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Harbour duties


A necessary activity

Dredging is a mundane but necessary activity if ports and channels are to remain open to ships.   Prior to steam, dredging was restricted to what could be done by men or horses.   Steam power meant that large quantities of silt could be moved, and ports and channels deepened considerably, and even created from scratch.   Three types of dredger have been developed.

The bucket dredger has a continuous chain of buckets with the hull arranged so that these can be lowered to the seabed.   The spoil which the buckets dig out is usually poured into a hopper which takes it away for dumping.   Bucket dredgers work well at deepening narrow channels but cannot be used in deep or rough water.

The grab dredger has a grab mounted on a crane.   The grab is lowered to the seabed and its operator scoops up a bucketful of silt which is raised to the surface and placed in the hold of the dredger or of a hopper moored alongside.   Grab dredgers are rather slow in action, but useful for working in confined spaces, for example in locks or along dock walls.

The suction dredger or sand sucker is the most efficient type of dredger.   It has a powerful steam or diesel-driven pump which sucks up spoil through a long pipe lowered to the seabed.   Unlike other types, the suction dredger can work on the move, which is useful when deepening a long channel.   Suction dredgers are not just used to improve channels.   They are also employed to dredge sand and gravel which is brought ashore to be used to make concrete for the building industry.

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