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1 people online in the last 1 minutes - 0 members, 0 anon and 1 guests. (Most ever was 29 at 13:36:32 Sat Aug 3 2002)

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eklawok
03:58:12 Tue
Aug 25 2009

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gravity dredge

I ran across a video on youtube of a guy using one of these contraptions. It works off the same principle as a siphon hose instead of using a pump motor. I had also come across a guy a few weeks back who claims to use one as well, but when the guy started telling me about how he was an alien clone (and I am not making that up to be funny, he was dead serious) I let his ...um...credibility go the way side. From this one guys video on youtube though, it seems that a guy could use this for a dredge or for anything that a guy needs running water for as long as you are downhill from where your intake is at. What do ya'll have to say bout this. I can't imagine that this is something new.

  
dragline
13:58:56 Wed
Aug 26 2009

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Re: Gravity Dredge

I designed and operated 3 or 4 gravity dredges back in the late 1970's and early 1980's. The smallest was 3 inch and the largest was 8 inch. The 8 inch gravity dredge monster employed up to 270 feet of aluminum irrigation pipe, but for the 3 and 4 inch versions I used class 100 PVC.

I did a patent search on gravity powered dredges while visiting the Sunnyvale patent library back in 1978 and discovered dozens of patents related to gravity dredging inventions stretching all the way back to the mid-1850's.

You can review my design and operational techniques for my 8 inch dredge here...
http://goldfever.com/g_dredge.htm

But understand that just because designing, building and operating a gravity dredge may be practical in some locations, it certainly is not suited for the majority of recreational or semi-commercial gold dredging applications. The biggest problem with gravity dredges is the need for a high enough stream gradient to power the rise over run, i.e. the head versus pipe length. Frequent clogs are another problem plaguing gravity dredging operations. Another problem involves the transportation and setup time and effort involved in installing or relocating a gravity dredge.

However, the power of a gravity dredge, depending upon the available head, can be as powerful or more powerful than a typical gasoline powered dredge.

Given the recent ban on California dredging imposed by the courts, legislature and governor, might gravity dredging be allowed where gasoline dredging is banned? While not a practical substitute in all situations, gravity dredging might possibly enjoy a explosion of active in California should it be a work-around to the tyrannical government laws restricting gasoline powered dredges there.



Timothy

[1 edits; Last edit by dragline at 14:00:08 Wed Aug 26 2009]

  
baub
15:39:01 Wed
Aug 26 2009

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Re: Gravity Dredge

I tried this about a year ago.

300 feet of 3/4 inch black plastic pipe, dropping about 30 to 50 feet, produced 5gpm. Enough for replacement water, but way short of the amount needed to power a dredge. As the source was limited, I didn't pursue the project any further
In the future I might revisit the site and use a large area pond further upstream and larger, 4 inch, hose. This should provide volume and pressure.

b

  
apostrophe
22:29:03 Wed
Aug 26 2009

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Re: Gravity Dredge

i'd hate to get a clog in the line.

  
dragline
13:10:22 Thu
Aug 27 2009

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Re: Gravity Dredge

Right. Clogs were a constant issue. With the 8 inch gravity dredge we were in continuous operation for about 12 weeks during the summer of 1979. Clogs occurred at a frequency of about 1 clog every 4 to 8 hours operation time. Some days we had no clogs and some days we had three or four clogs (arrrrrggghhh!).

But because we used quick-fasten hose clamps on the adjoining couplings the time to remove the clog was about 15 minutes. When clogs occurred the suction at the nozzle quickly dropped. We'd then lift the nozzle to air to lose prime and walk the length of the pipe banging on the pipe until we located the clog. We'd then uncouple the pipe fitting or fittings where the clog was located and lift the pipe up and bang on it until the clogged rocks fell out of the pipe.

After clamping the couplings back in place we'd close the watergate, put the nozzle back under water and then wait for the pipe to refill with water. When the bubbles stopped exiting the nozzle we knew the pipe was full and we opened the watergate and resumed operations.

Clogs were a constant problem, yes, but after welding a protruding cross on the nozzle out of 3/8" steel rebar it reduced the size of rocks entering the 8 inch nozzle. This addition significantly reduced the frequency of the clogs. Another factor involved with the frequency of clogs depended upon the operator. Some operators took more or less effort to avoid long pointy sharp rocks likely to get wedged into the pipe.
[1 edits; Last edit by dragline at 13:10:52 Thu Aug 27 2009]

  
eklawok
02:40:56 Sat
Aug 29 2009

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Re: Gravity Dredge

From looking at the diagram, you are not only sucking the water, but the material going through the sluice at the upper end of the siphon. What would happen if a guy was to use the siphon effect for running a dredge like a standard gas dredge? And what I mean by this is on a gas dredge, your material that you are sucking up dosen't go through the water pump. The water that is being pumped creates a venturi effect that sucks the material through your suction nozzle on a second hose. Could a guy run a gravity dredge the same way and just have a shut off valve to cut the flow when the line gets clogged? (p.s. forgive me if I have my facts wrong about the set-up of a gas dredge, I have never ran one before)

  
overtheedge
04:24:29 Sat
Aug 29 2009

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Re: Gravity Dredge

Caution, nomenclature!

Dredging is removal of material from below the water lie.

A dredge does above.

Dredges for heavy metal recovery use a sluice box at the outfall from the dredge.

Yes, you can use a siphon with several 10's of feet fall as the dredging mechanism.

A poorman's dredge might be a 10' section of poly pipe (nozzle end) and then several 4" diameter sections of pipe and dump the outfall into a river-robber sluice or even a 10-20' length of 4" corrugated poly pipe aka poop tube. Gotta make sure the pipe joints are sealed or lose suction.

Use a nozzle restriction to limit the rock size going down the pipe. I read of a fellow using nylon cord with washers every 10' running through the pipe. Get a plug, pull on the cord to free it. Supposedly works most of the time.

This is an area of interest for me too.

  

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