What affects buoyancy? Weight vs. Size

[JcoldwaterIL]

Buoyancy is afected by both size and weight.


The simplist way i can think to explain it is..

If an object weighs more then the volume of water it displaces, it sinks.

If an object weighs less than the volume of water it displaces, it floats.

If an object weighs the same as the voulme of water it displaces, it is neutral

 

[JahJahwarrior]

units of weight per unit of size is a unit of density, correct?

This deals with being more or less dense than water, not size or weight. A very heavy steel chunk that is hammered to be thin and wedge shaped, with nothing in the middle is a boat and floats, but a very heavy steel ball is cannon ordinance and often sinks things along with itself. On the same note, a very large piece of foam and a very large piece of lead. One will float,one will not.

 

[Frasermcd]

I would love some input from other "experienced" divers on this.

I have personally found that my bouyancy has changed hugely with experience. I used to be a 22lb diver in a 3mm. I am 6'2" with average body fat (a little bit of a beer gut). I was at eye level in salt water with an empty tank and a normal breath"

Now I have found in the same gear in the same environment I can happily descend with 8lbs, I havent gained or lost any weight.

Now I have two theories,

a) Over time your body "changes" or adapts to being underwater and becomes less bouyant?
b) my breath conrol now is far superior to how it used to be and I am able to maintain bouyancy unconciously by holding either larger or smaller lung volumes whilst breathing to fine ture my bouyancy. I find that now, I barely ever add air to my BCD after descent unless I get to 30m + when A small amount is needed to offset the neoprene compression.

I would love some input on what it may be, anyone else had a similar experience.

Also I have a buddy who must be 280 lbs and shaped like a sumo, yet he dives in a 5mm and uses no lead at all? now i am confused!!

 

[jbd]

Now I have two theories,

a) Over time your body "changes" or adapts to being underwater and becomes less bouyant?
b) my breath conrol now is far superior to how it used to be and I am able to maintain bouyancy unconciously by holding either larger or smaller lung volumes whilst breathing to fine ture my bouyancy. I find that now, I barely ever add air to my BCD after descent unless I get to 30m + when A small amount is needed to offset the neoprene compression.

Theory "a" is incorrect. It just isn't what happens.

Theory "b" is what happens with edperience and practice but is not really the issue.

Go back and read the post by NW Grateful Diver in this thread regarding "fear floats" etc, etc. This more correctly addresses the changes you have noted in your weighting changes.

 

[BLD_THRST]

I agree with Frasermcd.... sometimes i do see big guys (not the muscular type) who use little or no weight at all.... do our body really "acclimatise" to frequent diving? maybe by some remote possibility that there is some metabolic changes to our body? hehehehe ok ok..... i am no expert......

I managed to decrease the lead weight i use during every dive trip but I am stuck at 2kg.... I started with about 6kg during my OW in the beginning of this year and I have been shedding the lead off until my AOW couple of months back. my gear has stayed the same.... if possible i am aiming not to carry weights at all..... wishful thinking?

 

[jbd]

Vry well could be wishful thinking to use no weights at all. Its possible also. The important thing is to use the correct amount of weight for you in the evironment and gear you are diving in. Doesn't make any difference how much or how little that is, just that its the right amount.

 

[Bill51]

Fear floats and experienced divers using less weight has much to do with the internal 10+ pound BCD we have built into our chest.

The average person has a total lung capacity of about 6 liters, the ability to exhale or inhale about 4.8 liters of that, and typically only uses about .5 liters during normal respiration. At 2.2 pounds of water per liter, that gives one a buoyancy change of 4.8 X 2.2 = 10.56 pounds of buoyancy for a full breath and a buoyancy change of only about 1.1 pounds for a normal breath. The problem comes when one breathes shallow with their lungs relatively full rather than relatively empty or more at the mid-point as one becomes more comfortable underwater and confident in breathing underwater. Shedding that first 5 pounds of required extra weight just by keeping the lungs at a more normal level of inflation is the big step for many new divers so they aren’t carrying too much extra lift in their internal BCD.

Women, shorter people, and people used to living near sea level typically have small lung capacities and will see less improvement as they get experienced than those with larger natural lung capacities.