SUCTION DREDGES
Suction dredges are particularly adapted for excavating sandy bottoms. One type used for dredging channels consists of a large steam vessel with large bins into which the dredged material is pumped. At each side of the boat there is a long pipe which may be let down into the water. Each pipe terminates in a drag or footpiece with grated opening which is designed to be dragged along the bottom as the vessel slowly steams ahead. Powerful pumps suck a stream of water up the pipes which carries with it a quantity of sand. The sand and water flow into the bins, the solid matter settling to the bottom while the liquid flows out over the top. When the settlings have filled the tanks, the drags are pulled up and the vessel steams out to sea. Here doors in the bottom of the bins are opened and the material drops through. The idea of opening up the bottom of a boat to empty it seems rather startling until we consider that the bins are sealed off from the rest of the boat and do not contribute to its buoyancy. The sand that is dumped out of them is much heavier than the water that takes its place when the bin doors are opened.