Centre of gravity & centre of buoyancy

[Smache]

But I think CB and CG might be a good way to think about rig setup to maintain neutral trim. If stability is promoted in the video by bringing the CB and the CG together, it speaks to bringing buoyancy closer to where the weight is: use of wings rather than drysuit for buoyancy, aligning weights on the rig in line with where most of the suit and wing buoyancy is, forgoing ankle weights in favour of p weights or tail weights etc…

I had an AOW student recently who had been diving with a single weight pocket on a cam band. In order to make the weight centered, the pocket had to be on the opposite side of the tank as their backplate, and put the their CG well above their CB. We switch to two weight pockets on the cam band so that they could be pushed up against the backplate - immediate improvement in stability.

The biggest difficulty is the CM (center of mass, which is the same thing as CG) and CB shift due to gas consumption and wing deflation. IMO, the best thing to do is to first trim with a nearly empty tank/wing -- this gets the total weight correct as well as the distribution for stable trim. (I.e., the CM & CB are aligned.) As a second step -- without moving any lead -- switch to a full tank and adjust the position of the wing for stable trim. The wing lift is then aligned with and cancels the gas mass as they both decrease throughout the dive.

100% agree. Weight and balance needs to be fine tuned with (nearly) empty tanks. At least in backmount, the wing will automatically be pretty close to the ideal position since it goes right below the tanks.

 

[Smache]

If the CB and CG are together you are neutral, which means you will maintain any attitude you put yourself in, but also have zero stability as any force that's not exactly horizontal or vertical through the centers will rotate you.

For stability you want the center of bouyancy as far directly above the center of gravity as possible in your preferred position. This is why the wing goes on your back instead of your chest.

And then there are divers in steel backmount doubles and a drysuit - the CG is above the CB. (The diver in the drysuit is positive, the tanks and wing are negative, for a net buoyancy of zero.) That's why diving that configuration feels like a little bit of a balancing act. (Not knocking it btw, I dive like this regularly.)

When you put it like that, zero attitude preference doesn't sound so bad.

 

[grantmac]

This is why drysuit divers with heavy heads will frequently dive head up, that lets you keep most of the floation in the top of the suit and above the center of gravity. Because once you level off the gas moves downward and makes a bad situation worse.
Shifting weight down makes getting level easier. My preference is somewhat negative fins and gaitors any time I'm in a drysuit plus I run my bcd for buoyancy compensation through the dive with suit gas for comfort and warmth.