Dredge No. 4

Y0B 1G0 Dawson City, Canada (Yukon)

Dredge No. 4 is a wooden-hulled bucketline sluice dredge that mined placer gold on the Yukon River from 1913 until 1959. It is the largest wooden-hulled dredge in North America.

History

Designed by the Marion Steam Shovel Company, the bucketline sluice dredge was built on site at Claim 112 Below Discovery from mid 1912 until the onset of winter.

The Canadian Klondike Mining Company began operating the dredge in May 1913. After eleven years of operation, it had cut its way to the Boyle Concession, where it sank in 1924. By 1927, it had been refloated and worked its way to Hunker Creek, where it could produce up to 23 kg of gold a day at claim 67 Below Discovery. It ceased operations in the area on 11 July 1940, and was rebuilt by the Yukon Consolidated Gold Corporation on Bonanza Creek, where it resumed operations on 11 September 1941.
The wooden hull of the original dredge was discarded, left in the pond where it sank, but all other parts were salvaged for use in the reconstructed dredge.[ It worked its way downstream on one side of the valley, then back up the other side, until being decommissioned on 1 November 1959.

The immediate success of the dredge resulted in the Canadian Klondike Mining Company ordering the construction of two more dredges the following year.

Operation

The electrically-powered machine required 920 horsepower (690 kW) while digging, and more when moving its gangplank. With its installed hydraulic monitors, the eight-storey dredge would cut into gravel banks, washing down the released material for processing. The machine created a dredge pond by virtue of its operation, its size dependent on the valley in which it was operating, but sometimes reaching 150 by 90 metres  It would rotate on two spuds, each 56 by 1.42 by 0.91 m and 18 m  long.

The 33 m digging ladder enabled the dredge to dig a cut with an average arc of about  This wide arc was possible because of the use of two spuds. It could reach 5.2 m above water level and 15 m below it, with each of the 72 buckets capable of moving loads up to 0.45 m³. Each bucket weighed 1,515 kilograms  each flange 347 kilograms , and each securing pin 225 kilograms .

Gold was recovered in a rotating 15.1 m long trommel screen with 2.97 m diameter and 12.5% grade. A pipe was suspended within the trommel, carrying water upwards to spray the incoming material, cleaning it and breaking up larger lumps. Finer material (gold, sand, and pebbles) was sieved through 1.9 cm holes in the screen, which rotated at 7.8 revolutions per minute, into a distributor box. From there, it flowed into sluice tables, long troughs with an area of 158.4 m2 which had a constant flow of water.
 About 75% of the gold was caught in 1.2 m coconut matting and steel riffles at the bottom of the troughs. A smaller distributor captured another 20% of the gold, and all remaining material was washed out into the dredge pond. The larger pieces of gravel were ejected from a 81 cm wide stacker at its rear, a 40 m belt moving at 1.81 m/s per minute. These tailings remain as a "vast, rippled blight on the landscape". Finer particles of gold and nuggets too large to fit through the screen holes would be ejected with the gravel.

In its 46 years of operations, the machine mined nine tons of gold, dredging 22 buckets of gravel every minute, about 14,000 m³.per day. Dredge No. 4 had a much greater capacity than other dredges operated in the area, and could sometimes operate through winter. In 1918, it began operating on 1 May, and continued until 3 April 1919, at which time it was stopped for repairs.
 
It worked without interruption from late April or early May until late November every year. At the end of each season, the buckets were removed.

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