# Epoxy/styrofoam Buoyancy

Aloha, I have a polyester board that I am thinking about having copyied in Epoxy . And I am wondering about the difference in Buoyancy. I know that it depends on a few different things but is there any “basic” number /percentage for how much more buoyant the Epoxy-styrofoam board would be? Thanks…

From the archives…there’s probably more, too: http://www.swaylocks.com/cgi-bin/discussion/archive.cgi/read/12642

Subtract foam density in kg/m3 from 1000 (density of water) to get kilograms required to fully submerge 1m3 of foam.

I usually just make the boards about 1/8 inch thinner. That usually is close enough.

I have heard numbers like 30% difference, between styrofoam and polyfoam.

It’s based on density of the finished board. Lighter weight boards float more.

Greg, I could not agree with you more. All I was saying was that I heard that styrofoam, was about 30% more float, everything else being the same. I thought that is what was being asked? The difference between styrofoam and polyfoam?

Thanks to all … I feel like an idiot . I teach SCUBA I should know this . Gonna Try to get Greg L. to make this board as I live in S. Florida and have heard so much about his Epoxy boards … Once again thanks to all this is a great web site… Aloha Don J.

Hate to bring this up… but buoyancy is a function of water displacement (i.e. the volume of the surfboard) and the specific weight of water. As Archimedes said, the force acting upward on something submerged is equal to the weight of the displaced fluid, in this case, water. So… for two boards of the same volume, one poly and one epoxy, they have the same buoyant force acting on it. Now if the epoxy weighs less it will float better, if its sitting in the lineup by itself. I havn’t seen too many surfboards out in the lineup without a rider. Once you sit on that board, theres a new situation. What are you? … 160, 190, 220 lbs? Epoxy vs. poly means nothing… what are we talking about here, a pound or two? That’s the difference of stopping for a muffin before you go surf.

THANKS Good point … If I have this board copied Maybe I should just leave it the same thickness … I’m 190 and it floats good now… Trouble is pressure dings … see the post above about that

Archimedes didn’t surf! I do. I made a board out of a blank I got from Greg and glassed it with his epoxy ( with additive f ) using 2x4 glass on the bottom and 6+4 on the top. The final result was much lighter and stronger than my clark foam boards of similar contuction. If greg will copy your old board, or better yet, upgrade your board I think you will be happy with the results. Just my opinion. You know what they say about opinions!

Shape 2 identical boards off a computer. One EPS epoxy and one Clark with conventional glassing. Ride both and tell me which one is more boyant. To say equal volumes equals equal float is absurd(wow, lots of equals there!). Glass a cement board and tell me how it floats. aloha, TW

Think of it this way - the density of PU foam is about 40 kgm3 (I think) EPS is about 32 kgm3 (2 lb/cubic ft in counries still clinging to the system of the ancient British empire) If you had two cubic metre blocks of foam floating in water, One pu and one eps, the pu block would take a 960kg wieght to submerge it, the eps block would take a 968 kg wieght. Not that much in it. Like Greg said earlier 1/8 th of an inch thinner would give you equal bouyancy. As far as weight goes I don’t know how polysterene compares to polyurethane or how Epoxy compares to polyester. An Epoxy/eps board may be lighter as well as being slightly more bouyant than a pu/poly of the same volume.

hate to disagree but we had two identical boards, one poly one styro-exact dimensions all the way around.BIG difference in the way they floated/rode.It was obvious the styro needed to be thinner to get same feeling as poly board.Actually, I personally hated the feel/performance of the styro board.

My experience has been that the RESONANCE, the “feel” of polyurethane foam vs. EPS are quite different from each other… and that`s regardless of thickness and choice of resin/laminates. Not necessarily bad or good, but different. I wish an independent third party one would accurately measure and quantify the disparities in vibration/frequency among the materials commonly used to manufacture surfcraft. A side benefit of such data might put an end to some of the disagreement about polyurethane/polyester and EPS/epoxy.

It has to do with the differing densities between the two players, water and surfboard (Archimedes was THE MAN). Therefore there are differences between the two differing technologies because of dissimilar weights (densities). Molded epoxies add yet a third. There are also differences in resonance, longitudinal stiffness and panel stiffness. Everyone has to decide what their personal preferences are. I did a long time ago and have never looked back.

Glad to see another appreciative of the contributions of Archimedes; I love surfing though I’m certainly not the expert around here – I have however studied Fluid Mechanics for eight years or so now and feel confident in my knowledge. Sorry Tom W, but I think if you look back over my post you will find that I don’t say anything about equal volumes meaning equal float – buoyant force is not the same thing, float is the combination of buoyant forces, minus weight and any other external forces. My point is that I don’t think there is going to be much of a difference – because of buoyancy – in the two materials. Now flex, resonance, material (viscosity and effects on fluid interface), shape and glass – all will affect performance, of course. Let’s not forget that the displacement of weight in the board (i.e. weight evenly dispersed vs. on the perimeter) will significantly affect the feel of the board (i.e. the second moment of inertia). Now I sincerely believe that how much this affects your surfing and how different it feels are not the exact same thing. There is placebo here, and if your mind wants to believe that the different feel is inferior, it will feel that way. I guess I just find it a little unnecessary to trim the dimensions 1/8” just for the weight, I don’t think a pound is going to affect your surfing any more than the fact that the board is made of completely different materials, with different mechanical properties. Not to mention the fact that now your changing the dimensions. Cheers,

Rook, while I do agree with much of what was said I have to assert that a pound does indeed make a difference along with the many other aspects you mentioned. Instead of thinking about it as mearly a pound of weight think of it instead as what percentage of weight that pound is in relation to the weight of the board. Then ask Dennis Conner what performance difference that percentage would make in the weight of an America’s Cup racing hull. Or ask Dale Earnhart Jr. what difference that percentage would make in his car. Lightweight is a very important aspect of performance.

Rook, while I do agree with much of what was said I have to assert that a pound does indeed make a difference along with the many other aspects you mentioned. Instead of thinking about it as mearly a pound of weight think of it instead as what percentage of weight that pound is in relation to the weight of the board. Then ask Dennis Conner what performance difference that percentage would make in the weight of an America’s Cup racing hull. Or ask Dale Earnhart Jr. what difference that percentage would make in his car. Lightweight is a very important aspect of performance.

What floats better, my 10 lb or my 8 lb bowling ball? Are we for real here? It isn’t that complicated, weight and volume!! Although, why do I float better at the same weight with a beer belly than when I was in good shape. Oh yeah, I had less volume with the muscle. Sluggo