a number of ore deposits at depths of as much as 
150 feet. Current recovery techniques include the 
clamshell dredge, the bucket-ladder dredge, and 
dredges employing air-lift or suction hydraulic 
systems. 
a. Clamshell Dredge Clamshell or wireline 
dredges use large grab buckets, clamshells, and 
other digging and lifting tools lowered to the sea 
floor on flexible steel cables. They have the 
advantage of being adaptable to work in water 
depths to 350 feet. Having flexible cables, they 
also can be used in areas of high currents or wave 
motions. The main disadvantages of this method 
are the cost of operations in deep water because of 
the cycle time (which increases directly with water 
depth) and uncertainty in continuity of successful 
withdrawals. Clamshell dredges have been used in 
Thailand to recover tin ore from depths of 90 to 
130 feet. 
b. Bucket-Ladder Dredge The bucket-ladder 
dredge employs an endless chain of steel buckets 
to dig into the bottom. The dredged material is 
drawn continuously up the ladder and dumped. 
The material is then fed to various screening and 
concentrating devices. Bucket-ladder dredges have 
good digging capability, making them especially 
useful for placer mining, but are limited to water 
depths of about 150 feet. Because of the rigid 
dredging ladder, this method is confined to pro- 
tected waters or fair-weather operations. Several 
bucket-ladder dredges mined tin deposits in 1967 
off Indonesia in 60 to 100 feet of water. 
Bucket-ladder dredges were first used in the 
United States in the late 19th century. They have 
seen major service in inland waters. By digging the 
mineral deposit at the bow and depositing all 
barren materials (waste) at the stern, the dredge 
automatically advances to new reserves. 
Figure 46 shows a front view of the bucket- 
ladder dredge, Yuba No. 21. Still in operation on 
the Yuba River, California, mining placer gold, it is 
one of the last gold mining dredges in operation in 
the United States. With a mining depth capacity of 
107 feet and 18 cubic foot capacity buckets, it has 
an excellent record of efficiency. 
Figure 47 is a rear view of the 14 cubic foot 
bucket-ladder dredge, Rasep, shown mining to a 
depth of 100 feet on the island of Singkep, off 
Sumatra. 
VI-184 
Figure 46. Front view of bucket-ladder dredge 
Yuba No. 21 (C. M. Romanowitz photo) 
Figure 47. Rear view of bucket-ladder dredge 
Rasep, shown mining to a depth of 100 feet 
off island of Singkep, off Sumatra. (C. M. 
Romanowitz photo) 
c. Hydraulic Dredge The hydraulic dredge uses 
either air-lift or suction techniques. Using air-lift, 
air is injected into the bottom of a pipe which is 
submerged more than half of its length in water. A 
density differential is produced in the pipe, forcing 
the column of air-water mixture to flow upward in 
the pipe. This flow creates a powerful suction at 
the bottom of the pipe, bringing up silt, sand, and 
gravel suspended in a large volume of water. The 
mechanism consists of two pipes which may be 
constructed on the site with limited shop facilities 
and which require only compressed air to operate. 
However, air-lifts are extremely inefficient when 
operated without the assistance of water jets and 
other devices. The depth at which airlifts can be 
used efficiently is a function of the cost of 
supplying compressed air at the depth of dredging. 
With the suction dredge, a movable suction pipe 
with a support ladder and a discharge line are 
mounted on a floating hull. When digging in 
