198 



A TREATISE ON DREDGES AND DREDGING 



As soon as the empty boat rights itself again, the workmen rim 

 the platform back into position and secure it there by the braces. 



The tugboats used for towing scows are too well known to be 

 discussed. They are generally screw propellers or sidewheel 

 steamers of great efficiency and light draft. Fig. 66 shows a 

 sidewheel steamer used for towing purposes in shallow rivers. 

 Fi.ir. 67 shows a screw propeller steamer built by the F. Smulder Co. 

 of Rotterdam for towing scows in deep waters. 



With the exception of the dumping scows the dredged materials 

 transported by means of any other scow to reach its final desti- 



FIG. 67. Screw-propelled Tug Boat. 



nation must be raised up. This is effected by means of different 

 machines, namely, the bucket elevators, the derricks and grab 

 buckets. 



The bucket elevator consists as a rule of a land dredge, of the 

 down-digging type mounted either on a fixed scaffold or on the 

 platform of a truck running on tracks. The machine consists as 

 usual of a ladder hinged to a tower at its upper end, while its 

 lower end is supported by a boom. The ladder is provided with 

 a double endless chain carrying steel buckets and revolving around 

 1wo tumblers at the ends of the ladder. An engine providing 

 power to the upper tumbler moves the buckets along the Judder. 



