Dredging by Bucket between Two Lighters.—Another plan of dredging, practised at an early period in rivers of considerable breadth, was to moor two barges, one on each side of the river. Between them was slung an iron dredging bucket, which was attached to both barges by chains wound on the barrels of a crab winch worked by six men in one barge and round a simple windlass worked by two men in the other barge. The bucket, being lowered at the side of the barge carrying the windlass, was drawn across the bottom of the river by the crab winch on the other barge; and having been raised and emptied, it was hauled across by the opposite windlass for repetition of the process. This process was in use in the River Tay until 1833.
Bucket Ladder Dredgers.—The earliest record of a bucket ladder dredger is contained in the first paper of the first volume (1836) of the Transactions of the Institution of Civil Engineers. This machine was brought into use at the Hull Docks about 1782. The bucket chain was driven by two horses working a horse-gear on the deck of the vessel. The buckets were constructed of 5⁄8 in. bars of iron spaced 1⁄8 in. apart, and were 4 ft. long, 13 in. deep, 12 in. wide at the mouth and about 6 in. wide at the bottom. This dredger raised about 30 tons per hour at the cost of 2½d. per ton, which covered the wages of three men working the dredger, eight men working the lighters and the keep of three horses. A dredger of this kind and power would only work in ballast, mud or other soft material, but the machine was gradually improved and increased in capacity and power by different manufacturers until it became a very efficient machine in skilful hands, excavating and raising material from depths of 5 ft. to 60 ft. of water at a cost not very different from, and in many cases less than, that at which the same work could be performed on land. With the powerful dredgers now constructed, almost all materials, except solid rock or very large boulders, can be dredged with ease. Loose gravel is perhaps the most favourable material to work in, but a powerful dredger will readily break up and raise indurated beds of gravel, clay and boulders, and has even found its way through the surface of soft rock, though it will not penetrate very far into it. In some cases steel diggers alternating with the buckets on the bucket frame have been successfully employed. The construction of large steam dredgers is now carried on by many engineering firms. The main feature of the machine is the bucket ladder which is hung at the top end by eye straps to the frame of the vessel, and at the lower end by a chain reived in purchase blocks and connected to the hoisting gear, so that the ladder may be raised and lowered to suit the varying depths of water in which the dredger works. The upper tumbler for working the bucket chain is generally square or pentagonal in form and made of steel with loose steel wearing pieces securely bolted to it. The tumbler is securely keyed to the steel shaft which is connected by gearing and shafting to the steam engine, a friction block being inserted at a convenient point to prevent breakage should any hidden obstacle causing unusual strain be met with in the path of the buckets. The lower tumbler is similar in construction to the upper tumbler, but is usually pentagonal or hexagonal in shape. The buckets are generally made with steel backs to which the plating of the buckets is riveted; the cutting edge of the buckets consists of a strong steel bar suitably shaped and riveted to the body. The intermediate links are made of hammered iron or steel with removable steel bushes to take the wear of the connecting pins, which are also of steel. The hoisting gear may be driven either from the main engine by frictional gearing or by an independent set of engines. Six anchors and chains worked by powerful steam crabs are provided for regulating the position of the dredger in regard to its work.
Barge-loading Dredgers used formerly to be provided with two ladders, one on each side of the vessel, or contained in wells formed in the vessel near each side. Two ladders were adopted, partly to permit the dredger to excavate the material close to a quay or wall, and partly to enable one ladder to work while the other was being repaired. Bucket ladder dredgers are now, however, generally constructed with one central ladder working in a well; frequently the bucket ladder projects at either the head or stern of the vessel, to enable it to cut its own way through a shoal or bank, a construction which has been found very useful. In one modification of this method the bucket ladder is supported upon a traversing frame which slides along the fixed framing of the dredger and moves the bucket ladder forward as soon as it has been sufficiently lowered to clear the end of the well. In places where a large quantity of dredging has to be done, a stationary dredger with three or four large hopper barges proves generally to be the most economical kind of plant. It has, however, the disadvantage of requiring large capital expenditure, while the dredger and its attendant barges take up an amount of space which is sometimes inconvenient where traffic is large and the navigable width narrow. The principal improvements made in barge-loading dredgers have been the increase in the size of the buckets and the strength of the dredging gear, the application of more economical engines for working the machinery, and the use of frictional gearing for driving the ladder-hoisting gear. It is very important that the main drive be fitted with the friction blocks or clutches before alluded to.
Up to the year 1877 dredgers were seldom made with buckets of a capacity exceeding 9 cub. ft., but since that time they have been gradually increased in capacity. In the dredger “Melbourne,” constructed by Messrs William Simons & Co. to the design and specification of Messrs Coode, Son & Matthews, about the year 1886, the buckets had a capacity of 22 cub. ft., the dredger being capable of making 37 ft. of water. The driving power consists of two pairs of surface-condensing engines, each of 250 i.h.p., having cylinders 20 in. and 40 in. in diameter respectively, with a 30 in. stroke, the boiler pressure being 90 ℔ per sq. in. The vessel is 200 ft. long by 36 ft. wide and 11 ft. 6 in. deep, and is driven by twin screw propellers. The gearing is arranged so that either pair of engines can be employed for dredging. The speed under steam is 7 knots, and in free-getting material 800 tons per hour can be dredged with ease. On one occasion the dredger loaded 400 tons in 20 minutes. The speed of the bucket chain is 83 lineal ft. per minute. The draught of the dredger in working trim is 7 ft. forward and 9 ft. aft. The efficiency of the machine, or the net work in raising materials compared with the power exerted in the cylinders, is about 25%. The dredged material is delivered into barges moored alongside. Contrasting favourably with former experience, the “Melbourne” worked for the first six months without a single breakage. She is fitted with very powerful mooring winches, a detail which is of great importance to ensure efficiency in working.
The “St Austell” (Plate I. fig. 3), a powerful barge-loading dredger 195 ft. long by 35 ft. 6 in. beam by 13 ft. deep, fitted with twin-screw compound surface-condensing propelling engines of 1000 i.h.p., either set of engines being available for dredging, was constructed for H.M. Dockyard, Devonport, by Messrs Wm. Simons & Co. in 1896. This dredger loaded thirty-five 500-ton hopper barges in the week ending April 2, 1898, dredging 17,500 tons of material in the working time of 29 hours 5 minutes.
An instance of a still larger and more powerful dredger is the “Develant,” constructed by Messrs Wm. Simons & Co., for Nicolaiev, South Russia. She is a bow-well, barge-loading, bucket ladder dredger, with a length of 186 ft., a breadth, moulded, of 36 ft., and a depth, moulded, of 13 ft. The bucket ladder is of sufficient length to dredge 36 ft. below the water level. The buckets are exceptionally large, each having a capacity of 36 cub. ft., or fully two tons weight of material, giving a lifting capacity of 1890 tons per hour. At the dredging trials 2000 tons of spoil were lifted in one hour with an expenditure of 250 i.h.p. The propelling power is supplied by one pair of compound surface-condensing marine engines of 850 i.h.p., having two cylindrical boilers constructed for a working pressure of 120 ℔ per sq. in. Each boiler is capable of supplying steam to either the propelling or dredging machinery, thus allowing the vessel to always have a boiler in reserve. On the trials a speed of 8½ knots was obtained. The bucket ladder, which weighs over 100 tons, exclusive of dredgings, is raised and lowered by a set of independent engines. For manœuvring, powerful winches driven by independent engines are placed at the bow and stern. The vessel is fitted throughout with electric light, arc lamps being provided above the deck to enable dredging to be carried on at night. Steam steering gear, a repairing shop, a three-ton crane, and all the latest appliances are installed on board.
The “Dérocheuse” (Plate II. fig. 12), constructed by Messrs Lobnitz & Co., is a good example of the dredger fitted with their patent rock cutters, as used on the Suez Canal. These rock cutters consist of stamps passing down through the bottom of the dredger, slightly in advance of the bucket chain, and are employed for breaking up rock in front of the bucket ladder so that it may be raised by buckets afterwards. This system of subaqueous rock cutting plant, on Messrs Lobnitz’s patent system, was effectively employed in deepening the Manchester Ship Canal, and removed a considerable length of rock, increasing the depth of water from 26 ft. to 28 ft. at a cost of about 9d. per cub. yd. A full and illustrated description of this plant, and of a similar plant supplied to the Argentine Government, was published in Engineering of August 17, 1906. An illustration of a bucket of 54 cub. ft. capacity constructed by Messrs Lobnitz & Co. is given (Plate II fig. 11), from which some idea of the size of dredging machinery as developed in recent practice may be obtained. In regard to the depth of water that can be obtained by dredging, it is interesting to note that the dredger “Diver,” constructed by Messrs. Hunter & English for Mr Samuel Williams of London, is capable of working in 60 ft. of water. In this vessel an ingenious arrangement was devised by Mr Williams, by which part of the weight of the dredger was balanced while the ladder itself could be drawn up through the bucket well and placed upon the deck, enabling a long ladder to be used for a comparatively short vessel. The “Tilbury” dredger, also constructed by Messrs Hunter & English, was able to dredge to a depth of 45 ft. below the surface of the water.
Hopper Barges.—To receive the materials excavated by barge-loading dredgers, steam hopper barges are now generally employed, capable of carrying 500 tons or more of excavation and of steaming loaded at a speed of about 9 m. per hour. These hopper barges are made with hinged flaps in their bottoms, which can be opened when the place of deposit is reached and the dredgings easily and quickly discharged.
Good examples of these vessels are the two steam hopper barges built for the Conservators of the river Thames in 1898. The dimensions are: length 190 ft., breadth 30 ft., depth 13 ft. 3 in., hopper capacity 900 tons. They are propelled by a set of triple expansion engines of 1200 i.h.p., with two return-tube boilers having a working pressure of 160 ℔. Special appliances are provided to work the hopper doors by steam power from independent engines placed at the forward end of the hopper. A steam windlass is fixed forward and a steam capstan aft. The vessels are fitted with cabins for the officers and crew. On their trial trip, the hoppers having their full load, a speed of 11 knots was obtained, the coal consumption being 1.44 ℔ per i.h.p.
Methods of Dredging.—In river dredging two systems are pursued. One plan consists in excavating a series of longitudinal furrows parallel to the axis of the stream; the other in dredging cross furrows from side to side of the river. It is found that inequalities are left between the longitudinal furrows when that system is practised, which do not occur, to the same extent, in side or cross dredging; and cross dredging leaves a more uniform bottom. In either case the dredger is moored from the head and stern by chains about 250 fathoms in length. These chains in improved dredgers are wound round windlasses worked by the engine, so that the vessel can be moved ahead or astern by simply throwing them into or out of gear. In longitudinal dredging the vessel is worked forward by the head chain, while the buckets are at the same time performing the excavation, so that a longitudinal trench is made in the bottom of the river. After proceeding a certain length, the dredger is stopped and permitted to drop down and commence a new longitudinal furrow, parallel to the first one. In cross dredging, on the other hand, the vessel is supplied with four additional moorings, two on each side, and these chains are, like the head and stern chains, wound round barrels worked by steam power. In cross dredging we may suppose the vessel to be moored at one side of the channel to be excavated. The bucket frame is set in motion, but instead of the dredger being drawn forward by the head chain, she is drawn across the river by the starboard chains, and, having reached the extent of her work in that direction, she is then drawn a few feet forward by the head chain, and the bucket frame being still in motion the vessel is hauled across by the port chains to the side whence she started. By means of this transverse motion of the dredger a series of cross cuts is made; the dredger takes out the whole excavation from side to side to a uniform depth and leaves no protuberances such as are found to exist between the furrows in longitudinal dredging, even when it is executed with great care. The two systems will be understood by reference to fig. 1, where A and B are the head and stern moorings, and C, D, E and F the side moorings. The arc e f represents the course of the vessel in cross dredging; while in longitudinal dredging, as already explained, she is drawn forward towards A, and again dropped down to commence a new longitudinal furrow.