Trim/buoyancy conundrum

[dfx]

I use 41 pounds with an AL80 and 7 mil farmer john wetsuit and am not overweighted. My legs/feet are like stones and fins negatively buoyant. I get horizontal by putting 4 pounds each in my BC shoulder pockets and hoisting the weight belt way high. Using suspenders makes this easy to do and maintain. Same procedure when I use my shorty with 18 pounds. Once in a while I use just the 7 mil top, and am really out of whack-- so I attach small boat bumper floats to my ankles (not when others are around.....). I guess you do what you have to do.

I understand that and everything else that has been said. The question isn't about getting into trim. I know what I have to do to get into trim and have no problem with doing that. The question is about staying in trim as you descent or ascend within the water column. Logically it doesn't even make a difference where you put your weights or how much weight you carry, it seems impossible to stay perfectly in trim in a (somewhat thick) wetsuit. The moment you descend or ascend, your buoyancy distribution changes and this changes your trim. It's a simple matter of physics. Yet nobody ever seems to even mention this.

 

[TMHeimer]

I understand that and everything else that has been said. The question isn't about getting into trim. I know what I have to do to get into trim and have no problem with doing that. The question is about staying in trim as you descent or ascend within the water column. Logically it doesn't even make a difference where you put your weights or how much weight you carry, it seems impossible to stay perfectly in trim in a (somewhat thick) wetsuit. The moment you descend or ascend, your buoyancy distribution changes and this changes your trim. It's a simple matter of physics. Yet nobody ever seems to even mention this.

I'm not a physics expert, but logic would say that all of a thick wetsuit compresses equally and at the same time, as you descend. So logic says that if all of the suit is now only half as "thick"/buoyant because you've descended some, your ablilty to stay in trim (horizontal) won't be affected. I have never noticed any changes in trim as I change depth. It is a puzzling question. I can't come up with anything else other than to try moving weights around as suggested. Perhaps you can get a good trim for depth and have it somewhat off when at the surface? Everyone's body is different as well and maybe that contributes in some way. Some fat people do sink and some skinny ones float. I have determined that in my case, the top half of my body is VERY buoyant and the bottom half very negative.

 

[Eric Sedletzky]

I ride my tank really low while wetsuit diving, kind of old school style.
I also use a big steel tank. This helps to spread the weight out over the body instead of having it all up forward.
I also dive with as little weight as I can get away with. I kept removing weight until I got to the point where I could hold a 15' stop at the end of my dive with no air in my wing and control my stop with my breathing alone. This means a lighter weightbelt and less pulling down your waist as the wing is pulling up. And of course the metal backplate and heavy steel tank helps to get weight off your weightbelt too.
I can cruise around and not worry about trim. I learned to point my body towards the direction of travel like a freediver or a fish would so maybe my idea of proper trim may be different from other peoples' definitions of proper trim.

 

[Steve_C]

Thinking back on it i think that when in a rest position I tend to extend my legs a bit more a depth then when at rest when shallow. But that is all subconscious.

Given a body position only weight changes away from your center of mass affect your trim. Adding or deleting weight close to your center of mass has little affect.

Wetsuit may compress evenly but the material is not distributed evenly relative to your center of mass.

 

[dfx]

I'm not a physics expert, but logic would say that all of a thick wetsuit compresses equally and at the same time, as you descend. So logic says that if all of the suit is now only half as "thick"/buoyant because you've descended some, your ablilty to stay in trim (horizontal) won't be affected.

Correct, as long as you don't compensate for this loss in buoyancy. But when you do, the added buoyancy is only around your torso. So the net effect is that you now have less buoyancy in your legs, but more around your torso.

I have never noticed any changes in trim as I change depth.

Right, but why not?

Given a body position only weight changes away from your center of mass affect your trim. Adding or deleting weight close to your center of mass has little affect.

Wetsuit may compress evenly but the material is not distributed evenly relative to your center of mass.

Well, yes, that's exactly the problem! If changing the weights away from your center of mass affects trim, then changes in buoyancy away from your center of mass will also affect trim.

Hypothetically speaking, you don't even need a wetsuit to see this happen. All you would need is neoprene booties. Of course the change in trim would almost certainly not be enough to actually feel it, but it should still happen.

Like I said, it's a bit of a theoretical question. I don't see how moving weights around would make a difference. It may put me back into trim at depth, but then I would be out of trim in the shallows. Maybe that's what people do? Set up your weighting so that you're head first in shallow water?

 

[mala]

I understand that and everything else that has been said. The question isn't about getting into trim. I know what I have to do to get into trim and have no problem with doing that. The question is about staying in trim as you descent or ascend within the water column. Logically it doesn't even make a difference where you put your weights or how much weight you carry, it seems impossible to stay perfectly in trim in a (somewhat thick) wetsuit. The moment you descend or ascend, your buoyancy distribution changes and this changes your trim. It's a simple matter of physics. Yet nobody ever seems to even mention this.

you are going to need the center of lift to be near your center of gravity.
hard to do in a bcd
easier in a wing.

if you have a very thick wet suit I imagine you could go feet heavy at depth due to compression but really if its that cold its drysuit time.

 

[dfx]

you are going to need the center of lift to be near your center of gravity.
hard to do in a bcd
easier in a wing.

How is a wing closer to the center of gravity? Does it wrap around your waist like a belt instead of being on your back, or what?

 

[mala]

How is a wing closer to the center of gravity? Does it wrap around your waist like a belt instead of being on your back, or what?

lets say your center of gravity is your tummy.
being horizontal in the water the wing would be directly above this.

the aircraft people call it pendulus stability.

 

[dfx]

lets say your center of gravity is your tummy.
being horizontal in the water the wing would be directly above this.

the aircraft people call it pendulus stability.

What kind of wing is that? All the ones I know are positioned exactly the same as the bladder of my BCD: on your back, shoulders to belly or maybe hips if you're short or have a long wing. (My BCD is fully back-inflate with a donut bladder btw.) So the air is evenly distributed across this length, and the center of that would be somewhere around your stomach (ie right below your ribs) maybe? I don't think that's a person's center of gravity. Maybe if you go head down, then most of the air ends up closer to your butt and that would put it close to your center of gravity. But then you're not in trim again.

 

[TMHeimer]

You said you were properly weighted, and I imagine you have fine tuned that as much as possible considering your number of dives. Could the problem be as simple as you still may be adding too much air to the BC while descending? Again, physics not being my forte, but whatever air you have in the BC at the surface (usually there is at least a little) will of course compress with depth. If you add the proper amount while descending it should only bring the amount of BC air back to where it was, no? If too much is added your top will now be more buoyant than your bottom. I could've missed something though.