276 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



of which could not be prevented, a drag consisting of twenty 

 armed chains, each 4 feet long, placed side by side, and attached 

 to timber bars 8 feet long connected to the hauling rope by half- 

 bridle chains was used on special occasions ; but for general 

 work, the triangular arrangement (Fig. 279) proved the best. 



The use of these appliances, combined with partial sluicing, 

 was continued until the effects produced were such that it was 

 considered safe to bring the sluices into full play by running 

 two at a time full bore, near the time of low water for an hour 

 and a half or two hours, according to circumstances, which 

 course soon reduced the bank in front to the required depth. 1 



Removal of Shoals by Propeller-Sluicing. 2 The practicability 



TINES 

 Fm. 277. 



HARROW USED IN CONJUNCTION WITH SLUICES 

 Fia. 278. 



of propeller-sluicing has been demonstrated by experiments made 

 on the Colombia River, with the view of removing the St. Helen's 

 bar, where the depth of water was 12^ feet. 



The vessel used for this purpose was of 2124 tons register, 

 330 feet long, 40 feet beam, and 16 feet draught. The screw 

 was 17 feet in diameter, with a pitch of 20 feet, and made a 

 maximum of 54 revolutions per minute. 



In the first instance, a channel 1200 feet long, 200 feet wi<l\ 

 with an average depth of cut of 6 feet, was sluiced out in thnv 

 working days. A second trial resulted in a channel 1GOO feet 

 long, 150 feet wide, and an average of G feet depth of cut, com- 

 pleted in eight working days. At a third trial, a channel of the 

 same length and width, but 10 feet depth of cut, was ninl<> 

 without difficulty. The material removed consisted of sharp 

 heavy sand. 



1 M.r.I.C.R, vol. xc. p. 308. 



TIM vol. Ixxxiii. p. 386. 



