40 A TREATISE ON DREDGES AND DREDGING 



combine a hard center with a softer exterior, which, while wearing, 

 enables a sharp point to be preserved. 



The ram is lifted by a wire rope wound upon a loose drum, 

 driven by a friction-clutch. It is allowed to fall from a height of 

 8 ft., and on an average of eight to nine blows penetrates the rock 

 to a depth of 3 ft. which is sufficient to allow of its being dredged 

 to a depth of 2 ft. 6 in. The machine is arranged so that it can be 

 moved on end and athwart simultaneously by chains worked by 

 steam winches. By means of sighting rods on board and ashore 

 the barge can be moved over uniform distances. The ram was at 

 first worked in positions 3 ft. apart and this distance was gradually 

 increased to 4 ft. 6 in., which was found to be close enough for either 

 hard or soft rock. 



The average quantity of rock which one machine has broken 

 is per week, working night and day, and allowing for all stoppages, 

 908 cu.yds. at a cost of 8.8d. per cu.yd. This is based on six months' 

 working; but as it is necessary to allow for renewal of rams, the 

 following is the actual cost per week of wages, coal stores and water, 

 and the estimated cost of repairs, full allowance being made for 

 renewal of rams and all contingencies: 



s d 



Wages 16 1 6 



Coal, stores, and water 6 10 7 



Estimated cost of repairs, renewal of rams, ropes, etc .22 00 



Insurance at 30s. per cent = 102 per annum 1 19 2 



908 cu.yds. at 12.3d 46 11 3 



The cost of one rockbreaker is about 6800 and if 4 per cent be 

 allowed for interest and 2 per cent for depreciation (the machine 

 being well maintained) the additional cost per cubic yard would 

 be 2.2d. making a total cost of about 14. 5d. per cu.yd. 



The quantity of rock broken is ascertained by the number and 

 depths of the penetrations of the ram in a given time, which are 

 carefully recorded and after dredging the amount of rock remove M! is 

 checked by soundings. 



Rock Dredging. Each of the hopper dredges has two sets of 

 buckets, one for rock dredging and the other for sand, clay or gravel, 

 having 50 per cent greater capacity. The bucket lips are of cast- 

 steel and the pins and bushes of manganese steel. The lips of the 

 buckets for rock dredging are set at an angle of about 27 degrees 



