

SEA-GOING HYDRAULIC DREDGES 109 



tinued during heavy seas the pump should be so designed as to be 

 able to fill the hopper in the shortest time possible. As a rule it 

 takes from 40 to 50 minutes to fill the hoppers of the capacity indi- 

 cated above. 



Several engines are required for the service of the sea-going 

 suction hopper dredges, the most important being those for the 

 purpose of propelling, pumping, lighting, steering, hoisting both 

 the suction pipes and the anchors, opening the gates of the hoppers, 

 etc. This kind of dredge being always in motion, should be 

 provided with powerful propelling engines which will insure to 

 the vessel a speed of at least 7 knots per hour when loaded and not 

 less than 10 knots when light. Each pump is furnished power by 

 an engine of the marine type with vertical cylinders and so designed 

 as to overcome 100 ft. of head of water and causing the runners 

 to make from 200 to 300 revolutions per minute. Electricity for 

 the ordinary illumination of the vessel as well as for the arc lamps 

 used on deck during the night work and for the searchlights is 

 provided by a high-speed engine belted to a dynamo. The hoisting 

 engines for lifting the suction pipes and the anchors are of the usual 

 duplex type with two horizontal cylinders. Similar engines are 

 used for the opening and closing of the gates of the hoppers 

 when no other methods are used, and the steering is also done by 

 a small steam engine. Steam is provided by marine boilers of 

 sufficient capacity for the working of all the various engines simul- 

 taneously. 



The manner of working with the sea-going hydraulic hopper 

 dredges is as follows: When the dredge has reached the site of 

 excavation the suction pipes are lowered until the drags or cutters 

 reach the bottom and then the steamer moves ahead at a slow 

 speed while the pumps are put in action. In this manner one or 

 two furrows are dug on the bottom according to the number of 

 suction pipes, along the course of the steamer, the width and depth 

 of these furrows depending upon the device used at the end of the 

 suction pipe. Mr. Babcock states that the U. S. dredges in New 

 York Harbor are each provided with two drags 5 ft. wide and 

 located 52 ft. apart. Two furrows are thus excavated, taking in 

 a load of 2500 cu.yds., approximately 1800 cu.yds. in place, in a 

 course 15,000 to 20,000 ft. long. The courses were laid out of such 

 a length that the dredge could get a full load in going up and back 

 once. This made a shorter trip to the dump and saved time by 



