Lift of a BC...

[jmasin]

OK, something tells me this may be a dumb Q... but another thread made me think.

Background - I'm a tropical diver who wears 8lbs and dives a little locally with 5mm and wears 12-16, so I've never really had dive situations that required a lot of lead.

But I'm confused on something. Why is there a need for high-lift BCs?

The reason I ask is I would expect if correctly weighted you are slightly negative to descend, you want to go neutral at depth, and then effectively neutral to ascend and do a SS.

Then you want to be positive on the surface.

So even if you are wearing 40lbs of lead, you are doing so to counteract your clothing/body's buoyancy. You don't need to oppose that entire 40lbs with BC lift... do you?

Is it a matter of surface safety?

Feel free to point out my stupidity if I'm missing something brutally simple...

 

[The Kraken]

Suppose you're diving large steel doubles and, for some unknown reason, you have to remove your rig at the surface and the tanks are full. There is the distinct possibility that a wing with 28 lbs. of lift capacity might not be able to support them.

In fact, I had a student diving a HP130, steel B/P, intergrated weights and a 32# lift wing and a 7mm fj/j. The wing would not support his rig because of the weight required for the 7mm that he had in his weight pockets. Had to have him put some on a belt. He wasn't happy with it, but there ya go . . .

the K

 

[jmasin]

Suppose you're diving large steel doubles and, for some unknown reason, you have to remove your rig at the surface and the tanks are full. There is the distinct possibility that a wing with 28 lbs. of lift capacity might not be able to support them.

In fact, I had a student diving a HP130, steel B/P, intergrated weights and a 32# lift wing and a 7mm fj/j. The wing would not support his rig because of the weight required for the 7mm that he had in his weight pockets. Had to have him put some on a belt. He wasn't happy with it, but there ya go . . .

the K

K,

Thanks, makes perfect sense. I just hadn't encountered/learned a situation like that yet.

Thanks!

 

[j.doe]

If I am diving a dry suit not all my wieght is easily removed. Then if you pop a drysuit seal and or loose suit boyancy you want to make sure that you can float ALL + a few pounds extra all your non dumpable weight. For me that is my 25lbs of non dumpable + 3 lbs of gear + 5 lbs extra for safety and that works out to be at lest 33lbs lift that I need bare minimum.

Also if you are diving in colder water where you have more rigging, lines, lift bags etc... you need a wing that can float all the gear with room to spare.

Just my 2 cents.

 

[jmasin]

j.doe,

Thanks. Again, makes sense. I appreciate the education.

 

[fire_diver]

Wetsuits compress at depth. If you need 25 pounds of lead just to get under the surface, you can become 15-20 pounds negative at depth just from the suit compression, plus the weight of unbreathed air.

 

[TSandM]

Big lift is associated with big negative things -- there is rarely a reason for a single tank diver to need 40 lbs of lift. (A very large man, with a very large tank, wearing a lot of thick neoprene is about the only situation I can think of.)

Wearing an HP 130, I start the dive 11 lbs negative from gas that I'm carrying that I intend to use. My dry suit is 28 pounds positive, and a lot of that is lift I can lose if the suit floods (it's unlikely to lose ALL of it). So, if I begin a rapid descent, as I might have to when trying to hit a target in current, and my suit floods on the way down, I could be 30 or more pounds negative at the bottom. I need wing lift to get me back up, but of course, I also have some lift I can generate with my fins. A 30 lb wing is not unreasonable in that situation.

Make that double steel tanks, and you've changed the equation.

 

[Blue Sparkle]

I learned about this from SB too (and from the vendor when I was buying my BC), so... good question!

There is nothing here in my post that hasn't been mentioned here already, but I like lists and it's a good mental exercise for me to put what I (think I) know into words

So, as I remember it, wing size/lift is relative to three things:

1) Compensates for weight of gas in full tanks (i.e. difference in buoyancy between full tanks and empty tanks).

2) Compensates for lost (or potential lost) buoyancy of exposure protection.

3) Is configured such that your rig will be able to float at the surface without you in it, even with full tanks.

Notes:

a) As you can see, numbers 1 and 2 are also things that you purposely wear more weight for when they are at their "opposite" point (i.e. fully "puffy" buoyancy, and fully light/empty tanks).

b) In regards to #3, it can change depending on how you distribute weight between your body and your rig.

c) And the reason you don't need to compensate for *all* your weight is that the weight you are wearing that is not accomplishing #1 or #2 is weight that is simply making your body neutral (if you are a naturally buoyant person). So in that sense it is "neutral" weight already, unless you have it distributed such that it influences #3.

*******
Okay, now I will see if/what I got wrong -- or left out -- because I'm sure that other SB'ers will let me know

As I mentioned, I first calculated this (special thanks to Tobin at DSS!) just before I bought my BC, but that was pretty simple since I was planning it for my immediate future diving: No exposure protection and an AL63 in summer Florida (with provision for a 3mm suit). I went along diving like that for the first 25+ dives with my first "very own" BC (whew, no more rentals).

Then this winter, I thought about it some more as Florida was quite cold (66º water at times), so more exposure protection, and I also had the opportunity to dive a steel tank. So I had to think about whether my wing would still be adequate overall, and/or how to distribute my weight in order to keep my rig floating at the surface without me in it. It's fun to think about/calculate

(And I knew I would be testing it immediately as I was nursing a case of knee tendinitis and so was doffing my rig at the surface at the end of every dive and then handing it up to the DM.)

Blue Sparkle

 

[Hatul]

As someone else pointed out, the wing should make the Scuba rig positive when removed so it does not sink.

Beyond that, it has to compensate for the changes in buoyancy from wetsuit compression and the weight of the air consumed, plus a few pounds of lift extra as margin of error.

Adam

 

[DiverLS]

It's not just the weight on the surface. I used to dive a neoprene drysuit. So I needed more lead to get off the surface. This was coupled with a Aluminum 80 tank. I'm 5'10" tall and lean so I sink, just slightly 'cause I'm female with only a bathing suit on in a pool. A new female clerk in a large dive store sold me my first BC, with a 15lb lift. The B/C fit my lean frame.

Well, 4 years later I wanted to take our dive club to the Rubicon dive at Lake Tahoe. A lot of O/W certified divers have died there over the years and I found out why. A Dive Master friend of mine wanted to take me to a dive on Rubicon before I took the club there.

As we decended over the wall, the planned dive was to 85'. Lake Tahoe is at 7,000 ft so that is the maximum depth on air. We started down feet first and I though no problem, when I get to 85feet, I'll just fin up. NOT. I kept going down with all my effort. I had my BC fully inflated. Luckily I was in a drysuit and as I was passing 115', I pressed the valve to fill the suit. Luckily that happened before I panicked. Drowning averted. I was overweighted. I did have my hand on the buckle to drop my weight belt if needed.

On the way back up I noticed the DM I was diving with knew everything I was going through. I was never in danger. But I did replace that B/C before my next dive.

So, it's not just the weight to support your rig at the surfact, it also needs to support you at your maximum depth. That takes practice.