[17] CONSTKUCTION AND WORK OF FISH-HAWK. 19 



A compositiou sheave (Plate VI) is inserted in the heel of the boom, 

 two revolutions of which are equal to one fathom of dredge rope, and 

 attached to its shaft is a register which accurately records the amount 

 of rope out at all times. 



SAFETY-IIOOKS. 



The safety-hooks (Plate VII) are designed for the purpose of detach- 

 ing the trawl when from nwy cause, such as fouling a rock, or wreck, 

 the tension on the dredge rojje reaches the limit of safety. It can be 

 adjusted to detach at any point between 3,000 and G,000 pounds by the 

 nut on the end of the central rod. In practice we have set it to 4,000 

 pounds, the breaking strain of the dredge rope being 8,700 pounds. 

 The spring and hooks being placed in the cylinder and the cap screwed 

 on, it is ready for use. 



The end of the dredge rope is spliced into the eye and the trawl 

 shackled to the hooks, which are held in position by their shoulders 

 pressing against the inner surface of the cylinder (Plate VIII.) 



The spring is compressed as the tension increases till, the limit of 

 safety being reached, the shoulders are released and the hooks open 

 freely, allowing the shackle pin to slip through, detaching the trawl and 

 relieving the rope from undue strain. 



The accumulator (Plate VIII) is designed to prevent jerking strains 

 on the dredge rope due to motion of the vessel in a sea-way, or working 

 over a rough bottom. It is copied from that used on board the Coast- 

 Survey steamer Blake, with slight modifications. It answers its pur- 

 pose admirably and is an almost indispensable adjunct when steel- wire 

 rope is used. 



The side rods and central shaft are of steel, the ends of wrought iron. 

 Twenty-six rublxir buffers, with a brass w^asher between each, are placed 

 on the central shaft under considerable pressure and secured by a nut 

 on the upper end. A swivel link at the lower extremity carries a lead- 

 ing block. 



The hubs of the brass washers are extended on each side, forming a 

 (.'ollar over which the rubber buffers ride free from contact with the 

 central shaft. This feature, introduced bj* Lieutenant-Commander 

 Sigsbee, U. S. N., placed the present form of accumulator far ahead of 

 all others for our purposes. 



DREDGING BLOCKS. 



The dredging block at the boom end and that seen hooked to the 

 accumulator (Plate VIII) aae all that are used. They, also, are copied 

 from those of the Blake, except that the diameter of sheave is reduced 

 from 18 inches to 12 inches, making the blocks much lighter. 



STEEL-WIRE DREDGE ROPE. 



This excellent rope was made by the John A. Eoebling's Sons' Com- 

 pany, Trenton, N. J. It is one and one eighth inch in circumference, 



