"Buoyancy and trim" stability: physics principles behind it
- Thread starter Roberto Inzerillo
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Exactly. When I started diving there was no BCD. So buoyancy was never neutral, and you could keep a constant depth only dynamically, kicking and also using hands in what was called "opposition movement".
This dynamic approach became obsolete after the BCD was introduced.
Still I sometime dive without BCD and possibly naked (no suit).
It is another way of diving, close to free diving (even closer if using a compact chest mounted CC rebreather instead of a bulky back mounted OC tank)...
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Stability is two fold: Gear balance and skill. The closer your gear is balanced, the less skill you need to be trim. With back mount scuba, there is always skill needed. The weight is on top of the buoyancy, so there has to be some balancing done and that takes skill. When you go to something like sidemount, you put the weight below the buoyancy, so less skill in balancing is needed.
However, it's important to get your natural balance as close to possible in either situation. That requires you to do the eyes closed test to see how you set naturally. 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.
However, it's important to get your natural balance as close to possible in either situation. That requires you to do the eyes closed test to see how you set naturally. 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.
OP
Roberto Inzerillo
Contributor
OP
Roberto Inzerillo
Contributor
Tell me if I'm wrong here ... at the end of the day, the only possible trim-stable (and at the same time granting trim and buoyancy equilibrium) configuration is the one where both forces have equal magnitude and are vertically aligned in such two points like here (i.e. Fb=buoyancy applied on the upper point and Fw=weight down):
right?
Well, everything beyond the basics gets swamped by factors such as an individual diver's physiology, position, muscular contraction and the ever changing amount of air in the lungs; environment conditions such as currents and wave action; the movement of air in wings and drysuits and its expansion and contraction in these as well as neoprene as depth changes.
In the end, there is no exact mechanical solution for trim because the moments are constantly changing. What you are looking for is a compromise that allows you to use your body to make the final adjustments without wasting too much energy.
Right.
But the legs and arms positions affect the trim significantly.
And my ideal trim is NOT with 90 degrees flexed knees..Nor with arms partially stretched forward-down.
My ideal trim is petfectly horizontal, with straight knees and ankles and with arms along the body.
Of course this depends strongly on your purposes, on the environment where you dive and on the equipment.
Your picture shows a trim which is suited to tech diving in US caves (no red coral on the ceiling).
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...
My ideal trim requires placement of buoyancy and weights different than the placement which provides the trim of your drawing...
OP
Roberto Inzerillo
Contributor
Almost every gear we attach to ourselves underwater (weight belt, wetsuit+hood+gloves, fins), each singular piece has uniform mass distribution. Hence they can be dealt with as discrete components, which is easy.
Exceptions (this is huge though): our body and the tank. Our body's center of volume shifts while breathing; the tank's center of mass shifts because we consume gas from inside during the dive.
Can you tell me more about them? Any number ... even approximate estimates? Would be cool to correctly grasp where those centers are placed (horizontally and vertically) under normal circumstances.
I understand that in case of the human body, the centers move around by moving our limbs positioning also. I'm happy to take into account a basic horizontal config with legs at 90° and arms slightly in front (almost superman style).
Yes and no. Mostly yes, but we must acknowledge some complexity.
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.
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.
OP
Roberto Inzerillo
Contributor
I agree, a compromise is what we're looking for ... always.
Still, it's good to know what's physically possible, what's not, what's the best possible realistic outcome, what's totally impossible, or what's too difficult to achieve to be considered feasible.
Somethimes a bit of math and physics helps a lot in this regard and makes doing those needed changes reasonable and meaningfull
I don't really like a trial and error approach if have a better option.
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