SUCTION DREDGES are extensively used where the material to be excavated is mud, sand, or other soft matter which can be mixed with water and pumped through pipes. The excavating ma chinery consists of a suction pipe which can be lowered through a well or over the side or end of the hull, a centrifugal pump for sucking or drawing a stream of water through this suction pipe, and a discharge pipe through which the pump discharges the water thus sucked in. In operation the end of the suction pipe is lowered to the soft bottom, and partly buries its end in the material. The pump is then started, and sucks a stream of water and mud through the suction pipe and discharges it into the discharge pipe. In most dredges of this type the end of the suction pipe is provided with revolving cut ters or with water jets to loosen the material, so that it will be more easily drawn into the suc tion pipe. The discharge pipe may lead to hop pers in the dredge. or it may be extended to the shore and discharge at some point where land is to he filled in or where the waste material will do no injury. The two suction dredges built dur ing 1900 for removing the 4n,000.nnn cubic yards
of sand and mud in the East Channel improve ment of New York llarbor are among the most perfect suction dredges of the hopper type ever built. These dredges have hulls inn feet long. with a beam of feet and a depth of 25 feet. The hoppers on each dredge have a capacity of 2S00 cubic yards. and occupy a space of 125 feet amidships on each side of the hull. Between the two rows of hoppers is the well for the suction pipe. Water jets are used to loosen the mate rial. The centrifugal pump has a capacity of 75,000 gallons of water per minute. The dredge is provided with its own propelling machinery. The four types of dredges which have been briefly described arc often modified in their construc tion and arrangement for special uses. and the features of One type are often combined with the features of another type to form a eombination maehine. The best descriptions of dredges are to lie found in the various engineering society period and the engineering papers of Europe and America.