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Post by RKC on Nov 11, 2008 11:31:06 GMT 12
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Post by RKC on Nov 11, 2008 11:32:53 GMT 12
From: RKCNZ Sent: 16/07/2007 4:19 p.m.
G'day VJG73 , Great job you have done with your dredge ... thanks for posting! I have a million questions ... however. I'm all for overpowering production dredges ... although I suspect you will be using most of the power available, even though it’s a 6" dredge. You have a significant height to raise the wash, and the bowl to power ... and the heaters to run. If you could still run it at half rpm you would be doing well. The only aspect that makes me a little nervous is that it might get top-heavy when running ... as it looks to be a narrow dredge. However, as you have probably already considered, it would be easy to add stabilizers if there are any problems in fast water. Using a bowl is a great idea to get that flakey gold. It looks like you have everything ready to get it in the water next summer. Looking forward to having you post here after you do the "sea trials". Regards, Rob (RKC)
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Post by RKC on Nov 11, 2008 11:35:33 GMT 12
From: VJG73 Sent: 16/07/2007 5:29 p.m. Hello Dredge has been out for extensive testing in the Waimak river here in Canterbury, I can safely say there is no gold there but we were just checking to see if everything works. The motor is governed to around 2600 revs which gives the bowel a 450 RPM speed which should give 50 G's in the back of the concentrator.(not that I'm willing to hop in and check) I went oversize with the pump as the Bowel requires about 60 gallons a minute to fluidise properly as well, water is require to venturie the 2mm minus material from the bottom of the screen to the top of the cyclone. You are right about the narrowness of the dredge I have some clip on allow floats off my 3 inch dredge which stabilizers things a great deal, i can still stand on one side without to many issues. I tried to keep the dredge balanced but did not allow for the incredible suck the pump creates on the pump inlet which initially gave a few balance issues.When floating dry the dredge sits around 50% up the floats. When running this changes to around 4 inches of free board. Calculating out the buoyancy this must equate to around 600 kg of operating weight. (wet) the lift is not that great. At around 4000revs with the concentrator disconnected the suck at the nozzle is scary to say the least, i could see limbs being lost when it all turns to custard.. Will post some photos of inside the bowel when i get a chance. Vaughan
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Post by RKC on Nov 11, 2008 11:38:04 GMT 12
From: RKCNZ Sent: 17/07/2007 4:34 p.m. G'day VJG73, I can envisage safety problems arising because of the suction created when running your dredge at a high rpm. Any accidents are always a problem but it would be a disaster to crush a couple of fingers at the start of the season. The 8" and 10" Precision dredges had considerable suction at the nozzle and were designed that way, but what they did to enable the nozzle to be efficiently and safely operated was to put a remote throttle control at the nozzle so the nozzle man could speed up or slow down the motor at will. Also, the nozzle on the Precision dredges had a rubber by-pass on the nozzle that could be employed to free any rocks (or fingers) that became jammed at the nozzle intake. However, a by-pass would not prevent the catching of fingers ... but would nevertheless be of advantage by allowing the quick release of any stuck fingers. The Precision advertising says of the by-pass ... "Rubber By-pass flap on top of gravel nozzle for by passing suction water. Enables diver to remove plugged debris from end of gravel intake without the need of idling engine". A dredging technique that has significant safety advantages (and with the added bonus of considerably adding to efficiency) is to have two divers under water. One operates the nozzle exclusively and the other manhandles any rocks too large to go into the nozzle. Dave McCracken has referred to the use of two men underwater in many of his writings about dredging and by reading his dredging books more can be learnt about this most useful and underutilized technique. This nozzle smg.photobucket.com/albums/v521/NZGOLD/Kalamath%20river%20dredging/?action=view¤t=goldtrip034.jpgsmg.photobucket.com/albums/v521/NZGOLD/Kalamath%20river%20dredging/?action=view¤t=goldtrip033.jpgis the best designed nozzle I've seen with a by-pass flap and which also has the reducer on the end of the nozzle set at an angle (see: img.photobucket.com/albums/v172/Coochee/nozzelOKOKwithGap.jpg?t=1184638264 ) which greatly assists when removing a rock stuck on the end of the nozzle. The only way to work with a dredge that has strong suction is to remain aware all the time ... however it’s when fatigued that accidents always seem to occur and fatigue is unavoidable when dredging on a commercial scale. So … I think you are going to have to be prepared for a bit of pain at some time. When I started dredging I had a bad habit of throwing rocks back and not letting go of the rock as soon as I should have and I damaged a few fingers that way. But, I only had to do it a few times and I soon learnt to let go at the right time. Looking forward to seeing more photos! Regards, Rob (RKC)
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Post by RKC on Nov 11, 2008 11:39:32 GMT 12
From: Roscoe Sent: 17/07/2007 8:33 p.m.
I reckon 3 inches teach you all about pain. When a rock comes a-flying from a foot away, you know you are going to cop it. Always worked with my fingers curled in around the edge of the nozzle and paid the price. Bigger sizes seem to just suck around the edges and have a null zone near the center. I tried the nozzle-unloader like on Tom Ash worth`s dredge (49-ER Mike`s forum), but finally went with a clapper type behind the hand grip. One knock and the stuck stone drops away. I still think we need to discard oversize close to the bottom and just send gold-infested gravel to the surface. Less hammering on your sluice, less scouring and more settlement time. With sorting on the bottom, throwing is too tiring, so I keep a stout wire basket and drag it away when it`s half full. Might even stick flotation on it as I get older, to ease the load.
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Post by RKC on Nov 11, 2008 11:40:24 GMT 12
From: Roscoe Sent: 17/07/2007 8:38 p.m.
Hi, VJ, I`d go for more flotation. I`ve upset and sunk a few when they wandered cross-current, or even through having one float climb on to a rock. Gravel load shifts and 'thar she goes'. Can you lower the shaker box down closer to the surface? Or the centrifuge?
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