The purpose of a back inflate or a wing is to assist in keeping a plane (horizontal) profile in the water. If the diver is weighted such that he/she is upright when on the surface, then the weighting won't work well when diving. It seems like simple physics to me. What am I missing?
You are asking a very good question. And, approaching the issue from the perspective of physics is 'spot on'. I really wish more divers would approach their buoyancy and weighting in that manner. I am not sure that you are 'missing' something, as much as possibly needing to give a bit more consideration to the effect of weighting, both total amount AND distribution, and the positioning of the external buoyancy and weight relative to the diver's physiologic center of lift (the thorax). I am working on some pictures, because this topic is not in frequently the subject of SB discussion, but that is taking time. So, some 'textual' thoughts in the interim:
1) I might suggest an alternate view to the statement that weighting which leaves the diver upright on the surface 'won't work well when diving'. I think it can work. I have several jacket BCDs. I use them in the pool for teaching, as much because they are inexpensive yet functional, as because of any buoyancy characteristics. I can dive in good horizontal trim using those jackets,
BUT that is primarily because I am properly weighted, I need to use VERY little air, if any, in the bladder underwater, and I have very little weight on my waist, pulling my legs down (see additional comment on that, below). But, the more over-weighted I am, and the more air I have to use in that jacket BCD, the less stable I become. So, the problem is primarily weighting - total amount and distribution - not the configuration of the badder.
2) To the extent possible, I prefer (and recommend to others) that both external buoyancy AND added weight be placed as closely adjacent to the physiologic center of lift as possible - the lungs, i.e. adjacent to the thorax. That has not always been the case with weight belts and BCDs, be they jacket or back-inflate, although that situation has fortunately changed to some extent over the years. Traditionally, divers have used weight belts, which place the weight well below the diver's physiologic center of lift ( which should be the primary lift during diving if the diver is properly weighted) and below the external center of lift (BCD), when horizontal. That alignment - weight below (i.e. more toward the feet) the center of lift when horizontal underwater - has a tendency to pull the legs of most divers down, because the lift in the BCD and lungs is seeking to align itself vertically with the weight. So, divers end up in a foot-low configuration, that I refer to as being a '45'er'. That produces a host of problems, which are the subject of another discussion. And, over time, since many divers were using jacket BCDs, and many divers were also over-weighted, jackets BCDs developed a bad rap, even though the problem was primarily one of weighting and weight distribution as much as bladder configuration. One other factor to keep in mind:- many divers using jacket BCDs also developed the habit of floating at the surface with much of their upper body above the water, even though it was unnecessary, simply because that where the bladder of their jacket BCD positioned them when it was (over)inflated. That bladder, when fully inflated, wanted to be on the surface, and that bladder was positioned closer to the diver's waist than their neck, so divers got used to sitting high in the water, when on the surface. In contrast, look at some episodes of the classic TV series, Sea Hunt, where you see Mike Nelson (who didn't use a
buoyancy
compensation
device) floating at the surface. His head is above water, and that's about all. And, that's all that is needed. You really DO NOT want to 'bob like a cork' at the surface.
3) So, along come back-inflate BCDs, which move the center of external lift into closer alignment with the diver's physiologic center of lift. That's good, and divers like it, and it does facilitate horizontal trim to an extent, although probably not quite as much as is often asserted. But, it is also more streamlined, and there is less of a tendency to roll, which jackets may create simply because of of the design and placement of the air bladder. Notably, at the same time that manufacturers were moving in the direction of back-inflate BCDs, they ALSO were moving toward weight integration. And, the majority of back-inflate, weight-integrated BCDs put those weights a) below the center of lift, but NOT as much below as weight belts did (so there really was some improvement in horizontal trim), and b) forward of the diver's vertical midline, when viewed from the side. Now you have a configuration which works reasonably well underwater, but has the potential for problems on the surface. The external center of lift is behind the diver (it is, after all, a 'back-inflate' BCD), the center of weight / gravity is in front of the diver (forward of the vertical midline), and those two entities seek vertical alignment - lift directly over weight. That is obviously not a problem underwater. In fact it is ideal. But, at the surface, it may be an issue. If there is little / no weight (which should be the case), and the diver doesn't (need to) use much air in the BCD to float comfortably (head only above water) on the surface, there isn't a problem. If the diver is a bit overweighted, that weight is in front of their vertical midline and it pulls their front down just a bit toward the water. The diver - who wants to float too high on the surface anyway - doesn't like that, and adds air to the BCD. All that does is make the problem worse, as the weight, and now added lift behind the diver, seek vertical alignment. So, back-inflate BCDs CAN create a sensation of the diver's face being pushed toward the water, WHEN the diver is over-weighted, and that weight is positioned forward of the diver's vertical midline, viewed from the side. Fortunately, over time, equipment manufacturers realized that there was a problem and they started equipping BCDs with trim pockets which moved weight to a) the back side of the diver's vertical midline, and b) a position more closely adjacent to the diver's center of lift (thorax, and wing)
4) Finally, let's add in another element - the metal (
negatively buoyant) backplate. What a wonderful addition to the diver's options for weighting! The plate sits adjacent to the diver's physiologic center of lift AND to the the external center of lift - the wing on the back. In fact, it is sandwiched between the two, so the surface flotation characteristics are better balanced. The plate also allows the diver to take weight off of the waist (weight belt) and/or away from the weight-integration pockets (forward of the vertical midline so a)horizontal trim is facilitated, b) the 'face down' tendency of fabric back-inflate BCDs is nicely offset.
This is an admittedly simplistic description - it doesn't consider the effect of cylinders (e.g. steel vs AL), buoyancy of fins, etc. But, it gets at the heart of the issue in many ways. And, not everyone's buoyancy characteristics are the same. Some people are 'leg heavy' (negatively buoyant lower body), some are 'leg floaty', etc. Each diver really needs to work out their optimal weighting, and their optimal weight distribution. One of the reasons that I like, and recommend, metal backplates is that they address one important component of weighting - distribution of weight toward alignment with the diver's physiologic center of lift, and external center of lift. I know what works for me, and am happy to outline why (all of the 'stuff', above). But, each diver should figure out what's best for them. I do like to see accurate information disseminated whenever possible.
