Can we put the eps = extra float myth to rest once and for all?

General Discussion
1

Somebody, please (PLEASE!) correct me if I’m wrong.

Flotation is about how much water a board displaces.

Flotation is not about how much a board weighs.

Flotation doesn’t depend on foam weight (except in the most miniscule way).

Let me elaborate.

If I weigh 150 pounds, and my board weighs 8 pounds and displaces X pounds of water, then for the sake of arguement, lets say I’ll float with the water’s surface at the level of my belly button.

If I weigh 150 pounds, and my board only weighs 4 pounds but displaces the same amount of water, then the net difference of the system is only minus four pounds. In other words, instead of a fixed displacement’s positive boyancy floating 158#, it is floating 154#. That is only a 2.6% difference, and I’m not sure any of us could tell. Could you tell the difference in boyancy if your shaper shaved an extra 26 thousandths of an inch off your next blank? That’s a 2.6% difference

Of course, all the above is assuming the surfer weighs only 150 and the difference between boards is a full 4 pounds. With a 180 lb surfer comparing a 7 lb poly to a 6lb epoxy, the change becomes even more miniscule.

Taking the surfer out of the equation, positive boyancy equals the amount of water the foam displaces minus the weight of the foam itself. A cubic foot of 1lb eps would only have 1lb less positive boyancy than a cubic foot of air. I don’t have the actual figures for boyancy, but when we’re talking about a trying to submerge a cubic foot of air, that one pound doesn’t amount to much.

So, I have to ask, why do so many surfers, and even so many shapers, continue to believe that certain foams and certain weights of foam are “more boyant” than others?

As differences in blanks, I can understand stiffer, more flexible, lighter, less inertia, heavier, more responsive, less environmentally aware, more likely to piss off your neigbor, less likely to delaminate…

But I just don’t get “more boyant”.

As I said to begin with, please correct me if I’m wrong. This is an itch I really needed to scratch, for some time now.

2

I’ve always followed that logic myself, but kept quiet about it :slight_smile:

I think Surftechs corkiness is attributed to greater bouyancy because so few surfers really know how flex works. When you try to kick stall a Surftech board - sink the tail - it squirts around under your foot but won’t submerge the way a pu/pe will. Its widely assumed that this is due to increased bouyancy, but I think its due to extreme stiffness. Same thing with laying out a turn & trying to bury a rail. A pu/pe (or a properly engineered epoxy) will bend & conform to the shape of the wave a bit, while the Surftech will stay straight as a board & give the feeling of staying on top of the water. Corky.

Surftechs are also sold & talked about as ‘paddling machines’ or ‘lineup equalizers’ (trading on the insecurities of weak paddlers). But I think they paddle so well because you’re not losing any energy to flex. What’s more efficient to ride up hill, a rigid road bike with 1" wide tires or a full-suspension mountain bike with 2.2" tires? Assume you’re in a tough gear - 42/16 or so and out of the saddle. The road bike will transfer nearly all your leg energy into forward progress while the mountain bike eats up half that energy in compressing the suspension, mushing the tires, rolling resistance…you get the idea.

I think the bouyancy thing is a smokescreen to cover other, truer concepts which are much harder to quantify & understand.

The proof, to me, is my d-cell covered surfboard. Anyone remember what I said about it? “All the weight of a pu/pe and all the corkiness of a Surftech.” No Flex. Explains it all.

Edit: I liberally use “Surftech” but also, of course, mean Boardworks, South Point, etc…as well as a couple of my own flexless failures at sandwich boards which ended up surfing just exactly as well as waterlogged Surftechs with not so nice a finish :slight_smile: Others - the wood ones, really - surf just like I hoped they would…

3

I am a simple person who likes a board that floats me , and paddles well , because I want my back , lower back , shoulders and spine to support my surfing god willing into my eighties at least . So my boards tend to float me.

4

the downward force of the weight of your body over the board is not constant as you surf. it’s how the board reacts to these fluctuations that you really feel the difference. as for the board itself, the factor to be examined is density. EPS foam is far less dense than PU, and i can even feel a difference between a superblue and a classic clark blank…especially when i pop into a wave. also, as most of these little differences translate to the environment, many are exponential in their effect. while i believe things like the surfer’s style play a far greater role in the degree to which these factors will make a noticeable difference, it’s still something that i put some consideration into.

5

What’s EPS again? :slight_smile:

When did Styrofoam become EPS?

And Surfboard Blanks become Urethane?

Lifeguarding 101

Some body types float some sink…

women have an advantage… at least some women problem is it gets in the way of paddling a surfboard

That’s all I know about floatation

other than using that deadmans float I learned to survive a plane crash or sinking in the middle of the ocean…

What can I say… it’s Friday

Hope you and everyone else has a good one J…

Been alittle rough around here lately

6

I agree…

Surfer’s are more sensitive and perceptive than we give credit to…

Even the slightest increase in ease of paddling is noticeable, especially if it’s apparent during the first few strokes.

Ever ride a heavy board? I mean with normal volume as you regular board, but just added weight. Easily notice 2 lbs extra, mainly in response, paddling and surfing. Prolly will notice .5 lbs.

I’m not advocating heavy or light boards, just that it’s noticeable on all performance aspects.

7

Good thread. I agree with you 100% about the extra floatation of the lesser weight @ the same volume. The science says the eps/epoxy will only float slightly more, dependant on reduced weight at the same volume. Or the same relationship. It could not be anything else because the water is only coming into contact with the outside skin in both cases, right? It seems well defined and easy to understand.

Funny thing is, just holding a styro board in the shorebreak and pushing down on the board from different surfaces, (btm, deck, rail) it seems to float more than a styro should.(wth?)

So the only think I can throw out there for discussion is: Could it be the more air in the foam and the cavities between the foam that seems to make an eps stick float more than the science says it should? – If in fact it actually does float more than the displacement theory says it should. (???)

8

You can’t separate weight and water displacement. The two go hand in hand.

If you take 2 identical boards in shape, flex, everything except board “A” weighs 4 lbs and board “B” weighs 8 pounds than board “A” will need to displace less water to achieve equilibrium than board “B”. That means board “A” will ride just that bit higher in the water than board “B”.

And although you may not be able to tell the difference sitting on your board in the lineup, even a beginning surfer will be able to tell the difference once up riding. There is 2 reasons for this.

One, is drag. Water is 800 times denser than air which means even the slightest difference in water displacement will be magnified once you start moving. And the faster you go the more obvious the difference is (even if it is just 2.6% as in your example). But as Soulstice has pointed out there are many times while up riding that the riders weight on the board is minimal, which means the difference in water displacement isn’t 2.6% but maybe 30 or 40% and thus the difference in drag is magnified even more at these times. And it is at these times when even a beginning surfer will notice the difference.

The second reason you can tell the difference is because the surfboard covers a much larger distance on the wave than your body does. Your body although moving down the wave, is relatively stationary compared to the surfboard which you are constantly moving all over the face of the wave. So an extra 4 lbs out on the end of your surfboard will be more noticeable than that 4 lb cheesburger and fries you ate right before you got in the water and is sitting in your stomach.

So I will argue that with an identical board EPS does = more float. And a noticeable one at that.

9
Quote:

Some body types float some sink…

What can I say… it’s Friday

Hope you and everyone else has a good one J…

Been alittle rough around here lately

All I know about float is, wider people seem to float better. I sink like a rock, but that is just because my concrete head acts like an anchor.

Thought you might could use a chuckle,

Hope things start going better.

10

Ahmmmmm… see, once opon a time, I studied naval architecture and marine engineering. And it’s all about volume.

Lets say you went to somebody with a shaping machine, had them make two boards, identical except one was standard 3.7 or so lbs /cu ft polyurethane foam and one was 2 lb EPS. Glass both with the same weight glass and they have the same volume. Lets call that volume 50 liters, and do this mostly in metric units, like everything else it becomes easier.

And lets spec this out at one ( EPS) board weighs in at 3 kg, the other one at 4.

Ok, they will both displace 50 liters of water, and with sea water that’s something like 52 kg - call it about 115 lbs.

Huh? but it’s supposed to float 155 lbs?

Yeah, but it doesn’t, really. Legs in the water, they displace pretty much their own weight, so it’s just the part of the body that’s out of the water, call it everything from the waistline up.

And the difference in buoyancy? Around 1 kg - call it a liter, 1000 cubic centimeters, 2.2 and small change lbs, a volume a little less than a quart. How much would you have to add to the board thickness to give 'em identical float?

Well, lets call the board about 8’6" long, about 22 wide - it evens out. That’s roughly 2.5 meters (250 cm) by .5 meters ( 50 cm) . And it has an area of ( again, roughly) 1.25 square meters or 12,500 square centimeters.

Awright, so how much thickness to add? Well, you got 1000 cubic centimeters to add over 12,500 so it’s ( 1000/12,500) or .08 centimeters. About one millimeter. 1/25th of an inch.The thickness of a matchbook cover, more or less . Which is also how much higher or lower the board is gonna float by itself.

Ya know, I kinda doubt Phil Becker could shape to those tolerances, let alone the likes of me. I am not real sure that a shaping machine plus hand finishing is gonna come consistently that close.

Nor can we actually notice it. As was said before, there may be a difference in flex, in resonance, but it’s not the buoyancy.

hope that’s of use

doc…

11

Yes I agree Bart, and I’d add that while differences in bouyant force between boards cored with lower density foams and PU may seem small - when you are on a wave you are always on the edge of instability, so differnces in bouyancy are exagerated by the effects of hydroplaning. One of the things I have noticed about my compsand board is how different it feels when you hit the white water or do floaters. I seem to glide up on top of the white water rather than sink into it. My compsand board has less volume than the PU board I used to ride, so I don’t think its has that much extra bouyancy. White water has a lower density than unbroken water so I think the lower density of my board is what is making for the difference. When I do those kinds of moves I’m usually unweighting so the board is behaving more neutrally and its relative density to the white water is what is coming into play.

12

Ok, a couple of things… as the thread title says, the myth of extra buoyancy is just that, a myth. The difference in buoyancy is tiny.

First off, a board that is in use on a wave is not displacing water, it’s planing on top of water. The amount of surface area that’s in contact with the water is a function of overall weight ( board plus rider) and speed ( the faster you go, the less area ya need in the water ) .

You hear a lot of people calling a particular shape a displacement hull…and it’s wrong. No surfboard is a real displacement hull. A paddleboard made for long distance competition, yes. But no surfboard that worked ever was.

Unweight the board, there’s an immediate difference in how much board is in contact with the water, no matter what it’s made of. What will be different in how the board responds is how it flexes or doesn’t flex. That’s what you’re feeling.

Now, if you can see that a particular board has less volume, it has less buoyancy. Want to know exactly how much less? Ok, get a kid’s wading pool or something similar and drill a hole ner the top edge. Fill it full, then put your board in, and push the board completely underwater. Save the water that overflows. That’s the same as the volume of the board and it weighs the same amount as the weight it will float ( board plus rider) .

Don’t have a wading pool? Or it’s full of six-year-olds? Okay, swipe weights from the weightlifting set somebody always has and never uses and put them carefully on the deck of the board until it submerges . That’s the buoyancy too, 1 cubic foot of salt water weighs in at around 64 lbs. Fresh water is around 62 and change per cubic foot. That’s its density.

Try that with a couple of different boards - you may be surprised by what ya find.

White water - well, it’s water plus air, and it has a density too. Which surely varies with a lot of things. But it’s not that thick either in terms of density or in terms of how deep the layer of white water is.

So, what’s happening when ya hit it? You unweight, the weight that’s on each square inch of planing surface goes down and if it goes down enough you go across it, if not ya don’t. But that’s a matter of area of a planing hull, not displacement or buoyancy. Flexibility of the board probably comes into it too. Rail shape, bottom shape, rocker, that too. But board volume and buoyancy? Nope.

hope that’s of use

doc…

13

If there’s any doubt at all, Google Archimedes Principle and put it to rest. Ounce for ounce, cubic foot for cubic foot, it’s gonna float the same. Guaranteed.

14

I agree with Ozzy.

Not that I am an expert but I always thought that polystyrene floated better because of the larger air pockets in

the foam.

Doesn’t floatation have to do with gravity?

Helium balloons float in the air because helium is a lighter gas than other gases in the air.

Air is lighter than water and so a ball with air in it will float on the water.

So my reasoning is the more air pockets in the foam of your surfboard the more it will float.

Ok so now someone is going to bring up a submarine!!!

I’m only Guessing.

15

to save you all a trip…

Submarines: How They Work - Archimedes’ Principle

hehe…

Archimedes’ principle is the law of buoyancy. It states that “Any body partially or completely submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the body.” The weight of an object acts downward, and the buoyant force provided by the displaced fluid acts upward. If these two forces are equal, the object floats. Density is defined as weight per volume. If the density of an object exceeds the density of water, the object will sink.

Cheers,

Ausitn

16

I float better when I have gas… If I am planning on ridding a thinner non EPS, XTR, ESP, FFA or T1-11 board I will eat tons of frijoles before going out… A air tight wetsuit helps retain the bio methane to make for even better float. My new wetsiut is lined with anti melting foil to prevent a melt a down…

17
Quote:
So my reasoning is the more air pockets in the foam of your surfboard the more it will float.

only in that a board full of air or more air has less mass (weight) per unit of volume (displacement). It is the same arguemnt that 2 boards of eaqual size and different weight will float differently. The heavier board will sit deeper in the water than the lighter board. But at the same time the math has shown that it is a negible effect.

The air only effects the weight portion of the amount of weight the displacement can support side of the equation. It does not have anything todo with float in and of itself.

18

Doc,

I unweight when hitting whitewater on my PU board too so that aint it. my compsand board has a different feel for sure - it seems to fly over sections and I come down a lot lighter and with more speed. You are right, I can’t discount different flex patterns as a cause for the difference but the board is about a kilo lighter than the Pu board it has replaced, so I can’t discount that factor either.

Regarding the so called myth of bouyancy I’m really not sure what you guys are on about. If you take two boards of equal volume, the one with the lighter core will weigh less and therefore will be more bouyant ie it will take more force to submerge it - which means it will take more weight to sink the rail or stall the tail, you will notice the difference just as sure as you will notice a flat deck or a thicker nose.

Look at tow-in boards - Aussie tow-in boards have tubes that can be filled with buckshot to increase weight when going from oz waves to Hawaiian waves. The difference in wave period Au max 18 secs to Hawaiian 25 secs means the same two-in board will be too bouyant in Hawiian conditions ie the upward lift from water rushing up the face of the faster moving wave + the upward bouyant force of the board prevents the rider actually getting down the face fast enough.So the weight (and overall density) of the board is increased and therefore bouyancy is decreased by adding more buckshot. Now try telling those guys bouyancy doesn’t matter.

19

Gentlemen,

Archimedes P is true but does not tell the whole story…

AP has to do with water pressure (Pascal’s Principle) acting on the object, it quantifies the force from the water to the object, that is only ONE force, there are others . It doesnt say anything about the other forces at play, like gravity for instance and the related effects of density variation.

Equal Volumes Feel Equal Buoyant Forces

Suppose you had equal sized balls of cork, aluminum and lead, with respective specific gravities of 0.2, 2.7, and 11.3 . If the volume of each is 10 cubic centimeters then their masses are 2, 27, and 113 gm.

Each would displace 10 grams of water, yielding apparent masses of -8 (the cork would accelerate upward), 17 and 103 grams respectively.

The behavior of the three balls would certainly be different upon release from rest in the water. The cork would bob up, the aluminum would sink, and the lead would sink more rapidly. But the buoyant force on each is the same because of identical pressure environments and equal water displacement. The difference in behavior comes from the comparison of that buoyant force with the weight of the object.

http://hyperphysics.phy-astr.gsu.edu/hbase/pbuoy.html#bcomp

now on a more practical note, my 2 inch thick 5.1 pound eps board floats me like my 2.4 inch 7 pound poly…at 41 y/o paddling and float is very important to me and i would NEVER ride a 2 inch poly…

Doc is correct about planning v displacing, but wrt float while paddling, duck diving and sitting (submerged board)…there is no comparison - EPS is lower density and therefore has more rapid upward float behavior than poly.

NO MYTH…its safe to say the myth guys just havent had the experiences i’ve had…my first eps shaped thin by my mistake, 2.4, and i was bummed that i wouldnt have the float i wanted…a pro builder told to me to finish it cuz i’ll proly work…this board was 0.2 thinner than my poly’s…THE BOARD WAS A CORK and i couldnt ride it well…so i reworked the board by peeling the deck lam off and shaping off another 0.2 of thickness…taking off more near the rails…only then was it right…

my current stick is only 2 inches thick…never in my dreams would i be riding something that thin…but in reality i am…

20

if you have two boards of identical shape and size,

one is made of EPS,

the other is made of concrete,

which will float better?