Hello fellow prospectors.
I have an idea about making a gravity dredge to try out on one of the rivers here. I already have 14 feet of 2 inch hose. My main question is that if I add 40 feet of 4 inch hose to the end of the 2 inch, will that help to increase the flow and suction of the nozzle for the 2inch hose? Or should I just add more 2 inch hose to the 14 feet I have now?
Replies
Roughly, 2 feet of drop for 1 psi.
30 psi to run a nozzle, but ya got to have volume too.
Been using a 3"dredge nozzle, 3" pipe @ 60 ft of drop for the past few years, but with the shortage of water this year
I'm changing over to a 3"siphon dredge. I have about 25 feet of drop available for this set up.
Nice quiet operation.
How long is your hose to be able to get a 60 foot drop, or even 25 feet ?
Thanks for your input.
Rob
For the gravity dredge I ran 600 ft of 3" pipe down a spring stream to the nozzle where I'm working.
The drop in that 600 ft is aprox 60 ft.
For the syphon dredge, going to use about 45 ft of 3" pipe to run from the nozzle to the sluice box some 25 ft below where I'm working.
I'm going to connect the two systems together at the 3" nozzle and that should greatly improved performance.
I completely agree with what MadJack said.
A gravity dredge uses the pressure from falling water to create the pressure to operate a venture nozzle on a conventional dredge in order to replace the gas engine for stealth or fuel saving reasons. In this case, the dredge is located in the lower pool and the gangue is drawn into the dredge at the lower pool.
In the case of the syphon dredge, a venture nozzle in not used and the water and gangue is drawn into the hose from the upper pool and carried down the hose to the sluice box at the discharge end of the hose and run through the sluice box in the lower pool.
The big difference is that you cannot use the syphon system to draw gangue from the lower pool unless you move the complete system down stream.
In both cases, be sure to firmly anchor the hose at the upper end so the weight of the water will not pull the hose down stream.
I hope this brings some clarity to your questions.
Dickb
Dickb, that makes sense. I think most (like me) just think about the gravity dredge as a syphon system, where no venturi or eductor is used at all. A syphon system doesn't need much in the way of head or pressures to work and is simple as it gets - you don't even need an actual nozzle, just suck the gravels into the hose end.
Thanks for the clarification.
As far as having too much flow into a sluice; I would think that issue could be resolved with a deeper or wider sluice box, baffle, or other engineering to accommodate the flow.
If you don't use a nozzle with a restrictor ring or other reducer, you will get rock jams. The you have to drain the hose, clear the jam and restart the siphon.
IF you can't keep your drop to less than 6 feet you probable will need the old-school header-box dredge sluice.
I feed my gravity dredge less than the normal 30% slurry to help avoid plugs.
Guys, if someone asks a question here, and you have not done it, PLEASE don't post your 'guess' as fact. I'm not a novice, I've been at it since 1979.
Also realize your water is not the same size, speed, depth as others here. This is the web, world wide!
p.s. I'm still learning too.
Adding 4" hose downstream of 2" hose will allow air to enter from the bottom and you will lose your prime fairly quickly. I beg you to try it and post your video here.
Are you doing a SIPHON or are you powering a jet tube (eductor)??? They are to completely different items.
Last summer we were running TWO 4" Gravity (siphon type) dredges with just 4' to 5' drop. (from the bottom of the upper pool)
We tried one spot with 6' of drop and you surely didn't want your fingers anywhere near the nozzle!!! Any more drop than 6' in under 100' of hose (think friction) and you will over-water your sluice and blow out picker sized gold.
I KNOW, because I do it!
Also, forget the idiots on youtube using corrugated drain-hose. Too much friction and it fills with gravel, good luck getting your cons out of that!
Here's a tip: buy your hose in 100' roll. You'll save on the cost over buying 40'.
Another tip: VARCO has 3" hose on sale for around $2 foot right now.
Last one: Cut your hose into 2-50' lengths, or 1-50 and 2-25' and use exhaust couplers to rejoin them when needed.
Adding 4" hose downstream of 2" hose will allow air to enter from the bottom and you will lose your prime fairly quickly. I beg you to try it and post your video here.
Are you doing a SIPHON or are you powering a jet tube (eductor)??? They are to completely different items.
Last summer we were running TWO 4" Gravity (siphon type) dredges with just 4' to 5' drop. (from the bottom of the upper pool)
We tried one spot with 6' of drop and you surely didn't want your fingers anywhere near the nozzle!!! Any more drop than 6' in under 100' of hose (think friction) and you will over-water your sluice and blow out picker sized gold.
I KNOW, because I do it!
Also, forget the idiots on youtube using corrugated drain-hose. Too much friction and it fills with gravel, good luck getting your cons out of that!
Here's a tip: buy your hose in 100' roll. You'll save on the cost over buying 40'.
Another tip: VARCO has 3" hose on sale for around $2 foot right now.
Last one: Cut your hose into 2-50' lengths, or 1-50 and 2-25' and use exhaust couplers to rejoin them when needed.
Thanks for your input. I wish I could buy a 100 foot roll but here in Colombia, hose is much more expensive. So you think if I just use 2 inch hose ( maybe 60 feet) and find a drop of 3 feet, I won't have to worry about blowing any gold out the sluice?
You will need at least 6 feet of drop. The answer to your initial question is yes it will add suction as long as it is running down hill. Level gets you nowhere, obviously but if you have the 6 feet of drop then forget the 2" and go with the 4" because unlike a motorized dredge, your suction will be limited to how much drop you have in the river. So even with a 4" hose with that volume of water, 2" rocks may be too big to flow through the 4" hose. The amount of drop and the size of the hose will make it or break it and a 2" hose is a waste of time for the effort it will take to get it set up.
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