calculations for lift bags????

[BVI Hunter]

Is there a formula I can use to calculate the size of lift bag for an object off the sea floor?

I assume depth is a consideration, anything else I can factor in?

Thanks!!!

 

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[ropesfish]

For displacement calculations seawater weighs about 63 lbs per cubic foot (7.5 gallons per cubic foot-roughly 8 lbs/cubic ft)
Remember to calculate the pressure on the bag if you are working much more than 33 ft (1 atm).
If you are building your own bags you can use the dump valve from a BCD to dump air,. You can change the spring to change the overpressure release point. Any bag over about 10 gallons ought to have a dump valve. If you don't have some way to get vent the excess air as you rise, the whole endeavor can go south pretty quickly.

 

[BVI Hunter]

Thanks!

I had planned to buy a propose built bag(s) so assume those will come with a dump valve?

just need to calculate the size I need to lift the object?

 

[ropesfish]

What are you lifting or what does it weigh?
Is it buried in sediment, partially buried or sitting on top of sand? There is sometimes a suction that you will have to overcome to float an object.
How deep is it?
Water temps?
The better that you define the conditions, the more specific we can be with advice.
B)

 

[BVI Hunter]

half buried in soft sand
approx. 6ft long
1 ft wide
1 ft deep
at a guess I'd say 1000lbs weight?
water 10ft deep
water - salt - 84'

 

[Mackaydon]

Boyles Law and Physics, in general, were never my trump suit in school. But I can give you some other suggestions that may compliment your effort. (1) Make sure the air bag is equipped with line or chain that can lift a load of about 2-3X the lift capacity of your device. (2) Use more than one bag; and (3) If you haul from 6 feet then into deeper water before ascending again, consider attaching a marker buoy to the object. I almost hesitate saying the obvious: Never stay or get under an object that is being lifted.
Don...

 

[Zodiacdiverdave]

Rule number one; more is better then less. If an item wheighs 1000 lps but is stuck to the bottom by concretion then you may need twice the lift just to free it.

Rule number two; DO NOT STAY UNDER THE LIFT WHEN IT STARTS TO GO UP. two stories, my buddies lifted a large, flat piece of a ships hull that wieghed at least 1000 lps. it went up 70 ft to the surface then broke free and came back down swinging from one side to the other like a giant pendulum that would have killed anyone that it hit. Fortunately everyone saw it coming down and scattered so nobody was hit by it.

Second story; I was salvaging brass from a WW1 wreck and sent up a lift bag with around a 150 lps for brass and bronze on it, I went on my way gathering up a load of lead when the bag of brass landed with in 2 feet of my head. The rope to the lift bag had broken and the load came down 40 ft right in front of me.
Please learn from our mistakes so we don't have to read about you in the papers.
ZDD

 

[ropesfish]

Something that shallow with that sort of shape..6' x1' x1' lends itself to using something besides a lift bag, like a single pontoon. with tow straps firmly attached so it won't roll and a couple of come-alongs...or if your budget will stand it: Lift Bags Salvage Tubes
Make the same sorts of precautions before the lift that you would if you were doing it on dry land...everything firmly attached including the slings under your object....everyone in agreement on the plan and in agreement that there is NOTHING important enough for anyone to get under the lifted object. If it falls, it falls. If you aren't dead, you can try again.
You can excavate underneath the object with a waterjet or a suction lift.

 

[Hangtown]

Lift Bag Size/Volume Calculator

 

[Jolly Mon]

I am guessing your object is not solid, but for a general weight idea, remember that a 6 foot cylinder with a diameter of 1 foot has a volume of 4.7 cubic feet.
Iron weighs between 442 and 491 lbs. per cubic foot, while bronze weighs between 481 and 514 lbs. per cubic foot.
So a solid tube of iron would weigh between 2077 and 2307 lbs., while a solid tube of bronze would weigh between 2260 and 2415 lbs. once out of the water.

 

[Jason in Enid]

I hate to be the "debbie downer" to you again, but PLEASE be careful! It is NOT a simple operation to lift objects and the bigger the item, more complicated it becomes. I know you aren't deep but that makes things somewhat more difficult.

You need to understand the process of filling multiple lift bags evenly, and you better know how to tie proper knots or use slings. Items tend to be stuck into the bottom and the force to over come that results in a run-away item, spilling air at the surface and then crashing back down to the bottom. SO you better be AWAY when you do it.

These types of operations typically involve compressors, manifolds, lots of hose, lots of strap and rope. It's easy for one problem to lead to something catastrophically wrong in a flash.

I would highly recommend talking to a local UW salvage person for some advice.

 

[BVI Hunter]

I am guessing your object is not solid, but for a general weight idea, remember that a 6 foot cylinder with a diameter of 1 foot has a volume of 4.7 cubic feet.
Iron weighs between 442 and 491 lbs. per cubic foot, while bronze weighs between 481 and 514 lbs. per cubic foot.
So a solid tube of iron would weigh between 2077 and 2307 lbs., while a solid tube of bronze would weigh between 2260 and 2415 lbs. once out of the water.

Thanks! - just what I needed !!

its an (almost) solid iron tube so now I can re-calculate given those weights!!

 

[BVI Hunter]

Something that shallow with that sort of shape..6' x1' x1' lends itself to using something besides a lift bag, like a single pontoon. with tow straps firmly attached so it won't roll and a couple of come-alongs...or if your budget will stand it: Lift Bags Salvage Tubes
Make the same sorts of precautions before the lift that you would if you were doing it on dry land...everything firmly attached including the slings under your object....everyone in agreement on the plan and in agreement that there is NOTHING important enough for anyone to get under the lifted object. If it falls, it falls. If you aren't dead, you can try again.
You can excavate underneath the object with a waterjet or a suction lift.

Good stuff- thanks!!

 

[BVI Hunter]

Lift Bag Size/Volume Calculator

many thanks!!!

 

[signumops]

Don't forget to subtract the object displacement from the lift volume, to wit: a cannon for example might displace 4 cubic feet of seawater or approximately 240 pounds. Even though it might weigh 1200 dead-weight pounds, it is actually 240 pounds buoyant.
10 feet is about .3 atmospheres, so a lift bag with an open end need only be filled two thirds full to reach full expansion at the surface. Otherwise, it will simply dump air if completely filled at the beginning of the lift.
A 55 gallon drum when filled with air will displace about 460 pounds of seawater, minus its own weight, say 40 pounds, ergo 420 pounds buoyant.

 

[Mud Skipper]

Don't forget to subtract the object displacement from the lift volume, to wit: a cannon for example might displace 4 cubic feet of seawater or approximately 240 pounds. Even though it might weigh 1200 dead-weight pounds, it is actually 240 pounds buoyant.
10 feet is about .3 atmospheres, so a lift bag with an open end need only be filled two thirds full to reach full expansion at the surface. Otherwise, it will simply dump air if completely filled at the beginning of the lift.
A 55 gallon drum when filled with air will displace about 460 pounds of seawater, minus its own weight, say 40 pounds, ergo 420 pounds buoyant.

A drum from Tortola Concrete costs 25-50 $ which is made from nylon durable and with two ratchet straps going to float the "stick" otherwise may I suggest a pillow bag over a parachute given such shallow water, the water column being 10 feet will have the "stick' bouncing along the sea floor the bag being a minimal of 4000 as a parachute would break surface if you break suction and you would then have to drag the"stick" deeper before getting going, prob is with the dock location you are possibly headed to you, would run aground "stick" in tow before making the dock, unless of course you are just trying to move it for another purpose. Enjoy.

 

[Mackaydon]

Is that target close enough to shore to extend a cable (like a tow truck's cable) and winch it in?
Don.....

 

[Southern_Digger]

I presume you are going to move it in closer to the beach so you can drag it out with a winch. That is how cannon were recovered here years ago off Ft. Lauderdale. Maybe a few of those in Dominica would be willing to lend you a hand. Would not hurt to ask--safety in numbers.

 

[BVI Hunter]

Is that target close enough to shore to extend a cable (like a tow truck's cable) and winch it in?
Don.....

No unfortunately not!!!

And that would attract FAR too much attention!!

 

[BVI Hunter]

I presume you are going to move it in closer to the beach so you can drag it out with a winch. That is how cannon were recovered here years ago off Ft. Lauderdale. Maybe a few of those in Dominica would be willing to lend you a hand. Would not hurt to ask--safety in numbers.

Will not be being dragged up the beach that's for sure!

whoever mentioned the "C" word? lol!