234 A TREATISE ON DREDGES AND DREDGING 



The Hydraulic Dredge. The hydraulic dredge has been used 

 extensively in removing sand-bars along the Mississippi River, and 

 the private concerns engaged in dredging sand for commercial 

 purposes have found these machines efficient. 



The hull consists of a scow of the light-draft square type, of 

 heavy construction, being, as a rule, housed over and arranged 

 with suitable quarters for the crew. The dredging end of the hull 

 is provided with a strong A frame to support the suction head, 

 which projects beyond the hull. The other end is provided with 

 means for the proper handling and anchorage of the dredge. The 

 dredging end is usually downstream see Fig. 77. 



The pump used is of the centrifugal type with a runner provided 

 with four curved blades. The runner is closely fitted to the casing, 

 and this is necessary in order that the discharge may not leak back 

 into the suction. In the operation of the pump in sand or gravel 

 dredging, the question of wear is an important one, and in the 

 latest designs the runners are of steel and large space is provided 

 in the discharge volute. Of course no sharp bends should be 

 allowed and the passages in the pump should be larger than the 

 suction pipe so as to prevent obstruction by stones. 



In many dredges special arrangements are provided for stirring 

 up or agitating the sand at the suction end. This is usually accom- 

 plished by directing strong jets of water into the sand, resulting hi 

 the upheaval of the latter, which is caught up by the suction and 

 drawn into the suction pipe. The discharge from the pump is 

 arranged so that the barge moored on the side of the dredge may 

 be conveniently filled. An open discharge trough with screen of 

 any desired mesh in the bottom is suspended over the barge so that 

 any stones or coarse gravel may be washed past and discharged 

 overboard. The operator whose duty is to feed the suction head 

 in the sand is situated so that he can see the barge and regulate 

 the depth of the suction head by observing the discharge 

 mixture. 



The centrifugal pump is usually driven by a compound steam 

 engine, of either the vertical or horizontal type. As a rule, the 

 dredges operated by the larger companies have a double equipment, 

 consisting of a pump on each side, so that barges may be loaded 

 on both sides. A powerful hoisting engine is provided for raising 

 the suction head, and steam power is applied to the capstans and 

 other handling machinery. As a rule the Mississippi River type 



