Does EPS Vs PU effect Board sizes ?

Discoweb1's picture
Discoweb1 started the topic in Friday, 19 Feb 2016 at 11:40am

Not sure if this is the right spot to ask this but I have been reading up a bit on EPS V PU foams but one question I have not yet found answered (although I am sure it has been somewhere) is can you ride an EPS board at a slightly lower size/ volume due to the weight difference between EPS foam and PU foam?

My thinking being that if the board is actually lighter, the bouyancy provided by the relevant size/volume of the board would be higher therefore you could have a smaller/lower volume board for the same bouyancy ?

Anyone got thoughts or experience in this ?

George

morg's picture
morg's picture
morg commented Friday, 19 Feb 2016 at 12:07pm

I like the lighter board, however haven't actually been able to go smaller. My understanding is that your overall buoyancy is determined by the surfboard's volume and the water displacement. Theoretically as the EPS board is slightly lighter than PU, then yes you could have a slightly lower size/volume board HOWEVER the reality we are only talking about grams, and not kilograms. A wet rashy, or too much wax on your board, or even an extra fin(s) eg quad vs tri probably weights more than the weight difference between EPS and PU foam.

stunet's picture
stunet's picture
stunet commented Friday, 19 Feb 2016 at 1:07pm

It's a valid question Disco Boy, but I've never been able to get to the bottom of it. However, I vaguely recall that it's not the 'lightness' that matters but the displacement....or something.

Yeah, like I said, I don't know.

Calling blindboy or Freeride!

50young's picture
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50young commented Friday, 19 Feb 2016 at 1:16pm
indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Friday, 19 Feb 2016 at 1:17pm

I agree with Morgan. maybe technically there is a more buoyancy in a EPS board but it's so small that you would never notice it if you shaped two identical boards.

It would be great if it was true id be able to ride boards of lesser volume, but reality is i can't :(

Just another EPS/Epoxy wife's tale.

stunet's picture
stunet's picture
stunet commented Friday, 19 Feb 2016 at 1:28pm

Shit, didn't realise Morgan explained it.

False alarm blindboy and Freeride!

wally's picture
wally's picture
wally commented Friday, 19 Feb 2016 at 1:53pm

Two identical shaped boards have identical buoyant force, irrespective of the materials used. They can displace the identical amount of water.
However, if one board is .5 of a kilo lighter, it will float a little higher.
The floating board just needs to sink enough to displace its weight in water. The lighter board does not have to sink as far.

However, once you put a 80 kilo person on a surfboard, then the effect of that .5 of a kilo weight difference becomes pretty minimal. About the same as if you decided not to have that burger for breakfast.

batfink's picture
batfink's picture
batfink commented Friday, 19 Feb 2016 at 2:01pm

That's all true, and I have done the same maths wally, but up and riding at speed the difference in weight of the board makes a definite difference to how it feels, I think.

A few of the shapers who use EPS exclusively suggest that you go a little less volume in EPS. My initial thoughts were incredulous, but I'm assuming chaos theory and that small differences in inputs can result in large differences in outputs (performance characteristics). I suspect that shapers are making that distinction based on feedback from customers and personal experience, and they would have a pretty good idea.

But my opinion is also hard to justify, in that I have never had exactly the same board, same dimensions, with different construction, and not many of us have, so possibly nobody knows. I've taken my cues from other people that I know who do shape boards, and assumed they have much greater knowledge than I do.

surfingbymyself's picture
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surfingbymyself commented Friday, 19 Feb 2016 at 2:05pm

I've long been curious about this, since I weigh nearly 100Kgs, and had an opinion which turned out to be wrong. I just did the maths and unless my assumptions are crazy, it would seem to make fark all difference.
I think the issue is total weight of board plus rider vs volume displaced.
So if I go through the maths for myself, using densities I found on a website about blanks, I get the following,
A PU blank at 35L will weigh 1.26KG (based on density of 36kg/m^3)
An EPS blank at 35L will weigh 840gm (based on density of 24kg/m^3)
I assume similar glassing weight (can someone tell me if this is valid?)
I assume board weight of around 4 kgs for standard shorty.
Me plus board is 104Kgs. Difference in board between PU and EPS (of around 400g) is less than .4% of total weight being supported by same displaced volume. As Morgan said, an extra fin, or a wettie vs boardies will have similar impact. Even more so, my weight varies by +-2kgs in a week.
If you scale it down to average surfer weight and volume, difference is even less.

Discoweb1's picture
Discoweb1's picture
Discoweb1 commented Friday, 19 Feb 2016 at 2:07pm

Thanks for the replies.

wally's picture
wally's picture
wally commented Friday, 19 Feb 2016 at 2:26pm

Batfink, I wonder if the difference felt is illusory.
But, in reality your weight is not evenly distributed over the length of the board. If you are standing on the back of the board, maybe the slight difference in the weight of the surfboard material sometimes has a small effect on the unweighted front of the board. That skittering.

Discoweb1's picture
Discoweb1's picture
Discoweb1 commented Friday, 19 Feb 2016 at 2:30pm

Thanks for the analysis surfingbymyself.

As a similarly proportioned surfer it would appear that if I attend to my morning ablutions prior to a surf that will influence my surfing buoyancy about as much as the difference between EPS & PU.

On the positive side, maybe a EPS board allows me to eat an extra burger now and then.

Thanks again.

50young's picture
50young's picture
50young commented Friday, 19 Feb 2016 at 2:56pm

great video by Bert Berg on volume, planing area etc, which all relate to your question and explains possibly BF post
https://vimeo.com/128338702

simba's picture
simba's picture
simba commented Saturday, 20 Feb 2016 at 8:09am

Firewire has a volume calculator that includes fitness and age.

simba

simba's picture
simba's picture
simba commented Saturday, 20 Feb 2016 at 8:12am

So does anyone think that eps isnt as good in bigger faster waves cause it seems to sit too much on the top of the water,i know they talk about this with mals just wondering if its the same in shortboards.

simba

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Saturday, 20 Feb 2016 at 8:22am

simba wrote: Firewire has a volume calculator that includes fitness and age.

IMO that thing is way way out id be ridding boards with an extra 10 even 15 litres if i went by that thing, i guess my EPS boards would then be sitting above the water. (which they currently don't)