Buoyancy & Trim - Advice Please!

[gabriel]

Can someone please give me some advice?

I am just getting into the diving groove and have about 35 dives. My buoyancy control is good, but I am having some difficulty setting up my trim.

Most of my diving is warm water, so I am wearing a 4mm wetsuit with a Pro-QD BCD and usually an aluminium cylinder. On my last few dives my weight set-up is as below:

a. Two small weights (2lb) in the small weight pockets at the top/back of the BCD.
b. 6lbs (3lb each side) in the integral weight pockets.
c. Remainder of the weight (2x3lbs) on a conventional weight-belt.

My problem is this, when finning along I can easily maintain a level posture, for example when examining a reef. However when performing other tasks (safety stop for instance) I struggle to maintain a comfortable position and it feels as if the natural tendency of my trim is to tip me head forward.

Any tips or advice on how to set up or move my weight so I can comfortably maintain any level or head-up/head-down attitude without constant finning and movement. How do the experienced guys maintain a constant position without motion?

Many thanks!

 

[dkktsunami]

Assuming that your weighting (buoyancy) is correct, then proper trim becomes a balancing act, a matter of shifting your center of gravity on your horizontal body toward the head or the feet. Since your head seems a little heavy you need to shift a small amount of weight toward your feet. This might be done by merely changing your body position a little bit while stationary - hands at your side rather than in front, feet back a little. If more shifting is needed you might lower your tank a little bit. If this doesn't do the trick move a little more weight from trim pockets to belt. If your buoyancy is correct just start shifting stuff around until balance is achieved. This takes practice.

 

[battles2a5]

As dkktsunami, you need to shift some weight below your cG. Think of yourself on a teeter-totter underwater, with the fulcrum on your center of gravity (which is different for everything, but usually between your upper waist and lower chest). The first thing to do is make a solid teeter totter, so arch your back and assume a nuetral position with your arms and legs. If you are teetering toward your head, you need to move some weight to the other side of the fulcrum. I would start with moving those 2lbs of trim weight from your shoulders to the belt. Then check again, and keep making progressively smaller adjustment until you have it dialed in. Try to only make one change at a time, and do them in small incremements. You will drive yourself crazy if you are makeing multiple changes every time you splash because you can't isololate the effects of any one change.

 

[DA Aquamaster]

As a guess I'd suggest moving the 2 lb weights at the top/back of the Bc to your belt or into the integrated pockets.

If you are tipping head down, move what ever is closest to your head down toward your feet. Moving things closer to your mid section shortens the moment arm those weights have in comparison to your center of gravity. The farther any lift or weight is from your CG, the more effect it will have on trim.

I am also guessing/assuming there is not much difference between the moment arm of your weight belt and integrated weights so switching weight from one to the other will have little impact on trim.

Conversely, some people resolve a head down trim problem with ankle weights as a couple pounds near the feet have along moment arm and a big impact on trim. The downside is that you are moving them up and down through the entire dive every time you fin and that is very innefficient.

Another option is to move the tank higher or lower in the tank band(s). It depneds on the tank you are using. For example an AL80 is 4 lbs positive when empty and only a pound or two negative when full. So moving the BC lower on the tank tends to help resolve a head down trim position for most of the dive (just not at the beginning). Most steel tansk on the other hand are 1-4 lbs negative when empty and 6-11 pounds negative when full depending on the specific tank and capacity. So moving the BC higher on the tank would help resolve a head down trim position.

Another option is heavier/non floating fins such as scubapro Jet fins.

 

[Garrobo]

Lose the weight belt and use an armless 2mm shorty.

 

[Straegen]

As someone mentioned ankle weights or a different fin could be the trick if your legs/feet are floating up. I and my instructor had a heck of a time getting me even until I tried a BPW which seems to distribute the weight and my buoyancy more evenly and some heavier fins that weren't as buoyant.

 

[don Francisco]

I strongly recommend against ankle weights unless absolutely necessary. Putting weight on the ankles raises the momentum and means extra work when finning.

Try to work with what you have, experimenting with moving weight up or down and seeing how you naturally trim out when going limp. Also remember than unconscious finning can affect trim.

One other thing to remember, is that as you concentrate or focus your mind on various tasks, your breathing pattern changes and this can affect both bouyancy and trim. It's especially common among newer divers, so you need to be ready to make adjustments.

 

[novalyne]

Silly question, but...does trim change depending on depth? When I'm diving deeper, I am pretty sure I'm nicely horizontal, but I always end up vertical during safety stops. Maybe it's because I'm busy concentrating on depth, the computer, etc...but I never stay horizontal. Does this have anything to do with having less air, etc. Should I even care?

 

[hudson]

Trim can vary by depth, because the neoprene will provide more buoyancy when shallow. The other factor is the tank will be more buoyant at the end of the dive.

OP: Everything has been pretty much covered, but you must also realize that your ability to adjust your trim will improve tremendously over the next 20 dives or so. I used to struggle a ton with weight placement etc, now I could care less and still maintain decent trim, not while completely stationary perhaps but with very small fin movements. So give it some more time instead of focusing too much on it if the first few adjustments don't help.

 

[pwl]

something to try on a shallow dive: just stop, do nothing, don't try to adjust your position & see what happens. chances are you'll tip to one side and/or up & down. reposition your weights accordingly as other people of described.

also - as you dive more, you will probably find you'll be able to do without the same amount of weight, which will give you opportunities to reposition the remaining weights again.