Sounds logical, but let’s see my particular case. I have a 7’3’’ hollow board that
weights around 20 lbs. It’s volume is about 2 cu. ft. equivalent to 128 lbs of floatation.
I weigh 162 lbs, that’s: 162 + 20 = 182 lbs. so, in theory, I’ll sink. But when I’m
sitting on the board in the water it floats below my belly line. So what is happening here?
you neglected to add the bouyant force of the part of you that’s underwater. So, working backwards, if you are (obviously) at equilibrium with the water to your belly button, there is a bouyant force pushing you and the board up with the same force that earth’s gravity is pushing you and the board down. If your board pushes up with 128 lbs, and you and board are pushing down with 182 lbs, the submerged part of you is pushing up with 182-128=54 lbs, and is displacing (54 lbs / 64 lbs per cu.ft.) .84 cu. ft. of water.
Hey Doc,
Regarding floatation: Maybe it’s a specific gravity thing. Without getting too mathematical (because I am unable), the specific gravity of a substance can easily be calculated by dividing its density (in grams per cubic centimeter) by the density of pure water (one gram per cubic centimeter). The brilliant Greek mathematician and inventor Archimedes discovered over 2,100 years ago that a body in water is buoyed up by a force equal to weight of the water displaced. Archimedes reportedly came upon this discovery in his bathtub, and ran out into the street without his clothing shouting “Eureka, I have found it.” Since one gram of pure water occupies a volume of one cubic centimeter, anything having a specific gravity greater than 1.0 will sink in pure water.
Sooo, at least with wood boards we don’t use ebody or ironwood.
Can anyone advise the extent to which results are affected by immersion in salt water?
Laconic,
Ahhh, now that’s a whole nuther subject.
Or subjects… salt water has a somewhat higher specific gravity, on the order of 5%, that varies considerably with temperature, salinity and so on. Some bodies of water, such as the US Great Salt Lake and the Dead Sea, have such a high dissolved salt content that a human can float on his back almost half out of the water. Tropical seas are generally saltier than more temperate seas, but there are most definitely variations.
Interestingly, water with a lot of solids in suspension ( such as river or current water with a lot of silt ) has a very similar effect. While this isn’t surfing, during the building of the Brooklyn Bridge ( New York Harbor, US) the cassions ( which operated in a higher air perssure equivalent to the pressure exerted by sea water of an equivalent depth ) used a water column as a kind of air lock for removing excavated materials. When this water column became filled with suspended river bottom silt particles the system failed rather nastily.
Interestingly also, unseasoned wood ( such as what Heyerdahl used in his balsa raft Kon Tiki ) tends to block or resist the intrusion of heavier salt water and thus retain flotation over time better than seasoned/dry wood. Just in the extremely unlikely case you were planning on building a solid wood board with no outer coating.
In a more practical sense… to find the volume of a surfboard, you can either put it in fresh water with enough weights on top to just sink it ( the weight, in kilograms, will give you a volume in liters ) or put it in a smaller container of fresh water that is completely full, push it under and collect what water overflows the container. Again, the volume in liters will give you a buoyancy in kilograms in fresh water. In both cases, multiply by around 1.05 to give you a nominal buoyancy in salt water. To convert to pounds, see the very handy Pocket Ref
doc…
well, it can explain why, but I also can lay down on the board with my body completely out of the water an it still floats.
Doc, thanks for the info. The differential in buoyancy will be reserved as a “fudge” factor since it’s unlikely I’ll be age regressing. Re the Pocket Ref, it’s sterile contents would not provide for the informative, humorous, eclectic examples as this interactive forum does. I ordered one anyway.
Lee DD, thanks for the input. I’m not a surfcraft maker and time constraints guarantee I won’t ever be, so input from those who are is appreciated.
you neglected to add the bouyant force of the part of you that’s underwater. So, working backwards, if you are (obviously) at equilibrium with the water to your belly button, there is a bouyant force pushing you and the board up with the same force that earth’s gravity is pushing you and the board down. If your board pushes up with 128 lbs, and you and board are pushing down with 182 lbs, the submerged part of you is pushing up with 182-128=54 lbs, and is displacing (54 lbs / 64 lbs per cu.ft.) .84 cu. ft. of water.
Wells - thanks. That is correct. Your body displaces buoyant force that prevents you from sinking when in the water.
Actually, Archimedes principles apply to both fresh and saltwater. The only difference is that the density of saltwater is greater than freshwater, thus giving you more buoyant force. Fresh water has a density of 62.4 lbs per cubic foot. So, for 2.0 cubic foot of board, you would get 124.8 lbs of buoyant force. The density of saltwater is 64 lbs per cubic foot. Same board, 128 lbs of buoyant force. Therefore, if you sat on your board in a freshwater lake, you would be sitting a little lower than in the ocean. If you sat in the Great Salt Lake, you might sit higher than in the ocean. Cool ain’t it?
“If you sat in the Great Salt Lake, you might sit higher than in the ocean. Cool ain’t it?”
The salinity of Great Salt Lake varies with water volume of the lake. Generally speaking, the salinity is noticeably higher than ocean water.
And yes, you definitely float higher in Great Salt Lake. Saltair, a now defunct resort, was a place where swimmers could test the waters…
Actually, Archimedes principles apply to both fresh and saltwater.
Actually… Archimedes priniciple applies to any fluid, so that would include water fully saturated with salt, steam, and yes… air.