"Buoyancy and trim" stability: physics principles behind it

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I agree, I'm almost quite over-simplifying, just to make it easy to discuss.



I agree, all that has to do with the equilibrium part only, though, not the stability of it.



I disagree. Let me explain what I mean and please tell me what you think about it.

I'm going to stress the term stable here, I'm trying to deal with the stability of the equilibrium regarding the trim, i.e. only what deals with the rotation.
Granted that I'm talking about a neutrally buoyant config in the first place at all times.

View attachment 618185
Fp=buoyancy force
Fn=weight force
Fp=Fn
Diver starts in an horizontal and steady position.

Case A. Fp is applied on a point on top of the diver. Fn is applied on the bottom. Fp and Fn are aligned (no torque).
Case B. Fp and Fn are still aligned but placed the other way around.
In both cases (A and B) the equilibrium (force and momentum) is granted.


Now, what happens if the diver experiences a change in his trim?
e.g. something, anything, makes him rotate clockwise a bit.
Well, those two forces are not aligned anymore => a moment (do you call it a moment, a torque or what?) is being generated.

In case A, Fp and Fn will generate a counter-clockwise torque which will push the diver back into an horizontal position, hence the starting config A is a trim-stable configuration. The torque resulting from the altered configuration will take the diver back to it's original equilibrium state (and the torque will eventually be zero again) .

In case B, they generate a clockwise torque which will keep pushing the diver away from its initial horizontal position, hence the starting config B is a trim-unstable configuration. The resulting torque will keep pushing the diver away from it's starting equilibrium. Id est to get back to an horizontal state (and counteract this torque) the diver has to actively do something (use fins, exhale, anything ...).

I see what you mean. Taken as a given that the two vectors are vertically aligned, there are two points of equilibrium -- one is stable, and the other is 180 degrees off of it, and is unstable. The point I learned from your diagrams is that you want your center of mass to be as low as possible (low as in, close to your belly), and your center of buoyancy as high as possible (as in, close to your back). So that the stable alignment is face-down, rather than face-up. That makes a great deal of sense to me mathematically, I would be curious to see what @The Chairman and other experienced instructors have to say about it.

Edit to add this paragraph: On second thought, it seems to me that regardless of which vector is above the other, you want the distance between them to be as small as possible. That's because whenever they are out of vertical alignment, the moment / net torque will be proportional to the horizontal distance between the two vectors. The closer they are together, the smaller that torque will be, with the ideal distance being zero. Theoretically, zero distance between these 2 would yield stable trim in all orientations. Since we cannot achieve that, aiming for them to be as close as possible is a good thing, with mass being below buoyancy better than vice versa, so that face-down is stable instead of face-up.

Having your wing on your back as opposed to wrapped around your waist would help to bring your center of buoyancy higher. However I think the thing with the tanks is misleading: it's true that tanks are very heavy, but they are not very negatively buoyant, and some are positively buoyant. @tmassey has a great thread documenting the relative buoyancy of various tanks here. The takeaway there for me is that a typical single tank will be "close" to neutral, full steels are around -8lbs, while empty aluminums are more like +3lbs. So the tank might not be the dominating factor here, compared to your body, your backplate, and your lead for example.

With negative tanks, I can totally see why sidemount would be more stable than backmount. With buoyant tanks, I would guess that the opposite may be true.
 
Yes the vectors prove that it makes sense for a sail boat to have a keel weight and it increases its stability. Clearly, if you want a relaxed, face down position, then you want the weights underneath you. I always try to keep any significant lead on my belt on my hips or forward of that. This is also why a BP/W is going to put you into a more face down position compared to a jacket BC when it is holding air and providing lift under your arms.

With respect to buoyancy, it may also be useful to acknowledge that the whole "thing" is UNstable. The system is unstable because as soon as you inhale, the volume increases, the diver will begin to move up and any air in the BC or the suit will immediately expand, causing more lift. So a diver learns to "balance" a system that is dynamically unstable with respect to buoyancy. The same can be said for the trim but it is more complicated. It can be stable to a point, then become unstable.

So I guess understanding the physics is useful, but it still requires some degree of athletic ability to learn to balance a dynamically unstable system. How do you balance a bike? I really don't know enough to explain it, you just have to learn to balance it. Diving and buoyancy control is more of a feel or balance thing, rather than an intellectual understanding of the vectors.
 
Quick question - when you say you want to avoid trial and error, are you going to apply a mathematical model to your dive gear then make small adjustments in the water?
Partially correct.
I'd like to have a correct theoretical understanding before doing any trial and error test, so that those tests (and adjustments) will then be based on true facts and not vague (and possibly incorrect) speculations. Such an approach is generally more successfull and avoids doing too many trial and errors in the first place.

I see it this way. The math/physics model is helpfull only (but then it's a huge help!) if it's simple and comprehensive enough to take into account the major aspects of the system and its mechanics, and if it provides practical solutions. Most of the times, modeling reality on the paper before doing any real life test proves to be more successfull then simply start trying and hope for the best :-(

Just think of a simple scenario: you start diving, nobody tells you about oxygen toxicity over a specific partial pressure value, you don't read any book, you don't ask any instructor, you don't have any theoretical model of what's happening in your body in such case, you dive deep and inhale oxygen over that limit ... well, don't tell me this kind of trial and error approach is going to be successfull!

Ok, this is an extreme scenario, but again, grasping what's behind a physical problem before "trying" it on yourself is usually a wiser move.

Seems like there will always be some trial and error and then you might need to factor out your steadily improving skills, not to forget adding new kit.
I agree.
 
Anyway, I'm positive I will eventually get it right.
Trim is the foundation of effortless diving. That you're concentrating on trim means that you'll definitely get it right. It's not hard: just essential. Experiment and have fun. I once found myself almost perfectly head down in a pool, with no hope of getting upright. I made it to the ladder and got upright and out. It was a good laugh and I'm glad I was in confined water. I feel the idea that it takes a hundred dives to get great buoyancy is a lie. Once you get your trim fairly horizontal, adjusting your depth by adjusting your breathing is a, ahem, a breeze.
 
I'd like to have a correct theoretical understanding before doing any trial and error test, so that those tests (and adjustments) will then be based on true facts and not vague
Not possible. Again, you're mixing archimedes principle and a center of buoyancy with a center of gravity that's almost assuredly on a different axis and plane. It's statically non-determinate due to all the variables. You have to get wet to get it right.
 
Okay, we have the physics sorted out, and I would like to address an aspect of the skill involved, an aspect that is purely biological and well beyond my knowledge. I am hoping someone with more knowledge of biology can help me out.

I want to start by talking about volleyball. (I was once a pretty good player, and I was certified to coach by the USVBA at a high level.) When a skilled setter receives a pass, the setter puts the hands into the shape of the ball in readiness. The ball hits all 10 fingers simultaneously, and they release the ball simultaneously, with the pressure from the fingers dispersed so that the ball arcs upward with no spin whatsoever. A relatively new setter can do that with passes arriving in a soft arc. A highly skilled setter can do that while falling to the ground to receive a wildly spinning, line drive, shanked pass. It takes many thousands of repetitions to achieve that level of skill.

Think about what that means. When that spinning, line drive pass hits the setter's hand, each individual finger somehow determines its best position and how much pressure to provide to create that perfect set. How on Earth does that happen? I know that a reflex differs from a conscious act biologically in that the nerve impulse travels only to the spinal cord and back rather than through the brain, but that does not seem like enough of an explanation. It seems to me that numerous subconscious decisions are being made at the same time as the fingers react to the stimuli of the situation. It reminds me of the fact that the arms of the octopus can act totally independently of one another, as if they are 8 different beings making 8 different decisions that are somehow part of a coordinated whole.

The same thing happens to a diver in trim. A couple years ago a diver started tech training with me, and the first few dives (backmount) were a disaster. He was literally rolling around with the doubles. His new drysuit was a mystery to him. He now has no trouble at all; in fact, he looks pretty darn good in the water. I would love to be able to claim that my superior instruction was the cause, but most of it is due to the many repetitions that trained his body to react to the stimuli of the situation, like a skilled setter in a volleyball match.

Experienced tech divers swimming in trim achieve much of that by mysterious (to me at least) subconscious decisions being made by the body parts as they react to the situation. Just as a skilled volleyball setter can react to an imperfect pass, the skilled tech diver reacts to different physical aspects of imperfect trim. The position of the arms changes. The position of the legs change. The torso reacts to balance issues. I have on several occasions used rental or borrowed equipment and struggled a little early on as my body tried to figure out how to adjust to the differences in what it was used to. The more experience you have, the better you--or your body--can do it.
 
Okay, we have the physics sorted out, and I would like to address an aspect of the skill involved, an aspect that is purely biological and well beyond my knowledge. I am hoping someone with more knowledge of biology can help me out.

I want to start by talking about volleyball. (I was once a pretty good player, and I was certified to coach by the USVBA at a high level.) When a skilled setter receives a pass, the setter puts the hands into the shape of the ball in readiness. The ball hits all 10 fingers simultaneously, and they release the ball simultaneously, with the pressure from the fingers dispersed so that the ball arcs upward with no spin whatsoever. A relatively new setter can do that with passes arriving in a soft arc. A highly skilled setter can do that while falling to the ground to receive a wildly spinning, line drive, shanked pass. It takes many thousands of repetitions to achieve that level of skill.

Think about what that means. When that spinning, line drive pass hits the setter's hand, each individual finger somehow determines its best position and how much pressure to provide to create that perfect set. How on Earth does that happen? I know that a reflex differs from a conscious act biologically in that the nerve impulse travels only to the spinal cord and back rather than through the brain, but that does not seem like enough of an explanation. It seems to me that numerous subconscious decisions are being made at the same time as the fingers react to the stimuli of the situation. It reminds me of the fact that the arms of the octopus can act totally independently of one another, as if they are 8 different beings making 8 different decisions that are somehow part of a coordinated whole.

The same thing happens to a diver in trim. A couple years ago a diver started tech training with me, and the first few dives (backmount) were a disaster. He was literally rolling around with the doubles. His new drysuit was a mystery to him. He now has no trouble at all; in fact, he looks pretty darn good in the water. I would love to be able to claim that my superior instruction was the cause, but most of it is due to the many repetitions that trained his body to react to the stimuli of the situation, like a skilled setter in a volleyball match.

Experienced tech divers swimming in trim achieve much of that by mysterious (to me at least) subconscious decisions being made by the body parts as they react to the situation. Just as a skilled volleyball setter can react to an imperfect pass, the skilled tech diver reacts to different physical aspects of imperfect trim. The position of the arms changes. The position of the legs change. The torso reacts to balance issues. I have on several occasions used rental or borrowed equipment and struggled a little early on as my body tried to figure out how to adjust to the differences in what it was used to. The more experience you have, the better you--or your body--can do it.
I think that the biological structure involved is called a "neural network".
Such networks are continuosly forming and disappearing and provide abilities impossible to achieve with sequential processing (rational or logic thinking).
I know this not as a biologist, but as a software developer; NNs are the cutting edge of current computer science technology, and is thought to be the way of building intelligent machines which replicate or surpass the capabilities of our neural system.
 
Balance is a learned instinct. However, you can only learn this by using it. The more you use it, the more natural it becomes and you will trust it more which allows you to use it and trust more, ad nauseum. These are rooted in our atavistic responses tied to our survival instinct. Yes, "learned instinct" is an oxymoron, but I don't know how else to describe it.

Most of our lives, our balance is tied to our contact with terra firma, either directly or through a structure(s). During our waking hours, much of this is through our feet. We learn to spread our feet or crouch when we need stability. It's only when we're on Scuba that we get to float above a surface. It's a completely alien experience to us. Our atavistic mind tells us a fall is imminent even while our reasoning mind is busy trying to codify what's actually happening. Our body hasn't quite learned how to correct for any imbalance while floating, so our atavistic side gives us flashes of terror as we over/under compensate. It's a freaking war of opposing wills within you.

We call these subconscious adjustments to balance "muscle memory". Unfortunately, muscle memory takes time to process and requires many successes for it to become truly instinctual. Not to over beat this drum, but that's why it's so incredibly important to get the OW student into proper trim so that they can start the muscle memory process. It amazes me that on the first pool day, students often resemble whirling dervishes, but let them sleep on it and they are incredibly calm the next day. It's why I won't cram all confined water learning into a single day. The more they get to practice being trim and neutral in confined water, the better they will perform in open water. It's a matter of eliminating those terror moments through muscle memory. This is why trim is the second skill I teach, right after mask clearing.

As a caveat, I am not an expert in neural or physical physiology. I am not a doctor and I avoid Holiday Inns. These are my observations in over 50 years of diving, almost 20 teaching and most of those teaching students with no kneeling.
 
The two-days effect is interesting to be noticed.
The build up of a neural network occurs in two phases (which can be repeated several times):
1) collect input data based on the experiences
2) re arrange the neural connections in a more advanced network.

Of course 1) happens when doing exercises in water.
It appears indeed that 2) occurs while sleeping...
So the day after the student will employ the freshly-updated NN, giving him better balance than the previous day...
 
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