Thoughts on bouyancy

[Dadvocate]

Hello, everyone!

I’d like to take some of my down time from diving before my rescue course kicks in to ask for some feedback on bouyancy and my practices.

To start with, I’d like to say that I typically dive with 3 or 4 kilos (6.6 – 8.8lbs) using a 3mm, long suit while wearing a Seaquest Balance BCD. In addition to this, I’ve taken to using Scuba Pro Jet Fins, which account for another 1.5 kilos (3.3 lbs).

When I dive at closer locations to Manila (those that don’t require flying), I take my own “beanbag” weights which are easier to use in my BCD than the bars are. I need to use these bars when I am at other locations, however.

I try to distribute the weights I use equally at four points in my BCD, one or two kilos in the back pouches of my BCD and two kilos in my integrated weight pouches. If the shop only has two kilo bars, I place all of the weight in my integrated pouches.

I tend to use 4 kilos on the days when surface currents can make it a little difficult to get down at the start, on the morning “Canyons” dive in PG for instance. I wear 3 kilos for everything else.

Here is my question for those in the know.

I have a psychological issue going on when I do wall dives. In Malapascua back in mid June, I did several dives on walls that were amazing. The problem was that I found myself always in slightly negative bouyancy when I wore 4 kilos and a slightly positve bouyancy at the end of the dive when I wore 3 kilos (this after purging excess air to compensate for a more bouyant tank).

I should say that this isn’t anything that makes me panic, and I am neither sinking nor ascending in an uncontrolled way. I know from reading another thread that continuing movement along a wall can negate negative bouyancy given the phyics involved, but when one stops, one begins to sink and that this is a sign of over weighting. I very much understand this concept, but I wonder if overweighting really is the culprit or if it is my personal hangup.

I’ve expericned this more since I’ve taken up photography on these wall dives because I now need an ever so slight finning to keep me stationary to take shots, but this finning can make the shot harder to take. There is also a nagging feeling I get that I should have better control of this without the need to pump air into my BCD (because I don’t need to on dives when there isn’t a wall drop off). Or so I tell myself.

I use air in my BCD to compensate for this when I get irriatated by this slight sink, which works well enough, but I have it in my head that I need to use more air in my BCD than that sinking feeling should require. Other times I can keep moving and the sink never becomes a factor in the dive at all.

My questions are these. Is what I am describing a typical issue for other divers still trying to master bouyancy? Am I sweating something not altogether important, or am I simply acknowledging that one does need to use air in BCDs to maintain bouyancy on some dives (wall dives) and not on other dives when all other considerations are the same, dives with a flat or sandy bottom for instance?

Thanks for any feedback you can give me.

Cheers!

 

[TSandM]

This is a very typical issue! And no, you are not sweating something unimportant. You are on track to master one of the biggest things that can make diving a joy, rather than a chore.

I know from reading another thread that continuing movement along a wall can negate negative bouyancy given the phyics involved, but when one stops, one begins to sink and that this is a sign of over weighting.

Okay. Here's a big part of the problem. You are confusing weighting with buoyancy. When you go into the water, you have attached to you various things which sink (negative buoyancy) and things which float (positive buoyancy) and things which can vary in their buoyancy (buoyancy compensator and wetsuit, if one is used). The buoyancy compensator, as its name suggests, is used to COMPENSATE for the loss of buoyancy as your exposure protection compresses, and for the fact that you will use gas during the dive, and become less negative as you remain underwater. Within broad limits (and, of course, depending on the BC you are using) you can compensate for SIGNIFICANT overweighting, simply by further inflation of the air bladder in the BC. It's not desirable to do this, because the larger the volume of gas in the BC, the more unstable your buoyancy will be, and the more drag you have. But if you are finding yourself negative when you dive, it's not the weight you are carrying that is at fault.

Much more likely as a culprit is your trim. If your body and your equipment are distributed so that you tend to assume a feet-down position in the water, then every time you kick, you are driving yourself toward the surface. In order not to ascend, you have to keep yourself a little negative. Then the net force upward and downward cancels, and you remain at the same depth (wasting a good part of the energy of your kick in the process). If you stop kicking, you sink.

If you redistribute your equipment so that you tend to balance in a horizontal position, then the energy of your kick drives you directly forward, and you can fine tune your buoyancy to neutral. Then, when you stop, you simply stop; you hover in place, stable and secure.

My guess is that your 4 kg weighting is probably correct for you, but you may need to move your tank up in the cambands, and use some of that weight in trim pockets to stay horizontal. (When I dive in the tropics, I use a stainless steel backplate (which is 5 lbs negative) and 2 lbs on the cambands. That balances me in a horizontal position without effort. If you need to take small trim weights to do this, you can put them in carry-on luggage, which is what I do.

Do keep working on this -- especially for a photographer, the ability to hover without finning is a really useful skill.

 

[don Francisco]

Dadvocate, You're trying to find a problem where none exists. The BC is just that - a bouyancy compensator - and it's perfectly logical to add or remove air to acheive neutral bouyancy. After all isn't why you're wearing it?

Bouyancy varies with depth, time (tank loses .07 kg per cubic foot of air consumed), breathing pattern and finning. Changes in any of these, if sustained, will call for adding or removing air from the BC, though you wouldn't bother adjusting for short term changes.

All other things being equal no weighting will be ideal for both the beginning and the end of the dive, when you're 2-3 kilo's lighter. You can split the difference, or adjust for either. I prefer to adjust so that I'm neutral at 10 feet with zero air in the BC, and 30bar in the tank, but you may prefer something slightly different.

On wall dives, I try to be neutral without finning, so photography is easier, and fine tune with adjustments to my breathing and ever so gentle finning where needed.

Don't expect that you can establish static neutral bouyancy. It's more like maintaining your balance on a moving train, constant small adjustments.

 

[spectrum]

I'm reading like TSandM. Your buoyancy is fine but your hydrodynamics, caused by trim are off a tad. Right now you a like an airplane with you body creating some lift as you swim. When you stop, you go into a stall and like a plane that goes to slow, down you go.

Picture your body as a see-saw and move things to get the balance just right. Remember that the task is to move weight, not add. When it is right you should know it and it's a breakthrough moment.

This may take some fiddling since you are entering the realm where the 5+ pound shift in cylinder buoyancy will change how you dive slightly as well as the various depths effects on neoprene. As perfect as your configuration will be there is a portion that will be adaptive by managing lung volume and nuances in how you swim.

Pete

 

[Dadvocate]

There seems to be truth in all three of your comments from what I see. I need to use some air my BCD sometimes and I also need to adjust for my trim to make the weight I am using-carrying work better than I currently am.

I think there is also a heartbreaking reality that is creeping into this issue of trim as well: my fins.

I love my Jet Fins because I have gotten good at moving myself around gingerly on reefs and even in larger wrecks and so on, but I can get a great amount of propulsion out of them when I need to, in current or in getting to the love of my life and my buddy.

If what you are saying is true, TsandM, I may need to consider the weight of these fins might also be a significant factor in “pulling” my legs down relative to the rest of my body. Mind you, sometimes I want a vertical profile when taking shots or in looking at that cool eel hiding behind a fan coral. At other times I can show with a horizonal profile.

When I am honest with myself about my fins, I know that all I need to do is allow the weight of my fins to bring my profile vertical when that is desired, whereas a small degree of effort on my part is needed to keep my profile horizonal.

Thank you very much for your description of how and when to put air into my BCD, TS and M. As I think back to my dives in Malapscua and earlier in Palau, I recall having that sinking feeling, then putting myself upright or vertical, shooting some air into my BCD, then going back to horizonal to see if things are better. This process might well have made the the issue of bouyancy control more troubesome than it needed to be. I don’t tend to stay vertical, but I have indeed gotten into the habit of going vertical to put air into my BCD.

In further reflection, I also see that those times when I have had to use 2 kilo bars in my integrated pouches, I have had more problems than when I was able to have half that weight placed in the upper BCD pouches.

I see that I have some tweaking to do based on this feedback. Thanks for reminding me of the function of my equipment, Spectrum and Don Francisso, and for driving home the fact that it is okay to have to fiddle still in order to achieve that wonderful “breakthrough moment”. I’ve already had some of those in this sport, and I hope to get that many more…

Cheers!

 

[ReefHound]

I think there is also a heartbreaking reality that is creeping into this issue of trim as well: my fins.
I love my Jet Fins because I have gotten good at moving myself around gingerly on reefs and even in larger wrecks and so on, but I can get a great amount of propulsion out of them when I need to, in current or in getting to the love of my life and my buddy.
If what you are saying is true, TsandM, I may need to consider the weight of these fins might also be a significant factor in “pulling” my legs down relative to the rest of my body. Mind you, sometimes I want a vertical profile when taking shots or in looking at that cool eel hiding behind a fan coral. At other times I can show with a horizonal profile.

This is true for many of us when diving wet, which is why my Jets are dry and I am now using OMS Slipstreams. They are essentially clones of Jets but much lighter, about neutral without the springs and about a pound negative with them.

 

[rongoodman]

Try keeping in horizontal trim with your legs bent at the knees and your fins flat. You'll be moving the negative buoyancy of the fins away from the end of the long lever which is your legs and closer to your center of gravity, minimizing its effect. If you need to go head up for a shot, just extend your legs a little. (It also sets you up for the frog and flutter kicks, backing up, and helicopter turns.) If the Jets are still too heavy, there are similar designs which are closer to neutral, such as the Slipstreams or DiveRites.

 

[don Francisco]

I love my Jet Fins because I have gotten good at moving myself around gingerly on reefs and even in larger wrecks and so on, but I can get a great amount of propulsion out of them when I need to, in current or in getting to the love of my life and my buddy.

If what you are saying is true, TsandM, I may need to consider the weight of these fins might also be a significant factor in “pulling” my legs down relative to the rest of my body.

Dadvocate, before chucking your jet fins check their bouyancy. Their weight in air is meaningless in the water, since it's partially or fully offset by their displacement. In any case, the net negative bouyancy is probably not that significant, unless you using steel heel springs.

Give yourself some time to dial things in before throwing money at this problem or giving up fins you like. dF

 

[ReefHound]

Are you talking about standard Jet fins or "Twin Jets". Twin Jets come in various colors in various buoyancy configs. Jets come in black (except a limited edition in white and yellow offered in Japan) and are all negative. They are made of rubber. Rubber is dense. I've never seen a pair of black Jets float. If yours do, I'd love to see a video clip of it.

 

[TSandM]

Try keeping in horizontal trim with your legs bent at the knees and your fins flat. You'll be moving the negative buoyancy of the fins away from the end of the long lever which is your legs and closer to your center of gravity, minimizing its effect. If you need to go head up for a shot, just extend your legs a little. (It also sets you up for the frog and flutter kicks, backing up, and helicopter turns.) If the Jets are still too heavy, there are similar designs which are closer to neutral, such as the Slipstreams or DiveRites.

Yes! Body position can have a BIG influence on what your fins do to your trim. When I dive in a 3 mil wetsuit, I still use my Jet fins with spring straps. I just move more weight to my back, and bend my knees more. I doubt you will have to give up your Jets.

Why do you go vertical to ADD air to your BC? Adding can be done in any position you like, and is best done with tiny puffs of air and some waiting time to see what the effect will be (new divers often get into trouble because they don't wait, and end up adding more and more, until they are strongly positive and have to dump a bunch). Venting, with some BCs, may require breaking trim (although in a well-designed one, it shouldn't), but adding can be done absolutely on the fly.