Body Orentation's Effect on Neutral Bouyancy

[Pudge]

I haven't seen this subject addressed on this board, but as new diver, I have noticed that neutral bouyancy at a given depth can be lost with a change the body's orientation from horizontal to vertical or vise versa and air must be added or bled from the BC to regain neutral bouyancy. Logically, it seems for this to occur the BC and/or body must be displacing different amounts of water based on the body's orientation in the water. I would like to know if others experience the same phenomena and the explanation for why there is change in bouyancy.

Pudge

 

[dlndavid]

This doesn't happen to me and it makes no sense that orientation would have anything to do with it. Maybe you hold more air in your lungs when in different positions.

 

[jonnythan]

If you go vertical your lungs and/or BC might move slightly upward in the water column, increasing your buoyancy.

Additionally, I've never seen a new diver keep their fins still for any length of time.. so if you're vertical, you're probably still finning downard a bit, increasing your tendency to move upward.

 

[Rick Murchison]

(edit... I see Jonnythan was posting while I was typing... so this is a little redundant)
Buoyancy is strictly a function of water displaced and weight.
However...
"Neutral buoyancy" is much like balancing a stick vertically on your finger... you have to keep moving your finger around slightly as the stick tries to fall off.
Likewise, if you are perfectly neutrally buoyant and get displaced upward, the gas in your BC & the bubbles in your wetsuit will start to expand and you'll just continue to go up. If you're neutral and get displaced downward the gas in your BC and the bubbles in your wetsuit will be compressed and you'll continue downward...
So, you have to do something to counter this instability, and that is timed breathing. If you're neutral and inhale, you'll get positive and start up - so you have to exhale to counter it. But since you're moving up, you have to exhale more than you inhaled to stop it; you have to get a little negative to counter the momentum. If you just stop breathing as you stop going up... you'll go down. So you have to start inhaling before you even start going back down to get neutral again... and so on.
Now, a change in position is going to cause some vertical movement somewhere - if going from horizontal to vertical then probably upward with your lungs and BC, so you can expect to start up and need to exhale to stop it. So while a change in body position itself won't change your buoyancy, a move upward of just a little bit with the BC or your lungs will...
Make sense?
Rick

 

[D_O_H]

Another factor to consider is that you move up or down more easily in the water column when vertical, as opposed to horizontal, since you're more hydrodynamic with respect to up and down movement in the vertical position. If you're a little bit positive, you're going to rise that much faster if you're in a vertical position.

 

[dlndavid]

Another factor to consider is that you move up or down more easily in the water column when vertical, as opposed to horizontal, since you're more hydrodynamic with respect to up and down movement in the vertical position. If you're a little bit positive, you're going to rise that much faster if you're in a vertical position.

Ok, now this makes sense. Surface area resistance.

 

[JasonH20]

On a similiar note, lately I've tried several times diving face up towards the surface. Inevitably I tend to sink when I do this. All I can think of is I must breath differently / shallow when I spin onto my back? However it is major cool doing this when you're swimming under something and watching your bubbles "pool up" and wrap around stuff.

 

[Pudge]

If you go vertical your lungs and/or BC might move slightly upward in the water column, increasing your buoyancy.

Additionally, I've never seen a new diver keep their fins still for any length of time.. so if you're vertical, you're probably still finning downard a bit, increasing your tendency to move upward.

You're right about that. I find that I kick more and move faster than the more experienced divers. I think a lot of that is due to having to overcome poor trim (overweighted/feet low) but that should get better now that I have my own back-inflate BC w/upper torso trim weight pockets. Properly weighted I was able to remain in trim without kicking in the pool.

I'll have to pay closer attention to the breathing.

Thanks for the help.

 

[Mako Mark]

It has a lot to do with the bouyancy of your breath.

When standing vertical you must draw air down into the lungs and this is difficult while exhaled air has natural bouyancy and reses with less effort.

In the case that it is easier to exhale rather than inhale you will bemore negative than if you are standing on your head and it is easier to inhale rather than exhale.

Anyone who has ever used a rebreather knows that a change in body position relative to the counterlung can give you chipmunk cheeks and nearly blow the mouthpiece out of your mouth.

 

[donacheson]

I haven't seen this subject addressed on this board, but as new diver, I have noticed that neutral bouyancy at a given depth can be lost with a change the body's orientation from horizontal to vertical or vise versa and air must be added or bled from the BC to regain neutral bouyancy. Logically, it seems for this to occur the BC and/or body must be displacing different amounts of water based on the body's orientation in the water. I would like to know if others experience the same phenomena and the explanation for why there is change in bouyancy.

Pudge

It's possible, but it's a small effect and it should always tend to move you toward the surface. For example, if you're truly neutral and horizontal and the air in your BC is spread out, tilting up or down will drive the air in your BC toward the end in the direction of the surface whence it expands a bit and makes you a bit more buoyant. However, that's a very small effect unless one has lots of air in one's BC.