Ghetto Lift Bag

[Bratface]

Several years ago 3 of my dive buddies and I constructed a lift bag out of a 30 gallon black trash bag placed inside of a net laundry bag. We used line to suspend it with. We took it to a lake and lifted a small boat motor 30 feet. It worked but I wouldn't rely on it. It made a great experiment.

 

[Rich Keller]

Would a large kitchen trash bag work like a lift bag or would it just break?

Put inside another type of bag that will provide the strength a trash bag does not have would work but control during the lift would be lost doing it this way. If the bag is strong enough to not break when it is full of water then it should be good enough but then how you attach the bag to the object becomes a big issue. What are you trying to salvage?

 

[DivemasterDennis]

If you have ever lifted objects from the water with a lift bag other than as a game, or pretend exercise, you know that is one area where you need proper, reliable and safe equipment. That includes proper lift bags, an important feature of which is a dump valve to control ascent rate from depth. I don't think you can do an efficient dump valve in a trash bag, with or without a gunny sack overcoat. If you are going to engage in this kind of activity, for fun or profit, I strongly suggest you acquire equipment designed for that duty.
DivemasterDennis

 

[Hickdive]

Would the trash bag filled with water on land rip? If the answer yes then it will rip as a lifting bag under water as the forces involved are identical.

Dump valves on lifting bags are NOT for controlling the ascent of the object. Objects don't get bent, divers do. The dump valve is for removing/repositioning the object/bag.

The correct way to control a lift is to tailor the volume of your lifting bags so that they are full with the object just positively buoyant, expanding air will spill from the bottom of the bags and keep the ascent rate constant.

If you need more control than that then have fewer bags filled to the brim so the object is still negatively buoyant and provide the last lifting effort with a winch or even
manpower hauling on a rope. Again, it won't run away because the bags are full and expanding air will spill from the bottom of them. You can add sealed pontoons or barrels to secure everything from sinking again once the object is at the surface.

It is much better to use 5kg too little lift but all full bags and haul to overcome the 5kg than to have too much bag capacity into which air can expand and cause a runaway.

If you need to apply excess lifting force to break an object free of the bottom suction then attach the break out bag to the object by ropes/slings long enough so that the bag itself is just under the surface of the water. Then, when it breaks free, the whole lot will only travel the distance the bag was underwater. You can then tow it a short distance so it doesn't settle back in its own hole, deflate the bag (using the dump valve for its intended purpose), lowering the object back onto the seabed for lifting as normal. This avoids dangerous/silly situations like shackles breaking and sending bags into the air or the object breaking free of the bottom and then accelerating all the way to the surface under grossly excessive and uncontrolled lift.

Finally, if someone tasks you with controlling an ascending lifting bag by ascending with it and operating the dump valve, tell them to go away and come up with a safe scheme of work or to do it themselves if they think it is a safe idea.

 

[Dsix36]

Would the trash bag filled with water on land rip? If the answer yes then it will rip as a lifting bag under water as the forces involved are identical.

Dump valves on lifting bags are NOT for controlling the ascent of the object. Objects don't get bent, divers do. The dump valve is for removing/repositioning the object/bag.

The correct way to control a lift is to tailor the volume of your lifting bags so that they are full with the object just positively buoyant, expanding air will spill from the bottom of the bags and keep the ascent rate constant.

If you need more control than that then have fewer bags filled to the brim so the object is still negatively buoyant and provide the last lifting effort with a winch or even
manpower hauling on a rope. Again, it won't run away because the bags are full and expanding air will spill from the bottom of them. You can add sealed pontoons or barrels to secure everything from sinking again once the object is at the surface.

It is much better to use 5kg too little lift but all full bags and haul to overcome the 5kg than to have too much bag capacity into which air can expand and cause a runaway.

If you need to apply excess lifting force to break an object free of the bottom suction then attach the break out bag to the object by ropes/slings long enough so that the bag itself is just under the surface of the water. Then, when it breaks free, the whole lot will only travel the distance the bag was underwater. You can then tow it a short distance so it doesn't settle back in its own hole, deflate the bag (using the dump valve for its intended purpose), lowering the object back onto the seabed for lifting as normal. This avoids dangerous/silly situations like shackles breaking and sending bags into the air or the object breaking free of the bottom and then accelerating all the way to the surface under grossly excessive and uncontrolled lift.

Finally, if someone tasks you with controlling an ascending lifting bag by ascending with it and operating the dump valve, tell them to go away and come up with a safe scheme of work or to do it themselves if they think it is a safe idea.

Are you kidding me? Your statements hold true when doing a big commercial lift, but not everyone is doing that. Although it is safer and easier to just inflate the bag and let the object go up, sometimes that is not an option.

Many recreational divers will use a lift bag for a found weight belt or a small anchor and such. I have brought up several anchors and a couple of weight belts while holding onto the lift bag. The main caution is to be prepared to let go and not let it tangle up on you.

Many technical divers will learn how to use a lift bag for redundant buoyancy and have to use it for their ascent including holding their deco stops.

 

[Spimon]

How ghetto are we talking?
I'd be thinking a dry bag - they come in a range of sizes and from very light nylon to quite heavy duty vinyl.

Add a PVC in-line valve of some sort in the top (or bottom, as the case may be).
Sandwich (maybe with a bit of rubber reinforcing and some rubber cement) the bag between two nuts on the male end of the valve.

Might even be able to open the valve just enough to get the ascent rate just how you want it.

Or is this getting a little too upmarket? Should I ghetto it up a little more?
I also assume you're not diving from a boat and can't just take a rope to the surface and pull up your prize?

 

[Rich Keller]

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Are you kidding me? Your statements hold true when doing a big commercial lift, but not everyone is doing that. Although it is safer and easier to just inflate the bag and let the object go up, sometimes that is not an option.

Many recreational divers will use a lift bag for a found weight belt or a small anchor and such. I have brought up several anchors and a couple of weight belts while holding onto the lift bag. The main caution is to be prepared to let go and not let it tangle up on you.

Many technical divers will learn how to use a lift bag for redundant buoyancy and have to use it for their ascent including holding their deco stops.

You would be much better off marking these items with a SMB then coming back and doing it right rather then riding a lift bag to the surface. For smaller items the same principle Hickdive describes can be achieved with gallon jugs to get the lift close to neutral then hauling the object up with a line from the surface. You could also use a continues feed line come along to haul the item to the surface from a boat without using any lift bags.

 

[Hickdive]

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Are you kidding me? Your statements hold true when doing a big commercial lift, but not everyone is doing that. Although it is safer and easier to just inflate the bag and let the object go up, sometimes that is not an option.

Many recreational divers will use a lift bag for a found weight belt or a small anchor and such. I have brought up several anchors and a couple of weight belts while holding onto the lift bag. The main caution is to be prepared to let go and not let it tangle up on you.

Many technical divers will learn how to use a lift bag for redundant buoyancy and have to use it for their ascent including holding their deco stops.

What, precisely, did you gain by 'controlling' the ascent of a weightbelt or a small anchor?

When is inflating the bag and letting go for small recoveries, like the above, 'not an option'?

A balanced rig on a membrane dry suit eliminates any need for shenanigans like using a lift bag for redundant buoyancy.

I've lifted lots of stuff, from weightbelts and anchors to scientific apparatus and small boats, with loads of spidge in between, and I or my buddies have made lots of mistakes in lifting. I even used to teach on Search and Recovery courses. But, if you want to carry on 'controlling lifts' by using the dump valve on the bag you just carry right on - just don't mislead people into thinking it's a good idea or ask someone else to do it.

The big problem with lifting underwater is that it gives people with very little concept of what they are doing the opportunity to lift some very heavy stuff. Put a 1 tonne weight in their garden and ask them to shift it and they'll go, "Oh, no need to get the
professionals in with a crane". Put the same weight in 30m of seawater and they're all over it like a rash.

Small stuff - bung a bag on it and fire it off. A decently designed bag, even oversized, won't tip over and sink on hitting the surface. No need to come up with it. Big stuff - a bit more thought required but a bag with a capacity one person could reasonably lift but fitted with a dump valve is clown-shoe stuff. Coming up with such a bag to 'control' it is like arriving at the dive site in the matching car.

 

[PfcAJ]

What, precisely, did you gain by 'controlling' the ascent of a weightbelt or a small anchor?

Control, probably...

 

[Hickdive]

Control, probably...

Why do you need control?