I’ve already searched for this but haven’t found an answer. Is there a general rule of thumb when comparing an epoxy board (Merrick Tuf Lite for example) to a PU board of the same shape/dimensions?
My main question is would a PU board of “X” thickness float the same as an epoxy board of “Y” thickness with all else being the same?
I hope that makes sense and thanks for any help.
due to epoxy and eps corre being lighter in weight than PU PE construction
I may not have worded my question well. If I'm trying to duplicate the buoyancy of one board (PU with a thickness of 2.75") with another board (epoxy) of the same size, what would the thickness be on the epoxy board?
thi
thinner eps board, or add soome weight components designed at the specific weight difference to even out the weight…such as what it appears TL2’s have done.
There’s a lot of disagreement on this topic, but my opinion is that they will be equally buoyant. Buoyancy is a function of volume. Same volume, same buoyancy. So if you’re sitting on your board, it will float you almost exactly the same. Any slight difference will come from the fact that if you build a lighter board from EPS you might get a little better “float” out of it due to the fact that it weighs less itself. But what, maybe a pound? How much did you eat and drink for lunch? What does that rashguard weigh filled with water? Get my point? Once the board is up and planing, a lighter board might be more responsive, but in terms of buoyancy, they’re the same, according to the laws of physics.
So why go thinner with EPS? Well… I glass my EPS boards heavier to make them stronger, so they’re stiffer. A thinner board gets you some of that flex back. How much thinner? My experience has been that I need to reduce the thickness of an EPS board by about 1/8" if it’s glassed with 6oz all around to start to get the same kind of flex feel as PU with 4oz all around. For me, it has nothing whatsoever to do with buoyancy. But a thinner board is less bouyant, so it’s harder to paddle, but duckdives better! So if you’re reducing thickness of an EPS board so it floats the same as a PU of the exact same shape and size, you’re actually making it harder to paddle than the PU, not the same.
Others will likely disagree…
[quote="$1"]
Buoyancy is a function of volume. Same volume, same buoyancy. So if you're sitting on your board, it will float you almost exactly the same. [/quote]
I agree, as long as foam density is the same. While it may be negligible, same shape/volume with differing core densities aren't likely to have the same buoyancy.
Please explain. I believe foam density has nothing to do with buoyancy. Buoyancy is a force. The force comes from the fluid an object is placed in, and acts upon the object. The force does not come from the object, it comes from the fluid. So the material the object is made from does not determine the bouyant force. Only the amount of fluid the object displaces does - it’s volume determines the force in any given medium, at a particular depth. Reduce the volume (thin the board) and it becomes less bouyant.
This is not just semantics. Many people think that because a standard density EPS board is a pound lighter than a standard density PU of the same volume, the EPS board must be thinned in order to provide the same float. The reality is, and this is the crux of my theory, that the difference in density of both boards is so great relative to the fluid they’re floating in, that the difference between the two different density cores creates (and you said it) a “negligible” amount of difference in what I refer to as “float,” not bouyancy… too negligible to even notice. To any mere mortal, they paddle the same. They duckdive the same (the PU may actually duckdive easier, beacuse it’s heavier… but negiligibly so). But they ride differently because once up and planing, weight matters. Put both boards on the surface of the water and they stay there. NO! you say? The PU will float lower in the water than the EPS! Ok, but the difference in displacement is, again, negligible… A one lb. difference in board weight = just under 16 fl. oz. of displaced water difference between the two baords. How much water did you chug before your session? Probably made up for the difference in “float” between the two boards. (yea, I know… water is neutrally bouyant in water. I’m just trying to make a point.)
This crosses over into what Ben Thompson is studying. I wish he’d chime in and give us his thoughts.
This has been talked about a great deal, when I first started posting on here it was a popular subject.
njsurfer is correct, if two boards are exactly the same shape, volume, and weight, the bouyant force they exert when submerged will be identical. If one board is lighter, the bouyant force will be increased by the amount of the weight difference. It doesn't matter if the core is heavy, light, or absent (hollow). Look at it this way - the water's not smart enough to know what's inside the board.
The determinants of the force are displacement and weight of structure (unless you change the liquid's density). So if one construction is going to be lighter than the other, you could make a slight decrease in volume to compensate. There's no ''rule'', i.e. take off 1/8''. It would depend on the size and shape of board. It could be calculated using CAD programs and some math, but you'd also have to know the exact weight difference that would be yielded.
Distribution of mass is another subject, and core density has a role there.
Here is the equation of the tension (T) required to restrain an object that wants to float up to the surface:
T = F(up) - F(down)
and
F(up) = d*v*g This is equal to the weight of the displaced volume of water
F(down) = mg The forge of gravity pulling down on the mass of the board
so,
T = dgv - mg
where:
d = Density of Water
v = Volume of the object
m = Mass of object
g = Acceleration of gravity
Example:
Say you have two unglassed foam blanks of equal volume, which are measured to be 40 liters. The amount of water displaced for both boards weighs 88.1 pounds. This is F(up).
Now lets calculate F(down) for each board.
Blank A is EPS and has a density of 2 pounds per cubic foot, and weighs 2.8 pounds
Blank B is PU and has a density of 4 pounds per cubic foot., and weighs 5.65 pounds.
The difference is 2.8 pounds! Now... a pint is a pound the world around. So, here's what you do if you normally ride an epoxy board, and it snaps. Go grab a a PU board of equal volume, drink three pints of beer, and go surfing. It will be exactly the same -- I promise.
It’s even simpler. Water has a density of 1.0 grams/cubic centimeter. If an object has a density less than 1.0 g/cm3 it will floats in water. Greater than 1.0 it sinks. If the volume of the boards is the same, but one has a denser core(more mass) it will not be as buoyant in water. Mike
Mike… can you make a sample calculation for us? Any stock shorbaord will do…
Hi nj surf,
I could probably come up with something, but I don’t measure the volume of my surfboards or the masses(or weight). The only way to measure it’s volume, as far as I know, is to build a displacement tank of some sort, push the board under the water, collect the water displaced, and measure the volume of the water displaced. The volume of water displaced is approximately equal to the boards volume because water has a density of 1.0 gram/cm3.
Think about this. Take 2 identical plastic water bottles and cap them. Those are your ‘boards’. Fill one board with sand and the other with air and put them in water. Which ‘board’ is more buoyant? The volumes are identical, but you have increased the mass of the ‘board’ filled with sand. Make sense? Mike
You guys are all missing the most simple fact.
EPS is lighter and holds more [b]air or gas[/b] thats why they float better.
Its the trapped air/gas that causes the board to float in the first place.
Hence when water displaces the air things tend to sink.
Look at modern day ships made from steel and concrete we all know none of these materials float unless they trap air.
Now if we could make strong hollow boards and fill them with helium!!!
[quote="$1"]
Please explain. I believe foam density has nothing to do with buoyancy. Buoyancy is a force. The force comes from the fluid an object is placed in, and acts upon the object. The force does not come from the object, it comes from the fluid. So the material the object is made from does not determine the bouyant force. Only the amount of fluid the object displaces does - it's volume determines the force in any given medium, at a particular depth. Reduce the volume (thin the board) and it becomes less bouyant.
This is not just semantics. Many people think that because a standard density EPS board is a pound lighter than a standard density PU of the same volume, the EPS board must be thinned in order to provide the same float. The reality is, and this is the crux of my theory, that the difference in density of both boards is so great relative to the fluid they're floating in, that the difference between the two different density cores creates (and you said it) a "negligible" amount of difference in what I refer to as "float," not bouyancy... too negligible to even notice. To any mere mortal, they paddle the same. They duckdive the same (the PU may actually duckdive easier, beacuse it's heavier... but negiligibly so). But they ride differently because once up and planing, weight matters. Put both boards on the surface of the water and they stay there. NO! you say? The PU will float lower in the water than the EPS! Ok, but the difference in displacement is, again, negligible... A one lb. difference in board weight = just under 16 fl. oz. of displaced water difference between the two baords. How much water did you chug before your session? Probably made up for the difference in "float" between the two boards. (yea, I know... water is neutrally bouyant in water. I'm just trying to make a point.)
This crosses over into what Ben Thompson is studying. I wish he'd chime in and give us his thoughts.
[/quote]
I wasn't clear in my response. What I meant by density, was two different weight foams. If I have a 2lb density core vs. a 3lb density core, then all else being equal, the 2lb is going to float higher in the water. If I had two boards of identical cores, but different glass schedule, the heavier glass job will be less buoyant (although that would likely be almost immeasurable just as the 2 to 3 lb comparison would likely be negligible). I guess the most obvious comparison would be a board shaped from foam and an identical board made from lead.
Personal experience has told me I can go thinner with EPS than PU, and I think this is because the EPS used has a true 2lb (or close to it) and the PU that I used in the past was somewhat denser. Just my opinion.
Keep 'em coming, guys. This is highly entertaining.
One more time - if the volumes and weights are the same, ''bouyancy'' is same. Doesn't matter if the core is helium or tungsten, although it would be hard to equalize the weights that way, lol.
Maybe we can build a hollow board and put enough compressed air (or helium!) inside to cancel out all the weight? j/k...
Say I shape a board that has 29L of foam. That’s about one cubic foot of volume. I glass that board and it weighs 6lbs. It will displace cubic foot of water when I sit on it. Another board of the same volume, but weighing in at 5 lbs. will displace the same amount of water when I sit on it. But will it float me any higher in the water? No… at least not to any significant degree. If I imagine I’m sitting on that 5lb board and I’m holding a pint of beer, or a one lb. weight… in my hand. Will I sink noticeably? I don’t think so, considering me and my board together weigh over 200lbs. Try it. Sit on your board, hold an extra pound of weight in your hand, and look down at your water line. I highly doubt you’ll see a noticable difference.
So I should thin that 5lb board and reduce it’s buoyancy so I floats me better… or at least like my 6lb PU? Nope… at least I sure am not. It will only make it harder to catch waves and keep my position fighting the current.
Wrong, a vacuum board wil float even better, but it has less air/gas trapped inside.
I make quivers for many guys who know how to surf. We just keep the shape exactly the same and no adjustments are made between material (PU or EPS).
Of course they surf different however that’s why we make them. EPS boards work well in the right conditions just as PU boards do.
As for myself I just make them the same volume.
I tried making the EPS boards slightly thinner and the rider would hate them. When I keep the thickness and maintained the volume without reduction it’s a love fest. (For the Board).
Sometimes people just plain over think.
I’m sure there are experts who think otherwise?
Kind regards,
surfding
That’s been my experience, too.