176 A TREATISE ON DREDGES AND DREDGING 



attachment of the hoisting rope form the claims of the numerous 

 patents, which are generally known by the name of the various 

 inventors. Fig. 54 shows the single-chain attachment to a whole- 

 tine bucket of the Cooper & Holdsworth patent. 



To operate the dredging bucket by the single-line method seems 

 at first simple and convenient. But when it is considered that all 

 the strain is thrown upon only one line, which is more liable to 

 break while the machine is working and possibly when the bucket 

 is under water, thus requiring the costly assistance of divers to 

 recover it, this method has decided drawbacks. Another objection 

 to the single-chain method is the fact that in order to discharge 

 the bucket it is necessary to raise it to a certain fixed height, 

 regulated by the distance of the opening gear or hook from the 

 boom head, thus when a false lift is made the bucket must be 

 brought to this fixed height before it can be reopened. A favor- 

 able consideration is that a single line does not require a special 

 boom, but it can be readily attached to the boom of any existing 

 crane. 



In the double-line system the bucket is suspended by two lines, 

 one called the closing or hoisting line and the other the opening line. 

 The latter is held in tension while the former is being lowered, and can 

 be stopped to allow the dredgings to be discharged, this being done 

 by allowing the entire weight of the bucket to come upon the opening 

 line. The engine operating the lines can be provided either with 

 one or two drums. In the latter case each line is attached to its 

 own drum and its movements are regulated by the operators by 

 means of levers. But when the hoisting engine is provided with 

 only one drum the opening line is connected by a series of sheaves- 

 to a weight moving in a vertical slide against the boiler. As the 

 bucket is lowered this weight is raised, thus keeping the bucket 

 open by the tension on the opening line. The total force exercised 

 upon the material to be lifted by the grapple dredge, being the 

 weight of the bucket plus the energy of descent, it follows that 

 in this kind of machine, this must be minus the effect of the counter- 

 weight, which is seldom less than 1 ton and for deep lifts as much 

 as 4 tons. By avoiding the use of weights to take up the slack 

 of the opening line, the whole energy of the bucket in its descent 

 is available to excavate the material, thus enabling a greater cut 

 to be taken. American manufacturers, as a rule, build only grab 

 bucket dredges provided with the double-line system, each ilne 



