Agreed; there simply is no objective test data. To postuate on a "No BC" configuration, one could argue that since it arguably would offer the smallest in cross sectional area, it should be better. However, in addition to cross-sectional area, there's also the drag coefficient in the calculation and it is entirely possible for a larger-but-more-streamlined shape to come out better overall. For example, consider
this old US Divers design.
Yes, but that's a very short answer to a very long explanation.
Basically, it has to do with trim, and the fact that being trimmed out horizontally doesn't mean that you're also trimmed out vertically.
To undertand trim, one needs to understand that it is the management of the diver's
Centriod locations for both his weight (CG) and his lift (CP). We normally just think about this in the
horizontal orientation, but there is also one's
vertical orientation.
In Engineering terms, whenever the centroids of weight (CG) and lift (CP) aren't in vertical allignment, the mismatch creates a "Moment Arm" based torque that tries to rotate the frame so the two are in vertical allignment. If this orientation isn't the rotation angle you want, you need to adjust trim to make it so...or you fight it, which burns air.
It is pretty straightforward & easy to do with a Wing for horizontal trim, due to the nature of the BC's design: the "rear" air bubble is an inherently more stable system for horizontal orientation, particularly as it pretty much automatically solves "roll" trim. However, when one rotates the diver by 90 degrees to now be vertical, its advantage now becomes a disadvantage, since what was "up" (and beneficial), it is now "out" to the side (to the right in the above illustrations), which serves to increase the moment arm length (too bad we're trying to make it be zero) more so than a Jacket design.
Sure, there's ways to fix this. Since our ultimate goal is zero out the moment arm, we just need to allign the diver's CG to be directly under his Lift (CP), just like we did in horizontal trim. For the vertical trim condition where the CP is behind the CG (eg, from a wing's bladder being behind the diver), you either need to move the CP forward, move the CG backwards, or combinations thereof. For example, changing to a more negative steel tank moves the CG backwards (thus, closer to the CP). Similarly, if one uses a drysuit and puts a bubble of air in it instead of in the Wing, that moves the CP forwards (closer to the CG).
Of course, for the warmwater diver in a tropical wetsuit who is diving with a dive resort's floaty AL80 tank, neither of these options are available for him to correct vertical trim. What can easily happen is that he runs out of 'adjustments' to make - - keep an eye out for divers who add a lead weight onto their tank strap, for example...and also for BP/W divers who fill their STA with lead.
Similarly, adding a negatively buoyant UW camera held in front of the diver is great for horizontal trim ("Heavy Stuff Down"), but it accentuates the potential 'face down' trim imbalance problem when the diver vertical and (often out of habit) is still holding the UW camera in front of him. Part of the solution can be as simple as "So don't hold it there": hold it in one hand, off to the side.
Which still leaves us all with one more way that one's vertical trim can be managed. Everything above simplistically assumes that the diver is a stiff, immobile board to keep the problem simple. However, what the diver can do is expend energy to tread water to counter whatever overturning moment is present ... and this motion obscures its presence.
So why isn't this seem to be a huge problem? Because while a diver can actively resist the torque by treading water .. he can do so for only so long until he fatigues out. For the generic common rec dives where the diver swims right back to a moored boat (or a good drift dive operator is able to follow flags well), the float times are sufficiently short such that the energy consumed isn't a big safety issue...but if you watch carefully the next time you're out on a boat, you'll probably notice some 'treaders' (watch from below water too, for constantly kicking fins on surface floating divers). Unfortunately, what this also means is that on long surface floats, this vertical trim issue very well can be fatal due to this requirement of a continued ongoing exertion.
Its an interesting subject; I hope this starts to explain the contributing factors involved.
- - -
In any case, I did finally get a chance to get down to my LDS to look at some Jacket BCDs for the OP as I had promised.
After looking at the toys, as well as a nice chat with the LCD Owner / Instructor, for ScubaPro equipment, I'd recommend consideration for the "
Glide Pro".
I found that several of the old school "simple" BCs didn't have trim pockets on the shoulderblades, which I wasn't sure if that was of interest or not. Overall, the Glide Pro is fairly similar to (but also cheaper) than the Classic. My LDS's Owner/Instructor claims that it is what he currently teaches in and likes it ...for what that's worth. I figure that it looks promising enough to be worth a quick test dive in the LDS pool while carrying my UW camera system to see how it does. The trick for me is to make sure to have it rigged to a cheap AL80 instead of one of their steel tanks they do classes in.
-hh
But considering we all know bottlenose dolphins can swim in the vicinity of 30 mph if they want to, I don't think it likely anyone would think I meant I could do this.....they cruise at more like 5 mph, and that was what I was talking about....
