Buoyancy of fat?

[Tagerisatroll]

I've lost 30lbs since I started diving again, and while theory would have me using less weight to mantain my bounacy, the actual change in my weight belt has been negligiable. I was 296 in may and now am down to 266, but my weight belt is only a pound lighter. I've had more succes in changing my setup to steel tank, and backplate to get the weight off the belt.

 

[redacted]

Body shape has just as much effect on boyancy as does its density.

Barring a shape where surface tension comes into play, I thought bouyancy was strictly a function of volume and densities.

 

[AggieDad]

Suppose you have an outboard motor weighing 300Lbs and displacing 2cf of seawater, how big a bag of fat would you need to make it slightly positively buoyant?

172 lbs

On this subject: My son is 6 ft 3 and weighed 225. He lost 50 lbs. Before he used approx. 4 lbs of weight with a HP 100 steel tank. We went diving the other day and he used no weight with the same tank. With 1000 PSI left he had to fight to stay down. If theory and the books are correct he should have been very negative!

??????

 

[pupdiver]

You are correct. Net buoyancy is weight density of water multiplied by the volume of the diver minus the weight of the diver (which includes the density of the diver).

When a diver loses fat, they are losing volume and weight. When you lose fat, you also decrease some of your other non-fat tissues, so you may or may not change your overall density as dramatically as you might think.

 

[Charlie99]

My son is 6 ft 3 and weighed 225. He lost 50 lbs. Before he used approx. 4 lbs of weight with a HP 100 steel tank. We went diving the other day and he used no weight with the same tank. With 1000 PSI left he had to fight to stay down. If theory and the books are correct he should have been very negative!

Not all that negative. It is just confirmation that fat density is more around 0.93 rather than the 0.7-0.9 some others have posted.

I looked around a bit for info on specific gravity of fat, and found various answers ranging from 0.918 to 0.94, with 0.93 being the most common figure. Using the 0.93 figure, losing 50 pounds of fat would reduce buoyancy by about 3.5 pounds in fresh water, or about 5 pounds in saltwater. That leaves a discrepancy of just a pound or so, since 500 vs 1000psi in an HP100 is about 1 pound.

Charlie Allen

 

[Charlie99]

My fat body sinks like a rock ... go figure ...

Perhaps you have other parts of your body that are extremely dense. Do you have to wear negatively buoyant jetfins to balance out extreme density at the other end? :1poke:

http://www.scubaboard.com/showpost.php?p=2156914&postcount=4

 

[MXGratefulDiver]

Perhaps you have other parts of your body that are extremely dense. Do you have to wear negatively buoyant jetfins to balance out extreme density at the other end? :1poke:

http://www.scubaboard.com/showpost.php?p=2156914&postcount=4

As a matter of fact, I do ... that explains much ...

... Bob (Grateful Diver)

 

[SNorman]

Body shape has just as much effect on boyancy as does its density.

I'd love to see the explanation behind this!

 

[ClayJar]

Body shape has just as much effect on boyancy as does its density.

I'd love to see the explanation behind this!

I think I found the explanation, particularly the part about "very small rocks".

Shape would indeed affect drag, but shape is *completely* irrelevant when it comes to buoyancy in a fluid of uniform density.

 

[Notso_Ken]

I've had more succes in changing my setup to steel tank, and backplate to get the weight off the belt.

Personally, I'd rather have the weight on my belt, on my hips, than in my tank, weighting down my spine. I've watched what my wife went through with 2 spinal disk surgeries, I don't want to do anything to increase the odds of me going through that.