Rapid Descent Rate

[L13]

I get that, and I understand the principal behind it.

My understanding was that while some air would need to be added at depth to maintain buoyancy, you should never be so over weight that the air is required to arrest the descent vs using kicks to arrest the descent and then air in the BCD to reduce the work of staying at the desired depth.

While you shouldn't be weighted more than you can kick back up without your BC, you shouldn't actually do it that way unless you have a BC failure. Swimming up to control your descent vs using your BC is a bad idea. Use your BC to reduce work during all phases of your dive.

The difference in weight of air in your tank between full and safety stop is approximately 4.5 lbs which requires approximately 2 quarts more air in your BC at the beginning of the dive than at the safety stop. As you descend from 15 ft to 65 ft, that air will compress to 1 quart, so you will have to add another quart to stay neutral. As you continue to descend from 65 to 130 ft, it will again compress to 1 quart and you will have to add another quart. If you were to ascend immediately to 15 ft, with the tank still full, you would have to vent 6 quarts of air at 15 ft to restore neutral buoyancy with 2 quarts left in your BC.

As you breath down the tank all those numbers will get smaller, and the extra air in your BC to compensate for tank air weight will drop from 2 quarts to 0. Suit compression and other similar effects will result in you needing to add even more air than listed above descending, and vent more ascending. The above numbers are just the effects on BC volume due to the weight of air in your tank.

Swimming up 4# of air plus 8# of suit compressing during the decent at the beginning of your dive is a bad idea, even though you can swim up 12# in an emergency. You would reach your target depth with a high CO2 loading and be more prone to narcosis, paranoia, and panic.

 

[tursiops]

My understanding was that while some air would need to be added at depth to maintain buoyancy, you should never be so over weight that the air is required to arrest the descent vs using kicks to arrest the descent and then air in the BCD to reduce the work of staying at the desired depth.

?? You are saying to stop the descent by kicking upwards, and then add air ro be neural? Why? What purpose do the kicks provide that is not provided by the air?

 

[johndiver999]

I often find it advantageous to descend rapidly. This means swimming down hard and trying to not inhale much in lungs until I hit about 20 or more feet, then I just try to take shallow breaths. Keeping the lung volume down helps a lot and not breathing in the most efficient manner for 30 seconds is not a big deal for me. Once I am down 40 ft or so, the wetsuit is crushed, any air in the Bc is crushed, any air bubble trapped in suit are crushed and I can breathe normally.

On descent I am probably 8-10 lbs negative and with a good vertical position, and a little kicking, I can move pretty fast. You want to be careful however, that you are able to instantly STOP your descent should your ear lock up.

The quickest way to do that is to take a big breath and simultaneously turn from a vertical to perfectly horizontal position, with your fins held horizontally. This makes a ton of drag and should give you a second or two to bobble the inflator, find it and press the button.

The idea that you should always maintain neutral buoyancy is in the text books, but is not really practical nor necessary if you are a good swimmer and you can equalize quickly. Finding yourself sinking, with a locked up ear and being way negative is good way to get hurt.

Once you are down 60-80 feet, the pressure change is much slower, so a faster descent rate is less stressful.

This is one example where being a decent freediver provides a big advantage, they know how to do a surface dive, how to clear quickly, how to descend vertically in a streamlined manner and how to flair out in a negative condition, when they want to. Of course they start out with full lungs upon descent.

This all assumes that the rapid descent rate is necessary AND that you have sufficient visibility to not crash into something on descent.

 

[Lorenzoid]

The wisdom I learned is control your descent, monitor your equipment to confirm it’s working properly, monitor your buddy, your environment, and your body (How do I feel? Do I need to equalize, etc.). Decelerate to your planned bottom depth.

 

[kinoons]

?? You are saying to stop the descent by kicking upwards, and then add air ro be neural? Why? What purpose do the kicks provide that is not provided by the air?

A degree of finer control and more immediate response than adding air to the BCD

If your descent is at a gradual pace, it shouldn’t take more than a few kicks to suspend that descent for a moment or two allowing the diver to add air to the BDC (or doing so simultaneously) to maintain the depth without any significant effort.

While you shouldn't be weighted more than you can kick back up without your BC, you shouldn't actually do it that way unless you have a BC failure. Swimming up to control your descent vs using your BC is a bad idea. Use your BC to reduce work during all phases of your dive.

The difference in weight of air in your tank between full and safety stop is approximately 4.5 lbs which requires approximately 2 quarts more air in your BC at the beginning of the dive than at the safety stop. As you descend from 15 ft to 65 ft, that air will compress to 1 quart, so you will have to add another quart to stay neutral. As you continue to descend from 65 to 130 ft, it will again compress to 1 quart and you will have to add another quart. If you were to ascend immediately to 15 ft, with the tank still full, you would have to vent 6 quarts of air at 15 ft to restore neutral buoyancy with 2 quarts left in your BC.

As you breath down the tank all those numbers will get smaller, and the extra air in your BC to compensate for tank air weight will drop from 2 quarts to 0. Suit compression and other similar effects will result in you needing to add even more air than listed above descending, and vent more ascending. The above numbers are just the effects on BC volume due to the weight of air in your tank.

Swimming up 4# of air plus 8# of suit compressing during the decent at the beginning of your dive is a bad idea, even though you can swim up 12# in an emergency. You would reach your target depth with a high CO2 loading and be more prone to narcosis, paranoia, and panic.

Again, I follow the principal behind what you’re describing. I’m not advocating trying to ascend from 100ft to 20ft by kicking alone.

Although I am still a fairly novice OWD, so my understanding still was that the BCD is intended to maintain your buoyancy at a specific level. Thinking about it more, buoyancy is dynamic based on many variables and will constantly change. Intuitively I knew this - air in the BCD expands as you ascend, exposure gear compresses at depth, etc, but I still was taught that the power inflator is not an elevator button, so using it to arrest or propel changes in depth is a foreign concept to me.

Thanks for the interesting insights

Back to the thread topic, as discussed the only big thing I can think of is squeeze, primarily ears, but other body cavities potentially as well. The faster you descend the more actively those would need to be addressed.

 

[L13]

A degree of finer control and more immediate response than adding air to the BCD

It gives no finer control.

If your descent is at a gradual pace, it shouldn’t take more than a few kicks to suspend that descent for a moment or two allowing the diver to add air to the BDC (or doing so simultaneously) to maintain the depth without any significant effort.

So adding air on the way down? that is what people are saying.

Although I am still a fairly novice OWD...

So listen to everyone else (not just me) with more experience telling you you are wrong.

For a short descent you way works okay. for a large descent you are making a big mistake if you wait to add air to your BC until you are stable at the new depth.

Going the other way, do you fin up to start your ascent and then, as your BC bubble grows, fin down to stop your ascent, and then (while fining down at your safety stop) vent air to reduce the work of maintaining your depth? Or do you vent air on your way up to reduce the rate of ascent, trying to reach neutral as you reach your stop depth?

You are confounding the advice not to use your BC to start going up with using your BC to stop going down.

Think of the two buttons on the BC as:

add air is down brake​

dump air is up brake (except for initially leaving the surface)​

​

Instead of:

add air is up elevator​

dump air is down elevator​

 

[Rilelen]

Another reason not to kick to stop your descent: if you are kicking to stay up, you're going to create powerful downdrafts with your fins. That water you are propelling downwards is going to hit the bottom and stir up whatever's there - sand, silt, etc. That might not matter so much if it's a rocky bottom, or if there's NO hard bottom* at your target depth (e.g., descending to 80' on a 400' wall), although there are other reasons why it's inadvisable, including many other folks have already mentioned.

* not putting ANY air into your BC to slow your descent with no hard bottom makes my skin crawl a little...it's a good way to get an unpleasant and potentially dangerous surprise should your inflator fail to work at depth

 

[rob.mwpropane]

I'm assuming this is in a wet suit?

In a drysuit I'm adding air on the way down to prevent squeeze. I very rarely add to my wing at all, just to the suit. The plus is it automatically vents on the way up. Because of this I never gain the speed down that I used to in a wetsuit. As they compress I would start to rocket down like a rock!

I never noticed anything adverse from dropping too fast, except as @tursiops pointed out, the ears.

 

[kinoons]

So listen to everyone else (not just me) with more experience telling you you are wrong.

The intent of this portion of my post…

Thanks for the interesting insights

…Was to signify my acceptance and understanding on how the use of the power inflator as you described, a break instead of an accelerator, is not only appropriate, but possibly very neccessary.

Sorry I didn’t make that clear.

 

[inquis]

The safest, most controlled descent is to add air to maintain a slow descent, then add a little extra to halt. (I'm ignoring the more advanced situations that would benefit from a rapid descent.) When your skills are better, you can descend faster -- but still in control -- much like you can drive faster while in control after skills improve from those of a beginning driver.

Kicking to halt a descent when you have a working BC is undesirable.

ETA: we cross-posted, sounds like you got the gist of things now.