Your body's buoyancy

[Tortuga James]

While at the YMCA pool yesterday, I decided to try an experiment. At the deep end, with just mask and dive socks and my fins (and compression shorts), I did a buoyancy check.

With full lungs, I floated with my mask half way submerged. When I exhaled, I sank like a rock.

I thought it might be that my fins were negative, so I took them off. Same thing exactly.

Am I negative, positive or neutral? Is that common for divers?

Then I took a fin down to the bottom and held it 3 feet up. It basically stayed suspended, maybe just fractionally negative, just sinking ever so slightly. Are most fins neutral?

I think I might have been narc'd in the pool.

 

[The Chad]

Sounds like your negative to me. There are all kinds of body types out there people on the leaner side tend to sink and if you have more body fat you tend to float. Stick a lean guy in a 7mil and he'll likely need to add some weight. As for fins they also come in different masses, some float, sink, even float, guess it just depends what you like. Hope you recover well from your pool narcosis!

 

[Walter]

You are neutral. Most are positive, a few negative.

 

[dsteding]

Funny thing:

I'm losing weight at a good pace, I was about 215-220 at 6'1" and am down to 205. In terms of trim, I've always had a feet-down tendancy, even in doubles. The last few dives that has started to shift to a more heads-down trim. My explanation is I am losing fat in the upper body that makes things floaty . . .

A buddy of mine apparently experienced a similar situation when he stopped lifting a lot (he was a bodybuilder). So, besides being positive, negative, or neutral, the distribution of our weight on our body affects trim.

 

[Charlie99]

Am I negative, positive or neutral? Is that common for divers?
.

Obviously you are ALL THREE, depending upon your lung volume. You just vividly illustrated how important lung volume is to buoyancy control, and how unless one specifies lung conditions (normal, exhaled, forced inhalation, etc.) buoyancy check can be off by 5 pounds or more.

Tidal Volume gives some typical numbers, in liters. Each liter of volume is 1kg or 2.2 pounds of buoyancy.

Yes it's very common in divers. When you try the same thing in saltwater, you will find that you will have to exhale more forcefully to sink.

 

[Nemrod]

Different body types have different--uh--flotation charactristics.

Generally, men sink and women float, but it really is about density and this means muscle/fat ratio. I know women who sink like a rock who were into triathlons and aerobics and I know men who bob like a cork because they are into McDonalds Drive Through.

When I was marathoning and triathloning etc I had a body fat ratio around 3%, I sank like lead. Know that I am old and fat I can almost float---almost. See, age has it's advantages.

N

 

[roakey]

Am I negative, positive or neutral?

D) All of the above.

By controlling the volume of gas in your lungs, you can be any one of the three.

Roak

 

[SteveFass]

Anyone recall seeing a chart quantifying the density of various factors? Can't recall where I saw that. Anyway, bone density was near the top of the list.

I'm not fat, I just have light bones.

 

[Charlie99]

Anyone recall seeing a chart quantifying the density of various factors? Can't recall where I saw that. Anyway, bone density was near the top of the list.

I'm not fat, I just have light bones.

I googled away once, trying to figure out how much buoyancy change someone would have from dropping 10 pounds of fat, or from converting 10 pounds of fat to 10 pounds of muscle. IIRC, the various sources had different numbers but most had fat in the 0.92 to 0.94 specific gravity range. I didn't find as much on muscle, but it was around 1.05. Anyway, I came to the conclusion that it took the loss of about 15 pounds of fat to get 1 pound more negative. In real life though, a combined exercise and diet program will also build up muscle mass, effectively trading out fat for muscle. In that case it was a bit more than 1 pound more negative for each pound of fat "converted" to muscle.

The bottom line conclusion is that by the time one has shed enough weight to make a significiant difference in buoyancy, you are probably also changing wetsuits, which probably will mask the 1 or 2 pound buouyance difference from the body weight change.