Using large lift bags??

[RonFrank]

So a guy had some lift bags for sale on the board. I picked up a super nice SS with a valve, and a flag. I also picked up a 250lbs lift bag.

I had no clue the thing would be as large as it is! He was selling some 1000lbs bags, and I can't imagine how large those suckers must be.

I any event, does anyone have a clue as to how one would use this thing? I could think of a few ways to supply air to it, but I think the thing would rocket to the surface long before it was full? Yes, I understand that the air will expand, but it's large enough that I doubt I could even get it 10% full before it would take off, and that would require some depth assuming I could even get it 10% full, and I have no desire to do this deep, in fact just the opposite.

I don't have any plans or need to use this at this time, but it would be cool to inflate it UW and give it a go.

I suppose if I attached it to something heavy on the bottom, that would allow me to fill it up.. hmmm.. maybe I just answered my own question.

 

[Doc]

Quite often, lift bags in that extremely large capacity are used in a calculated array of similarly large bags, positioned around a large object at precise locations. They are utilized on concert, not necessarily at full volume of inflation. First comes neutral buoyancy, then comes a gentle lift.

On a large lift, a 250# bag would be used for "tweaking". I don't think they all have bleeders on them, but the ones that I have used did. Better than a bunch of blue polyethylene barrels, anyway.

You now own the coolest SMB Safety Sausage on the boat. Don't tell these guys about it: http://www.scubaboard.com/forums/ad...s/304442-re-using-air-bcd-dire-emergency.html

 

[akscubainst]

1000 lb aren't that big.

We usually use them to stabilize things while the 4,000 lb bags do the lifting. Fill gas comes from an air compressor on the surface and everything is filled really slow and controlled.

 

[HowardE]

A #1000 open bottom lift bag can be filled with a slung 40 or even a 20 no problem (or even your back gas, if you have the gas for it). The Carter #1000 is 52" x 50" and uses only 16 cubic feet of air at the surface.

closed bottom bags, salvage tubes, etc can be filled with hoses run from surface supplied compressors, and can be worked in pairs, or multiples as stated above.

When lifting large object, like concrete blocks, or other heavy items, the point of neutrality is fairly apparent. Once the object is buoyant, it shoots pretty quickly, caution is required.

 

[kosap1]

I suppose if I attached it to something heavy on the bottom, that would allow me to fill it up.. hmmm.. maybe I just answered my own question.

Yes you did just answer your own question. As you fill it, the bag will become closer and closer to neutrally buoyant, assuming that it is connected to a heavy object. Then once you get it neutrally buoyant you can lift it up and swim up with it. Just be careful on the way up to vent air keeping it neutrally buoyant so that it does not get away from you.

A #1000 open bottom lift bag can be filled with a slung 40 or even a 20 no problem (or even your back gas, if you have the gas for it).

Once the object is buoyant, it shoots pretty quickly, caution is required.

First, don't use your back gas to lift an object. To be safe it is best to use an alternative cylinder and regulator. If you are filling it and the reg gets caught on the bag it will be the alternative reg, not your reg that is attached to your bc, which can then pull you up in an unsafe manner.

Second, there should be no "shooting". The objective is to make the item to be lifted neutral, so that you can then easily lift it using your fin kicks.

However, as others have said if you are lifting items big enough to need large bags there is a whole different protocol to follow.

 

[RonFrank]

Don't tell these guys about it: http://www.scubaboard.com/forums/ad...s/304442-re-using-air-bcd-dire-emergency.html

These threads come up periodically. My thought is always, how do you get to a place where breathing air from a BC becomes appropriate?

Damn, I hijacked my own thread!

 

[Cave Diver]

Damn, I hijacked my own thread!

 

[Cave Diver]

Don't use nitrox to fill it unless the bag has been properly O2 cleaned first. :lookaround:

 

[Gilldiver]

When lifting large object, like concrete blocks, or other heavy items, the point of neutrality is fairly apparent. Once the object is buoyant, it shoots pretty quickly, caution is required.

If just getting enough lift to be neutral was how things work in the real world, this would be easy.

Once you get to objects large enough to be thinking about 250-1,000+ pounds of lift, you also need to be thinking about bottom suction. Course sand is not much of a deal as long as you clear it all off, but muds are totally different. If you have a nice sticky mud bottom you may need to have 2 or 3+ times the objects weight to break it free on a standard lift. Don't be in the way when it does, it'll hurt.

For lifts like this, you need to guess how much lift is needed for the weight, set that amount of lift, and then undercut the object by hand or with a water or air lance to break it free. I used to lift 250-500 pound mushroom anchors (anchors, chain, tackle etc.) out of a muddy harbor and if you had more lift then you needed by even 100 pounds, the disk of the mushroom just wanted to catch you as you dug under it. If it broke free quickly, there is enough energy going to the surface to break your jaw. Add in some current to give a lateral velocity and it makes it all that much worse.

 

[HowardE]

First, don't use your back gas to lift an object. To be safe it is best to use an alternative cylinder and regulator. If you are filling it and the reg gets caught on the bag it will be the alternative reg, not your reg that is attached to your bc, which can then pull you up in an unsafe manner.

Double 85's on a shallow dive, with a long hose and an inflater hose doesn't seem unsafe (depending on the circumstances). Caution and careful planning are essential when lifting something big.

Shoots was maybe a poor choice of words? My bad.

If just getting enough lift to be neutral was how things work in the real world, this would be easy.

Once you get to objects large enough to be thinking about 250-1,000+ pounds of lift, you also need to be thinking about bottom suction. Course sand is not much of a deal as long as you clear it all off, but muds are totally different. If you have a nice sticky mud bottom you may need to have 2 or 3+ times the objects weight to break it free on a standard lift. Don't be in the way when it does, it'll hurt.

For lifts like this, you need to guess how much lift is needed for the weight, set that amount of lift, and then undercut the object by hand or with a water or air lance to break it free. I used to lift 250-500 pound mushroom anchors (anchors, chain, tackle etc.) out of a muddy harbor and if you had more lift then you needed by even 100 pounds, the disk of the mushroom just wanted to catch you as you dug under it. If it broke free quickly, there is enough energy going to the surface to break your jaw. Add in some current to give a lateral velocity and it makes it all that much worse.

Pete,

I was being somewhat general with my answer, not so much of a how to, but more like trying to answer the original question. Of course every technical lift requires careful planning, and knowledge of the bottom structure, and how embedded the object is in that structure. Any technical lift should be entered into carefully, and with a clear plan.