The physics of buoyancy

[krusher74]

I keep reading in surf articles from major surfers/shapers what from my research seem to be myths not based in physics.

The latest is this article from US blanks http://usblanks.com/promer-surfboards-a ... snowdonia/

Particularity the sentence "but opted for EPS for the added buoyancy needed for freshwater"

From diving forums etc I found the flotation of sea water compared to fresh water and within normal surf temps 5c to 30c there is less than 0.5% difference made to a floating object.

30L of PU foam, 30L of EPS and 30L of Mc donalds frys all have the same buoyancy they are 30L,

The weight of a board will change how much a board sinks below the water surface, but if the EPS board was 5lbs, PU board 10lbs , and MC donalds frys board 15lbs we are still talking a less than 1% difference in float

So from all the physics I can research, this myth of the 30L EPS board having more buoyancy than the same 30L PU board has no science behind it.

any physics guys out there ?

[rodndtube]

I am not a scientist, but not sure that 30L stuff is physics... seems like chemistry to me! The physics in the difference between EPS and Poly is the flex & toughness (bend and not break) and float per volumetric inch, i.e., the differences in plan shape and dimensions one can achieve with the same 30L goal.

[Poobah]

A 30 liter board made from french fries will displace 30 liters of fluid when the board is completely submerged. The displaced fresh water would weigh 30 kilos, and the saltwater about 30.8 kilos. That's less than a two pound difference. Can you offset that difference by shaving a couple pounds off the board by using EPS foam? It does seem possible. Maybe it matters more to a surfer standing up.

I don't think water salinity makes much difference for us prone riders. Does anybody notice their buoyancy difference during red tide? Do you compensate for the extra plankton? How about surfing in a river mouth on an outgoing tide? Can you feel the change in buoyancy?