Don Janni:
We might want that "keel" effect if we want a balance such that we're forced into a prone position. That's not what I'm after though. What I want is for the center of gravity and the center of buoyancy to be as close the the same spot as possible so there are little of no apposing forces to create torque on any axis. That way I'm balanced such that I can maintain any position I want to be in. If the tank is where most of the weight is then a wing that wraps around the tank is a good start. Other gear carried and our boddies and suits enter into it too of course.
IME, the absolute worst situation is to be overweighted with all the weight below the bc like a wet suit diver, deep enough to have his suit compressed and has all his weights on his hips. He inflates the bc to compensate for suit compression. The result is a drastic shift in where the buoyancy is centerd and a big time HEAD UP diver. A diver who's very well trimmed at 20 ft can sometimes be a real mess at 100 ft. That's why I don't dive deep in a heavy wet suit, if I have a choice, and if I do I take steps to even things out at depth. It's also the reason why I don't think single AL 80's make any sense for cold water diving. You just have to put that much more weight on the belt which is too far below the bc for many wet suit divers. While a front to back inbalance isn't the best situation it's not much of a problem in a horizontal diving position though it would make it hard to swim on your side or stay vertical. Who wants to do that anyway? Now there might be divers with issues that don't fall into this (like divers with really floaty behinds) but about 90% or more of the divers I see are head up/foot down. Part of it is a skill issue, part may be overweighting but the rest is built into the equipment configuration that they were sold on early in their diving (wet suit, AL tank bc on upper body and weights below). It might work fine in the tropics where suit compression isn't an issue but then you don't really need any bc at all. Around here those divers just leave a trail of silt everywhere they go and getting a plate on their back (which is under the bladder and up high) is a good step in the right direction for them. This concept of managing the centers of buoyancy and gravity to control trim is really pretty simple to understand and manage. They should really teach it to divers from the start. It would save so many divers so much trouble and clean up our waters so much that it would be the greatest thing since sliced bread. As it is most divers have to wait til their cave training to see a text that illustrates something that should be in every OW manual. Unfortunately it's all kept a big secret. That's why cave divers have such great trim...they read chapter 6 of the NACD text (I don't really remember which chapter it is, I made up the 6).
The keel effect you mention though is one of the things that many side mount divers really like. The tanks, lights and everything else are below the bc yet not much below making any position easy to maintain.
