Sparky:
In going from a Balance to a bp/wing, you should be removing at least 6 lbs... Because that weight is now in the plate. (And positioned differently.)
In fact, you're likely to actually remove more than just six pounds... The Balance actually has some inherent buoyancy in it which the bp/wing does not have. Additionally, the SS hardware and single tank adapter (STA), if you use one, has weight to it as well. It's not uncommon to hear people on this board remove 8 to 10 pounds when they switch to a bp/wing.
Have you tried checking your weighting? There's a variety of ways to do it, but perhaps the best known way is the way that PADI outlines in their OW manual.
Before you can address where to place your weights, you need to address how much weight you need... And frankly, from what you're describing, it sounds like you may be overweighted.
Of course, I reserve the right to be completely wrong about this...
You may already be correctly weighted, and you may have already addressed these things. But it sounds to me like you might be carrying too much weight.
In a 7 mil suit, it's common for people using a Luxfer S080 AL80 tank to wear about 5 pounds on each side of a weight belt when they use a "standard" backplate and light "standard" STA. Your requirements may vary, but 16 additional lbs certainly seems like a lot of weight.
Wingtip once bubbled...
You're center of gravity has moved when you're on your back in that manner.
I was hoping that someone else would chime in on this one - I've already taken more than my fair share of this thread. But since nobody has...
Sorry, Wingtip, but that's simply not correct.
Unless you've dropped weight (or picked up weight) during your dive, your center of gravity does not change, no matter what position you are in. The only way that this could happen is if you moved your weight around... And if that's happening, then you need to secure your weight better on your body.
It is possible, however, for your center of buoyancy to move. A jacket style BC is a good example of this. In fact, Scubapro actually brags about it when it comes to selling their Classic Plus BC. They claim that the "wraparound bladder" allows the bubble in a partially inflated BC to always travel to the top of the BC... Which is true. This, they claim, allows the bubble to never interfere with the position of the diver.
This isn't entirely correct... It's really a sales pitch, tinged with half-truth.
Those who have experienced an incorrectly configured bp/wing know that if the wing is too large for the tank, it can "taco" over the tank. The situation is made worse if the diver is overweighted, because in order to be neutral, a larger air bubble must be in the wing for the diver to be neutral. When the diver "tilts" to one side or the other, the bubble rushes to the high side of the wing, which effectively moves the center of buoyancy away from the diver's center of gravity... Which can force the diver over to one side. In the article I cited above, I called this action an "opinionated BC."
A simple solution is to do what Scubapro did... They wrapped the bladder around the diver, effectively preventing the bubble from ever getting that far from the diver's center of gravity. In effect, it works, and this is their sales pitch.
The problem is that it's not the diver that changes buoyancy throughout the dive... Sure, the exposure suit changes with depth, but by comparison to the tanks, not much. It's the tanks that change buoyancy as they empty over the term of the dive.
Since it's the tank that's changing, the device used to compensate for that - the air cell - needs to be as close to the tank as possible. The design that does that best is the "wing" part of a BC.
Additionally, a wing design helps to maintain a solid position of the center of buoyancy... With a wraparound air cell (which wraps around the body of the diver), every position change by the diver results in a shift in positioning of the air bubble inside the partially inflated BC, which creates a center of buoyancy that moves considerably. Now, the diver must manage not only his buoyancy and trim, but must do so while the center of buoyancy moves all around.
If the diver started with poor trim in the first place, this might not be such a bad thing. But I suggest that a better solution would be for the diver to perfect his/her buoyancy and trim, and have it "locked" in place by equipment that does not allow for the shift of either the center of gravity or the center of buoyancy.
These subtle differences in gear design and configuration is what helps highly skilled divers to "hover" like a genie... Sure, it's to do with skill, but if you're a new scuba diver without that skill, it can appear impossible to accomplish. That's because there's subtle differences in gear design and configuration which help the diver to make the most of their skill... Which is developed after many dives in gear that does not allow for a shifting center of buoyancy.
You can be completely neutral at any given depth, roll over and with the bp down below your body at the same depth find yourself negative.
This is not correct, either, Wingtip. Sorry. While a diver's body position in the water column can shift the center of buoyancy if his gear allows for it, it cannot change the forces of gravity or buoyancy. If he/she was neutral to begin with, then he/she should be neutral in any position.
I suggest that one of two things is happening, if you find that you do not have the same buoyancy in all positions...
1. You are negative to begin with, and using your hands and fins to maintain depth. Once you flip over, you can't do this any more, because your hands and feet are pointed in a different direction... And so you sink.
2. On flipping over, you exhale, reducing how much gas you have in your lungs (reducing the amount of water displaced by your body) and you sink.
The solution to both of these problems is to work on your buoyancy skills. The common suggestion is to fold your hands and cross your ankles and use your lungs to change your depth up and down about a foot, while in a horizontal position. If you can't do this, then you need to use a combination of proper weighting, properly placed weights, gear that maintains a center of buoyancy no matter how big the "air bubble" is inside of the cell, and of course, practice. The emphasis should be placed on the last solution: Practice.
It's not necessarily overweighting.
THAT is correct.
Overweighting, in and of itself, isn't usually the cause of trim problems. But it will make a small problem worse, and it requires more skill to manage it. To keep things as simple as possible, start with correct weighting, and then address the trim.
