Dredgers come in a variety of forms, types and sizes. Their purpose is to remove silt or sand from harbours, rivers or entrance channels to harbours, in order that seagoing vessels have sufficient depth to navigate safely in these confined spaces. They are also used in underwater mining operations.
Whilst there are still examples of bucket dredgers around, they are a dying breed and will not be discussed. The main type of dredger used today is the suction dredger, either with or without a cutting device at the mouth of the suction pipe. There are also floating grab dredgers which are used for the more solid types of material on the sea bottom. The material dredged from the bottom is usually deposited either in a hopper barge or hopper tanks in the hull of the dredger. When the latter are filled, they are taken out to sea and dumped. The suction dredger uses a wide diameter pipe which is lowered onto the sea bed and the sand/silt is sucked up by a wear resistant centrifugal pump. This is then dumped into the hopper tanks. In the case of the suction dredgers with a cutting tool at the end, the tool is used to break up the bottom material and feed the pieces into the suction tube. Besides being dumped into hopper barges/tanks, the sand is sometimes pumped ashore through large diameter pipes. In this way badly eroded beaches can be restored.
As far as mining is concerned much more sophisticated equipment is used, since the ore or mined materials have to be separated from the sand/silt which is sucked up. An example of underwater mining is that conducted off the west coasts of Namibia and South Africa, near the mouth of the Orange River. The object here is the recovery of alluvial diamonds.
As stated earlier, the shape, size and type of dredger varies considerably and it is not intended to discuss them all. The characteristics stated below are for a typical self-propelled suction dredger used to maintain the depths of harbours already dredged:
- The superstructure is usually situated down aft. This includes the bridge, engine room, accommodation, stores, etc.
- The suction hose or pipe is fed through the bottom of the ship and has special control facilities to position the mouth of the pipe wherever it is required.
- Onboard the dredger is a powerful wear-resistant centrifugal pump which is connected to the pipe.
- Running the length of the vessel between the superstructure and the forecastle is a hopper tank/hold into which the sand/silt is pumped. When this tank/hold is filled to the designed capacity, the dredger takes the material clear of the harbour and dumps it in deeper water.