Archimedes Schmarchimedes

[Kirstie]

Originally Posted by Conor

So for an example, an object weighing 1000kg with a volume of 700L is sunk in fresh water at 30m.

What size lifting bag would be required to make it neutral?
What volume of gas would be required to fill the bag at that depth?
If you have a 12L cylinder with you for lifting the object, what pressure would it need to have to contain enough gas to do the lift?

Sorry if all that is useless but it was interesting for me

Can't I just get a reeeaaaallllyy big liftbag, fill it from a 15l pumped to 250 bar and hope for the best?


Or get lots of divers to each grab hold and fin really hard?

[Conor]

That would be the BSAC solution



(sorry)

[Finless]

Originally Posted by Kirstie

Can't I just get a reeeaaaallllyy big liftbag, fill it from a 15l pumped to 250 bar and hope for the best?


Or get lots of divers to each grab hold and fin really hard?

Or get a big straw and suck really hard! "No such thing as a sucking force" HA!

[Kirstie]

Originally Posted by Finless

Or get a big straw and suck really hard! "No such thing as a sucking force" HA!

Or fill the object full of air, tie a piece of line to it and float it up like a balloon!

Hang on, that's almost a liftbag.

[David G]

Have a look at pages 16 to 18 of this document:

http://www.bsac.org/uploads/document...otes/srins.pdf

and use the first 10 or so slides in this presentation:

http://www.bsac.org/uploads/document...R/recovery.zip


Hope this helps.

David.

[Janos]

Kirstie,

Think of it this way:

There are forces acting on the Thingy that make it go up.
There are forces acting on the Thingy that make it go down.

If the total force acting up is exactly equal to the total force going down then it is neutrally bouyant.

So. Forces [1] going up = upthrust = volume of object * density of seawater
So a 2 litre Thingy will have an upthrust of 2.06kg in seawater.

Forces going down. Well if our thingy is made of lead it will have a density of about 11 so a total mass of 22 kg (2 litres x 11kg per litre).

So the forces going down are way bigger than the forces going up so it will sink like a stone. To make it neutral you need a lift bag of 22kg-2kg capacity. Ie a 20kg liftbag, which has a volume of around 20 litres.

Janos

[1] = Strictly speaking, Force isn't measured in kg, but in Newtons (N). To convert to Newtons you need to multiply by 10. If you don't know what a Newton is ignore this.

[Nick Kay]

Originally Posted by Kirstie

Surely, in the real world, if you have to lift an object from the seabed, you take down the biggest liftbag you have, fill it completely full of air and let it go? Do people really sit about calculating how much air they need to the nearest bar? Do they buggery!

Yes, they'd do calculations because they'd want to control the lift

Don't forget that you'd need more lift dependent on the "stickiness" of the bottom (Finless is bound to respond to that statement) - or the degree that the object has settled / become embedded

[Kirstie]

Originally Posted by Janos

To make it neutral you need a lift bag of 22kg-2kg capacity. Ie a 20kg liftbag, which has a volume of around 20 litres.

Janos

That's very clear, but why 20kg minus 2kg?

[Kirstie]

Originally Posted by Nick Kay

Yes, they'd do calculations because they'd want to control the lift

Don't forget that you'd need more lift dependent on the "stickiness" of the bottom (Finless is bound to respond to that statement) - or the degree that the object has settled / become embedded

Ah yes, I remember some mention of bottom stickiness. But how do you determine that, or isn't it that relevant?

[Buoyant Babe]

Originally Posted by Kirstie

Ah yes, I remember some mention of bottom stickiness. But how do you determine that, or isn't it that relevant?

I thought you knew all about bottom stickiness? It's what brought you and Turbs together in the first place, isn't it?