"Buoyancy and trim" stability: physics principles behind it

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But the legs and arms positions affect the trim significantly.
Agree

My ideal trim is perfectly horizontal, with straight knees and ankles and with arms along the body.
...
My trim is aimed to maximum speed against strong current in open water, such as channels in the reef (Maldives) or places with strong currents in the Mediterranean, or internal rivers.
For such purpose you need long free diving fins and a very efficient flutter kicking technique. If you flex your knees you will never reach the required thrust and speed...
That's an interesting note.

I dive in the Mediterranean sea too (you know), but still I mostly like to dive close to the seabed to inspect small creatures without disrupting their habitat; so I'd like to stay close but feet up, steady and ready to back away. So, I tend not to dive with strong currents.
But it did happen sometimes and it was fun too ... and I totally agree, flutter kicking with straight legs made the difference, trying to frog kick against strong current was a pain in the ass and not that much efficient at all :-( I'm still not good at that anyway ...

My ideal trim requires placement of buoyancy and weights different than the placement which provides the trim of your drawing...
Tell me more about it, I'm curious :)
 
Yes and no. Mostly yes, but we must acknowledge some complexity.
I agree, I'm almost quite over-simplifying, just to make it easy to discuss.

There are two separate statements here: one is that the forces must have equal magnitude, which has to do with a diver's buoyancy. If the buoyancy force equals the weight force (in magnitude only), then the diver will be neutrally buoyant, and neither sink nor float.

The second statement has to to with good trim: if the diver's buoyancy force and their weight force are in the same vertical line, then they will neither rotate head-down nor head-up.

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

When you combine both of these, you are describing the goal: solid buoyancy control, and a distribution of buoyancy and mass such that the diver's trim is stable in the desired position (typically horizontal). Which is exactly as you say.

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.

trim_stab_01.jpg

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 ...).
 
Stability is two fold: Gear balance and skill. The closer your gear is balanced, the less skill you need to be trim.
Good point, I agree and at the end of the day I'm aiming at that at the moment, getting a good gear balance in order to dive as effortlessly as possible. But aside a few rules of thumbs I didn't read much about it.
Skill and training will be needed anyway, I'm aware.

With back mount scuba, there is always skill needed. The weight is on top of the buoyancy, ...
Always?!! Please tell me more about that.

When you go to something like sidemount, you put the weight below the buoyancy, so less skill in balancing is needed.
I kinda got that early on ... my first teacher is one of those; those sidemounter "glide" underwater without any shame!! ;-D

Once you get to the point that without any movement or adjustment, you stay within an inch or so of where you started, then you can start to work on stability.
I'm way off from that point at the moment :-(
 
Always?!! Please tell me more about that.
The tank always has weight, and if it's an aluminum tank, one end has buoyancy. Most (not all) aluminum tanks are heavy on one end and buoyant on the other. When people have light feet and they slide that tank back, they are making matters worse as it's not intuitive. That buoyant butt becomes the gentlest of levers. But, all the weight of the tank and regulator are on the top while most lead weights are quite close to the BC or on the waist. Especially with a steel tank, the 'natural' position is with the tank down and the floaty things like any exposure suit going up. We have essentially balanced an anchor on top of us. In side mount, they become more flush with the diver and usually just a tad low, but the BC is still on top.
I'm way off from that point at the moment :-(
Gross adjustments are easy. Get in the water and get comfortable by swimming a bit. If you have a camera trained on you for this, the better. Now get nice and level and close to the ground, with that film rolling. Close your eyes and relax. Let the bad weight distrubution have its way with you and count to fifteen seconds. This is your first snapshot of what is wrong with your natural equilibrium. Shift gear, shift bouyancy and try it again. Hopefully, you're closer than before and you keep massaging your gear to get it so that you can hover effortlessly, eyes closed an inch or so from the bottom for a minute or more.

Having your eyes closed is essential. Your body will adjust if it sees you going out of trim because you want trim. But fighting for your trim all dive long is exhausting so work on that natural equilibrium first. Make it easy. Trim should be relatively effortless, and having a natural equilibrium will help you defeat bad habits like hand sculling and constantly fidgiting. Your SAC will drop and people will accuse you of not breathing. It's my intense belief that this should be taught FIRST in OW and not left to last. Students should be trim and neutral through their whole class and look like a world class diver before they graduate. No, it's just not that hard and because they're in control, they learn faster and the course is easier on them and the instructor. Kneeling, standing or lying on the bottom should never be allowed. Ever. [/soap box]

BTW, I always reccomend a cavern course for all my students. A great cave instructor will square your trim away, because it's not just a good idea in an overhead: it's a safety thing.

This is a great conversation, and I'm going to come back and look closer at your force diagrams. However, I am travelling all day, taking a nephew home to Central Florida, dropping off some special cut covers for my ex's recessed lighting as well as to look at her closet shelves that fell and then back up here. I need to get going so that I can make it back to cave country by night fall. Don't fret if I don't answer quickly.
 
Clearly the natural solution is to increase the fat content of your lower body and the muscle content of your upper body, like a manatee.
 
That's what I was looking for!!! Very nice indeed! And so well explained. It's been a pleasure reading your post. Respect!!

Much of it I've already understood by myself but you added a few precious details here and there that made my day. I appreciate it soooo much. That's food for thoughts.
Hello. What were the "Precious Details." that you derived from this?
Thanks.
Cheers.
 
Gross adjustments are easy. Get in the water and get comfortable by swimming a bit. If you have a camera trained on you for this, the better. Now get nice and level and close to the ground, with that film rolling. Close your eyes and relax. Let the bad weight distrubution have its way with you and count to fifteen seconds. This is your first snapshot of what is wrong with your natural equilibrium. Shift gear, shift bouyancy and try it again. Hopefully, you're closer than before and you keep massaging your gear to get it so that you can hover effortlessly, eyes closed an inch or so from the bottom for a minute or more.
Will try that whenever possible.

Having your eyes closed is essential. Your body will adjust if it sees you going out of trim because you want trim.
I'm aware.

It's my intense belief that this should be taught FIRST in OW and not left to last. Students should be trim and neutral through their whole class ...
Sadly enough I've not been so lucky to get this kind of training ... like, at all :-(
I mean, the word "trim" has been eventually pronounced ... but no specific training or correction has been suggested or offered :-(
Anyway, I'm positive I will eventually get it right. In the meanwhile I'm learning something new every day :)

BTW, I always reccomend a cavern course for all my students. A great cave instructor will square your trim away, because it's not just a good idea in an overhead: it's a safety thing.
Interesting point :)
 
Roberto,

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?

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.

Anyway, it's a good fun discussion!
 
https://www.shearwater.com/products/perdix-ai/

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