78 A TREATISE ON DREDGES AND DREDGING 



The dredging machinery is driven by gearing through a brake 

 transmission to avoid the danger of fracturing any part of the 

 machinery in the event of a sudden shock arising from contact with 

 unusually hard material. 



The engines are arranged to be driven together or separately, 

 so that the power of one or both can be applied to the screws or 

 to the dredging machinery. 



The principal dimensions of the engines are as follows: 



Inches. 



Diameter of high-pressure cylinders 20 . 08 



Diameter of low-pressure cylinders 34 . 26 



Length of stroke 19 .69 



Each engine is provided with a separate starting gear driven 

 by a special motor, so that the engines can be turned at slow speeds 

 down to two revolutions a minute, to facilitate the operations of 

 mounting or dismounting the bucket chain. 



The boilers, two in number, are of the ordinary marine type, 

 registered to a working pressure of 114 Ib. per sq.in., and of ample 

 capacity to drive the main and all the auxiliary machinery. 



The gearing which transmits the motion from the engine to the 

 bucket shaft is so arranged that its speed can be varied independently 

 of that of the engine. 



This arrangement is desirable in consequence of the variable 

 nature of the material which has to be lifted. 



The bucket ladder is central and is placed in the middle of the 

 hull; it is so hung that the bucket wheel at the lower end can be 

 raised clear of the water when the vessel is being propelled; its 

 length is sufficient to dredge at a depth of 33 ft. below the surface. 



It is mounted in such a manner that it can be easily shifted aft, 

 when it is desired to remove the mud lying against the foot of quay 

 walls in a slight depth of water. 



The bucket chain is made of links of soft untempercd steel 

 connected by cast-steel bolts; the bolt holes are lined with soft 

 steel rings which can be easily renewed when worn. 



The buckets are made entirely of steel with a reinforced cutting 

 edge; their capacity is about 1 cu.yd., they can be driven at speeds 

 varying from 10 to 16 buckets past the discharge channels per 

 minute, according to the nature of the ground being excavated. 



The winches for lifting the ladder are driven by a separate 

 compound engine of sufficient power to raise the ladder when the 



