THE HOPPER SYSTEM OF DREDGING.

Otago Witness, Issue 1341, 11 August 1877, Page 17

THE HOPPER SYSTEM OF DREDGING .

TO THE EDITOR.

Sir — As you gave prominence in your ship* ping columns to the launch of the hopper dredger Willunga, for Adelaide, will you be good enongh to find a place for the enclosed, shewing her actual working, in an early issue. You will see that the actual cost of deepening the channel in an exposed situation by hopper dredger is only 8d per cubic yard, while by their old plant it costs them 3s. There should be no bar harbonr on the New Zealand coaat, when they have got such an effective and cheap remover of bars.

The following is the extract relating to the Willunga, hopper dredger, of 1000 tons burden, of Adelaide :—: —

" The report of the first month's work of the hopper dredger Willunga was considered by the Marine Hoard as being more satisfactory than it was perhaps expected it would have been. During ?• period of 26 days, representing' 265 hours, 24,247 cubic yards of sflt had been raised at the outer bar and deposited in the sea. The time employed in raising was 189 hours, against 76 hours in discharging. The average amount of stuff raised during each of the 189 hours was 171 tons, and the discharge into the ' sea was at the rate of 425 tecs for each of the 79 hours. The hopper dredger, including the raising and discharging operations, was thus shown to have cleared the channel of 122. tons par hour, and 1220 tons per day. The total cost of the month's work was L 799 lls 9d, made up of L559 14s 4d for raising stuff, and L 239 17s 5d for discharging the same. The expense per cubic yard or a ton and a third was 5Jd for raising, »nd 2Jd for discharging^ 8d per cubic yard for deepening bears a marked difference on the price usually paid in this Colony; The cost of dredging per cubic yard at Port Pirie, for instance, for the same month, was Is 5Jd, while by No. 1 , Dredge, at the iiiner bar, it was 3s per cubic yard, 9Jd of ' jwhich was for bringing the silt to the wharves;" The Willunga must be notified as working at " considerable disadvantage, having been sunk by one of the Melbourne steamers, and was [under water for nearly two months. This occurred immediately after arrival, via Suez Canal, from the Clyde. The machinery necessarily 'being stiff, also the dredging being often done on the outer bar, with a considerable roll of the ,sea to contend with. Under more favourable circumstances, no doubt the dredging and depositing will be done for 6d per cubic yard. The Greenock, a duplicate constructed sirce the launch of the Willunga, is dredging and depositing for the Greenock Harbour Trust at 4d per cubic yard, which under the old style, with stationary dredgers and punts, cost them Is 8d per cubic yard. I would recommend the hopper dredger to the especial notice- of the people of Temuka in opening the mouth of tbe Opihi River, and the Christchurch people for rendering the Heafchcote navigable, at a very low cost. — I am, &c,

W. G. Jenkins.