The 'hydrodynamics' are all already accounted for. For example, the camera's actual mass is 15lbs, but because it displaces only 13lbs of water, its net effective ("felt") weight in water is the aforementioned -2lbs.
I agree with this, but there's more to it than that...you're missing resistance to torque due to density of water.
Viscosity effects is generally similar to having a high Moment of Inertia value in that it can mask small forces. But masking is not the same as eliminating. Pragmatically, it has its place, and in this circumstance, the torque is of sufficient magnitude that it is not being masked (by this or other noise sources).
For an automotive analog, I've already cranked up the radio, but we can still hear "bad noises" coming from the engine...so much for trying to ignore the problem.
First, how the secondary mass is supported and "distributed" by the primary body is irrelevant,
Really....tell that to all the bridge designers and civil engineers...their jobs just got a lot easier...
It is irrelevant within the context. Here, the context is of calculating the net Moment Arm length. To review, dig out your Undergraduate Statics textbook.
if I hold the lanyard off my chest by the same distance the camera was held before, the resulting torque is identical; if I flip the lanyard around and hold it to my butt, I shorten the moment arm and reduce its torque.
This I do agree with...but the net effect of the system is not solely related to the torque which is what I'm trying to point out
It sounds like we have a definitions problem/disconnect: which "system net effect" are you talking about?
My discussion has been on the effective "system trim" not while horizontal UW during a dive, but on the surface after a dive, and the tendency for forward (face down) rotation with the Wing-based system I have, which does not have a weighted SS backplate or steel tank: my primary application is for a minimal-weight system for travel, intended to be used with single AL80 tanks. I've been experiencing sufficiently unacceptable (to me) forward rotation that I've been seriously considering going back to a Jacket. Part of this forward rotation may be due to the contribution of the UW camera, but eliminating the camera is not a pragmatically acceptable option.
What's fundamentally wrong IMO is that the location of the BC's lift is too far rearward
Are you sure you're not hunched over in the water...you know bent at the waist....if that's the case, adding more weight isn't going to do squat except make things worse
I can visualize what you're describing. In my specific circumstance, I don't think its a potential candiate, but I'm only 95% sure, so I'll have to doublecheck it to be absolutely certain.
It does, however, raise the question of system's performance objectives. My criteria and preference is for a system that minimizes the diver's energy input requirement for him to remain stable and upright on the surface, post-dive. This would mean that a system that is unduly sensitive to body position (rigid vs a more relaxed one) is a 'bad thing'. It would appear that for a lightweight travel system that will use single AL80 tanks, this requirement appears to be better performed by a Jacket-style and not a Wings-style BC.
Now granted, as a design criteria, this is to a degree a YMMV, but in its defense, considering that the majority of diver deaths occur at the surface and often involve diver physical debilitation and/or fatigue, a system which has a greater tendency to permit the diver to become face down is pragmatically going to need huge benefits elsewhere to justify this trade-off element. Given that no one has published actual drag performance data, that claim remains open and TBD. Given that no BP/W can be bought for under $200 (the retail price point of the basic commercially available Jackets), arguing price becomes an uphill battle as well. Its also hard to argue a safety factor against style...about the best you can do is to argue one safety factor against another.
FWIW, I think my experience has been similar to what gedunk has described:
"The jacket might not be as streamlined or as easy to trim out but it did keep a head up position better at the surface...IMO, there is some good value to that, especially for newer or less active divers."
I'm carrying a big UW camera that can only accentuate that tendency. Besides, small variations in gear streamlining are pragmatically insignificant, so the justification to change BC's for this benefit is very weak. FWIW, I've previously publically offered to sponsor an Undergraduate Senior Engineering Design Project through a Colleage at Steven's Institute of Technology, if Halycon and other claimants of "ours is the mostest streamliningest BC's" were willing to provide a 1-year (school year) equipment loan, but it hasn't happened yet. If I were cynical, I'd say that its because they know that the streamling differences are small and pragmatically lost in the rest of the system's noise.
-hh
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