Compressed air dredge?

[H'Islander]

Hey folks - relatively new to diving - I'm in Connecticut and have concentrated my diving mostly on old freighter wrecks in CT/NY/RI waters.

We dive wrecks for a few reasons, spearfishing, lobsters, and trinkets (bottles, brass, the usual suspects) that have spilled from wrecked cargo vessels - Was wondering if anyone has tried to design a dredge using compressed air bled into say, a 3" pvc pipe, to clear silt and debris from around wreck pockets?

I'm not talking reef destruction, I'm thinking 2-3 foot square areas known for sometimes giving up a bottle, a textile spindle, a brass coat hook, etc...

Am I on track or totally clueless?

 

[captain]

It would take a large surface compressor to supply enough air for any meaningful work.

 

[H'Islander]

So....what should I design into the unit? I totally value the opinions of folks with experience....need to know what is required...

Not being completely without a clue from a physics perspective, the expansion of compressed air in a shallow (up to 3 atmospheres) water, I would have thought that 90 psi in a 3 or 4 inch dredge would realize expansion to an exponentially high value and thus create a formidable suction as the reciprocal effect...but in all honesty, I'm assuming/guessing here = I'm not a physics major, just some minor practical exposure and thinking about what I learned in my open water training...any help is greatly appreciated!!

 

[Couv]

Here is a simple one to make, you should be able to figure out the parts required by looking at the photo:

http://uncw.edu/nurc/systems/toolmanual/AIRLIFT.pdf

But Captain is right (he always is) if you have a lot of material to move, you'll want surface supplied air.

Couv

 

[knotical]

Air Lift Device

 

[Gilldiver]

Those designs are fine with 2 mods:

1) Make sure that the working end is narrower then the rest of the pipe. If you use a 3" PVC pipe, use a 2" to 3" reducting cupling on the suction end. This is to make sure that if something gets stuck, it is at the working end in front of you. If it gets stuck in the pipe above the air entry, the whole system will be heading to the surface almost immediatly.

2) Put a suction breaker at the working end. This can be a thick piece of rubber or a section of plastic pipe that covers a hole in the rig set between the air entry and suction end. Use screws to hold one side and your control hand to hold it down. This bearker is so that if something gets stuck at the suction end, like your hand, you open the breaker with the other hand and free the suction end.

To drive a suction dredge you can use either water or air. Air is easier for most divers to run as you can do it off of a SCUBA tank. But you will need more air as you go deeper or use a larger diameter pipe. I have used twin 80's on a 2" ID dredge down to 120 feet and got 12-15 minutes work out of them.

 

[Couv]

Good info Pete,

One other small design improvement I saw once was on a large bore (6-8 inch i.d.) pipe. Instead of just one air nozzle, it had a second smaller one positioned a couple of inches above the primary air nozzle. I think the idea was to break up the larger bubbles into smaller ones and supposedly making the suction more effective. I never used that one so I don't know if it really helped or not.

Pete, do you have any ideas about that design?

Couv

 

[H'Islander]

That's great info - I really appreciate the help! I intended to use something like an Atrium Grate on the working end, maybe alter it to remove every other span or widen the gaps a bit if necessary. Also intend to use a gas engine contractor's compressor that will deliver 90 psi sustained, I think that should give me the work needed to dredge the light silt and shell fragment debris from the one wreck that I have in mind.

 

[Couv]

If there's gold or anything valuable involved, we will want our consultation fee. ;-)
Good luck and keep us posted.

c

 

[HowardE]

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