Damselfish:
I don't think I'm the only person this is heard from. BIs face planting people is often mentioned, then quickly put down as a fictional thing that doesn't really happen. Just maybe, sometimes it does...?
Yes, it does happen. Anyone who believes this to be 'fiction' is welcome to give me a call and we'll head over to my local diveshop's pool for a demo...if I rotate face-down, they owe me dinner at the Black Forest Inn.
The physics are straightforward: the BC is a lifting force, and it is a vertical vector up. The diver's mass and the rest of his gear (weightbelt, etc) gets expressed as a vertical vector down.
When the buoyancy vector and the mass vector are the same total magnitude, then the diver is neutrally bouyant.
However, this is not the only thing that's happening. We're also concerned about what we call 'trim'.
Unless these two vectors are
also perfectly alligned on the same vertical axis, then the horizontal distance between the two of them creates a "Moment Arm" that is the source of a torque. This torque is trying to rotate the diver to the point where these two vectors are vertically alligned, with the cnter of buoyancy (CB) located directly over the center of gravity (CG)
The magnitude of the torque is: (Moment Arm length)*(Force magnitude).
Now if we have our gear well sorted out, we will have had minimized both these variables, so that the created torque will be small and thus, pragmatically negligible to the diver...hence, "good trim".
However...
First, we have two basic conditions: diver horizontal and diver vertical. Its easy to minimize the moment arm for ONE of these conditions...its a lot harder to keep it minimized for BOTH. Generally speaking, we tend to optimize trim for the horizontal postion. Unfortunately, the "face down" issue is for when the diver is vertical, usually after the dive...empty tanks, and regulator out.
Second, we also have a lot of diver-to-diver variation in gear selection that will influence things. Back Inflation is part of the example here, and tank selection can play a big part: a single AL80 will act very differently from dual Steels.
The bottom line is that the optimium rig depends a lot on your rig and what you're doing, and what you believe is important.
For example, my current rig is an easy rec configuration that includes a fairly negative UW camera system, and while its UW horizontal trim is fine, it has what I personally consider to have an unacceptable amount of "face down" torque in the vertical (surface) position, despite moving weights rearward to try to shift my CG to be closer to my CB. Overall, this undesirable torque is the additive effect of:
- BI BC (positive on rear)
- AL80 tank (positive on rear)
- my UW camera (negative on front)
And FWIW, its not that I can't manage the torque that my rig creates while on the surface after a dive...its that because it does take a non-zero amount of energy to counteract, and because I have been in situations where the seas didn't allow the "simply roll onto my back" resting position while the chase boat didn't find us for a half hour...my personal trade-off choice is that surviving the surface drift after the dive is a higher priority for me than mere 'trim' comfort during a dive...
[/uote]I've actually bought several pockets intended for harnesses and tried them on the strap but none of them have worked very well for one reason or another. I'm using the least objectionable of them currently. When I get around to it I'm going to make the perfect pocket...[/QUOTE]
Let me know what you find. The next step for me before selling this BI BC is to place a weight on the tank strap; I've already cleaned out my front WI pockets and filled the rear trim pockets, plus put the balance on a weightbelt that I put on the small of my back to try to move my CG rearward to be closer to my CB. I could ditching my UW camera, but that's not really an option IMO.
-hh
