Grapple dredges are operated in deep water. The operation of dipper and grapple dredges is intermittent, while a ladder dredge digs trenches and other narrow cuts continuously. It uses sharp-edged buckets carried on an endless chain or wheel. The assembly makes a first cut to the desired depth. Then a power plant draws it along, and it continues the cut as it moves.
Mechanical dredges are unable to transport materials for long distances.
They are not self-propelled and are low in productivity. Their advantage lies in their usefulness in dredging in restricted locations such as ditches and jetties, and they can be equipped with devices to treat and drain water from dredged material in mining operations. The suction heads are outfitted with high-velocity water jets for softening the bottom, but they cannot cut through hard or compacted material. They are suitable only for dredging soft materials. The hydraulic cutterhead dredge is similar to a dustpan dredge but is equipped with a rotating cutter at the end of the suction pipe.