Duck dive for descent?

[Zinman]

Newly certified diver here.

Can you duck dive from surface to descend with all your gear on your back?

I recently floated up to the surface during a recreational dive (my buoyancy control not established yet), (I am very accustomed to duck diving in non scuba environments) - but found I couldn't duck dive or even swim downwards from surface as the gear kept weighing me down in the opposite direction. Was the first time I tried it, as deflation on BCD wasn't working.

 

[scubadada]

@Zinman

Do a proper weight check. With your cylinder near reserve pressure (500 psi in an AL80), you should be neutral at your safety stop with no air in your BC. You should be able to do a nice, controlled, slow ascent to the surface. If this is not the case, adjust your weights. With a full cylinder, you should be a little over 5 lbs negative at the beginning of a dive due to the gas that you will consume during your dive. You should have no trouble descending and will need to put some gas in your BC as you approach your bottom depth to become neutral. Practice.

I find the butt dump the most reliable way to get all of the air out of my BC. Butt up and side with the dump up, it becomes second nature.

In Boynton Beach, West Palm, or Jupiter, I often want to make a relatively quick descent to hit the wreck or reef on a drift dive when the current may be quite brisk. I jump with little to no gas in my BC, start down head first, pull my butt dump to make sure my BC is empty, kick down, add a few bursts of air to my BC as I near my bottom depth. I have a very good idea of how much air to add to my BC to end neutral. This is often done while deploying the line from my reel for my surface marker flag.

 

[inquis]

one can be perfectly weighted for the end of the dive and still need to kick down at the start.

I disagree. A wetsuit will regain about 31% of it's buoyancy, at most, between a 15 ft safety stop (where everything would ideally be neutral at reserve pressure) and the surface. (This from Boyle's Law). My 7mm is on the larger / more buoyant side at XXL, and that delta is 4.7 lbs. I carry more than 5 lbs of gas over that reserve that I haven't yet breathed (AL80) and can easily lose about a pound of buoyancy by a more-than-normal exhale (a total of more than -6 lbs).

I'm sure some 8-10 mm wetsuit users might need to duck dive that first descent, but adjusting the neutral depth (shallower) would be smarter. This avoids corking in the final ascent and would therefore be included in the definition of "perfectly weighted". It also would obviate the need to duck-dive.

That's not to say it's not useful to duck-dive to speed things up, but that's a "choose to", not a "need to", in my view.

 

[boulderjohn]

I am going to add a heretical notion. The quest for perfect weighting is not necessarily a good thing. A few extra pounds beyond perfection allows for a quicker descent and makes it easier to dump a little burp of air as needed. With a nearly empty BCD, you have to get that tiny bubble of air in just the right place to get it out.

Note the phrase "a few."

 

[Zinman]

It does paint a picture. It is a picture I saw often while working with new divers.

What I have frequently seen with new divers is that they are swimming (at best) at a 45° angle, kicking constantly to maintain their depth. They think they are neutrally buoyant, but if they were to stop kicking, they would sink. They also think they have all the air out of the BCD, but they don't. Their body position and their kicking eventually brings them shallower, which causes the air they don't think is in their BCD to expand. They start to ascend even more, so they go completely vertical (while still kicking) to try to dump air, which makes things worse.

In many cases, these people are already overweighted, which is why they have so much air in the BCD, which is expanding as they rise. If that is the case, adding more weight will make it worse.

Hi there, this is exactly it. You’ve nailed it.

I am utterly confused at this point whether I am over or under or correctly weighted (as I’ve had most people say under & so did the dive guide). I only add 2-3 tiny bursts of air in BCD. My instinct would be - at my next dive - keep my 4kg weight as is (I am 60kg). Or perhaps make it 5kg. And when the tank gets to 120 bar (ish) - so half way, have no air in BCD, like release it all (or what I think is “all”), just use lungs and see how I go? Just try different configurations and see what the body does.

 

[lowwall]

Edit - this is not to descent. I descent as taught, feet first, exhale, deflate BCD.

This is for when I float up during the dive and when BCD is already deflated and exhaling won’t bring me down. Exploring an alternative, rather than being stuck on the surface. I couldn‘t duck dive cos’ the weight of the gear was pulling me behind my shoulders (towards my back) and duck dive requires a curl forward. If that paints a picture.

You are using a conventional jacket BCD, right? I think you've got a bunch of gas left in the front air cells of your BCD.

You should be able to roll over onto your stomach. Get your butt up a little and use your rear dump.

Even if you don't have any gas in the BCD, your inability to maneuver in this situation (along with what you've posted on your other threads) suggests you need to work on very basic skills. Before you hurt yourself or create an emergency situation for others, please get some further instruction or at the very least get enough practice in a pool that you have basic buoyancy and maneuvering nailed down.

You can deal with imperfect trim by giving yourself extra space, but you have to be able to hold a depth, change depths, and change directions and your physical orientation to safely dive. Right now you seem to be having trouble with all of these things.

 

[dmaziuk]

Hi there, this is exactly it. You’ve nailed it.

I am utterly confused at this point whether I am over or under or correctly weighted (as I’ve had most people say under & so did the dive guide). I only add 2-3 tiny bursts of air in BCD. My instinct would be - at my next dive - keep my 4kg weight as is (I am 60kg). Or perhaps make it 5kg.

Start with getting horizontal. Most men and many women are leg-heavy and need weights up higher than where you normally have them in BCD pockets or on the belt. If you can borrow some "bullet" weight that clip on to shoulder/chest D-rings, that's the easiest way to figure out your trim. You can also attach the "bricks" with a loop of bungee cord.

 

[NDonahue]

I disagree. A wetsuit will regain about 31% of it's buoyancy, at most, between a 15 ft safety stop (where everything would ideally be neutral at reserve pressure) and the surface. (This from Boyle's Law). My 7mm is on the larger / more buoyant side at XXL, and that delta is 4.7 lbs. I carry more than 5 lbs of gas over that reserve that I haven't yet breathed (AL80) and can easily lose about a pound of buoyancy by a more-than-normal exhale (a total of more than -6 lbs).

I'm sure some 8-10 mm wetsuit users might need to duck dive that first descent, but adjusting the neutral depth (shallower) would be smarter. This avoids corking in the final ascent and would therefore be included in the definition of "perfectly weighted". It also would obviate the need to duck-dive.

That's not to say it's not useful to duck-dive to speed things up, but that's a "choose to", not a "need to", in my view.

You are clearly an analytical person. I often joke with my German colleagues about their frustration when a solution works in practice but not in theory. That may be applicable here.

I can state with very high confidence that in my warm/warmish water set-up* that I will not drop below the surface without full exhalation and active diving effort, and yet I am ultimately perfectly neutrally buoyant at 5m with 30bar in my cylinder and an empty wing. An inverted initial descent position with an open wing generally ensures that I have no air trapped in the wing. I suspect there are confounding factors** at play.

I don’t dispute your analysis, but my example reinforces the universal point that empirical data is more valid than predictive modeling. I’d be happy to host you for some dives and we can solve this conundrum if you’re interested. I’m always looking for an excuse to dive and maybe we can contribute to improving the accuracy of buoyancy.cc
______
* setup: 6’3” male, 210lbs, 3 mil full or 5mil shorty, al bp/ cordura covered wing, mk19evo, 2 G260s, rubber hoses, neutral OMS slipstream fins, al80 and 5kg of lead attached only with bungees (no weight pockets), al reel with neutral line, neutral dsmb, no light (typically high visibility in red sea beyond the depth I’m willing to dive on a single tank (30m)).

** Possible confounding factors (none of which individually should be significant enough to impact initial vs safety stop buoyancy): I dive mostly in upper Red Sea with relatively higher salinity; I naturally breathe more shallow when diving (versus surface breathing); the buoyancy characteristics of the specific neoprene in my suits; I hang my suit in direct sun between dives and therefore rarely enter the water w/ a waterlogged suit; etc.

 

[Zinman]

You are using a conventional jacket BCD, right? I think you've got a bunch of gas left in the front air cells of your BCD.

You should be able to roll over onto your stomach. Get your butt up a little and use your rear dump.

Even if you don't have any gas in the BCD, your inability to maneuver in this situation (along with what you've posted on your other threads) suggests you need to work on very basic skills. Before you hurt yourself or create an emergency situation for others, please get some further instruction or at the very least get enough practice in a pool that you have basic buoyancy and maneuvering nailed down.

You can deal with imperfect trim by giving yourself extra space, but you have to be able to hold a depth, change depths, and change directions and your physical orientation to safely dive. Right now you seem to be having trouble with all of these things.

Thank you again. Very useful piece of advice, as mostly I am just told "dive more" but that doesn't seem sensible with the way my skills (or lack of) currently sit. I have discussed it with my LDS, and have asked to arrange a pool session.
And I use BP/W BCD

 

[inquis]

my example reinforces the universal point that empirical data is more valid than predictive modeling

As you say, there must be confounding factors at play. It would indeed be fun to investigate -- the Red Sea is definitely on my "Someday" List.

 

[tursiops]

You are clearly an analytical person. I often joke with my German colleagues about their frustration when a solution works in practice but not in theory. That may be applicable here.

I can state with very high confidence that in my warm/warmish water set-up* that I will not drop below the surface without full exhalation and active diving effort, and yet I am ultimately perfectly neutrally buoyant at 5m with 30bar in my cylinder and an empty wing. An inverted initial descent position with an open wing generally ensures that I have no air trapped in the wing. I suspect there are confounding factors** at play.

I don’t dispute your analysis, but my example reinforces the universal point that empirical data is more valid than predictive modeling. I’d be happy to host you for some dives and we can solve this conundrum if you’re interested. I’m always looking for an excuse to dive and maybe we can contribute to improving the accuracy of buoyancy.cc
______
* setup: 6’3” male, 210lbs, 3 mil full or 5mil shorty, al bp/ cordura covered wing, mk19evo, 2 G260s, rubber hoses, neutral OMS slipstream fins, al80 and 5kg of lead attached only with bungees (no weight pockets), al reel with neutral line, neutral dsmb, no light (typically high visibility in red sea beyond the depth I’m willing to dive on a single tank (30m)).

** Possible confounding factors (none of which individually should be significant enough to impact initial vs safety stop buoyancy): I dive mostly in upper Red Sea with relatively higher salinity; I naturally breathe more shallow when diving (versus surface breathing); the buoyancy characteristics of the specific neoprene in my suits; I hang my suit in direct sun between dives and therefore rarely enter the water w/ a waterlogged suit; etc.

Ok, I will accept your explanation of confounding factors....but point out that my empirical data disagrees with yours; I am heavier at the beginning of a dive that at the end. I assume you are too.