Buoyancy Inverted

[boulderjohn]

When I am teaching students in the pool, over the course I hover in different positions, going from one to another. I definitely notice a difference in buoyancy depending upon my physical orientation. I even warn my students about this on CW #4. If they do the hover in a vertical position (head up) and then proceed to swim about for a while, they will be too buoyant and have to dump a dab of air.

 

[The Kraken]

The buoyancy remains the same, but the diver has reduced his cross sectional area of resistance to the water giving gravity the upper hand in the equation.

Keep in mind that when you are neutrally buoyant in a horizontal position, you present a far greater surface area to the water with respect to the directional pull of gravity.

the K

 

[fnfalman]

Interesting. As I recall, gravity is a function of mass and distance. Shape of the mass was not an element of the equasion..

BUT, if a body is close but not quite neutral (like divers which are breathing), then surface area and friction may come into play and effect acceleration.

Gravity alone doesn't dictate your establishment of neutral buoyancy. If you were in a vacuum then size and shape wouldn't matter. However, you are in a fluidic environment (water) and that fluidic environment has drag and drag is dependent on not just the frontal area but also shape.

It's no different than sky divers who go into the frog position has a lesser terminal velocity than sky divers who go head down.

 

[laguna230diver]

When you are "horizontal" you may notice that your feet are slightly lower than your body indicating a slight overweighting. When head down, a fin pivot or other posture correcting measure is more noticeable hence you dont do it and sink.

 

[jim2386]

"I have noticed the exact same thing. I don't know if it has to do with a change in lung volume with posture, or if the buoyancy cell is lower when you invert and is therefore slightly compressed. It's not a big effect, but it's consistent and noticeable."


Based on physics, I'm all but certain this is the answer. When you're level, everything on your body is under the same pressure...wetsuit....BCD....lungs etc.

When you go inverted, your legs are at a lower pressure than your chest and BCD. Therefore, your BCD gets compressed and you lose a small amount of buoyancy. My trick usually is to control your breathing. You may need to add a tiny bit of air to the BCD, but right when I go inverted, I descend a few inches, I'll inhale until I stop moving. After that it's all just a matter of breathing control to stay still.

It's a blast......now in a drysuit things get a little more tricky

 

[redacted]

Gravity alone doesn't dictate your establishment of neutral buoyancy. If you were in a vacuum then size and shape wouldn't matter. However, you are in a fluidic environment (water) and that fluidic environment has drag and drag is dependent on not just the frontal area but also shape.

It's no different than sky divers who go into the frog position has a lesser terminal velocity than sky divers who go head down.

It is quite different from skydivers. If you are truly neutrally buoyant, that means that the particle that is the diver is experiencing the same forces as every other neutrally buoyant particle or molecule that surrounds you. Friction (drag) may dampen movement from random forces and those will be effected by surface areas, but the effect of gravity that may move you up or down in a non-random fashion are zero. Theoretically any way, as the result of breathing rhythmically moves the body through very small values of positive and negative gravity effects.

 

[hudson]

It is quite different from skydivers. If you are truly neutrally buoyant, that means that the particle that is the diver is experiencing the same forces as every other neutrally buoyant particle or molecule that surrounds you. Friction (drag) may dampen movement from random forces and those will be effected by surface areas, but the effect of gravity that may move you up or down in a non-random fashion are zero. Theoretically any way, as the result of breathing rhythmically moves the body through very small values of positive and negative gravity effects.

Indeed. And at least part of the answer is definitely breathing, if we are talking about rolling inverted. When you roll, whatever feedback loop you use to regulate your breathing to stay close to neutral most likely disappears due to the change in orientation. Given the likely reduction in BC volume and perhaps the downward force induced by the tank swinging around, you are likely moving down slightly when you complete the roll. But now you cannot easily compensate with your breathing volume as you are not used to the new orientation and it snowballs from there.

It seems to me the easiest solution is to anticipate this and inflate as you roll.

 

[elmer fudd]

If you're neutrally buoyant to begin with and you're not up near the surface where a foot in either direction would affect your buoyancy, then if your buoyancy changes when you roll over it's because of something different that you are doing. Maybe you instinctively breathe more shallowly while "inverted" or when you kick it pushes you downward. But if you start with true neutral buoyancy, just rolling over shouldn't make any more difference than if you descended a foot and a half or so.

 

[Hatul]

I have found that even when I have achieved mean buoyancy, when I go inverted (which I really enjoy), I descend. I have paid attention to not moving my fins, keeping my breathing rate steady, etc. Whats the issue?
Good Diving

Interesting observation. I think it has to do with wetsuit compression. When horizontal the compression on wetsuit is roughly the same everywhere because it's roughly at the same depth.

Now turn upside down and part of the suit is below the center of buoyancy and part is above. The part below includes your chest and head and possibly arms. The part above includes legs and pelvis. There is much more compressible material below the center of buoyancy than above, and the net effect is loss of buoyancy you start to ever slowly to sink.

Once you sink a bit the compression increases even more and you sink faster and before you know it you're sinking fast.

Adam

 

[Crush]

I have noticed the exact same thing. I don't know if it has to do with a change in lung volume with posture, or if the buoyancy cell is lower when you invert and is therefore slightly compressed. It's not a big effect, but it's consistent and noticeable.

I hope he comes back to clarify . . . I was reading "inverted" as lying on one's back, which is where I have seen the buoyancy difference.

I might suggest that TSandM's experience is due to a small pressure differential between her lungs and her mouth/reg (assuming a perfectly horizontal posture, her reg will be higher than the mean of her lungs) which forces a subtly larger volume from her lungs upon exhalation (and a smaller volume into her lungs with inhalation) when on her back as opposed to on her front. This might make her less buoyant on her back.

As for being less buoyant feet-up vs. feet-down, I cannot rationalize this unless the act of inverting ones self drives one's BC slightly deeper, increasing compression.

Have fun diving!