alex" <al.m_at_3web.net> wrote: >> My physics knowledge is already rusty, I wonder what did they mean by reduced floatation. Heavy load with little displacement, like VHF + water bottle and/or sandwich and/or compact camera can easily weigh over 2 lb in total, reducing available floatation force from 17.5 to less than 15.5. And what if somebody just had a good lunch and, consequently, needs more floatation ? :-) >> Alex, the business about making PFD pockets small enough that they can not be loaded with much dense stuff (such as shotgun shells, which contain metal loads -- were lead, now often steel shot), is so that when submersed, the PFD still provides "enough" flotation to the human inside the PFD. When stuff which is less dense than lead is submerged, the buoyant effect it provides offsets more of its weight, so that the net effect on the human is smaller. For example, the effect on the buoyancy of a PFD system of stuffing a water bottle in a pocket, assuming the bottle is fully submerged when the PFD (and person) are in the water is ... nil. This is because the water is neutrally buoyant in water. It "weighs" nothing when submerged. On the other hand, something of great density, such as lead shot (about 8 times more dense than water, IIRC), being of lesser volume than the same weight of water, will displace less surrounding water and hence not be neutrally buoyant when completely submerged. In fact, the shot will have a negative buoyant effect, decreasing the effect of the PFD system. You've got to be careful about confusing the weight of something with its buoyant effect. For example, consider the effect of adding several pounds of helium gas to your PFD system. If the helium is at normal atmospheric pressure (as in a balloon, not a steel tank), the more helium, the higher you will float (if the helium is completely submerged). This is because helium is much less dense than water, so its buoyant effect, when submerged in water, is very positive. To confuse the waters further, ponder on why regulations do not mandate more flotation in a PFD for heavy guys like me, or lesser flotation for someone missing a leg (answer below the sig). -- Dave Kruger Astoria, OR -- Because humans are very close to neutral buoyancy. The basis for 15 (or 17) lbs of flotation in a PFD is to keep about 15 lbs of "human" out of the water ... a weight something like a human head and neck ... so the human can breathe. Same thing for a "heavy" lunch. Food is nearly neutrally buoyant in water. Cheese floats. So do bread, salami, an orange, etc. Avoid lead sandwiches, and you'll be OK. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Wed Jun 02 2004 - 06:30:27 PDT
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