The question about the SeaO2 PFD reminded me of an earlier question I had in mind:
Why so few buoyancy “sizes” for PFDs?
Seems like there are only two categories: child and adult (“over 90 lbs”). If 90 lbs is a good dividing line, why are there no cats for “90 to 180 lbs” and “180 to 270 lbs”?
As it is now, the adult PFDs have more bulk and buoyancy than necessary for many people, and possibly barely enough for the heaviest paddlers.
Since this is a safety issue, I question the “two sizes fits all” mentality.
Body fat
A PFD supplements a person’s own natural bouyency.
A person’s bouyency is largely a factor of their body fat percentage. A person’s weight is really not a major factor. A 90-pound PNG with 20% body fat and an bigger average fit guy with 20% body fat will float equally well.
As body fat percentage goes up, less supplemental floation is needed.
I had an uncle that weighed 375 lbs.
He floated with his head & shoulders above the water. I sink like a rock if I don’t tread water.
Maybe, but…
– Last Updated: Oct-31-05 4:31 PM EST –
then the categories should be grouped according to body fat percentages, e.g., "under 10% body fat," "10% to 20% body fat," "20% to 30% body fat, " and so on.
But I suppose that would be too hard to bear for some people's sensitivities. Even more offensive to them than going by weight. Plus the varied accuracy of different methods of measuring body fat % could lead to someone placing themselves in the wrong category.
Also, I think that absolute weight still matters. If it didn't, then a PFD that could hold up 50 lbs of lead could also hold up 500 lbs of lead. Unless the PFD is *overkill* for the 50 lbs, it won't work for the 500 lbs.
500 lb cinder block
ou need to compare your 500 lb cinder block (or even your 50 lb) to a 500 lb lump of lard. Both weigh 500 lbs, but the lard needs no help to float.
Keep your head up…
– Last Updated: Oct-31-05 4:30 PM EST –
The purpose of a PFD is to keep (or help keep) one's head above water.
It's possible that what is needed to do this is less variable for adults than adult body mass.
How much do the weights of adult heads vary? I bet it's less than body (ie, torso weight).
Not really
Both cases are extreme: cinder block and lead have no fat at all, and lard is all or mostly fat.
But it is possible to have a 100 lb person with 5% body fat using the same PFD as a 250 lb person with 5% body fat.
Seems like overkill for the light person OR not enough for the heavy person. Even without the buoyancy itself being a factor, the same volume of buoyant material on a smaller person could just get in the way, due to probably shorter limbs and torso.
13 lbs.
the human head is reported to weigh about 13 pounds and probably doesn’t differ significantly between one human and another.
Body size not weight.
My guess would be that the “over/under 90 lbs” specifications have more to with body size and proper fit than flotation requirements.
…Mike
No comments about fatheads
PFDs are not attached to the head or neck; they are attached to the torso (of greatly varied body fat %), which exerts a downward force on the head.
Otherwise, the argument favoring only 2 buoyancy categories might make sense.
Density not weight
– Last Updated: Oct-31-05 5:02 PM EST –
Buoyancy in water is not determined by weight (or more accurately mass) but by density; if the density is less than water you float if greater you sink. It's the density of the engine block or cement block that makes it sink, not it's weight.
Density = mass divided by volume
Fat is composed of greasy molecules (long chain hydrocarbon acids ) think of it as oil floating on the water. The four hundred pound fat person "weighs" almost nothing in water. Musclular people are more compact (less volume for the same amount of weight therefore higher density.) A typical life jacket will support about 15 pounds of a lead weight. A life jacket can only overcome about 15 pounds worth of an engine blocks mass. The PFD is just giving extra volume to make you more buoyant.
Standardization
I doubt it’s just a matter of sensitivity. The Coast Guard has certain requirements and the PFDs divide into simple categories adult/child. Inspection is simple, a boat needs one suitable PFD or each occupant. If you have a system based on body weight or fat, how would you assure each person was wearing the appropriate PFD?
Remember it is about a simple standard on required floatation. No official looks a kayaker and decides, ok you have on neoprene gloves and boots, a drysuit, and a tuilik., since you already have 26 lb of added buoyancy you don’t need to add a PFD. Whether it’s a tee shirt in Florida or a drysuit in Maine, everyone (like it or not) has the same PFD requirement.
Ralph
density …
that’s why huge steel boats which are as long as a city block can actually float. physics … crazy huh?
I’ve heard
that about 10 lbs of the head is Fat 
Brain floats
That’s why Greenland techniques and all rolling instructors have you keep your head down and take it out of the water last. It helps you in watre and fights you out. But then, there is that breathing thing…
Another reason I MUCH prefer saltwater. Float head out without PFD.
Stop wiggling!
You gave an analogy - I answered - then you switch back to more realistic stuff!
Why the nitpicking over PFD labels? Is your paddling season over up there? Who cares about what scale the numbers are on? Just go with what fits you and your paddling needs.
Most Type III and V PFDs don’t keep you turned face up without conscious effort anyway. Buoyancy aid, not lifesaver. You want real buoyancy? Stay with your kayak!
The minimum flotation requirements may be a bit much for small folks and minimal for some larger paddlers, but there are enough options available to find decent ones to fit just about anyone (at least anyone who’s willing to shop around and pay for good ones vs. discounted and limited big box store stuff).
My wife and I have some…
that are made by a Canadian Company that you can open up and take differnt layers of the high density foam out.
The only problem is they are not US coast guard approved.
They are great for racing since we remove almost all of the floatation which leaves very little bulk to get in the way. Of course with all the flotation gone, they wouldn’t hold a non swimmer up!
But for your needs, they would be great.
Needless to say, in rough water races or the unknow, we use our regular CG approved ones.
Cheers,
JackL
lead sinks
The difference between a 50 pound block of lead and a 500 pound block of leak is 450 pounds of lead.
Lead sinks, because it is more dense than water.
The difference between a 50 pound person and a 500 pound person would be about 100 pounds of bone and muscle and 350 pounds of fat.
Bone and muscle are more dense than water and sink, but fat is less dense and floats. The net result is an overall decrese in density, and an increace in bouyancy.
Design Flotation
Many Pfds are listed as having some number of pounds of flotation.
Certainly that minimalist rodeo vest with 16 lbs of flotation is going to be less boyant than the Hi Float with 27 lbs.
http://www.nrsweb.com/shop/product_list.asp?deptid=956
I guess it’s up to us to figure out how much flotation we need.
Tommy
weight and floatation - it depends
Most of the time it is a simple matter of FAT FLOATS.
Today I saw a woman in line for continental breakfast in a motor scooter seat, she was loading up on fruit and fruit juice but I didn’t bother to see what else. I grabbed a cranberry juice and went back to work pushing a cart of water bottles around. I felt thin for a minute until I met up with normal sized people in the hall and remembered I probably weigh only about 50 lbs less than the woman in the cart.
I did think about the two guys from p-net who have expressed their dislike of fat people.
I then gave thanks to whatever God I believe in that I am was not the one in the cart, instead I was pushing a book cart fileld with bottles of water from floor to floor and room to room. This is what I got my masters degree for? But that is another topic.
Before I took up the long hours of water aerobics ten or eleven years ago, I floated with my shoulders well out of the water. Now I float with just my nose out of the water. I call it progress.
Hopefully one day I will need that $95.00 Lotus for safety reasons, not just to be legal in the water.