Centre of gravity & centre of buoyancy

[datho87]

I came across this great video of spacesuit design at the NASA natural buoyancy lab, and thought it provided a good framework that could apply for diving trim.


If you go to the 27:40 mark, they discuss the centre of gravity (CG) and the centre of buoyancy (CB), and how they use them to achieve space-like “trim” in the spacesuit.

I know these concepts are understood somewhat intuitively by our diving community, but just thought that it could help structure our thinking around trim (especially in a drysuit) - that being, the need to achieve trim on a horizontal plane by modifying our centre of gravity and our centre of buoyancy in relation to one another.

Of course there are different considerations in diving, such as shifting bouyancy with different trim angles, overweighting, gas compression at depth etc. Would love to your thoughts

 

[Smache]

Great video rec! I do think there's value in understanding the physics behind neutral buoyancy and neutral trim.

Here's a riddle that I like to ask friends: Take an empty aluminum 80 with a bolt snap around the neck of the tank and clip it to a horizontal line - it'll float above the line with the neck of the tank pointing downward. Now, put a bolt snap on the very bottom of the tank (just a thought experiment) and clip the bottom of the tank to the line instead. Will the tank float above the line or hang below it?

If you think you know the answer, how would you explain it to a new diver?

 

[Mobulai]

Will the tank float above the line or hang below it?

In a perfect world I wanna say hang below, but I suspect some bowing (up) action from the line and eventually some minor inbalance would create a rotating moment (chaotic and hard to predict)

How would I explain it? Just as I would to a non diver: draw balance of forces (tank and valve weights, buoyancy); apply static analysis

 

[lizardland]

I know these concepts are understood somewhat intuitively by our diving community, but just thought that it could help structure our thinking around trim (especially in a drysuit) - that being, the need to achieve trim on a horizontal plane by modifying our centre of gravity and our centre of buoyancy in relation to one another.

Of course there are different considerations in diving, such as shifting bouyancy with different trim angles, overweighting, gas compression at depth etc. Would love to your thoughts

As you say, it's kind of what divers do intuitively but done in this case in a very quantifiable way. And probably that assessment costs more than the average rebreather diver's entire kit and a year of diving I wonder how it compares to just faffing about with some weights in a pool in terms of efficacy? I guess it's like the old cliche of America inventing the Fisher space pen, Russians just used pencils.

The obvious big difference is they aren't dealing with suit buoyancy changes. I'm assuming the spacesuit is designed to run at a positive pressure so unlike a drysuit the air isn't going to shift when the diver changes their trim.

 

[datho87]

As you say, it's kind of what divers do intuitively but done in this case in a very quantifiable way. And probably that assessment costs more than the average rebreather diver's entire kit and a year of diving I wonder how it compares to just faffing about with some weights in a pool in terms of efficacy? I guess it's like the old cliche of America inventing the Fisher space pen, Russians just used pencils.

The obvious big difference is they aren't dealing with suit buoyancy changes. I'm assuming the spacesuit is designed to run at a positive pressure so unlike a drysuit the air isn't going to shift when the diver changes their trim.

Absolutely agree that gas shifts whereas I believe the spacesuit air pockets remain the same as it’s maintains its overall shape. (But I might be wrong).

But I think CB and CG might be a good way to think about rig setup to maintain neutral trim. If stability is promoted in the video by bringing the CB and the CG together, it speaks to bringing buoyancy closer to where the weight is: use of wings rather than drysuit for buoyancy, aligning weights on the rig in line with where most of the suit and wing buoyancy is, forgoing ankle weights in favour of p weights or tail weights etc…

This is all a bit geeky and probably unnecessary, but it’s got me thinking about how best to weight my rig

 

[lizardland]

Absolutely agree that gas shifts whereas I believe the spacesuit air pockets remain the same as it’s maintains its overall shape. (But I might be wrong).

But I think CB and CG might be a good way to think about rig setup to maintain neutral trim. If stability is promoted in the video by bringing the CB and the CG together, it speaks to bringing buoyancy closer to where the weight is: use of wings rather than drysuit for buoyancy, aligning weights on the rig in line with where most of the suit and wing buoyancy is, forgoing ankle weights in favour of p weights or tail weights etc…

This is all a bit geeky and probably unnecessary, but it’s got me thinking about how best to weight my rig

I think moving CoG & CoB closer together probably is taught, just maybe not in that explicit a way and maybe not that well. I'd guess horizontal trim is a lot harder than trying to balance out walking trim because you've potentially got forces acting fairly long ways apart and forces that like to move/change (suit, lungs, etc). Which probably isn't helped by the closer you are to horizontal the more unnatural it feels. At least to me, I was always fighting the feeling of face-planting.

But yeah, I think anything that helps how newer divers think about it is a positive. Plus I've always been willing to sell a child into slavery if it got me a couple of hours in the neutral buoyancy tank with some interesting kit.

 

[inquis]

The biggest difficulty is the CM (center of mass, which is the same thing as CG) and CB shift due to gas consumption and wing deflation. IMO, the best thing to do is to first trim with a nearly empty tank/wing -- this gets the total weight correct as well as the distribution for stable trim. (I.e., the CM & CB are aligned.) As a second step -- without moving any lead -- switch to a full tank and adjust the position of the wing for stable trim. The wing lift is then aligned with and cancels the gas mass as they both decrease throughout the dive.

In reality, it may not be that simple due to the discrete placement options of the wing. You may have to shift the tank slightly and start from step 1.

 

[dray_gnv]

When I saw the new wing that Halcyon is releasing, with weight pockets, it seemed like an equipment solution to a skills problem. But this offers some insight into why that may not be quite true.

 

[lizardland]

When I saw the new wing that Halcyon is releasing, with weight pockets, it seemed like an equipment solution to a skills problem. But this offers some insight into why that may not be quite true.

If it was Apeks then I'd just automatically think it was ******** and wait to be proved differently. It may be a good idea. The problem with v-weights and p-weights was always you either had to be practical enough you wouldn't kill yourself making it or wealthy enough that you could pay the diving tax on something someone else had made.

If it's Halcyon then it might work but it will only take proprietary weights that cost more than the wing did.

 

[lowwall]

But I think CB and CG might be a good way to think about rig setup to maintain neutral trim. If stability is promoted in the video by bringing the CB and the CG together, it speaks to bringing buoyancy closer to where the weight is: use of wings rather than drysuit for buoyancy, aligning weights on the rig in line with where most of the suit and wing buoyancy is, forgoing ankle weights in favour of p weights or tail weights etc…

If the CB and CG are together you are neutral, which means you will maintain any attitude you put yourself in, but also have zero stability as any force that's not exactly horizontal or vertical through the centers will rotate you.

For stability you want the center of bouyancy as far directly above the center of gravity as possible in your preferred position. This is why the wing goes on your back instead of your chest.