ar. Various kinds of apparatus
have been designed in the shape of grabs or buckets for dredging
purposes. These are usually worked by a steam crane, which lets the open
grab down to the surface of the ground to be excavated and then closes
it by a chain which forces the tines into the ground; the grab is then
raised by the crane, which deposits the contents either into the hopper
of the vessel upon which the crane is fixed or into another barge.

The Priestman grab has perhaps been more extensively used than any
other apparatus of this sort. It is very useful for excavating mud,
gravel and soft sand, but is less effective with hard sand or stiff
clay--a general defect in this class of dredger. It is also capable of
lifting large loose pieces of rock weighing from 1 to 2 tons. A
dredger of this type, with grab holding 1 ton of mud, dredged during
six days, in 19 ft. of water, an average of 52-1/2 tons and a maximum
of 68-1/2 tons per hour, and during 12 days, in 16 ft. of water, an
average of 48 tons and a maximum of 58 tons per hour, at a cost of
1.63d. per ton, excluding interest on the capital and depreciation.
The largest dredger to which this apparatus has been applied is the
grab bucket hopper dredger "Miles K. Burton" (Plate I. fig. 9),
belonging to the Mersey Docks and Harbour Board. It is equipped with 5
grabs on Morgan's patent system, which is a modification of
Priestman's, the grabs being worked by 5 hydraulic cranes. It raised
and deposited, 12 to 15 m. at sea, 11 loads of about 1450 tons each
with a double shift of hands, at a cost of about 1s. 5d. per cub. yd.
of spoil, including the working expenses for wages of crew, fuel and
stores. Mr R. A. Marillier of Hull has stated that "the efficiency of
these grabs is not at all dependent upon the force of the blow in
falling for the penetration and grip in the material, as they do their
work very satisfactorily even when lowered quite gently on to the
material to be cut out, the jaws being so framed as to draw down and
penetrate the material as soon as the upward strain is put on the
lifting chain. Even in hard material the jaws penetrate so thoroughly
as to cause the bucket to be well filled. The grab is found to work
successfully in excavating hard clay from its natural bed on dry
land." It is claimed on behalf of grabs that they lift a smaller
proportion of water than any other class of dredger.

Si