Trim adjustments after new BCD

[BubbleBubble7]

Ok, so I've always been pretty good with my buoyancy. After switching my BCD recently, I'm noticing an odd quirk that Id like to offset. I have a Zena now, and I love it, but I find that I become incredibly positively buoyant every time I shift into a vertical position. It's not a huge issue as I spend 98% of my dive horizontal but it would be great if i could figure out how to go vertical for a few seconds (say, on a safety stop) without drifting up. Thoughts?

 

[Landau]

Changing your position won't change your buoyancy. You still have the same amount or air in the BCD. It might feel more buoyant though.

Perhaps you are always swimming downwards a bit when you think you are horizontal. I'd suggest having a buddy or instructor take a look while you are swimming.

 

[Landau]

Or inflator gets triggered somehow when you are vertical?

 

[rongoodman]

Even if you're neutrally buoyant, as soon as you go vertical, any fin movement is going to propel you upwards in the water column. Nervous feet, perhaps?

 

[stuartv]

If you are rotating to a vertical position from your hip or knee area, then your BCD will be a foot or so more shallow than it was when you were horizontal. That would make you a *little* more positively buoyant. If you are also over weighted, the extra gas in your BCD that is compensating for that extra weight would result in the increase in buoyancy being even greater. If you are also relatively shallow (say, at at safety stop), then that will also make the increase in buoyancy even bigger.

So, make sure you're not over weighted. Your Zena could be less buoyant than your old BCD, so you might not need as much weight as you are used to.

And, be aware of how you are going vertical. If you can, rotating in place, around your center of gravity (i.e. somewhere around your stomach) will make things easier than if you rotate up from the feet or knee area. Maybe try exhaling as you rotate from horizontal to vertical, to keep your center of gravity from moving upwards.

 

[BubbleBubble7]

Thanks! I never have air in my BCD when under water (I dump it to descend and then fully ascend before adding at the top) so I doubt I'm over weighted. I've also seen video of myself and I'm not much of a kicker/mover. I wonder if maybe I'm not slightly underweighted and I'm noticing the vertical thing because I'm most likely to be in that position while on a safety stop (shallow and less air in the tank). Now that I think about it, tt's possible I picked up a little chubbiness around the time I switched into the new BCD and I'm blaming the wrong thing. That extra 4-5 pounds this past winter might be it : / Ugh.

 

[stuartv]

Forgive the following if you already know this.

Let's just assume for the sake of discussion that you are using a single aluminum 80 as your tank.

An AL80 is 1.5# negatively buoyant when full. It is 4.4# positively buoyant when empty.

If you are weighted perfectly then that means with an empty BCD and a nearly empty tank (i.e. at the end of a dive) you will be neutral. Backing up to the start of the dive, that means you would be 5 - 6# negative with a full tank.

If you are wearing a wetsuit, it will compress some, which means you'll be even more negative when get down to whatever depth you're diving.

All that is a long-winded explanation of why you would be at least 5 or 6 # negative (more with a wetsuit) at the start of your dive.

If you are starting with an empty BCD and not adding ANY air to it, then you are either really good with your breathing, to continuously compensate for being 5 or more # negative, or you are constantly finning to hold your depth, oorrrr.... you are not carrying enough weight and you are ending the dive too light.

If you are using a wetsuit, you'd be more than 5 or 6 # negative and if you are using a tank that is bigger than an AL80, you'd be another increment more negative. With a 100, you'd start off 8# negative, just for example.

 

[BubbleBubble7]

I suspect I'm a bit light at the end of the dive. On closer inspection of my log I noticed I was down to 800-900 PSIs on some of my more recent dives....that's rare as I usually tend to end around 1200-1300. That plus a couple of extra pounds of fat would have become more noticeable towards the end of the dive. Time to work a little harder in the gym.

Damn. I really wanted to blame the new BCD.

 

[gfaith]

@BubbleBubble7

My extra small buddy/wife dives with a Zena, and she loves it. We did move the trim pockets up to the high cam tank band but other than that hers is a stock XS. She also add little air to the BCD unless we dive around 80-100 feet.

What happens if you are finning along in good trim horizontal and you just stop finning. Do you go up or down; head or feet first?

 

[BubbleBubble7]

@BubbleBubble7

My extra small buddy/wife dives with a Zena, and she loves it. We did move the trim pockets up to the high cam tank band but other than that hers is a stock XS. She also add little air to the BCD unless we dive around 80-100 feet.

What happens if you are finning along in good trim horizontal and you just stop finning. Do you go up or down; head or feet first?

I typically stay in the same spot, but I might tip a little bit forward/down. My tendency is to be a little more head down in the water (as opposed to many other people who are butt down/hips dropping).