To echo Mike Daniel, the lightness of the board/ density of the blank has so little to do with how the board floats. If the board weighs 7 pounds or eight pounds when compared with the sixty or so pounds of water it displaces is about 10 percent difference. Or in other words, you will float about two ribs higher.
The lightness of the board has to do with how easily you can “twirl it around” while riding it. Or conversly, how little it will “twirl around” when chop and bump hit it.
What it floats like while sitting still means next to nothing compared with how the shape allows it to plane on the water.
I work with you engineers all day long, and you guys can really loose the “forrest from the trees” =)
Hey everysurfer, heh heh, I are engineer, and there's no one named Forrest around here. Perhaps Forrests are more common at the computer you sit near. Two ribs higher will be a good bit more float it you're used to sitting on a 6-0 x 18 x 2.375" board.
The OP directed his question at density. One cubic foot of anything will provide a buoyancy equal the the weight of one cubic foot of whatever it's submerged in. Period. If it's submerged in salt water, than 64 pounds of buoyancy as originally suggested is about right. If you should ever find yourself submerged in mercury, the buoyant force will be about 847 pounds, which will be good to know, for a very short time.
How much the cubic-foot object floats is a little different, because the object has some weight, which causes some submergence.
AAARGH!!! The spell-check police got me again!!! I’m trying to learn to type without looking at the keys, and I’m not doing such a good job!
"If you should ever find yourself submerged in mercury, the buoyant force will be about 847 pounds, which will be good to know, for a very short time."
That brings to mind an old question: When is an ounce a pound?
Answer: When it's one fluid ounce of mercury. (Not quite, but very close...)
Which will float the best? Whichever is lightest…not because the buoyant force is greater but because the weight is less. Think about a cubic foot of iron. It’s going to sink because the gravitional force (weight) pulling it down is greater than the buoyancy. A cubic foot of foam floats because it weighs less than the water it displaces, just as a helium rises because it is less dense than air. Since saltwater is about 64 lb/cu ft, choosing a foam that 2.0 lb/cu ft instead of one that’s 2.2 lb/cu ft won’t result in a huge difference in floatation as the buoyancy is 30x > weight (I’m trying to be careful with lb_f vs lb_m).