DREDGING partment of civil engineering. It is proposed in this article to describe the improvements which have been made in dredgers and in dredging operations since the publication of the article in the ninth edition of this work (vol. vii., published in 1877), and to give the cost of dredging operations in various instances. Bucket-Ladder Dredgers.—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 material close to a quay or wall, and partly to enable one ladder to work while the other was being repaired. Of late years, however, bucket-ladder dredgers have generally been constructed with one central ladder working in a well, and this is the type now generally followed. Such dredgers are frequently made with the bucket ladder projecting either at the head or stern of the vessel, to enable it to cut its own way through a shoal or a 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, in the strength of the dredging gear, in the application of more economical engines for working the machinery, and in the use of frictional gearing for driving the ladder-hoisting gear. It is very important that the main drive be fitted with friction blocks or clutches which shall slip and prevent breakage in the machinery when hidden obstacles causing unusual strain come in the path of the buckets. Up to the year 1877 dredgers were seldom made with buckets of a capacity exceeding 9 cubic ft., but since that time they have been gradually increased to 14, 16, and 18 cubic ft. capacity, while in the dredger Melbourne, constructed by Messrs William Simons & Co. to the design and specification of Messrs Goode, ■Son, k Matthews about the. year 1886, the buckets had a capacity of 22 cubic ft., the dredger being capable of making 37 feet of water. The driving power consisted of two pairs of compound 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 !b per square inch. The vessel was 200 ft. long by 36 ft. wide and 11 ft. 6 in. deep, and was driven by twin-screw propellers. The gearing was arranged so that either pair of engines could be employed for dredging. The speed under steam was 7 knots, and in free-getting material fiOO tons per hour could be dredged with ease. On one occasion the dredger loaded 400 tons in 20 minutes. The speed of the bucket chain was 83 lineal ft. per minute. The draught of the dredger in working trim was 7 ft. forward and 9 ft. aft. The ■elfieiency of the machine, or the net work done in raising materials compared with the power exerted in the cylinders, was about 25 per cent. The dredged material was delivered into barges moored alongside. Contrasting favourably with former experience, the Melbourne worked for the first six months without a single breakage. She was fitted with very powerful mooring winches, a detail which is of great importance to ensure efficiency in working. The St Austell (Fig. 1), another powerful barge-loading dredger, 195 ft. long by 35 ft. 6 in. beam by 13 ft. deep, fitted with tvvm-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 William Simons & Co. in 1896. This dredger loaded thirty-five 500-ton hopper barges in the week ended 2nd April 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 Devolant, constructed by Messrs William Simons & Co. for Aicolaieff, South Russia. She is a bow well, barge-loading, bucketladder dredger. Length 186 ft., breadth, moulded, 36 ft., depth,
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moulded, 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 cubic 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 lb per square inch. Each boiler is capable of supplying steam to either the propelling or the dredging machinery, thus allowing the vessel always to have a boiler in reserve. On the trials a speed of 8£ 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 manoeuvring, 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 3-ton crane, and all the latest appliances are installed on board. The Majestic, constructed by Messrs Lobnitz & Co., is a good example of a 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 afterwards be raised by the buckets. In regard to the depth of water which can be made 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 with a comparatively short vessel. The Tilbury dredger, constructed by the same firm, is able to dredge to a depth of 45 ft. below the surface of the water. Hopper Dredgers. — In places where barge - loading dredgers are inconvenient, owing to confined space and interference with the navigation, and where it is further necessary to curtail capital expenditure, hopper dredgers are convenient and economical. The vessel for this type of dredger is made of sufficient length and floating capacity to contain its own dredgings, which it carries out to the depositing ground as soon as its hopper is full. Considerable time is of course occupied in slipping and recovering moorings and in conveying material to the depositing ground, but these disadvantages are in many instances counterbalanced by the fact that less capital is required for plant and that less room is taken up by the dredger. If the depositing ground is far away, the time available for dredging is much curtailed, but the four-screw hopper dredger constructed by Messrs W. Simons & Co. for Bristol has done good work at the cost of 5d. per ton, including wages, repairs, coals, grease, sundries, and interest on the first cost of the plant, notwithstanding that the material has to be taken 10 miles from the Bristol docks. It can lift 400 tons of stiff clay per hour from a depth of 36 ft. below the water-line, and the power required varies from 120 I.H.P. to 150 I.H.P., according to the nature of the material. The speed is 9 knots, and four propellers were provided, two at the head and two at the stern, to enable the vessel to steam equally well either way, as the river Avon is too narrow to permit her to be turned round. The largest bucket hopper dredger in the world is La Puissante (Fig. 2), constructed by Messrs William Simons & Co. Ltd. for the Suez Canal Company, for the improvement of Port Said roads. It is 275 ft. long by 47 ft. beam by 19 ft. deep. The hopper capacity is 2000 tons, and the draught loaded 16 ft. 5 ins. The maximum dredging depth is 40 ft. and the minimum dredging depth is only limited by the vessel’s draught, it being able to cut its own way. The bucket ladder working through the well in the stern weighs, with buckets, 120 tons. The buckets each have a capacity of 30 cubic ft., and raised on trial 1600 tons per hour. The dredger is propelled by two sets of independent triple-expansion surfacecondensing engines of a combined indicated horse-power of 1800, working with steam at 160 lb pressure, supplied by two mild steel multitubular boilers. Each set of engines is capable of driving the buckets independently at speeds of sixteen and twenty buckets per minute. The bucket ladder is fitted with buffer springs at its upper end to lessen the shock when working in a sea-way. The S. III.— 67
