Physics 101:
The density of sea water = 64 pounds per cubic foot
For every cubic foot of water you displace, you earn 64 lbs of buoyancy
Given the above facts, there must not be any rigid foam in existence that can support more than 64 pounds per cubic foot.
Thoughts?
Buoyancy is a force exerted on an object by a fluid. Weight is a measure force exerted on an object by gravity.
Buoyancy is a pushing force. Gravity is a pulling force.
Bouyancy is determined by volume, not mass. Weight is determined by mass, not volume.
Volume is measured in cubic units. Weight is measured in lbs.
What was the question again?
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Thoughts?
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If a 300 pounder comes to me looking for a board, I'm going to put enough foam underneath him to float him, no matter what the foam's buoyancy is.
Thanks for the comments. You can float a 300 pounder because you are increasing the boards volume. Heck, we can float a two story building with enough volume.
My question was more directed at the pounds per cubic foot of floatation.
Foam is less dense water, so it floats.
Picture a surfer sitting on a shortboard…he sinks the board about a foot underwater. As NJ_surfer said, there are two forces at work: gravity and buoyancy, and these forces must be in balance if the surfer+surfboard system is in equilibrium. Gravity affects everything in the system while buoyancy only affects those things that are in the water.
Buoyancy Forces = Gravity Forces
Surfboard + Portion of surfer underwater = Weight of surfboard + Weight of surfer
The surfer will sink a shortboard completely underwater. From that point, the only thing that changes is how much of the surfer’s volume is underwater. Once enough of the surfer is underwater so that the flotation of the board plus buoyancy of the surfer match the weight of surfboard + surfer , he stops sinking. The foam doesn’t have to support 64 lbs/cu ft, that’s buoyancy force upon it. If you mean support as in strength, that’s what the fiberglass is for. If you mean support as in float, your last statement is non sequitur. On what do you base that assumption? The fact that you don’t see 128lb surfers sitting on top of the water on 2 cu ft surfboards? Not sure if we’re addressing your question here…
Maybe we should defer to the expert: Any object, wholly or partially immersed in a fluid, is buoyed up by a
force equal to the weight of the fluid displaced by the object. - Archimedes
I’m a little slow, so help me out…
What do you mean by pounds per cubic foot of flotation?
Hey iclypso, I think Archimedes got screwed in the end. So what you guys think is how well a surfboard performs for a person is all based upon displacement? So far, foamtheory you have expressed you have no clue to what is really going on. Since your so bright, the scientific method of inquiry just refuted your brilliance.
I love a good narcissistic response.
I am trying to determine the maximum amount of floatation that can be achieved on a surfboard based on a single cubic foot of material. For example, EPS can float approximately 55lbs per cubic foot. Are there other foams in use that can float more than 55lbs/ft3?
a bag of helium works
herb
ya,polyethylene or polyproplylene foam will.
herb
Now I get it. Lots of other materials will provide the kind of flotation you’re talking about… some are rigid foams, others are not. But flotation in only one property, and there are other mechanical properties to be concerned with if you’re talking about something you can glass. They can even make polyurethane foam in such light densities that they feel almost weightless (something like .5lbs/cm3), but in terms of structural integrity, it’s not appropriate for surfboards.
Thanks for the replies Herb and NJ. I have heard EPP foam is a pain to glass due to lack of mechanical bond. So lets say the target is a surfboard appropriate rigid material that has similar properties to PU, but with more floatation.
Is polypropylene or Polyethelyne heat sensitive like XPS is?
Where did you come up with a density of 55 lbs/ft^3 for EPS foam? Typical densities of EPS used in surfboard manufacture are on the order of 1 to 3 lbs/ft^3, Hence the floatation in seawater is closer to 61 to 63 lb/ft^3 (assuming that the foam is self-sealed so that no water penetrates into the foam).
OOPS…
sorry… meant to say PU as light as .5lbs/ft3 in the above post.
Depends on whether or not the term “foam” is interpreted to include a single cell (which could be a real challenge to shape 
and what definition of “rigid” is assumed.
You sure of that density? …Perhaps meant 0.5 (or 5.0) lb/ft^3 ?
1 inch = 2.54 cm
1 ft = 12 inches = 12 x 2.54 cm = 30.48 cm
1 cu-ft = 30.48 x 30.48 x 30.48 cm^3 = 28,316 cm^3
Wt of 1 cu-ft of foam with a density of 0.5 lbs/cm^3 = 0.5 lbs/cm^3 x 28,316 cm^3/cu-ft = 14,158 lbs/cu-ft
Polypropylene and Polyethelyne are both heat sensitive in that heat is commonly used to fuse two pieces of these foams together. Unfortunately, heat can also warp a board (rocker change) or cause the skin to separate from the core foam. These are common problems in bodyboards left in a closed vehicle on a hot day.
The volume of the board and the weight of the board are the sole determinants of bouyant force when the board is submerged. The type of foam inside the skin doesn't matter. Look at it this way, the water's not smart enough to know what is inside the skin.
So, to answer your question, there is no foam that will provide a bouyant force greater than the weight of the fluid displaced by the object.
This subject has been flogged repeatedly here....
edit to add: that was a good post for #2 by iclypso, welcome to sways.
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...the water's not smart enough to know what is inside the skin.
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...and the checkered flag goes to
my general idea of what I think I know gets totally blown out of the sky (and the water) at the strangest moments.