The Natural
Contributor
Ok so lets say I find a 40lb porthole that I want to lift to the surface with my lift bag. What rope/line should I use and what would be proper technique?
Lift Bag Usage
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If you'll just e-mail the GPS coordinates to me, I have a buddy who will be happy to bring it up for you. He'll even e-mail a photo of it to you after. 
theskull
theskull
OP
The Natural
Contributor
Smartasses everywhere. :lol:
I'd use my sissy line (3/8" poly) as the line (double or triple loop). As for technique, this is one of those things you really should get someone to show you, first in a controlled lift under supervision, then practice a few times before doing it for real.
Lifting is full of ways to hurt yourself or someone else - not just with the object, but with poor buoyancy control of the bag as well.
Rick
Lifting is full of ways to hurt yourself or someone else - not just with the object, but with poor buoyancy control of the bag as well.
Rick
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What these folks are trying to tell you is that using a lift bag properly is an acquired skill ... and not one you can learn by reading about it. You have to use the bag just as you do your BCD ... and just like learning to use your BCD, it requires some practice.
You don't actually use the bag to "lift" the object ... you use it to make the object neutrally buoyant, and you swim it up, letting air out of the bag just as you do your BCD to remain neutrally buoyant on the ascent. Let out a little too much, and the object sinks back down ... accelerating with each foot that it sinks as the air in the bag gets squeezed. Look out below ... the object ain't gonna stop till it hits the mudline. Conversely, let out too little air and the object starts to accelerate surfaceward as Boyle's Law works the other way and the air in the bag expands. The problem then is that when the bag hits the surface ... if it's a standard open-cell lift bag ... it'll collapse, lose air, and the object will sink back down ... and you really won't want to be underneath it when it does.
You don't start with a 40-lb object ... start with something a bit less aggressive ... like a 10-lb weight. Find someone to show you the proper way to handle the bag, and practice till you get the technique down. Then go recover your porthole.
... Bob (Grateful Diver)
You don't actually use the bag to "lift" the object ... you use it to make the object neutrally buoyant, and you swim it up, letting air out of the bag just as you do your BCD to remain neutrally buoyant on the ascent. Let out a little too much, and the object sinks back down ... accelerating with each foot that it sinks as the air in the bag gets squeezed. Look out below ... the object ain't gonna stop till it hits the mudline. Conversely, let out too little air and the object starts to accelerate surfaceward as Boyle's Law works the other way and the air in the bag expands. The problem then is that when the bag hits the surface ... if it's a standard open-cell lift bag ... it'll collapse, lose air, and the object will sink back down ... and you really won't want to be underneath it when it does.
You don't start with a 40-lb object ... start with something a bit less aggressive ... like a 10-lb weight. Find someone to show you the proper way to handle the bag, and practice till you get the technique down. Then go recover your porthole.
... Bob (Grateful Diver)
Someone gave me a lesson, and I've done it a few times, but I'm not an expert, that's for sure.
With that said, I think (using Bob's knowledgeable advice), with a little practice, you can learn to use a life bag by yourself (diving with a buddy, of course). That is assuming you have your own buoyancy skills down.
And my less-than-two-cents is to go slow. And if it gets away from you and sails up and out of site, move out of the way. Chances are you'll be seeing it pass by again very soon.
With that said, I think (using Bob's knowledgeable advice), with a little practice, you can learn to use a life bag by yourself (diving with a buddy, of course). That is assuming you have your own buoyancy skills down.
And my less-than-two-cents is to go slow. And if it gets away from you and sails up and out of site, move out of the way. Chances are you'll be seeing it pass by again very soon.
OP
The Natural
Contributor
I was just curious as to procedures. I've deployed a lift bag before, just never actually had to LIFT something yet. Just curious as what type of line to use, because of tensil strength for heavier objects, best ways to attach the object etc.
The underwater strength of the line isn't usually the issue - you want to make sure the line you use is adequate for hauling the object aboard the boat or ashore after you get the bag to the surface - if you do that, then the line's strong enough underwater. Attach the object as close to the bag as is feasible - remember the object is hanging down below the bag. In deep water this isn't much of an issue, but if you're bringing the object ashore rather than to a boat you want the object to stay off the bottom as long as you can - makes the final haul out of the water shorter and easier.The Natural:
Use bowline knots.
The other biggie... never attach yourself or allow yourself to get attached to or entangled with the liftbag or object.
Rick
OP
The Natural
Contributor
Rick Murchison:
Yup, I always do a last minute check before inflating any SMB to make sure I'm not going to go to the surface with the bag.
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