Trim is a function of the relative locations of the diver's
center of lift (the point around which all those pieces and parts of the diver's body and rig that are positively buoyant are balanced) and the diver's
center of weight (the point around which all those pieces and parts of the diver's body and rig that are negatively buoyant are balanced). Of course those points are dynamic - they move a little as the diver breathes, adds or removes gas from the BC or dry suit or counterlung, shifts gas in the dry suit (and even the BC - some are worse than others), uses gas from the tank(s), and with wet suit compression changes with depth.
If we can arrange things so that the center of lift and the center of weight are co-located, all is lovely, and we can remain in any position we desire without effort - pitch and roll forces will be non-existent no matter what position we choose, and remaining there is effortless.
However... getting those points together is seldom possible, and adjusting trim to make position maintenance easy can be very frustrating, even counter intuitive.
The first step to take in the trim adjustment process is to discover what the relative positions of these centers of weight and lift really are:
- establish neutral buoyancy, where able to control depth precisely with just breathing.
- while maintaining a constant depth, lock arms in position with hands together, and lock legs in position with feet crossed. DO NOT KICK or SKULL.
- WAIT - (yeah, it's a real PITA and very un-natural) - DON'T MOVE - until your position stabilizes in both pitch and roll. Where you end up may surprise you, especially if you're wearing (a) steel tank(s).
The position in which the body stabilizes will be with the center of lift directly above the center of weight, and now adjustments to the rig to improve trim can be made that make sense.
There are essentially two positions: on your belly or on your back. What to do depends on this "final position" and goes something like this...
If on your belly, then when in the prone position the center of lift is higher than the center of weight, and all the recommendations folks have made so far will make sense - to get the center of weight directly below the center of lift in the desired prone position is achieved by adding or shifting negatively buoyant stuff (like weights) towards the high end/side and/or shifting a positively buoyant item toward the low end until that balance is reached... the center of lift is directly above the center of weight when prone. This is a stable arrangement - any displacement from that position will naturally swing back toward it.
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BUT, if the diver's stable position is somewhere on the back, we have an entirely different kettle of fish!
In this case, when prone, the center of lift is
lower than the center of weight, and the trimmed out position will be where that center of lift is
directly below the center of weight. This is both
unstable (any displacement will continue to roll or pitch in the displaced direction until the diver has arrived at the lift-above-weight position), and staying in the trimmed position requires the diver to apply constant little balancing forces with fins - or, gasp!,hands - to keep the weight from "falling off the bubble" and adjustments to the trimmed position are precisely backwards from what seems right - that is, since the trimmed position is weight-directly-above-lift, if the diver's feet are low then it's because the center of weight is too far forward and the diver must add or shift weight toward the feet to get the feet up to the desired trimmed position.
And so forth...
Y'all have fun now, ye hear?
Rick
