Average rates for a day, or longer periods, are further reduced, for both kinds of dredgers, by detentions for shifting anchors, moving out of the channel for passing vessels, and other contingencies, not present in a mere trial of speed. The St. Lawrence dredgers, however, often raise 4800 cubic yards in twelve hours, or an average of 500 tons per hour, while, according to the published reports for a recent month, two of them raised an aggregate of 117,525 cubic yards of clay, giving an hourly average of 336 tons per dredge for 69 hours of duty per week.
As a combined steamship and dredger which can be turned out complete on the Clyde for export, the Otago dredger is said to be the largest and the best thing yet built, but as a machine to dig a channel, one of these St. Lawrence dredgers is better still.[300]
Another comparatively modern machine is known as the La Châtre dredger, 92 feet long, and 20 feet width of hull. It has an engine of 50 H.P., which works the chain of buckets. The material falls 2⅔ feet from the buckets into a long steel shoot 2¼ feet in width and depth, and semi-circular at the bottom, extending out 15⅓ feet from the axis of the dredger, and supported by twenty-four steel cables from shear-legs 80 feet high, standing on two iron pontoons fastened to the dredger; a pontoon on the opposite side, weighted with 32 tons of ballast, counterweights the shoot. The material is drawn along the shoot (which has a general inclination of 1 in 20, increasing close to the dredger) by water pumped into the shoot, at least double in volume the amount of material. The dredger, shortly after starting work, lifted and transported 183 cubic yards of excavation per hour. It cost about 10,800l. Another dredger deposited the material from the buckets on a divisor formed of two sets of revolving sharp blades, turning in opposite directions, which cut up the large pieces and discharge the material on gratings of sharp blades, through which it falls, mixed with about 85 per cent. of water, on a sheet-iron inclined plane, along which it is conveyed to the pipe of a suction pump. This Dumont 1-foot pump, specially designed for silt, stands with its engine on a pontoon alongside the dredger. Another similar pump draws along the silt discharged by the first, and discharges it into a 1-foot iron pipe. The silt is deposited from 650 to 1000 feet away, at a height of 16 to 20 feet, with a velocity of about 13 feet per second. The mound formed at the outlet of the pipe has a very flat slope, but the settlement is rapid and complete. The dredger was able at once to lift and transport 130 cubic yards per hour, and this amount will probably be eventually raised to 160 cubic yards. This dredger is said to have cost 12,800l., with its accessories.
In the construction of the Amsterdam Ship Canal, the excavations had to be deposited on the banks some distance away from the dredgers; and after being raised by the ordinary bucket dredger, instead of being discharged into barges, they were led into a vertical chamber on the top side of a sand-pump, suitable arrangements being made for regulating the delivery. The pump known as Burt and Freeman’s was 3½ feet in diameter, and made about 230 revolutions per minute; it drew up the water on the bottom side, and mixing with the descending mud on the top side, the two were discharged into a pipe 15 inches in diameter. The discharge-pipe was a special feature in this work, and consisted of a series of wooden pipes jointed together with leathern hinges, and floated on buoys from the dredger to the bank. In some cases the pipe was 300 yards long, and discharged the material 8 feet above the water-level. Each dredger and pump was capable of discharging an average of 1500 cubic yards per day of twelve hours. A centrifugal sand-pump, designed by Mr. Hutton, was also used on those works.
At Hull, the cost of dredging on the Humber, including everything except interest on capital and depreciation, is stated to be 2·1d. per ton. The material is mud, varying in consistency, and it is discharged about 1½ miles from the docks by steam hoppers, and by ordinary mud-barges and tugs.
On the Clyde, the average cost, including everything—depreciation, interest, and carrying in hopper barges 27 miles—is as follows:—Very hard clay, boulders, and sand, 30·15d. per cubic yard; hard silt, gravel, and sand, 24·17d.; silt, clay and sand, 8·49d.; silt, gravel, sand, clay, and mud, 8·08d.; and silt and sand, 7·94d. per cubic yard.
On the Tyne, the cost varies from 2d. to 6½d. per ton, according to the nature of the material. One dredger has dredged over 1,000,000 tons in one year, and, including discharging a distance of 17 or 18 miles, the cost per ton was a little over 3½d.
The cost of removing the bar at Carlingford Lough, including everything—Parliamentary expenses and insurance of plant—was about 1s. 9d. per ton. Taking the cost for one season, it was 1s. 4d. to 1s. 5d. per ton, or 2s. to 2s. 3d. per cubic yard. The material was hard clay and boulders.
At Aberdeen, the cost of dredging and transporting about 2 miles beyond the bar, including insurance, but not depreciation and interest, is 1s. 2d. per ton for dredging, and 2·9d. for discharging, giving a total of 4s. 1d. per ton.
On the Wear, at Sunderland, the total cost of dredging, including every item of depreciation and interest, is 2·37d. per ton. The material consists of sand, gravel, and clay.