By MR. A.A. LANGLEY.
This paper, lately read before the Institution of Mechanical Engineers,
London, is a description of the construction and working of a dredger
on M. Bazin's system, as used by the author for the past three years in
dredging sand and other material in Lowestoft Harbor. The dredger is
represented in its general features on next page, Fig. 1. The total
length of the hull is 60 ft., with 20 ft. beam. In the after part of the
hold is placed a horizontal boiler, A, which supplies steam to a pair
of inverted vertical engines, B. These engines drive, through belts and
overhead pulleys, a centrifugal pump, C, which discharges into the open
trough, H. The suction pipe, D, of this pump passes through the side of
the dredger, and then forms an elbow bent downward at an angle of 45
deg. To this elbow is attached a flexible pipe, E, 12 in. in diameter
and 25 ft. long, made of India-rubber, with a coil of iron inside
to help it to keep its shape. At the lower end of this pipe is an
elbow-shaped copper nozzle which rests on the bottom, and is fitted with
a grating to prevent stones getting into the pump and stopping the work.
The flexible tube is supported by chains that pass over the head of a
derrick, F, mounted at the stern of the dredger, and then round the
barrel of a steam winch. By this means the depth of the nozzle is
altered, as required to suit the depth of water.
A man stands at the winch, and lifts or lowers the pipe as is required,
judging by the character of the discharge from the pump. If the liquid
discharged is very dark and thick the nozzle is too deep in the sand
or gravel; if of a light color the pipe must be lowered. The best
proportion of water to sand is 5 to 1. When loose sand is the only
material to be dealt with, it can be easily sucked up, even if the
nozzle is deeply buried; but at other times stones interfere with the
work, and the man in charge of the flexible tube has to be very careful
as to the depth to which the nozzle may be buried in the sand. The pump
is shown in Figs. 2 and 3. The fan is 2 ft. diameter, and has only two
blades, a larger number being less efficient. The faces of the blades,
where they come in contact with the sand, are covered with flaps of
India-rubber. Small doors are provided at the side of the pump for
cleaning it out, extracting stones, etc. The fan makes 350 revolutions
per minute, and at that speed is capable of raising 400 tons of sand,
gravel, and stones per hour, but the average in actual work may be taken
at 200 tons per hour. This is with a 10-horse power engine, and working
in a depth of water varying from 7 ft. to 25 ft. The great advantage of
this dredger is its capability of working in disturbed water, where the
frames of a bucket dredger would be injured by the rise and fall of the
vessel.
[Illustration: THE BRAZIN SYSTEMEM OF DREDGING.]
Thus at Lowestoft bucket dredgers are used inside the harbor, and the
Bazin dredger at the entrance, where there are sand and gravel, and
where the water is more disturbed. The dredger does not succeed very
well in soft silt, because, owing to its slow precipitation, it runs
over the sides of the hopper barges without settling. Nor does it do for
dredging solid clay. It gives, however, excellent results with sand and
gravel, and for this work is much superior to the bucket dredger. The
experience in working was then described, showing that a great many very
discouraging failures preceded successful working, about a year being
expended in getting good results.