simbrooks:
Given every item of equipment and the posibility of placing i am sure we could make anyone in any type of BC (wing, BI or jacket) fall on their face the key is to balance it - which really isnt that hard even with an AL80 near empty.
True, it can be done. The question is just if the solution remains fairly conventional, or if we need to reach into our bag of tricks for some "non-standard" items to solve it, and if there's a particular BC design that needs that little bit of extra help more often than the alternatives.
Consider the following:
An empty AL80 is +2 lbs, and because of its 4" radius, will have its center of buoyancy around 4"-6" behind the diver's back. Call it 5" behind the back for easy math.
A Nikonos V with dual SS-200 strobes is around -2lbs, and will generically have its center of mass in front of the diver's chest. Call it 5" in front (again for easy math).
Anthropometrically, 5th/50th/95th percentile values for the Thorax (Chest) of an adult American male are 8.25", 9.50", and 11.04". A nice round value in the middle is 10". We need to know this because the above values are measured from chest and back (not middle).
Assuming that the diver's center of mass is in the middle of his body, then we get the following torque values that this diver is exposed to...when his body is vertically oriented:
AL80: (5"+10"/2)(+2lbs) = +20 inch-lbs
N-V: (-5"-10"/2")(-2lbs) = +20 inch-lbs
FYI, I'm using textbook "right hand rule" for my coordinate reference system, but any one that you could use will reveal that these torques are additive: the net effect is +40 inch-lbs torque.
Note that we've not included the BC yet, but this diver has undoubtedly been torqued "face down".
You can either use science and mechanics to solve or create (also a misapplication of science) problems.
...continuing to apply no-BS science to examine the problem, to look for solutions:
First, let's assume that the BC is providing +8lbs of lift and the diver's still vertical, such as he would be at the surface, where its generally desirable to not be face-down.
If the BC design is a horsecollar, then its buoyancy is right on diver's chest:
HC: (-5") (+8lbs) = -40 inch-lbs
--> Net torque: +40 inch-lbs - 40 inch-lbs = 0 inch-lbs ...diver now has zero rotation.
If the BC design is a jacket, then the net buoyancy is centered on diver's center:
JKT: (-0")(+8lbs) = 0 inch-lbs
--> Net torque: +40 inch-lbs - 0 inch-lbs = +40 inch-lbs ...diver rotated face down
If the BC design is a Back Inflation, the buoyancy is behind diver's back. Assume its the best low profile bladder anyone has ever seen, but is raised by 1" by the STA:
BPW: (1"+5")(+8lbs) = +48 inch-lbs
--> Net torque: +40 inch-lbs + 48 inch-lbs = 88 inch-lbs ...diver rotated face down
worse!
One solution would be to eliminate the camera:
HC Net torque: +20 inch-lbs - 40 inch-lbs = -20 inch-lbs ...diver rotated face-up.
JKT Net torque: +20 inch-lbs - 0 inch-lbs = -20 inch-lbs ...diver rotated face-down
BPW Net torque: +20 inch-lbs + 48 inch-lbs = +68inch-lbs ...diver is still face-down.
The way that we normally counteract this torque is by finning. Since the fin's on its own moment arm...let's call it around 3ft...we can calculate the amount of effort (continuous kicking) it will take for the diver to counteract the torque and keep his face out of the water:
BC -- No camera -- With camera
HC none none
JKT ~1/2 lb ~1.1 lbs
BPW ~1.9 lbs ~2.5 lbs
YMMV on when this finning becomes noticable. Or fatiguing on longer surface floats.
Getting back into the Science, if we try to solve the problem instead by adjusting our gear, its pretty straightforward to calculate how much of our weightbelt's mass we would have to displace rearward. The Horsecollar doesn't have a problem, and the Jacket's pretty easy...the worst case is the BPW, because its needs to generate -88 inch-lbs of counteracting torque. This could be done by:
-20lbs @ 4.4" moment arm length (would put it nearly to the divers back...consider it to be approximately a weightbelt in the small of the diver's back). Of course, if you're warmwater diving, you don't have a 20lb weightbelt, so this solution won't be enough by itself.
-10lbs @ 8.8" moment arm length (puts it 3.8" behind the diver's back, such as a weighted backplate). Of course, if you're warmwater diving, this now means that this weight has to travel in your checked baggage (50lb/bag airline weight limit may apply).
-6lbs @ 15" moment arm length (puts it 10" behind the diver's back, which is roughly the far side of the AL80 tank): this is the "weight on the BC tank strap" solution.
So now the question (at least for me) is: "who wants to recommend a good quality BC tank strap weightholder that's also big enough to accomodate a 5lb or 6lb weight?" If no such product exists, then IMO there is no good solution.
-hh