96 



A TREATISE ON DREDGES AND DREDGING 



with openings on the lower face and also with scrapers. This end 

 attachment of the pipe, being dragged along the bottom, scrapes 

 the soil, which enters into the box at the openings and is drawn 

 into the tube by the force of the pump. Fig. 25 shows the Allen 

 scraper or drag as designed by Mr. J. P. Allen and used in the 

 U. S. dredges " Manhattan " and " Atlantic " in New York 

 Harbor. 



When the hydraulic dredge is designed to work through hard 

 soils the end of the suction tube is provided with a cutter. This 

 consists of a number of knives (6 to 8) united by suitable disks 

 or rings at one or both ends. The knives may be either straight or 

 spiral, mounted around and concentric with the end of the suction 

 pipe, and encased in a cast-steel hollow shell provided with open- 



FIG. 25. Allen Scraper Used on the U. S. Dredge "Manhattan." 



ings. The blades are for the purpose of slicing off or excavating the 

 material and feeding it into the interior of the shell through the 

 openings, whence it is drawn into the pump. The efficiency of 

 a cutter depends on the form of the blades, the angles at which 

 they are set and whether they are straight or spiral, and on the 

 openings between them and at the bottom. It would be almost 

 impossible to determine absolutely the best form of cutter suitable 

 for any material. According to the experience of Mr. James H. 

 Apjohn, M. Inst. C.E., a cutter with its straight knives set at an angle 

 of 26 to the tangent of the circle round which they were placed 

 and overlapping each other to a slight extent, worked very well 

 in loamy soil, but when the clay was reached the openings U-1 \\een 

 the blades of the cutter clogged with the tenacious, plastic clay, with 

 the result that the proportion of clay found in water discharged 

 through the pipe line was exlrenielv small. Another cutter with 



