65 replies to this topic

[David Evans]

I just have to ask about the position in the water having anything to do with buoyancy. You still have the same volume, displacing the same mass - how would they become more buoyant? I have a feeling I'm missing something. Need help here . . .

Hawk:

It has to do with Boyle's law and Archimedes' principle.

Picture two divers that are otherwise neutrally buoyant and horizontal facing each other, both diving a conventional hose rig. One of the divers goes OOA, and the two divers then assume a vertical position to do the OOA "grapple".

Depending on the center of gravity, it's likely that both divers will move their torsos upwards in the water column while dropping their knees (legs are negative, torsos are positive).

This action will cause the air in both BCs to expand slightly, thereby displacing more water, and thus causing them to become positively buoyant. It's also likely that the heavier breathing pattern induced by the OOA will exacerbate the problem. And controlling buoyancy on the way up is more challenging when in a vertical posture due to the relative lack of hydrodynamic resistance.

The big issue, IMO, is that with two divers facing each other like this it makes communications VERY difficult. It's a challenge to communicate things like safety/deco stops, or remaining air, etc. And if you are in a situation where it would be preferable to swim a distance before ascending, this task is VERY difficult using a conventional hose rig.

I understand that some may prefer to ascend vertically, so like I've said before, more power to ya! I prefer a horizontal ascent, not just because it's easier to control the ascent, but also because the body is exposed to equal amounts of ambient pressure in this posture. While this isn't a big deal at depth, it can become more important shallow... I'm 6'3, and at depths less than 10', there can be a pretty large pressure differential between the depth of my feet and the depth of my head in a vertical posture.

I'm certain that someone's gonna mention that I'm over thinking things... that's fine... I'm not trying to win converts here. I'm simply explaining my own diving practices.

P.S. David, I like how you described the hose as being "Only 20 inches" longer - that sounds better in a discussion than saying "over a foot and a half" longer.

Hehehehe.... I've spent almost 20 years in marketing... whaddya want from me?

-d

Edited by David Evans, 12 September 2005 - 12:09 PM.

[ScubaHawk]

Depending on the center of gravity, it's likely that both divers will move their torsos upwards in the water column while dropping their knees (legs are negative, torsos are positive).

This action will cause the air in both BCs to expand slightly, thereby displacing more water, and thus causing them to become positively buoyant.

Ah-ha I knew I was missing something - Didn't think of them moving up in the water column - I was thinking to analytical and not enough practical - Thanks.

[Walter]

A "tech" charter is quite a bit more expensive as the operator can only make one trip out that day (to accomodate the additional surface interval and length of time that divers will be decompressing).

Except "tech" doesn't always mean decompression.

Insurance and agency-wise, SSI's certs will not go into the realm of "tech" diving This is a difference between them and a couple of other major training agencies

It's still useless. SSI has a list of their specialties. That's what they teach and nothing else. It does not matter if someone's definition (and there are several conflicting definitions) has one or more of them as "tech." YMCA, for example states it does not teach "tech" diving, yet by some definitions, its wreck course is technical.

When I read the word "tech" in a diving context, it immediately says "overhead" to me... meaning that there's a hard ceiling involved, whether it be deco or the roof of a wreck or a cave.

Not by all definitions. NAUI's definition includes rebreathers or switching gases during the dive.

The same could be said for the word "advanced".

Only because most advanced classes have been cut until they are a joke. There are still some good advanced courses to be found.

Or "no-deco".

Not at all. No deco means no required staged decompression stops. It has meaning.

Depending on the center of gravity, it's likely that both divers will move their torsos upwards in the water column while dropping their knees (legs are negative, torsos are positive).

This action will cause the air in both BCs to expand slightly, thereby displacing more water, and thus causing them to become positively buoyant. It's also likely that the heavier breathing pattern induced by the OOA will exacerbate the problem. And controlling buoyancy on the way up is more challenging when in a vertical posture due to the relative lack of hydrodynamic resistance.

Depending on depth, this moving from horizontal to vertical may have a large effect or almost no effect. Exhaling while switching position will negate this effect. It's really not an issue. I've brought up lots of OOA victims over the years, so this is not theory, but practical experience. In either case, starting an immediate ascent is not wise, so there's no reason to immediately switch to a vertical position even if it is your intention to ascent vertically. First, you need to establish contact with the victim and make sure he is calm before moving. Next, you need to control buoyancy. It is likely the OOA victim will forget to vent his BC in either position. It is also likely that unless you have a grip on his BC, he will start ascending before you are ready. It's extremely unlikely the victim will ascend horizontally regardless of what you do. If you are having difficulty controlling buoyancy in a vertical position, you might consider practice in that position.

The big issue, IMO, is that with two divers facing each other like this it makes communications VERY difficult. It's a challenge to communicate things like safety/deco stops, or remaining air, etc. And if you are in a situation where it would be preferable to swim a distance before ascending, this task is VERY difficult using a conventional hose rig.

Communications is easier when vertical and facing each other. It's much easier to see facial expression, eyes and hand signals. You are in a situation where it is preferable to be holding onto the victim and keeping them very close to you to prevent a panicked ascent in which the victim dies.

I'm 6'3, and at depths less than 10', there can be a pretty large pressure differential between the depth of my feet and the depth of my head in a vertical posture.

This is only an issue in a dry suit and then it's only a matter of comfort. In a rescue, you won't notice your feet getting squeezed.

[David Evans]

This is only an issue in a dry suit and then it's only a matter of comfort.  In a rescue, you won't notice your feet getting squeezed.

It's not a drysuit issue, it's a getting out of the water with as few bubbles in my bloodstream as I can issue.

Edited by David Evans, 12 September 2005 - 01:37 PM.

[Walter]

Then you are imagining a problem that doesn't exist.

[David Evans]

Walter:

Thanks for clearing that up for me. It's been worrying me for years.

[Walter]

Don't let it ever bother you again.

[ScubaHawk]

totally off topic -- how are ya'll getting multiple quotes in one post - If Walter (or anyone) can pm me with this, I's sure applicate it.

[annasea]

totally off topic -- how are ya'll getting multiple quotes in one post - If Walter (or anyone) can pm me with this, I's sure applicate it.

Henry,

Check this THREAD for various methods.

[Walter]

The girl is good.

[David Evans]

Walter:

You no nothing about my diving practices, about the mixes I breathe, about my decompression strategies, nor about my typical profiles.

The greatest pressure changes on the body occur when you exit the water. The body undergoes a 50% ambient pressure change between 15 feet and the surface, and the blood takes a full 3-4 minutes to pass through the circulatory system, which is needed for the alveoli to capture the bubbles that are created during an ascent (and they are created, even in typical recreational profiles).

As I mentioned, I'm over 6 feet tall. If my head is at a depth of 10 feet in a vertical posture, my feet are at a depth of 16 feet. That's roughly a 20% pressure differential between my head and my feet in a vertical posture at that depth. The shallower I go in that position, the greater the pressure difference. By maintaining a horizontal position in the water, and doing a glacially slow ascent, I'm doing my body (MY body - MINE) a favor.

Between 30 feet and the surface, I ascend no faster than 10 feet per minute, and stop for at least a minute every 10 feet starting at 30 feet, and that's after doing whatever deep stops and gas switches may be involved for the profile I'm doing. I want to give my body every chance to effectively offgas underwater, where it can do so most effectively. This may seem unreasonable to you, but then, I'm not asking you to dive that way.

It may not be how you dive, but you know, that's really of no concern to me, is it? It's how I dive, and that works well for me. I happen to have a pretty solid understanding of what works best for me and the way I dive. I won't lose any sleep whatsoever wondering what you may think of that.

You dive your way, I'll dive mine, and with 3000 miles between us and diving in two different oceans, I bet it won't cause either one of us any problems at all.

Edited by David Evans, 12 September 2005 - 11:41 PM.

[Walter]

Chill out David. Dive anyway you want, I've never told you otherwise. OTOH, if you think horizontal vs vertical makes a difference in your decompression status, you are mistaken, not that I expect you to believe it.

[drdiver]

By maintaining a horizontal position in the water, and doing a glacially slow ascent, I'm doing my body (MY body - MINE) a favor.

Very good point. Offgassing efficiency is improved by a horizontal profile.

[Walter]

There's absolutely no evidence to suggest that.

[drdiver]

Blood circulation is better in a horizontal position. Better circulation, better offgassing. Simple enough.