebound of the EPS. If you go to http://www.acp-composites.com/ they have some pretty interesting materials that can be used to make a stronger lighter more flexible surfboard.

I also got a chance to see the cloth that a high tech Austrian snowboard Co. was using for their boards. It was by far the nicest cloth I’ve ever seen. Super flat and tight, it looked like fine linen fiberglass. I’ve tried to find their source but I can’t read German. It was some kind of super high tech Quadrax cloth that felt like it was maybe 6 or 8 oz.

i kinda disagree

i feel that the components should have similar characteristics

(ie deck and bottomskins should have similar scheduals)

that way

every thing moves together in harmony

and no component is stressed anymore than any other

Great thread, this is something that keeps my mind busy for long now.

I have been shaping in XPS only for 10 years now, so I don’t know the difference for PU.

I started disliking PU in my windsurfing days, it wears too much, the bubbles get crushed. For my first board, a windsurfer, I used XPS, bent and glued to fit the rocker. I prefer XPS over EPS because the closed cells retain their shape better than the open cells of EPS and I hate repairing dings (never do). The only drawback is when the pressurized cells break, they will cause delam’s, but a super light laminate with many holes lets the air out safely. For my surfboards I used the same technique. It gives the blank some extra stiffness against bending and makes shaping different because if you sand off the top layer the foam starts bending back creating nice concaves.

The biggest improvement in speed I got from two strands of carbon on the deck next to the stringer.

Carbon has bad tensile strength but good compression strength. It has a fast reaction time and springs back to the original position without oscillation and energy loss. When chop hits the nose, the board won’t swing and push the tail down in reaction and create more drag, but it takes the impacts and flexes back with no energy lost.

The flex from the twist creates a variable rocker, this is only needed in turns on one side of the board so longitudinal stiffness combined with twist is enough for me, the carbon also working as a spring to give the energy back I put in it. Thin rails make the board twist better. I think in PU these things can only work for a few weeks, because the foam will breakup and lose its characteristics.

This is a bit different from the regular theory, but please provide me with arguments so our next boards can all benefit from new techniques and ideas!!!



The main problem with pupe is that the riding charactersitics of lightweight boards (flex and feel) is very dependent on the foam (If I recall its about a 60/40 or 50/50 weight split). Unfortunately, pu foam fatigues and ages very quickly, so that early majic is lost quite fast with moderate to heavy use.

Im glad Greg is contributing again. One thing Greg forgot to mention was foil (he has mentioned it in the past). For my own purposes, I use taper down tail schemes for proper flex characterstics, similar to some of the clark Stewart longboard blanks. Its basically the opposite of what you commonly see in pupe shorties, a pushed forward foil. Also one can use a more evenly dist fore-aft thickness flow. (btw, Bernie my immediate goals are to get flexcompsand like performance from a vac-less construction technique. Ive grown to really hate vac baggin, almost as much as I hate finish sanding). There’s something to be said about balance too…a #1 core with heavy glass seems out of balance, just an opinion. I agree with Toesidereefer. However, while there are performance benefits to assym, they come at some structural/strength risks. Flat bottoms are good.

Rusty P had an interesting and noteworthy comment in the current TSJ fin article. He was discussing feedback he’s getting from a boat trip with lots of new eps 4 finners.

He mentioned specifically “surfboard specific” EPS blanks (not just hw blanks cut from wall insulation foam - proly meaning Marko), epoxy and cambered (vector) fin setups (quads) as a “quantum leap” in surfboards. Maybe thats true for production/for-profit boards (I told a bud recently that the best most advanced surfboards are the least profitable), but the quantum leap lies elsewhere IMO, theres just no money in it…perfect for the backyarder looking for a lift.

And yes, Im glad Greg also mentioned buoyancy…how long has Greg been building them, 20+ years? Its not just me who thinks so…Pioneers think so.

2 identical boards shaped, glassed, finned the same way. One Pu with wood stringer, one EPS 2lb with a PVC stringer. The EPS board will out perform the PuPe everytime. You don’t even have to make it thinner or change the rocker etc. Just shape it like you have been shaping all your boards.

I continue to let friends surf my now 3 time broken EPS 7’10", and the claim that it is the best board they have ridden. It’s just a regular surfboard shaped out of different materials. Nothing special in the design, and it is turning into a real beast.

Durability is a issue, you need more glass on an EPS, but if durability isn’t an issue, a lightweight EPS is a very fun way to surf, and it will allow the average surfer to do above average manuvers on a wave.

My small to medium wave boards are pretty much all EPS now.

Damn good post, craftee–I’ll come back to it againa after I make the pancakes

Part. interested in your “basically the reverse” comment on foil, and in your “evenly dist thickness flow”

Can you go into a tiny bit more?


If you want to check out some tight weave fiberglas cloth go to (thayercraft.com).

I am on my third $14 three yard sample pack.Some great deals to be had and strong light cloth.


Wow thanks for the link! That’s exactly what I was looking for! I’m having a hard time believing that basic 4 and 6 oz. E is maximizing the mechanical properties of the epoxy. There has got to be a better way that wouldn’t be an arm and a leg to use. Aside from all the mechanical properties it would just be nice to try to preserve the boards feel for as long as you can.

You can download a sample Excel software tool at Vector Lam Blue that has data built into the program that will take your lay-up and calculate cost and performance. It is engineer stuff and I haven’t spent the time learning it but it is an easy way to see the effects of different lay-up schedules with epoxy.

In regards to Concaves I had a shaper give me a simple explanation once on how concave can stiffen a board’s reaction to flex. Take a piece of paper and hold it flat. The first thing you notice is that it is like a noodle in that it bends and twists in almost any direction. Now curve the paper on each side just as you would in putting a concave on a board and see how is stiffens the paper.

We need to give one of those aerospace engineers a surf lesson and get him hooked. I’m sure that they have a supercomputer that could provide a pretty detailed analysis of what is going on while the board is being ridden. Funny thing though most people wouldn’t care they only want to know if their board is done yet?!

I love this stuff

it makes me so happy to read the words here…

newbies take note

this is really important stuff going on here

I hope everyone is soaking this in…

cause it’s getting beyond all the hype and BS

regarding the change occurring now…

the thing I’ve learned the most over the couple years and ten’s of thousands I’ve thrown away playing with all this new vacuum bagging stuff(cause it can be expensive if you don’t know what you’re doing), is that you really need to nail the design part of the art of shaping before you start monkeying around with drastic material or design changes. This isn’t something most of us can to get overnight although we all here dream about it. And that’s the illusion that something like swaylocks creates.

The thing that comes across when that light finally clicks off in the heads of of all us dreamers looking for surfboard nirvana, is the greatest success comes from the smallest amount of change and in keeping things simple versus trying to jump the universe with some crazy radical design change. To get where Bert and CMP are at today takes decades of small RD changes over a very long time. Same thing with any great shaper… They got the shaping thing down first before starting to mess with their designs too much…

Keep it simple and do two so you always have a control sample(like Resinhead did) to compare what’s going on…

that’s what Dave(Meecrafty) and Mike(Sabs) taught me with what they were willing to share.

The question that’s been bothering me the most is how the ego most of us have developed here thinking that our feable attempts will be that much of an improvement than working directly with a guy like Loehr, Brewer, Griffin, Arakawa, Merrick or any of a hundred others in a true surfer to shaper relationship to have them craft a board specifically design for our surfing style and breaks we surf…

Keep it simple and understand that most of this is for fun.

If you want to get to design nirvana overnight your best chance is to go to a seasoned pro, not an over night pro but an open minded one that’s seens all the fluff come and go and had their designs in the feet of many testers over many wave situations.

This is the best thread…

we’re finally getting back on track again and I love it…

Hey, when I go to Cal-Poly, I’ll hook you guys up with all the computer stuff, just tell me what you want. They’ve got fluid simulations, FEA, etc, so I’m sure we can find out some cool stuff.

Hi Toesidereefer,

Can you give me the link to the site where that Exell tool is,

Google can’t give it to me, just what I was always looking for

Thanks, Souly