Floating on the surface

[FPDocMatt]

Let's say you are only wearing a bathing suit, find yourself on the surface with no land or boat in sight, and want to survive until help arrives. I'm trying to figure out the physics of the whole floating on the surface thing.

Here's what I've figured out so far: Buoyancy force is the weight of the water you displace. This is counteracted by your weight pushing you down. If you are vertical, then depending on these factors, some portion of your head will be out of the water. If you are sufficiently buoyant, the portion of your head out of the water will include your mouth. But most people aren't that buoyant. So the logical thing is to lie on your back, with your back arched so that the top of your head and your arms and legs are under the water, but not your face. Then your mouth and nose can be out of the water, even if the rest of you is under the water. In other words, by maximizing the amount of your body which is under water, you maximize your buoyancy. You just don't want the part under the water to be your mouth and nose.

This brings me to the question, Why are some people more buoyant than others? I've heard people talk of fat being less dense than muscle, etc. Can we infer that, if person A is more buoyant than person B, then person A has a larger percentage of body fat than person B? Or is it more complicated than that? Because I'm pretty lean, but I seem to require more weight than the DM's think I should need.

 

[elan]

I have never heard about such measuring equipment as a DM

 

[Doppler]

BMI... true BMI is a starting point. We also have 'swim bladders' built-in... our lungs of course but also our gut... we can fine-tune our buoyancy by sticking our gut out. And also, there is something to be said about being able to shed ballast as we become more relaxed in the water.

 

[TSandM]

It makes reasonable sense that bone density and bone mass affects this, too. Bone is heavier than water, muscle is close to neutral, and fat is lighter. I require much more weight than anyone ever believes I do, and my theory is that it's low bone density and very small bone mass.

Surface area to volume ratio is going to make a difference, too -- I would suspect that very round people float better.

 

[GrumpyOldGuy]

I though the original idea of BMI and properly measuring it was a ratio of the amount of water you displace vs... actual weight. The BMI equivalents used based on weight and height are an approximation to avoid the need to dunk in a tank.

Anyway, back to the point. Besides you actual BMI, if you can relax and breath off the top half of your lungs (so they are full or nearly full), you can float better. If you panic or nervous, you breath deeper and your lungs spend more time empty and you swallow water and start a downward spiral. You can also play the reverse, breath off the bottom half of your lungs (at least on initial descent) and use less weight if you are calm and relaxed. Not to mention the excess movements an excited person does that sometimes contribute to the need for extra lead or an ill fitting wetsuit or any number of other factors that vary from person to person.

 

[FPDocMatt]

I though the original idea of BMI and properly measuring it was a ratio of the amount of water you displace vs... actual weight. The BMI equivalents used based on weight and height are an approximation to avoid the need to dunk in a tank.

Because I didn't know this (until I read your post), I looked up BMI on Wikipedia, and learned something.

I thought that since the denominator of BMI was meters squared, that BMI was your weight divided by your body surface area. I suppose the square of your height might be an approximation of your BSA, but it's not intended to be BSA at all. According to Wikipedia, it's just that the radio of weight to square of height is proportional to a person's fat percentage. That is, the higher your BMI the higher body fat percentage you probably have.

Anyway, I like your idea, but I wonder if it's what BMI was originally intended to be.

When you talk about dunking in a tank, are you thinking of a person's density? Because that's exactly how you'd measure density. And density is what you need to know to figure out someone's buoyancy. Less dense people are more buoyant, since they take up more volume per unit of weight.

I suppose BMI might be inversely proportional to density. Hm, have to think about that one.

 

[GrumpyOldGuy]

You are correct, the dunk tank was used to measure density. From this I believe you can derive fairly accurate values for fat vs lean tissue (muscle/bone) since we know what the density of each type of tissue is and can solve for the unknown variable.

I am not sure what BMI was originally intended to be. Besides the dunk tank, I also remember skin fold calipers were also used to calculate BMI at one point. The current height/weight calculation is simple and cheap, I just wonder how reliable it is. Probably good enough to tell me to hit the gym tonight....

 

[k ellis]

As for having to use more weight then your divemaster thats a fairly common thing that alot of divers ask. Water people tend to be very comfortable in the water so thats a big help. Manyu students marvel at how the insturctors have got it down to where we use almost no weight if any at all. Many of the students require a minimum of 8 pounds or better to be able to sink.

Nerves play a tremendous role in buoyancy too as people who are nervous will breathe in quiet a lot faster and deeper then veteran divers. Thats one of the biggest reasons why divemasters and instructors use significantly less.

As far as body mass you have to remember muscle is solid and so is bone mass which make them less buoyant. For this reason professional swimmers and people who run alot will have less buoyancy. Body fat plays a significant role in making a person buoyant so the more fat is on a person the more likely they are to float higher.

Now to answer the question people like myself carry our weight very well. Many times I have been diving in Cozumel where I used 8 pounds of weight in the sea and the divemasters would beg and plead for me to use at least 10 to 15 pounds. (Dont know why but hey) They in turn usually are awestruck that even 8 pounds is often over weighted for me (No I was not in a wet suit at the time I was swim suiting it)

In short our bodies are unqiue to us. A person can be the same height and wear the same size clothes as we do so they would not look different in size but one may have a higher BMI then the other and thus require 2 different amounts of weight for the same dive wearing the same gear.

 

[Mike]

Wow, what a timely thread.

There has been an ongoing discussion of this topic in the Bonaire forums for about a week with some vehemently in denial scuba diving weight requirements can vary from person to person.

http://www.scubaboard.com/forums/abc-islands/413042-buddy-dive-bad-attitude-5.html#post6259420

 

[miketsp]

Well given that this is a hypothetical scenario on a diving forum as it's most unlikely that a diver would not have some other flotation aid even if it's just a piece of neoprene or an SMB let's see what comes into play.

Factors:
% and volume of muscle
% and volume of intra-muscular fat
% and volume of sub-cutaneous fat
% and volume of lean tissue
bone size and density (variation between individuals, races and with age - see Bone densitometry )
and finally
residual lung volume.

All these variations lead to huge variations between individuals.
In my own case, even though I appear average, I need a lot of lead to sink me. Something like 7 pounds just to go snorkeling in a bathing suit.
So in your scenario I can just lie on my back without doing anything, even in a fresh water pool, and wait for rescue. In salt water even better. My only concern would be wave height and getting sun-burned.
I have friends, of similar build, who would be either treading water or having to keep their lungs well inflated while lying back.

Edit: I've seen published density figures for muscle varying from 1.04 to 1.2, for fat varying from 0.65 to 0.92 and bone anything up to 2.5 all in gm/cc.