Trim/buoyancy conundrum

[dfx]

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.

Hmmm, if I added too much air to my BCD, then I wouldn't be neutrally buoyant.

 

[ScubaSteve]

How big a problem is this? Since you are claiming to be leg heavy, my recommendation for something to try would be to adjust/move your weights so you are comfortable in trim (knees bent ~ 90°) in deeper water and in shallow water, extend your legs farther out so that are preventing the lighter legs you have created by moving the weight around. I did a couple hundred dives on my 2pc 7mm suit in cold water and never noticed a significant issue as you have described. I moved weight around so that I was comfortable throughout the dive and enjoyed the dives. YMMV

 

[victorzamora]

Let's assume for a moment that what you've said is true. Your weighting is spot on, and you have no flaws in posture that is causing this. Let's assume you TRULY have no air in your wing when shallow with an empty tank when you're doing your weighting test. Let's also assume you really have ZERO motion when you're doing trim tests, and let's assume "trim" posture is consistent across depths.

The best way to fix it for a square profile is to set your trim for bottom, and deal with foot-light at the beginning of the dive. Then, you can extend your legs while you're shallow to bring your feet down (especially if diving negative fins). If not diving a square profile, setting your trim for somewhere in the middle would be good. Slightly off on both sides, but you should be able to compensate for it using posture.

 

[TMHeimer]

Hmmm, if I added too much air to my BCD, then I wouldn't be neutrally buoyant.

Good point. Perhaps you're just adding a little too much air--enough to make your top half rise just a little, yet with finning going on you're not really going to pop to the surface. Just a theory.

 

[dfx]

How big a problem is this?

Not a big problem at all, more of a small annoyance. Like I said, it's more of a theoretical question than one asking for advice. But it left me scratching my head.

People always get told, work on your trim, trim is important. Then you get all kinds of advice of how to fix certain issues (like even here in this thread), move your weights around, use heavier or lighter fins, move your tank or up down, etc. But it seems that given a thick enough wet suit (and my 5 mm is definitely enough to make me feel it), trim is only something that exists at a certain depth. Change your depth enough and your trim goes out the window. Yet nobody ever seems to mention that. Which made me wonder if there's something that people do to compensate for that, maybe without even realizing it? Or maybe people just live with it and so aren't really in trim (just "kind of" in trim)? Or maybe you need to be tall and skinny (like I am) to actually feel the effect?

It made a curious mind wonder.

 

[victorzamora]

dfx, I'm actually having a reasonably hard time determining exactly why it would matter *much*. It seems you're almost too foot-heavy at the surface, and this is exacerbated by depth. That's all I can come up with.

The solution? Setting your trim foot-light near the surface would allow you to keep your legs further back to compensate near the surface, and then bring them up the closer you go.

 

[nimoh]

There are things you can do to compensate for small weight changes such as the tank getting lighter or neoprene compressing. For example, if you have negative fins, extending your legs will move your center of gravity further down/back. Many divers are able to do this automatically and prevent loss of trim problems, and it comes with practice. I'm not quite all of the way there yet, but working on it.

 

[danvolker]

Not a big problem at all, more of a small annoyance. Like I said, it's more of a theoretical question than one asking for advice. But it left me scratching my head.

People always get told, work on your trim, trim is important. Then you get all kinds of advice of how to fix certain issues (like even here in this thread), move your weights around, use heavier or lighter fins, move your tank or up down, etc. But it seems that given a thick enough wet suit (and my 5 mm is definitely enough to make me feel it), trim is only something that exists at a certain depth. Change your depth enough and your trim goes out the window. Yet nobody ever seems to mention that. Which made me wonder if there's something that people do to compensate for that, maybe without even realizing it? Or maybe people just live with it and so aren't really in trim (just "kind of" in trim)? Or maybe you need to be tall and skinny (like I am) to actually feel the effect?

It made a curious mind wonder.

I was 2nd post in this thread...and I did suggest the amount of wetsuit material ( size of leg) would be an issue if this gets to be large.... long legs will qualify, and the long lever effect without buoyant wetsuit rubber ( at depth) will mean that your now negative legs will be quite different for trim than at the surface. Again, eliminating a weight belt and putting weight in the center zone of the backplate area will help.... I'd pick fins that were neutral--not heavy fins like Jets....and I WILL push freedive fins again, because the huge wing surface WILL hold your legs in trim, with even the tiniest of forward motion.

Or get one of these
[video]http://content.bitsontherun.com/previews/Snp3jfQ3-C3PNGnB0[/video]

 

[DivemasterDennis]

Possible considerations: lighter fins; rearrange weights placement; reduce weight so you need less air in the bcd ( I think this is the most likely fix.), focuis on and improve body control and awareness in the water. I'll keep my response this short. Other have elaborated on some of these things.
DIvemasterDennis

 

[TMHeimer]

Question: If you dived with a thick wetsuit and lots of weights, but no BCD to add BCD air to the question: With the 7 mil farmer john you have 14 mil on the core area and 7 mil on extremeties. At 33' (10m) your torso is now at 7 mil and extremeties at 3.5 mil. Now do the areas where more weight is attached become more important. Ei.- do the weights take on more importance as the difference between thicker and thinner wetsuit areas decreases?