Trim Affected by Water Type?

[northernone]

In real diving, likely not. When your wet suit shrinks, your volume and that of your water/displaced body change the same. But your water 'body' loses mass and you do not. Say you wore neoprene pants. The displaced water loses mass in the leg area.

I will drop by to say I do experience this in a very real way when I dive the upper half of a 7mm farmer john. My fins and bare legs are relatively unaffected by increasing pressure regarding buoyancy characteristics and my core (with long sleeves and hood) compresses and becomes negatively buoyant. Of course my bcd will counteract that but in a negative free descent I start out trimmed and end up feet up as my trim changes with depth. The bcd addresses this issue.

So in summary, I am now claiming changing DEPTH does effect trim. Depending on if the exposure suit is balanced.

Imagine an air filled balloon on a stick with a wax ball on the other end. Afix a weight in the middle to achieve perfect trim at 10fsw so the buoyancy of the balloon and the opposing wax ball are rendered balanced and neutrally buoyant.
Now, bring it down to 200ft. What happened to the trim?


Does this mean in a minuscule way the same is true salt to fresh?

 

[MichaelMc]

I believe just changing fresh to salt changes our buoyancy, but not our trim. If we fix the buoyancy by adding off center ballast, then we are neutral, but out of trim.

Assume the water we are in in each place has uniform density, 63.9 lb/ft3 in one place, 62.4 lb/ft3 in the other. Then the only thing, for trim, we care about the water is its geometric center. Which is the same regardless of that uniform density. Also, water is generally considered incompressible, so its density does not change with depth, due to pressure anyway.

 

[NAM001]

Nemo.... The answer is yes you willhave to relocate the weight along wth the compensation for the buopyancy. Most people don't have to change location because the change is so little. BUT .... take some one belly button to head is neut and belly dwon you are neut with FW weighting. Now you go salt and both upper and lower become light. adding all the weight on the hips will keep the legs down but the upper will be light. If your wights are located say center chest the effect may be less because the fw and sw placement was in the center of the mass. going salt will move the center towards the legs but for normal people it will not move far and is un-noticed. If however you have a pencil top and an elephant legs then all the sw lift will be felt the most on the legs the most.

 

[Professor Nemo]

Nemo.... The answer is yes you willhave to relocate the weight along wth the compensation for the buopyancy. Most people don't have to change location because the change is so little. BUT .... take some one belly button to head is neut and belly dwon you are neut with FW weighting. Now you go salt and both upper and lower become light. adding all the weight on the hips will keep the legs down but the upper will be light. If your wights are located say center chest the effect may be less because the fw and sw placement was in the center of the mass. going salt will move the center towards the legs but for normal people it will not move far and is un-noticed. If however you have a pencil top and an elephant legs then all the sw lift will be felt the most on the legs the most.

Thanks, KWS. That makes sense.

 

[DevonDiver]

I am now claiming changing DEPTH does effect trim. Depending on if the exposure suit is balanced.

Does this mean in a minuscule way the same is true salt to fresh?

Only in that you'd reach a given pressure at a very marginally shallower depth.

Assuming a very imbalanced exposure protection:

Your trim at 2 ata in salt or fresh water would be identical.

Your trim at 60ft in salt or fresh water would differ imperceptibly.