You said
better posture (relation of body parts) will fix most trim (body orientation).
Better dive posture will fix most of your trim problem.
Gravity does not care about your posture, all it cares about is the relation of your centers of buoyancy and mass.
I argue that your 'posture fixes most trim problem(s)' approach/suggestion ignores weight distribution’s big impact. Which is a disservice to novice divers. It does seem divers report holding their legs up but still rotating tail down. Suggesting that these frustrated divers may have good posture, but being tail heavy is what is pulling their tail down.
I proposed assuming you already have that better
posture. Leaving nowhere better-er to go on
posture. I did not say “perfect“
trim.
you have better dive posture already. Your posture is in fact ideal.
My point was that even if your posture is ideal, you trim may still be pushed toward vertical by gravity. If they are aligned very close to vertical, then the tipping forces will be very little. If they are very out of line those forces will be greater.
Like pushing on a wrench
It is no more complex than pushing on a 12" wrench attached to a nut. In the diver case, gravity is trying to rotate the diver/nut from the initial position. The wrench (gravity) does not care at all about the shape (posture) of the nut (diver) beyond how it affects where the two opposing forces are -- the points where one force is pushing one way (center of mass) and a different force is pushing another (center of buoyancy).
* If the wrench handle is pointing straight up and you push down on it, straight at the nut, nothing much will happen. For the diver, this is where it is easy to keep yourself in a horizontal position, as gravity is not fighting you much, and not consistently in any one direction.
* If you put the wrench end two inches off to the side and push down, you can turn the nut, if it is not stuck. In the diver case, you can compensate to prevent this by pulling legs in (shifting the end of the wrench back to above the nut). But you use up some of your ability to adjust for currents etc. Or you can fight the rotation by sculling.
* If you put the wrench so it is 12" to the side of the nut, and push straight down, you can turn a rusty nut. In the diver case, gravity will yank your ass toward vertical despite all your leg pulling in and skulling.
All that matters is if you are pushing straight in line with the nut (aligned vertically), or have some moment arm on the nut, pushing down 12" to the side of the nut. The shape of the nut (diver) does not matter, aside from how it affects the centers of mass and buoyancy, the ends of the wrench.
Diving
I agree that the skydiver posture is a very useful one. I use it. It puts your legs in an ideal spot for frog kick or to influence body orientation by shifting your mass slightly. But all
gravity cares about is the resulting relation between center mass vs center buoyancy.
If you are in perfect
posture, and your legs are still sinking,
across several dives, at the mid point of your gas usage, then you can not fix your rotation by getting in the posture. As you already are. You can use up the dynamic adjustment range by pulling in your legs. But that is not better posture. That is using the adjustment range your posture gave you to compensate. By making the posture
less ideal, as you are using up that adjustment range. For something that is not dynamic, as it has appeared on all of your last few dives. At the average point of the mass distribution swing caused by your gas use.
Shifting lead
So if you are already in a better posture, yet tail heavy over several dives, shifting lead will fix that. This is nothing more than all the talk people have of shifting to heavy/lighter fins, moving bands up/down. Just doing it with lead as that has fewer effects on the rest of your gear. For a rec. diver! Tech divers with no lead, I leave to them. If they are in better posture already, they have no better-er option there, leaving canting their bubble, pulling legs in, or sculling. But rec. divers typically have lead. So do not need to mess with those options.
Posture
How your posture matters is to make pulling in your legs shift your center of mass horizontally (a very useful thing) and make it that a frog kick pushes you horizontally and not some mixture of horizontal and vertical (also very useful). And to keep your legs off the cave/reef floor, again very useful.
Center of mass above vs below.
All this assumes your center of mass is about that of your buoyancy, so even if you have them close, you need a little bit of constant correcting to keep level. As the system is unstable, trying to flip you. If mass is below buoyancy, then gravity works to keep you wherever mass is below buoyancy, keeping you belly down. Pushing down on a, closed end, wrench that is directly below the nut, keeps the nut where it is, despite any fingers, acting on just the nut, trying to turn the nut this way or that. If the fingers on the nut succeed a bit, then the wrench end moves further to the side getting a bit of moment arm to turn the nut back to the starting point.
Posture (a beautifully shaped nut) vs. Mass Distribution (an uncaring wrench)
In verbal pictures
- So let's say the center of the bolt is where buoyancy is pushing up. The nut/diver is fixed around the bolt. The wrench/gravity is trying to turn the nut/diver.
- The analogy is inexact, as the diver does not rotate around their center of buoyancy (the bolt). But they rotate, which is enough.
- Gravity is pushing down at the end of the wrench.
- Say for simplicity, the diver has heavy fins on. So moving them has a bigger effect on the center of mass than on the center of buoyancy. Yes, they have an effect on center of buoyancy. But, unless they are neutral, they have a bigger affect on one than the other, so let's look at their net effect. So moving the legs in or out at the knee affects, in net, 'just' center of mass.
- We could think of the legs as a crows foot extension on the wrench or bending it a bit. As they bend in or extend, they shift the divers center of mass a bit, so they shift where gravity is pushing down on the wrench. Now, the lower leg and fins, the part that we can move closer or further from the body, are not really that heavy, relative to the mass of the diver, gear, and lead. They have a good range of motion, but they aren't moving much mass. So, to me, they may be able to move the wrench end a little bit one way or the other, but not hugely. They change things a bit, but the nut is still going to turn if the angle / moment arm is big enough.
- Sculling is the fins, like claws, clawing on the bolt in one direction or another to prevent the nut from turning. They are a force separate from gravity that is actively trying to keep the nut in its original position.
- Now, the nut/diver is much better shaped/postured than your common nut, just a beautiful nut/posture. Could not ask for more. About which gravity (and the wrench) cares, not the least. Beyond where that puts the wrench end points.
- The tank starts full, putting the mass (wrench end) in some spot, and goes to empty, shifting it that to a different spot. But likely not a bigger change than the legs could adjust for. If they were not busy with other work.
- Over several dives, gravity keeps winning by pulling the butt consistently down. The wrench is angled enough off vertical (mid way through the tanks effect) that the legs plus the sculling can not keep the nut in its original position.
- But, we have some lead. And shifting lead moves the end of the wrench. Remember, the wrench end (plus the legs extension) is the center of mass.
- Now, moving lead has a big effect. Moving 4 lb. from your hips to the top of your back or your shoulders has a big effect on your center of mass. So if we have a modest amount of lead, and can spare some from the belt area, then we can have a significant effect on the end of the wrench, center of mass.
- So, moving lead has few costs. Not much movement is needed. It does not diminish the ability of our legs to adjust to dynamic changes. As it is rather dense, it barely affects center of buoyancy. It does not use energy like sculling.
- After moving some lead, ashore after a few dives, the wrench end (center of mass) is over the bolt (center of buoyancy) at the mid point of the tank use. The legs can default to a midway bend point. Maximizing their usefulness for dynamic changes. A good body to leg posture keeps the legs up and off the reef/cave floor, in position so that kicks push us horizontally and do not also change our depth.
- That's my story. For rec. divers. YMMV.
Way too long. Sorry.
I get posture's benefits: dynamic adjustments, horizontal propulsion, vertical drag, and not hitting/kicking the bottom.
I do not get why some people do not get simple shop mechanics / playground physics.
And I feel that is a disservice to novice divers.
A good reference for new divers:
99 Problems....and your TRIM is one! %