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A vessel fitted with mechanical means for deepening harbours or clearing the entrances to rivers by removing part of the bottom. It is mostly self-propelled but some are towed into position. The ‘hopper’ dredger is fitted with an endless chain of buckets which scoops up the bottom and discharges the contents of the buckets into hoppers or dumb barges secured alongside it, though some have holds in which to put the spoil. These have hinged doors on the bottom so that the dredger can dispose of the spoil out at sea. Another type is the ‘bucket’ dredger where the endless chain of buckets operates through a central well so that the area of the bottom being dredged is immediately below the vessel. A ‘dipper’ dredger has one bucket attached to an arm. These types are always stationary when operating.

With the modern ‘suction’ dredger the bottom is removed by a vacuum pipe which sucks up the silt or mud before discharging it into the dredger's holds, a process which is done while the dredger is under way. Some employ cutters to loosen the bottom which can then be sucked up. The hovercraft principle has also been developed for small dredgers of up to 150 tonnes for use in very shallow waters and along the banks of rivers and estuaries.

Subjects: Maritime History.

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