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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