Dredge is a device for scraping or sucking the seabed, used for dredging. A dredger is a ship or boat equipped with a dredge. In American usage any floating vessel equipped with dredging equipment is called a dredge.
These operate by sucking through a long tube, like some vacuum cleaners. A plain suction dredger has no tool at the end of the suction pipe to disturb the material.
A trailing suction hopper dredger (TSHD) trails its suction pipe when working, and loads the dredge spoil into one or more hoppers in the vessel. When the hoppers are full the TSHD sails to a disposal area and either dumps the material through doors in the hull or pumps the material out of the hoppers.
A cutter-suction dredger's (CSD) suction tube has a cutter head at the suction inlet, to loosen the earth and transport it to the suction mouth. The cutter can also be used for hard surface materials like gravel or rock. The dredged soil is usually sucked up by a wear resistant centrifugal pump and discharged through a pipe line or to a barge. In recent years dredgers with more powerful cutters have been built in order to excavate harder and harder rock without blasting. The two largest cutter suction dredgers in the world are Deme's D'Artagnan (28,200 kW total installed power), and Jan De Nul's J.F.J. DeNul (27,240 kW).
This process functions like a cutter suction dredger, but the cutting tool is a rotating Archimedean screw set at right angles to the suction pipe.
This uses the Venturi effect of a concentrated high-speed stream of water to pull the nearby water, together with bed material, into a pipe.
An airlift is a type of small suction dredge. It is sometimes used like other dredges. At other times, often an airlift is used handheld underwater by a diver. It works by blowing air into the pipe, and dragging water with it.
A bucket dredger is a dredger equipped with a bucket dredge, which is a device that picks up sediment by mechanical means, often with many circulating buckets attached to a wheel or chain. Some bucket dredgers and grab dredgers are powerful enough to work through coral reefs to make a shipping channel.
A grab dredger picks up seabed material with a clam shell grab, which hangs from an onboard crane, or is carried by a hydraulic arm, or is mounted like on a dragline. This technique is often used in excavation of bay mud.
A backhoe/dipper dredge has a backhoe like on some excavators. A crude but usable backhoe dredger can be made by mounting a land-type backhoe excavator on a pontoon. The two largest backhoe dredgers in the world are Tauracavor and New York. Both feature a barge mounted excavator.
A water injection dredger injects water in a small jet under low pressure (low pressure because the sediment should not explode into the surrounding waters, rather it is carefully moved to another location) into the seabed to bring the sediment in suspension, which then becomes a turbidity current, which flows away down slope, is moved by a second burst of water from the WID or is carried away in natural currents. Opposition claims that Water Injection Dredging is not a natural way of dredging while the side of the WID claims otherwise.
These dredgers use a chamber with inlets, out of which the water is pumped with the inlets closed. It is usually suspended from a crane on land or from a small pontoon or barge. Its effectiveness depends on depth pressure.
This is a bar or blade which is pulled over the seabed behind any suitable ship or boat. It has an effect similar to that of a bulldozer on land.
This is an early type of dredger which was formerly used in shallow water in the Netherlands. It was a flat-bottomed boat with spikes sticking out of its bottom. As tide current pulled the boat, the spikes scraped seabed material loose, and the tide current washed the material away, hopefully to deeper water. Krabbelaar is Dutch for "scratcher".