144 A TREATISE ON DREDGES AND DREDGING 



In the year 1867 Major McAlester designed the U. S. dredge 

 "Essayon" using the propeller " Enoch Train " as a means of stirring 

 up the material. The dredge boat "Essayon" was a double-ender 

 provided with two strong and powerfully driven-screw propellers, 

 one at each end, driven by separate engines. She was provided with 

 watertight compartments so that when empty the vessel drew 

 16 ft. and when full of water 24 ft. The total depth of the vessel 

 from the spar deck to the bottom of the keel was 26 ft. Her spar 

 deck was nearly flat and clear of obstruction from both ends to 

 near amidship, to facilitate the addition at any time of any other 

 device which would be found necessary. It was provided with a 

 single pilot house located amidship. The screw propellers were 

 16 ft. in diameter, describing circles equal to the minimum draft 

 of the vessel. The propellers were provided with four blades and 

 their ends were shaped so as to readily cut away compact mud. The 

 dredge " Essay on " was built by the Atlantic Works, Boston, Mass., 

 at a cost of $223,000. It was found very efficient. The depth of 

 water on the bar at Pass a T Outre was increased from 11 ft. 6 in. 

 to 17 ft. 8 in. and kept at such a depth. But long delays to the 

 work were caused by repairing the blades of propellers, which were 

 easily and frequently broken by obstructions encountered. This 

 fact suggested some modifications, which were introduced on a 

 second dredge of the same type, called "McAlester," in which the 

 propellers were more solidly built. A plow was placed in front 

 of the excavating screws and also a large scraper, 18 ft. across and 

 10 ft. high, to be lowered directly in front of the plow, to make 

 the machine more efficient and prevent the breaking of the blades 

 of the propeller by coming in contact with obstructions. 



Stirring by Jets. To stir up sand-bars by forcing a jet of water 

 upon them was suggested by Messrs. Scott and McClintock. They 

 constructed a machine for forcing jets of water at high pressure so as 

 to dislodge the sand and mud. Although this method did not work 

 well at the delta of the Mississippi River, where the material varied 

 from sand and mud to gravel and clay, yet it was successful in other 

 cases where the bars were formed by very fine sands. In the Report 

 of the Chief of Engineers for the year 1882 is given a description of 

 a hydraulic excavator which was used in the improvement of the 

 Mississippi River and employed in grading the banks for receiving 

 a revetment. The excavator consisted simply of a powerful steam 

 pump placed upon a scow and furnished with the necessary boiler 



