A common misperception is that gravel is sucked through the pump. Nozzles sizes may be up to 2" smaller than hose sizes.Ī gold dredge is powered by a motor and pump. What a dredge manufacturer calls a 5" dredge may be a 4" intake nozzle dredge for permitting purposes. This is a very important thing to remember when working with permits. Therefore most dredge permits refer to the nozzle size, not the hose size, when imposing rules and restrictions. The volume of material processed is determined by the nozzle opening, not the inside diameter of the hose. A 5" dredge may have a nozzle opening of 4" for this reason. However, nozzles are used on dredges which restrict the intake to a size smaller than the inside diameter to help prevent clogs. So a 5" dredge has a main suction hose with a 5" inside diameter. Manufacturers rate suction dredges by the inside diameter of the main suction hose. Floats are also needed because there is rarely a nice gravel bar in just the right place to set up a dredge without floats. Every inch of vertical lift above the surface of the water robs power/suction from the system. This is because a dredge has just enough power to lift water and gravel to just above the surface of the water. Dredges almost always need to be mounted on a set of floats. A sluice box works because gold is 15 times heavier than sand and gravel and so is easily trapped by the riffles and carpet in the sluice box. Material is transported from underwater to the surface and run through a sluice box to capture the gold. A suction gold dredge is basically an underwater vacuum cleaner.
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