172 



A TREATISE ON DREDGES AND DREDGING 



stern and the second at the bow of the boat; steam being provided 

 by a stationary engine located amidship. This type of dredge is 

 preferred by South American engineers and contractors in the 

 improvements of harbor and rivers. (Fig. 49.) At Liverpool was 

 once employed a hopper dredge provided with four grab dredging 

 apparatus of the Priestman type. 



There is a large variety of buckets used on a grab dredge, but 

 for the sake of classification they can be grouped into two types, 

 namely, the clamshell and orange peel buckets. 



The clamshell bucket derives its name from its shape, 

 resembling the shell of a clam. The scoop or bucket is 

 divided into two segments pivoted at their upper inner corners 



FIG. 49. South American Grab Dredge. 



and is supported, raised or lowered and opened or shut by means 

 of chains passing to proper winding drums on the deck of the boat. 

 The dipper, when in motion, in old machines, was steadied by a 

 pair of long poles securely fastened to it and passing through rings 

 or ears fixed at proper points on the boom. Improved models of 

 buckets have made the guide holes unnecessary for dredging pur- 

 poses, although this old arrangement is still found on dredges of 

 small capacity. 



The bucket, as a rule, is constructed of two steel scoops, pivoted 

 together at their upper inner corners, arranged so as to open and 

 close at will, and forming when closed an ordinary bucket for the 

 raising of the materials from the bottom. The scoops forming the 

 bucket are made of different shapes, depending upon the quality 



