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Dredgers and Hoppers

The entrances to many ports must be kept free from silt, and without the ugly but efficient dredgers and hoppers that keep the channels clear even the greatest ports might fall into disuse


IN THE HEART OF LONDON opposite the Houses of Parliament, a grab dredger is seen in the photograph on the left. This vessel, the Port of London Authority dredger No. 8, flies a large flag to show that dredging operations are in progress. The existence of the Port of London is dependent on the activities of its dredgers.

A DREDGING OUTPUT of 750 tons an hour is given by the bow well self-propelling bucket dredger Oliver Bury, shown in the centre photograph. She is 165 feet long, with a beam of 37 feet and a depth of 12 ft 6 in. She dredges to a depth of 48 feet and her bucket capacity if 21 cubic feet. The bucket dredger is the descendant of the Dutch mud dredger, invented in the seventeenth century. The mud dredger had a series of flat paddles operating on an endless belt. In the modern version buckets replace the paddles.

AN INDIAN SUCTION HOPPER DREDGER, the Balari, is seen in the lower photograph at work on one of the many bars in the river Hooghly, below Calcutta. The bed of this great river is constantly shifting and, although dredgers are always busy, a special service of pilots is necessary to keep pace with the continual altering of the navigable channels. The Hooghly is the western arm of the delta of the River Ganges and flows into the Bay of Bengal.

THERE is a type of vessel so ungainly in appearance that it has been called the ugly duckling of the shipbuilder’s family. This type is the dredger. Appearance, however, is not everything, and the dredger is essential to the prosperity - and even the existence - of many great ports, especially when these are situated some distance up a river.

Navigable channels liable to be choked by deposits of silt have to be continually dredged, or they will become useless. In some instances a river is so completely cleared by human agency that its character is changed. The classical example is the River Clyde, birthplace of so many fine vessels, including the RMS Queen Mary.

In days gone by, the Clyde resembled other Scottish rivers; it had a rapid flow from source to mouth, and comparatively little depth even at Glasgow. It was a good trout and salmon stream, but of little apparent use to anyone who might propose to make Glasgow one of the great ports of the world. Were the Clyde in its original state to-day, it would be impossible for vessels of any but the shallowest draught to approach Glasgow. There would be no shipyards on the Clyde, no great ocean liners steaming majestically down past Dumbarton Rock.

The Clydeside shipping industry owes its existence to the dredging of the river, and the continuance of that industry is made possible only by dredging, incessantly repeated.

What is true of the Clyde is true elsewhere. In the Netherlands, the muddy mouths of the Rivers Maas and the Waal have to be subjected to constant dredging to keep them clear for navigation.

Without this labour the great seaport town of Rotterdam and her lesser sister Dordrecht might well sink to the insignificance of the “Dead Cities” of the Zuider Zee. Only a little farther south, exactly the same thing happens with busy Antwerp on the muddy River Schelde.

In England, on the Thames, the Mersey, the Humber or the Tyne, the dredgers are busy keeping the fairways open. Some idea of the dredging out of the Tyne will be gathered from an interesting fact. When the famous High Level Bridge at Newcastle-upon-Tyne was built by Robert Stephenson in 1849, it was possible to found the piers on simple piles in the bed of the river. Rather more than fifty years later, when the companion King Edward Bridge was built, the river had been so deeply dredged that piles were out of the question, and deep caissons had to be sunk.

A dredge, in its simplest form, is nothing more than a shovel or scoop. A single shovel, even a giant power-operated shovel, would be an inadequate instrument for turning a shallow river into a channel deep enough to float a ship of many thousand tons burden. The obvious step forward from a single scoop is a large number of scoops working in endless series and operated by powerful engines. That is the principle of the present-day bucket dredger.

THE FIRST STEAM-DRIVEN HOPPER on the Thames was the Thames Conservancy Hopper No. 1

THE FIRST STEAM-DRIVEN HOPPER on the River Thames was built at Gravesend, Kent, in 1876. She was the Thames Conservancy Hopper No. 1. Her name was later changed to Thalia, and in 1901 she was in use as a powder hulk in the Thames.

The bucket dredger is no longer alone in the field of modern dredging, for the suction dredger also is extensively used nowadays. The bucket dredger, however, holds its own for certain types of dredging, as a soft river or canal bed is necessary for the successful working of the suction dredger.

The forerunner of the bucket dredger was the “bag and spoon” apparatus, which dates back to the latter part of the sixteenth century, if not earlier. For river clearance, this consisted of a perforated leather or network bag, mounted on a ring at the end of a long pole or beam, resembling in its general principle a giant shrimping net. The beam in its turn was mounted on a fulcrum at its centre, in the same way as the beam of an old-fashioned steam engine, and the whole equipment was mounted on a small barge or pontoon. For light work the balanced beam was unnecessary, and the “bag and spoon” could be operated from a long pole by one man.

An old Dutch print, dated 1565, shows this early form of dredger employed in the removal of a mudbank. The dredger had the beam and fulcrum and was worked by three men. The boat was propelled by an enormous single oar in the stern, and was held steady during operations by four stout anchor cables. The “bag-and-spoon” principle survives to this day in certain small water-courses which an up-to-date bucket dredger would be unable to penetrate, because of her size.

In the second decade of the seventeenth century a notable advance was made with the introduction of a grab-dredge, mounted between two rigidly attached pontoons. The grab is extensively used to-day, not only for dredging in small rivers and canals, but also for the handling of coal and coke. This early grab had the double jaws so well known to-day, though its construction was of the simplest order, and motive power was supplied by a man working a treadmill.

Netherlands Origin

The immediate precursor of the modern bucket dredger was the Dutch mud dredger. This was probably invented by J. J. Nieng, who was Burgomaster of the town of Hoorn in 1632. Adam Clippens of Ghent, a contemporary of Nieng, has also been claimed as the inventor. With the then incessant trouble arising through the silting up of muddy rivers in the Low Countries, it is not impossible that the mud dredger was invented independently and almost simultaneously by these two men.

The early mud dredger involved the use of a series of flat paddles operating on an endless belt which was worked through a system of gears by a large capstan on board the dredging vessel. The capstan in its turn was worked by either horses or men, according to the size of the dredger.

The belt ran over a rigid frame that was let down into the bed of the river by a derrick, and the up-coming paddles passed through a U-shaped wooden chute incorporated with the supporting frame. The down-going paddles were on the upper side of the frame, the chute for the up-coming paddles and spoil being on the underside. The frame was lowered through an oblong well in the hull of the dredging vessel. Dredgers of this kind were used for many years, and steam power was applied to them in the last century.

Although the mud dredger answered its purpose well enough in waters where the bottom consisted of soft mud or even flaky clay, it was almost useless for shingle or for a coarse gravelly bed.


SELF-PROPELLING BOW WELL BUCKET DREDGER. The Delver, 217 tons gross, was built at Renfrew, Scotland, in 1912, for the Union Government of South Africa. She is seen in this photograph with the Cunard liner Franconia, 20,175 tons gross, in the background. The Delver operates at Cape Town and is 128 feet long, with a beam of 25 ft 2 in and a depth of 9 ft 6 in.

From the mud dredger with paddles to the bucket dredger was but a short step. In the modern dredger the old paddles are replaced by strong iron buckets, working in a contrary direction to that of the paddles on the old mud-dredger -that is to say, returning on the upper side of the frame. The chute is dispensed with as the spoil is retained in the buckets by the natural law of gravity. Though the power-driven bucket dredger is an essentially modern invention, yet the idea of substituting buckets for paddles is of considerable antiquity, and is a natural and obvious combination of belt-and-paddle and “bag-and-spoon”.

The modern bucket dredger, however, has many accessories and improvements never visualized by the early Dutch inventors. Powerful tackle is installed for the hoisting of the bucket frame or, as it is called, the “ladder”, up into the well of the vessel when it is not in use. The spoil is separated from the water which comes up with it in the buckets, so that the spoil can be shot into a hopper without much superfluous water, which is ejected overboard.

All the dredging equipment has to be of the strongest and most robust description. The first application of manganese steel, invented by Sir Robert Hadfield in 1882, was made in the construction of a bucket dredger for the Ribble Navigation Department of the Preston (Lancs) Corporation. The bucket chain had manganese steel pins. Fifteen months after the chain had been installed the apparatus was overhauled, and the manganese steel pins were found to be almost unworn. The ordinary pins wore out after about four months of service. A typical modern dredger may be driven by a 200 hp triple-expansion steam-engine working at 150 revolutions a minute and delivering sixteen buckets of spoil a minute.

A typical sea-going dredger of large dimensions is capable of dredging to a depth of seven fathoms, and can deliver 850 cubic yards an hour. For propulsion and operation of the buckets the vessel is equipped with two 450 hp triple-expansion steam-engines.

There are many variations of the suction dredger. Soft spoil is sucked up by a power-driven rotary pump from a nozzle which is pressed into the river bed. In one type of suction dredger the nozzle may be incorporated with a conical revolving cutter, which breaks up the spoil. In another type the nozzle is replaced by a shallow bell in which a horizontal rotary cutter is installed, the suction pipe and the driving shaft of the cutter both passing out of the top of the bell. In yet another type, cutter and suction pipe are separate, the cutter being a series of scrapers or bladed wheels let down over the bows of the dredging vessel, and the suction pipe trailing from the after part. This is known as the Bazin sand pump.

The Bazin sand pump was used for some time in the dredging of the Suez Canal, and was claimed to be capable, when working from a depth of approximately two fathoms, of dealing with 3,000 cubic metres (over 3,920 cubic yards) of sand a day. The first automatic-discharging suction dredger was built in 1895 for river and canal dredging in the Netherlands. The suction pipe was situated amidships, to avoid the possibility of its being damaged against a jetty or wharf-side adjacent to the ground chosen for the depositing of the spoil. A second rotary pump actuated the discharge.

Spoil may be tipped either into the vessel’s own hopper or into a special hopper barge lying alongside. In the hopper the bottom opens and is pressed to again by the water as soon as the spoil has dropped.

the Port of London Authority Hopper No. 5 refitting at TilburyAnother useful though scarcely beautiful type is the London County Council sludge vessel, indispensable to the health of the capital. The great sewer outfalls at Barking and Crossness - to take London alone - produce an enormous amount of solid refuse or sludge which must be disposed of. The big sanitary ships go down the Thames estuary, carrying the sludge to Barrow Deep in the North Sea, about ten miles east of the Nore Lightship.

A vessel of this class may have a gross tonnage of over 1,500, and be capable of carrying 1,000 tons of sludge in four huge, rectangular tanks. Well out to sea, special valves are opened and the unwanted cargo of these vessels is rapidly discharged into the water, about ten feet below the surface. It is quickly diffused by the action of the ship's propellers, the whole operation taking only a few minutes.

So we cannot look at the ugly sanitary vessel, or at the ungainly hopper or bucket dredger, without feelings of real respect. On the one depends the health of the city, and on the other two the activity of the seaport, and even its existence.

IN DRY DOCK AT TILBURY, Essex, the Port of London Authority Hopper No. 5 is refitting. In the background is the P. and O. liner Narkunda, 16,632 tons gross, also in dry dock. The Port of London Authority hoppers carry mud dredged from the bottom of the London River - the name given by seamen to the River Thames. In the hopper the bottom opens and is pressed to again by the water as soon as the spoil has been dropped.

[From part 24, published 21 July 1936]