104 A TREATISE ON DREDGES AND DREDGING 



per second, and different kinds of material require different veloci- 

 ties of flow for the most efficient work. Material like clay or soft 

 mud can be transported at a slower velocity than sand or gravel 

 materials, which tend to precipitate quickly. A high velocity of flow 

 means a greater friction in the pipe and pump and consequently 

 greater expenditure of power. A fluid mixture of sand or mud and 

 water is heavier than water alone, and therefore takes more power 

 to pump it against a given head, and also the friction in the pipe 

 is greater. 



European engineers call the hydraulic dredges, suction dredges; 

 but Mr. Robinson advocates calling them hydraulic, from the fact 

 that the suction principle is not employed at all in dredging, but 

 simply in conveying the debris from the bottom to the discharge. 

 Such an explanation is necessary in order to prevent confusion in 

 quoting from Engineering and other European papers, in which 

 these machines are described under the head of suction dredges. 



In hydraulic dredges a very large volume of water is pumped 

 in connection with the material, consequently a large percentage of 

 the power is wasted in useless work. They are therefore subjected 

 to the same criticism as the high-tower ladder dredges. But also 

 there the consideration of the economical disposal of the dredged 

 materials will show in the end that they are not as wasteful as they 

 appear at first. In fact the debris can be conveyed to a great 

 distance by the simple action of the pump, and the large quantity 

 of water carrying in suspension the debris allows the even deposit 

 of the materials over a large surface. Besides, the suction dredge 

 working continuously day and night is found to be both efficient 

 and economical, more so than other types of dredges. 



Hydraulic dredges can be grouped into sea-going and those used 

 for channel or river improvements; the latter group can be divided 

 according to Mr. Robinson in regard to their feedings as follows: 



(a) Lateral feeding or ship-channel type with floating discharging 

 pipe. 



(6) Forward feeding or Mississippi type with floating discharging 

 pipe. 



(c) Radial feeding with spud anchorage and floating discharging 

 pipe. 



These different types of hydraulic dredges will be described 

 in the following chapters. 



