Gosh durnit Uncle Pug, I havent had the time to reply to this note and you beat me to my points.
When not moving (static), your big two contributors to buoyancy are, in order:
1. Weighting
2. Breath control
Breath control is something you get a handle on by just playing with it. Theres enough hysteresis in the system to give you the appearance of perfect, non-moving neutral buoyancy once you practice it.
Once you start moving (dynamic) things change however, in that the big two become:
1. Weighting
2. Trim
Breath control is a distant third because you can overcome the delta buoyancy of breathing by very subtle changes in your kick, so subtle that youre not even aware of them.
The fact that trim is so very important is why Im a backplate bigot. Ill come back to this later.
The most buoyant part of ones body is the chest, where these huge flotation chambers called the lungs are located. The most negative portion of the body is the most muscular, the legs. Strapping any kind of weight just between the two, like a weight belt or even integrated weight pockets adds even more negative buoyancy to the end of your body that is ALREADY negative, giving, Id estimate over 90% of divers a head up attitude in the water. As Uncle Pug stated, in this attitude when you start swimming not only do you get lift off your chest forcing you up, but your fin kick thrust is down. To counteract this you have to modify your kick such that some thrust is keeping your legs up. Not only is this a waste of your propulsive energy, in some cases you need to dump air from your BC to keep you from going up due to the downward thrust thats keeping your legs properly trimmed.
You need to distribute your weight such that your natural body position is horizontal, not vertical. Many jacket BCs these days come with trim pockets which hold a pound or two, which may work if youre only diving in warm, tropical waters with only a skin. You need to move a significant amount of your weight over your back to be correctly trimmed, so the proper thing to do is to take a significant amount of weight off your belt and put it high on your cylinder. One easy way to do this is to thread a weight on the tank band before you tighten it over your cylinder. You still end up with plenty of ditchable weight, but now your weight is balanced about your center of mass (if you correctly distribute it).
As UP says, if you can be perfectly still and perfectly horizontal, youve nailed your trim. You can start, stop, hover closely to a reef without touching it, etc. without ever touching your BC.
My apologizes, NetDoc, but Im going to take a poke at you.
NetDoc stated to practice fin pivots. By all means jump in the pool and try and do a fin pivot. If you can successfully do a fin pivot, youll know for sure that your trim is all wrong. Youre trimmed feet down if your feet remain on the bottom as your body rises. If your entire body, from fintip to head, comes off the bottom of the pool as a unit, youve got your trim right. One suggestion I give to people is to throw a handful of coins in the pool and go around picking them up. If you can drift down to the coin on an exhale, pluck it off the bottom without any part of your body touching the bottom, and then rise away in a horizontal position on the next inhale, youve got it. Not only do you know your trim is right, but youre learning what you can do with your breathing. To make it more challenging, shorten your reach to the coin until its only inches in front of your mask when you pick it up. Next time on a reef youll be able to closely scrutinize that feather duster without anything touching the reef.
Youll also see a bonus in a drop in air consumption because youre not using any thrust to maintain correct trim, all your thrust is used for forward movement, so you expend less energy.
A huge amount of blame for poor trim I place squarely on the shoulder of instructors and divemasters. Ever go on a led scuba dive where the instructor or divemaster is trying to be a good buoyancy example at the safety stop by hanging vertically, motionless in the water with a smug look of this is how its done!?
To me they might as well have a huge neon sign hanging around their neck that says: I have no clue.
Not only does this demonstrate that they have horrible trim, but a vertical position is the worst position for gas exchange due to the hydrostatic head thats developed between the top and bottom of your lungs. The alveoli at the bottom are subjected to a greater pressure (about .5 psi) and dont expand as readily as the ones near the top. In a horizontal position this pressure difference is more than halved, and more alveoli can be used for effective gas exchange.
Now for the backplate diatribe. For all the above reasons, this is why backplates work so gosh darn well. A SS backplate puts somewhere between 5 to 11 pounds of weight right at your center of mass. A steel cylinder adds even more. All at your center of mass. The absolutely worst combination in terms of trim is a jacket BC and an AL80. Guess what the majority of divers dive in?
The jacket BC is a very poor solution to the buoyancy problem. It adds no weight on its own and it forces you to put all your weight lower than your lungs, though they pile a hack solution upon a hack solution by introducing integrated weights and trim pouches (can you say thousands of square inches of material to increase drag?).
A backplate is simple, maintainable, modular, more streamlined and puts the weight exactly where its needed, not exactly where its not needed, as in the case of a jacket BC. In the last five years theres been a huge groundswell in backplate followers and backplates are starting to creep into the mainstream, though in the guise of tech gear. Theyre not tech gear, theyre just as effective on a warm reef as a couple thousand feet back in a cave. They make diving easier, simpler and more enjoyable. Because of many shops requirements that instructors and divemasters can only wear what the shop sells, most professionals cannot wear a backplate system when teaching at this point in time. When backplates become a little bit more mainstream maybe theyll have that option and can begin teaching students correct trim, rather than just static buoyancy control.
Roak
