What affects buoyancy? Weight vs. Size

[BLD_THRST]

Ok. Hopefully this doesnt sound stupid. :blinking:

From what I know, buoyancy is affected by the volume of water displaced by the size of the diver in water despite their weight i.e. the bigger the diver is, the more buoyant. However, fat is a relatively less dense tissue than muscle, which again will increase buoyancy.

Assuming that a diver has a good control over their buoyancy, the amount of lead to overcome their positive buoyancy during their dive will stay the same as long as their size remains the same. This also is assuming that there is no change in their gear.

In other words, if I want to reduce the lead weight used, I must shed body fat and / or get smaller... heheheh my reasoning is, the max u can ever go to reduce yr natural buoyancy is a full exhale and the optimum lead is still required to descent due to the amount of water u displace.... only when yr body size and the negative buoyancy of yr gear is in sync, then no weight is required?

Am I right to say that as long my fitness level and size stays the same, the lead weight used will also the same? Ignore wear and tear of equipment especially neoprene on worn out wet suits.

This question arose as I notice that most DMs which use little or no weights are thin...

 

[BLD_THRST]

I am confused..... I have to re-read Archimedes again.... is it right that everyone is neutrally buoyant without any gear and the lead is to offset the aggregate positive buoyancy of our dive gear?

then why do i notice that bigger size divers have a tendency to use more lead weight assuming that there is little difference in the dive gear....

confused.....

 

[victor]

Could it be that the bigger size divers have bigger wetsuits?

 

[BLD_THRST]

Assuming a normal 3mm suit, the extra neoprene should not make much of a diff, i think.... hehehehe i saw these few divers (diff divers on diff occassions) that were wearing shorties but using weight belts full of lead...... Their average height is about 5'9 to 6ft.....

for me, i can comfortably use about 2kg of lead with a 3/2mm full suit, neoprene socks, gloves, 18AL tank ..... my height is about 5'8 and about 80kg..... basically, wat i am curious is, assuming my gear stays the same, and if buoyancy is affected by weight, i can do a quick ballpark weight check on the trusty ol' weight scale before a trip.....

 

[Walter]

I am confused..... I have to re-read Archimedes again.... is it right that everyone is neutrally buoyant without any gear and the lead is to offset the aggregate positive buoyancy of our dive gear?

then why do i notice that bigger size divers have a tendency to use more lead weight assuming that there is little difference in the dive gear....

confused.....

I'm not sure you ever read it in the first place. Archimedes never mentions if people are postive, neutral or negative. People fall into all three groups, most are positively buoyant. Bigger people are often bigger because they have a higher % of body fat which makes them more positively buoyant.

Am I right to say that as long my fitness level and size stays the same, the lead weight used will also the same? Ignore wear and tear of equipment especially neoprene on worn out wet suits.

Generally, yes. This will not apply if you change anything about your gear, Switch tanks, you could need more or less lead. The same for a different BC. BCs with padding are more positively buoyant and will require more lead. Metal back packs will substitute for some or all of the lead.

Could it be that the bigger size divers have bigger wetsuits?

That's one factor, but not as important as you might think. Some big body builders are negatively buoyant even in a wet suit while someone weighing the same who is fat will be very positively buoyant even without a wet suit.

 

[Luis H]

Substance Specific gravity:
Fat: 0.7-0.9
Freshwater: 1.0
Seawater: 1.03
Muscle: 1.08
Bone: 1.9

In fresh water anything with a specific gravity above 1.0 will have a negative buoyancy. In salt water anything with a SG above 1.03.

Different body types will have different combined SG. A lean muscular person will sink even in salt water, where some one with excess body fat will float (even in fresh water). This is the case even with no gear or wet suit.

The addition of a wet suit can compounds the buoyancy problem slightly for a large obese person, since that person will be wearing more neoprene (specially with a "normal" 7mm wet suit).

 

[MXGratefulDiver]

Substance Specific gravity:
Fat: 0.7-0.9
Freshwater: 1.0
Seawater: 1.03
Muscle: 1.08
Bone: 1.9

In fresh water anything with a specific gravity above 1.0 will have a negative buoyancy. In salt water anything with a SG above 1.03.

Different body types will have different combined SG. A lean muscular person will sink even in salt water, where some one with excess body fat will float (even in fresh water). This is the case even with no gear or wet suit.

The addition of a wet suit can compounds the buoyancy problem slightly for a large obese person, since that person will be wearing more neoprene (specially with a "normal" 7mm wet suit).

A nice rule of thumb, but there's more to it than that.

One of the biggest reasons that a newer diver generally requires more lead than a comparably equipped (body and equipment) experienced diver is because ... "fear floats". Your ability to relax in the water will affect how much lead it takes to sink you. That is mostly due to unconscious movements you may make with your hands or feet, or due to the fact that you're retaining some air in your lungs while you're breathing. As you get more comfortable in the water, you'll relax, breathe more fully, and stop that unconscious kicking or sculling while you're trying to descend (most new divers overload with lead to overcome a difficulty in descending). Your breathing will become more regulated, and you will more naturally expel all of the air from your lungs as you are beginning your descent.

Gear selections other than exposure suit can also impact ... sometimes dramatically ... how much lead you need. Some larger, bulkier BCDs can require quite a lot of lead to offset all that foam padding that looks so comfortable. Some fins float, others sink ... and that will impact not just your trim, but your overall buoyancy too. Gloves, hood, camera, add-on pockets, and many other accessories can impact your buoyancy ... not only because of the materials they're made of, but also because they can sometimes become "air traps". All of these need to be factored into how much lead you will need.

Certainly the proportions of fat, muscle, and bone will impact our density in the water ... but gear choices and our "mental" state will usually have far more impact on our weighting requirements than the overall density of our bodies.

... Bob (Grateful Diver)

 

[terrasmak]

... is it right that everyone is neutrally buoyant without any gear and the lead is to offset the aggregate positive buoyancy of our dive gear?
.

I wish , well kinda . In fresh water with lungs full i float at eye level. Kinda sucks not being able to do a dead mans float. Salt water is a lot better.

 

[SparticleBrane]

I'm slightly overweight but I'm still negative. Well...only when I want to be. I'm positive if I want to be as well. All with no equipment on. Try and warp your mind around that.
As it turns out, if I exhale everything I sink like a rock. If I have a decently full breath in my lungs, I won't. Simple as that.

 

[donacheson]

Ok. Hopefully this doesnt sound stupid. :blinking:

From what I know, buoyancy is affected by the volume of water displaced by the size of the diver in water despite their weight i.e. the bigger the diver is, the more buoyant. However, fat is a relatively less dense tissue than muscle, which again will increase buoyancy.

Assuming that a diver has a good control over their buoyancy, the amount of lead to overcome their positive buoyancy during their dive will stay the same as long as their size remains the same. This also is assuming that there is no change in their gear.

In other words, if I want to reduce the lead weight used, I must shed body fat and / or get smaller... heheheh my reasoning is, the max u can ever go to reduce yr natural buoyancy is a full exhale and the optimum lead is still required to descent due to the amount of water u displace.... only when yr body size and the negative buoyancy of yr gear is in sync, then no weight is required?

Am I right to say that as long my fitness level and size stays the same, the lead weight used will also the same? Ignore wear and tear of equipment especially neoprene on worn out wet suits.

This question arose as I notice that most DMs which use little or no weights are thin...

Weight and buoyancy are both large forces whose difference determines whether a diver is positive, neutral, or negative.

A diver's weight is his/her weight with all gear on scales in air. It's a force directed downward toward deeper water. Buoyancy is the force, directed upward toward shallower water, equal to the weight of the water displaced by the diver and all gear - that's Archimedes principle. A diver increases weight by adding lead and increases buoyancy by adding air to a BC.

Slimming down decreases a diver's weight and also decreases a diver's buoyancy, but since fat is less dense than muscle and bone the effect is to decrease buoyancy more than weight.

Unfortuanately, it's common terminology to use the word "buoyancy" to refer to the difference between weight and buoyancy, hence a lot of confusion. Personally, I prefer the terms "light", "neutral", and "heavy" instead of "positively buoyant", "neutrally buoyant", and "negatively buoyant". Achimedes rules: buoyancy is a large force upwardly directed.