adder70 once bubbled...
I see a lot of thought went into your article.
Thanks.
Yes, you're right.
One difference of opinion I have concerns actual experience. While you show the bladder of a back inflate BC evenly full, I know from personal experience that a Zeagle ranger will fill in areas. If you add air in a slightly head down attitude, the air will migrate to the "bottom" of the bladder, near your waist. This will assist in easily maintaning a head down or horizontal position. You can even go slightly head down, fill some, and go SLIGHTLY head up and add some more to get virtual trim pockets (maybe the ranger has designed trim pockets, I don't know) of air for a neutral buoyancy distribution at horizontal or slightly head down. It took a bit of practice in the beginning, but now it's more or less automatic. (Or it was until I stayed dry (destitute!) for the last 3 years!)
Yeah, for the sake of simplicity I didn't include the shifting of the air inside of the BC bladder... I took the approach of absolutes - i.e. fully inflated or fully deflated - to express my point. Using the same thought process, it's not difficult to theorize where the center of gravity and the center of buoyancy would lie, though, if the bladder was only partially full.
I will point out, though, that a partially inflated bladder worsens the problem I speak of, and only goes to prove the validity of the theory. With a partially inflated bladder, the "bubble" inside of the bladder now can move around to the bladder's highest point. This moving bubble creates a center of buoyancy which no longer "stays put" but changes location. Now, not only does the value of the amount of buoyancy change with the amount of air in the bladder, but the actual center of buoyancy (the green dot in my illustrations) moves as well. While you point out accurately that the movement can therefore produce less distance between the center of gravity and the center of buoyancy, it's nonetheless yet another thing that the diver must manage, and adds to the task loading.
...Which is why minimizing the movement, and keeping the bubble as much in the same place as possible has an advantage over a BC that allows the bubble to move around the body of the diver.
This whole thought process brings out another point as well... Not only is there an advantage in minimizing the bubble movement, but there is an advantage in minimizing the force which the bubble produces. In other words, managing the smallest, most stable bubble is easier - and less task loading - than managing a big bubble that moves around a lot. The way to do that is to swim with as little weight as necessary to keep yourself negative in the lightest of situations... In other words, at the end of the dive... And use a BC with a bladder that's as compact as possible. This has obvious advantages in terms of streamlining as well.
One last thing to drive this point home... Let's not forget that both of these issues... The size and location of the bubble... Are not only unstable, but are exponetially unstable. That is, if the bubble gets a little "out of place" from where it's supposed to be, then the instability is worse. If, for example, a horizontal diver, with a bubble centered over his back accidentally gets a little "head up" in his body position, the bubble moves to the top of the bladder and thus makes his tendency to go even further "heads up" worse. Likewise, if he gets a little head-down, then the bubble moves to the bottom of the bladder and the tendency is for the new location of the bubble to make his "head-down" situation worse. If a balanced and horizontal diver who's neutral in the water column accidentally goes down a little, then the bubble compresses, making him more negative than he was before. If he goes up a little, the bubble expands, and he has the tendency to float to the surface... Because he's more positive than he was before.
The point is that not only are these factors unstable, but exponentially so. For this reason, it pays to minimize the effects by using as little weight as is necessary, using the smallest bubble necessary to remain neutral, to keep the bubble as stationary as possible in the bladder, and of course, keep the center of buoyancy and the center of gravity as close together as possible.
Simply put, a rig that's been "tuned" in this way is more easily managed... And the result is a diver who's trim and buoyancy is "spot-on."
...Which always seems to amaze the newbie diver. They look at those people hovering with perfect control and think, "Wow... That's amazing... I'll never be that good." The truth is that part of the reason why they're "that good" isn't solely because the balanced diver has more experience... It's also because he has less issues to manage.
Could this be called the "secret" to the perfectly balanced diver?
To a degree, the answer is yes... Because this stuff was not taught in "normal" scuba classes.
...Which is something that I can't understand... Perhaps PADI or NAUI feel that this sort of information overwhelms the average newbie diver, but studies have shown otherwise. ...And I can't understand how anyone can expect the newbie to master this basic skill without all of the information.
...Which is why I wrote the article... As a sort of "introduction" to where proper trim and buoyancy comes from. If we all knew this information, we'd be better informed for making decisions on what type of gear to purchase and dive with.
That said, I have an intro certification but can't say I did a great deal of research into equipment, at least concerning BC vs BP/wing setups. I simply got a reasonable BC that would be serviceable for moderately technical diving situations.
Well, it sounds like you're defending your BC choice a little, which makes me think that maybe my article sounded a bit like an attack. I didn't mean it that way... But rather an explaination as to why I chose the gear that I did. Having read the article, at least you can be aware of some of the "issues" - whether they're problems or not for you (which apparently they aren't) - that some BC's can have. Armed with this knowlege, it's easy to adjust your rig - no matter what you dive with - in order to achieve improved trim and buoyancy. The way you do that might be different than the way I did that, but it's my hope that at least it shed a little light on the "issue" so that you can adjust for it... And end up with improved trim and buoyancy control.
