DREDGE, a machine for excavating mud, rock, or other solid material from the bottom of a body of water. Essentially the dredge consists of a scoop of some sort to pick up the material to be excavated, and a derrick-like mechanism to lift the scoop with its load up to and above the water level, and dump it. There are many ingenious modifications of this essen tial idea, by which the movements of a shovel, of a grasping hand, or of an ice-tongs are imitated with great precision and with im mense power at points 40 feet below the water level. The machinery which operates the dredge is built upon a scow or boat which is floated over the place where the work is to be done. The dredge is held in position by heavy posts called °minds) which go down loosely through openings in the deck of the scow and rest on the water bottom. There are usually three of these, two at the shovel or bow end of the dredge and one (sometimes two) at the stern. The bow spuds are much larger, having to withstand the strains of the working shovel In some large dredges the bow spuds are built up timbers upwards of 40 inches square and over 50 feet long, with a heavy iron shoe at the bottom. To the shoe is bolted a "foot') of timber covering an area of 10 by 12 feet to give a substantial bearing on a soft mud bottom. To add to the grip on the bottom some dredges have tackle attached to the bow spuds by which the scow may be hoisted upon them above its natural floating level, thereby shifting a part of the weight of the dredge and its machinery upon the spuds, and thus pressing them down the more firmly. When the dredge has finished the work it can reach from one position, the spuds are raised clear of the bottom, and the machine is moved by a tug boat, or by pulling on chains attached to anchors.
Dredges are of two general classes: inter mittent, and continuous. The former operates a single bucket or scoop, and alternately digs and lifts the load; the latter digs continuously, raising the material at the same time. To the intermittent class belong the dipper dredge and the grapple or grab dredges. To the continu ous class belong the ladder dredges, the hy draulic or suction dredges and the scouring dredges.
Dipper The dipper dredge is similar to the steam shovel used in earth exca vation on land, except that it is worked from a float, and that the dipper or bucket is much larger — up to 15 cubic yards capacity. The dipper dredge is the type almost universally in use in the United States, and is effective in depths of less than 50 feet of water, and in the greatest variety of bottom material. It will dig economically in stiff clay, hardpan and even soft rock. For such work the dipper is armed with teeth of appropriate size and shape, and a small bucket is used. Generally the material raised is placed upon a scow moored beside the dredge. In some instances the handle of the dipper has been lengthened so that the spoil may be dumped upon a nearby bank. With a dipper of six cubic yards capacity the output of one of these dredges warking in ordinary earth is about 5,000 cubic yards per day of 10 hours.
Grab These are of two models, with, respectively, the clam-shell bucket and the ((orange-peel') bucket. The former has a bucket composed of two great scoops which close to gether at the bottom like the valves of a clam shell. They are hinged at the top. They are spread wide open as they descend, close upon the material at the bottom, and remain closed as the bucket is lifted until released to drop its contents upon the scow tender alongside. The orange-peel bucket is of hemispherical shape, divided into three or four sharp, curved triangular sectors, resembling the sections. of peel removed when peeling an orange in the usual way. The orange-peel bucket is not much in use in the United States, but the clam-shell dredge is of the greatest usefulness in many locations, particularly for picking up chunks of broken rocks, and for excavation in water too deep for the dipper dredge. The buckets have to be very heavy, as there is no other force but their weight to drive them into the material they have to dig. One advantage which the grab bucket has over the dipper is the speed with which. it gathers its load and drops it. The largest of these buckets have a capacity of 12 cubic yards, and their output on a long job averages about one bucket per minute, two thirds full.