Struggling to understand buoyancy and trim wearing my gear

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jadairiii

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They are head heavy with AL plate and all their lead in hip pockets. So switching to a steel plate will move some of their movable ballast higher not lower. Making things worse.

The center of mass of that plate is higher than the center of mass of the lead on their hips. It is all grade-school playground see-saw physics.

Your are making assumptions based on the narrative of a student struggling with proper trim. The OP may not be head heavy.

But, always "fix" the obvious first. Having all that weight on the hips will create other problems down the road. Get the rig squared away and then address the "grade-school playground see-saw physics" later.
 

MichaelMc

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Your are making assumptions based on the narrative of a student struggling with proper trim. The OP may not be head heavy.
Sure....

Which part of adjusting the rig did you think was not involved in the physics?
If you move six pounds of lead or steel up, you need to move six other pounds the same distance down.
 

jadairiii

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Sure....

Which part of adjusting the rig did you think was not involved in the physics?
If you move six pounds of lead or steel up, you need to move six other pounds the same distance down.

You are putting the cart before the horse. Forget everything the op said, just focus on balancing his rig first (balanced rig....where have I heard that before.....?). 18 freaking pounds in weight pockets on the waist is not a "balance rig".

And again, you are assuming his "heavy head" is a weight issue. It may not be. Always start with the basics and work the problem from there.

I can make myself "head heavy" by adjusting my head position and leg position only. There is more to the "physics" of horizonal trim than just shifting lead ballast.
 

MichaelMc

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You are putting the cart before the horse. Forget everything the op said,
What I am doing is listening to what the student has reported. Not just saying we always start with steel.

There is more to the "physics" of horizonal trim than just shifting lead ballast.
Yes. And it is all simple physics.
Yes:
- head tilt matters, some.
- leg and arm position matters and provides dynamic adjustment, assuming they are not neutral...
- Tank selection matters, HP80 vs HP100 vs AL80
- Not taking a huge inhale and holding all that air, matters. If they are feet heavy, while the OP is head heavy.
- But lead position matters a lot, and it is the thing that does not mess with other aspects of the gear.

And yes, lots of lead on the belt is a physical pain in the lower back. But they need to get somewhat close and can then adjust some off of there as possible.

Now, that they had an HP80 is likely a very big cause of their issues. Getting rid of it will likely solve the issue. But if they are ever to dive that tank, they need to adjust other things.

And their moving past this:
I didn't realize the integrated pockets were not "1-size fits all situations" type of thing..
Which is a deficiency of their instruction.
 

MichaelMc

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@jadairiii, what you said about getting in the pool and listening to their body is spot on.

But it is not just 'holding their shoulders' properly or some such. Not that that is a quote from you.
 

wnissen

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Just to make the physics a bit more concrete, here is a notional force diagram of a slightly head-heavy diver. Assuming the body itself, and the uninflated plate/wing are neutral, I plotted the various sources of positive and negative buoyancy in red and blue, respectively. The length of the lines is the distance from the (again, very approximate) fulcrum. So the area is roughly proportional to the rotational torque. I can see this setup being head-heavy, with very little body fat in the spot where the weights are, and the steel 80 riding high so you can reach the valve.

Those feet and fins are waaay out there, too. One pound on the fins is worth ten in a waist pocket, torquewise.

Force Diagram(1).png
 

MichaelMc

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Excepts from slides I created while teaching a class in bouyancy and trim:

You as a skin diver with just fins, and weight vest and belt:
Arms and legs can move in and out giving a range of placement of their negative ballast (or positive ballast).
Lungs are variable volume and thus variable lift, thus shifting the center of volume of the body. The point it is 'suspended' by the water around.
The size of the arrows (crudely) represent the amount negative or positive.
The horizontal lines represent the range you can move that arrow over while in the water.
Screen Shot 2022-01-11 at 1.47.08 PM.png


As a scuba diver with suit, BC and tank.
The BC is variable volume. Increasing your buoyancy and shifting your center of lift.
By tilting the upper body, the tilt of the BC bladder is changed and thus its point of lift, shifting your center of volume.
In the water you can move arms and legs (and head a bit).
On land prior you can shift lead about (or swap between lead and a steel plate.)
Screen Shot 2022-01-11 at 1.48.14 PM.png


A thick suit may make legs or arms positive, and its buoyancy and likely trim will shift with depth.
 
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