Does Loss of weight reduce buoyancy ??

[decompression]

Sorry to disagree with you decompression, but in my own anecdotal experience, as my weight of the years has varied from 195 to 220, the variable being fat, not added lean muscle mass, I find that changes of 8-10 lbs impact weighting by about a pound in salt water. Add 8 to 10 lbs of fat tissue, add about a pound of weight. Lose the same, drop the same. I think most people have a similar experience. Recently on an open water weekend (fresh water) we had a student diver who was in his mid 40's, about 6 feet tall, and weighed 275. He used 30 lbs in fresh water with a 7 mil full wet suit. We had another diver, about the same weight, but 6'6" tall with very low body fat percentage. He did just fine with 20 lbs in a 7mil full wetsuit, and could perhaps have used a little less. No way would the "rounder" diver been properly weighted at 20 lbs.
DivemasterDennis

No harm in disagreeing. Physics is physics. There are so many variables (lung capacity/VO2 max, trapped air in equipment, breathing rate, etc) that this is why people believe this. It certainly is believed by many. Through the many hundreds of students I've put through and thousands of people I've dove with there are rare cases that the larger people require more weight, when you look at all the variables, dive experience, comfort level, anxiety etc, those can be contributing factors. Lung capacity alone can account for more than the 1lbs of weight from 10 lbs of body fat. I assume the 2 students you mention wore identical size/type wetsuits, had identical gear and breathed the same under water. Being an engineer, I default to science. I'm sure you've told OW students that when they get more comfortable they can take some weight off.....the reality of a balanced rig calculation really hits home when it is presented, as they say, numbers don't lie.

 

[3D diver]

I had pretty much the same weight reduction and hoped to shed lead too...but bought some thicker drysuit underwear and ended up having to add weight.

D'oh!

I'm pretty warm though.

 

[spectrum]

Physics are physics and the OP was asking about his own personal change in make-up. That pretty much normalizes the many variables and suggest a shift equal to something like 10% of his loss. Barely so if he only lost weight and maybe a tad more if he also added muscle.

Once you start going person to person all bets are off.

Pete

 

[Nemrod]

You may have been more muscular even at the 210 pounds than you thought but at a fit 170, yes, you will find you need less weight.

Oh, and congrats on your weight and fitness program.

N

 

[Hillmorton Scubie]

I am surprised that there is no agreement or consensus on this question, especially with the amount of instructors on scubaboard.

Thanks for the encouragement on the weight loss - MyFitnessPal.

 

[vladimir]

I am surprised that there is no agreement or consensus on this question, especially with the amount of instructors on scubaboard.

Thanks for the encouragement on the weight loss - MyFitnessPal.

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).

 

[tracydr]

After years of being around 210 pounds, I have spent the last 8 months getting down to a lean fit 170 pounds. Previously I have used 9 pounds of lead on my belt to sink with my full suit.
Will I find any difference in weight required on my belt,and if so, any idea how many pounds.
Or is it an old fable without scientific basis, that says body fat makes people more positively buoyant.

Thanks

You will need to do a weight check if it's been awhile since you last dived. But, that's the case if anyone hasnt dived in awhile.
i suspect, you won't need as much lead and you will be more efficient with your air. Congratulations on your weight loss!

 

[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).

Good point.

Specific density of fat .9 g/ml
Specific density of muscle 1.06 g/ml
Specific density of water....1.0 g/ml

.1 g/ml difference between fat and water..........water does not displace water. Especially if it is pure water (human body) to sea water. There would be a slight change if fat was swapped for muscle.

 

[Rhone Man]

Ok, seriously, the fat vs muscle really is more of a mistruth except in extreme cases. Human tissue is mostly what? Water. Is water buoyant in water? No. Yes, fatty tissue is less dense than muscular tissue but generally tissue is 60% water. The reason you can use your body weight to determine lead requirements is that it gives a rough size of exposure suit which is the greatest factor in lead requirement (research the concept of "balanced rig"). I wish instructors would be more thorough when discussing this topic. I'm glad a few agencies have embraced the "balanced rig" in their teachings as it would greatly assist people in diving safer. Bottom line, the reduction in your lead requirement will be minimal.

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.

 

[GrumpyOldGuy]

I am surprised that there is no agreement or consensus on this question, especially with the amount of instructors on scubaboard.

Thanks for the encouragement on the weight loss - MyFitnessPal.

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 ----------

Good point.

Specific density of fat .9 g/ml
Specific density of muscle 1.06 g/ml
Specific density of water....1.0 g/ml

.1 g/ml difference between fat and water..........water does not displace water. Especially if it is pure water (human body) to sea water. There would be a slight change if fat was swapped for muscle.

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