Does Loss of weight reduce buoyancy ??

[CindyMac]

Just my own observations:

I am "fluffy" and my triathlete brother is very lean (baby bro is ripped!). In their pool, I could not get down very far at all, unless he pulled me down (10ft). He, however sinks. Me, I float with ease, him no so much! In my gear, it takes a good amount of weight to sink me (I'm not telling).

I am working on being less "fluffy".

 

[vladimir]

Lets finish up the math.

40lbs ~= 18 kg (OP fat loss)
1KG water = 1 liter (the metric system is nice here)
1000 ml=1 liter (the ratio you are using is g/ml, lets use liters)

so using you ratio of .1g/ml we apply to the OPs weight loss

.1 g/ml * 1000 ml/l = 100 g/liter= .1 kg/liter
.1 kg * 18 kg =1.8 kg ~= 4lbs

This is close, but wrong. I just did it correctly a few posts above. He was displacing 20 kg of water with 18 kg of fat and 2 kg of lead. He lost the fat, and, for the purpose of simplicity, let's say the lead displaces no water. So now he is wearing 2 kg of lead (not 1.8) and displacing no water. He needs to take 2 kg off his belt.

And yes, good point, if he was displacing seawater it was more like 20.5 kg of water, so take 2.5 kg off the belt.

 

[Tigerman]

I am not sure. I used to dive with an instructor who was also a power lifter, and looked like a power lifter - solid muscle everywhere. He would dive in a 3mm fullsuit with no lead. I have never seen anyone else who came close. I am comfortable guessing his miniscule BMI was the reason.

His BMI was probably not miniscule at all, it was probably indicating he was overweight or even obese - Ive never had as high BMI as when I was lifting weights 2 days and training mma 3 days per week and I was by no means extreme at the weight lifting - too much muscle is a bitch when you try to have cardio as well

Problem is that BMI only account for height and weight, it does not account for how youre built.

 

[Nemrod]

I had a similar weight loss I guess now four or so years ago. I had taken a different job, same company but went from on my feet constantly to in a desk constantly and to top it off, my beloved dogs passed on causing depression I suppose. I went from 175 to nearly 215 at 5'11", ashamed to admit. At that time using a Body Glove 5/3 suit I needed 18 pounds. Now, at barely 162 pounds dripping wet, I use 12 pounds, with the same brand and model of suit. Other equipment is identical.

Oddly, having been a marathon, triathlete, cyclist, swimmer for much of my life, my swim speed over a mile at 215 pounds was better than it is now. But, between then and now I also had a pit bull attack while cycling resulting in a badly broken femur bone, a significant blow to my head and some discussion amongst medical folks at to my having had a blood clot induced stroke as well. So I have that excuse, perhaps all of the titanium hinders.

I would say the OP would need to start about 6 pounds less than previous at his "fat" weight. He may find though that the reduction in girth of course also results in a smaller suit size so there is less neoprene covering less bioprene so the reduction on a case by case basis could be around 4 pounds.

The real benefit is better fitness and health and I am a believer and always have been on the BMI charts. Most people, frankly, need to loose weight, some more than others. Power lifting and weight lifting and typical men's sports do not produce good aerobic fitness, at least please combine those with some activity that results in a sustained aerobic output for 30 to 40 minutes and three times per week (or a lot more).

N

 

[decompression]

Consensus is tough because too many people don't answer you (the OP) and your question, but rather either respond to each or spout off the standard answers (do a weight check) that are correct, but not a response to yours\ question.

I think 3-4 of us concluded about 4lbs (10%) due to the fat loss itself based on the density of fat, in theory.

But that does not really factor in added muscle is you have been working out or lost is you did just a diet and were not working out, which could swing it more or less (since muscle has a density close to 1.06, replacing fat with muscle has a bigger effect). Lastly, density is expressed as a ratio to fresh water, and diving salt water messes up the calculations a bit. There may be other factors that you have not disclosed unintentionally that could effect the final number. Experienced divers / instructors know these add up and thus jump to "do a weight check" rather than babble endless out what is should be theoretically.

---------- Post Merged at 10:22 AM ---------- Previous Post was at 10:10 AM ----------



Lets finish up the math.

40lbs ~= 18 kg (OP fat loss)
1KG water = 1 liter (the metric system is nice here)
1000 ml=1 liter (the ratio you are using is g/ml, lets use liters)

so using you ratio of .1g/ml we apply to the OPs weight loss

.1 g/ml * 1000 ml/l = 100 g/liter= .1 kg/liter
.1 kg * 18 kg =1.8 kg ~= 4lbs

A significant change in body fat, this case 40lb reduction, results in a net loss of 10% positive buoyancy. 4lbs it is, therefore (1 lb of lead for every 10lb of body weight)....OP is now 170lbs he should require appx 17lbs of lead just for his body to just sink in fresh water.....mmm?

 

[boat sju]

Have you guys all been remembering to factor in the actual change in water volume being displaced by the OP's body as it has physically become somewhat smaller. Unless he bulked up, then I take that back.

 

[GrumpyOldGuy]

A significant change in body fat, this case 40lb reduction, results in a net loss of 10% positive buoyancy. 4lbs it is, therefore (1 lb of lead for every 10lb of body weight)....OP is now 170lbs he should require appx 17lbs of lead just for his body to just sink in fresh water.....mmm?

The delta in density between fat tissue and water can be used to predict the delta of the amount of weight the OP needs to achieve the same state of buoyancy, if all other things are the same. He should do a weight check just because it been close to a year since he last dived, who know what else changed. I assume he is just curious is to what he should expect.

That 10% has nothing to do with predicting how much weight he needs. No one said you need 10% of your body weight in ballast. It just happens to be about 10% which is similar to a rule of thumb some people use is coincidence.

 

[decompression]

I can't give you a consensus, but I can give you the correct answer. Human fat has a specific density of 0.903 g/ml. Muscle has a specific density of 1.06 g/ml. If you swapped 20 kg of fat for 20kg of muscle, you could drop about 3.2 kg off the weightbelt. Just losing 20 kg of fat, with no change in muscle, makes you 2.2 kg less buoyant.

Your 18 kg of fat was displacing ~20 liters of water (18/.903), which weighed, obviously, ~20 kilograms. You added 2 kg of lead (18 kg of fat + 2 kg of lead = 20 kg of water) to get neutral. You can take that off your belt now (neglecting the minimal displacement of water by lead for simplicity).

So,
20kg (fat) = 2.2 kg (positive buoyancy/lead required) converted to 9 kg (fat) = 1 kg (pos/lead required)

So,
170 lbs (77kg)= 7.7 kg (positive buoyancy) = 17 lbs to be neutral in fresh or

77kg displaces 85 kg of water = 8kg of positive bouyancy to overcome, thus requiring 8kg to be neutral?

However, if fat is .9 and water is 1 then the difference being .1 * 18 = 1.8 L = 1.8 kg
He is now displacing 1.8kg or 3.96 lbs less........

40 lbs body weight = 4lbs positive buoyancy

170 lbs/40= 4.25
4.25x4lbs= 17lbs

It's just not making sense to me.

 

[vladimir]

So,
20kg (fat) = 2.2 kg (positive buoyancy/lead required) converted to 9 kg (fat) = 1 kg (pos/lead required)

So,
170 lbs (77kg)= 7.7 kg (positive buoyancy) = 17 lbs to be neutral in fresh or

77kg displaces 85 kg of water = 8kg of positive bouyancy to overcome, thus requiring 8kg to be neutral?

However, if fat is .9 and water is 1 then the difference being .1 * 18 = 1.8 L = 1.8 kg
He is now displacing 1.8kg or 3.96 lbs less........

40 lbs body weight = 4lbs positive buoyancy

170 lbs/40= 4.25
4.25x4lbs= 17lbs

It's just not making sense to me.

It seems like you went out of your way to unnecessarily complicate it. Let's stick to round numbers and metric units. The figuring I did is only relevant to the fat, not the whole diver. You seem to have generalized it to the whole diver, who has fat, muscle, bones, fluids, etc.—we have no idea what his specific gravity is (until we do a weight check). Hopefully we don't have a diver with 170 pounds of fat.

Let's examine the fat portion of a diver weighing 100 kg. Let's say he has 27 kg of fat. How much water does the fat displace? Well, the specific gravity of fat is .9, so .9 kg of fat displaces 1 kg of (fresh) water. There are 30 x .9 kgs of fat, so it displaces 30 kg of water. The fat on its own would float, because it displaces a weight of water that exceeds its own weight.

Archimedes' principle is a law of physics stating that the upward buoyant force exerted on a body immersed in a fluid is equal to the weight of the fluid the body displaces. In other words, an immersed object is buoyed up by a force equal to the weight of the fluid it actually displaces.

The net buoyant force is 3 kg. The weight of the water displaced exerts a 30 kg buoyant force, and that is offset by the gravitational force of the weight of the fat, 27 kg. To achieve neutral buoyancy, we have to add 3 kg (and displace no water)*. If he loses those 27 kg of fat, he will have to take the 3 kg of weight off his belt to remain neutral. If you're looking for a simple rule of thumb, multiply the lost fat by .11 (not .1) to find the change in buoyancy.

As has been correctly noted, this is all theoretical and subject to a number of simplifications, but it might be a good starting point for an in-water weight check. And it is a good review of some basic physics.

*for simplicity's sake, we assume lead displaces no water, but of course it does. Its specific density is ~11.34, so 1 kg of lead displaces ~88 ml (g) of water.

 

[ben_wilson3301]

I can say me at 6'-4" and 185 vs my instructor 5'-10" and 225. When we are both in the pool, no exposure suit, identical bcd, I need no lead, he does. Not much, but fat does float, I would agree however... XL neoprene suits float much more.