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D R E D G I N G

dredger can deliver the dredged material either into its own hopper or into barges lying on either side. The vessel attained a speed of 9| knots per hour on trial. The coal consumption during six hours steaming trial was T66 tb per I.H.P. per hour. Dredgers fitted with Long Shoot or Shore Delivering Apparatus.—The first instance of dredgers being fitted with long shoots was in the Suez Canal. The soil in the lakes was very variable, the surface being generally loose mud which lay in some places on sand, but frequently on more or less hard clay. Mr Lavalley employed shoots 230 ft. long, supported on pontoons connected with the hull of the dredger. The sand flowed away with a moderate supply of water to the shoots when they were fixed at an inclination of about 1 in 20, but when the sand was mixed with shells, these formed a coating which prevented the stream of water from washing out the shoot, and even with an inclination of 1 in 10 the material could not be delivered. A pair of endless chains working down the long shoot overcame the difficulty, and also enabled hard clay in lumps to be dealt with. One dredger turned out about 2000 cubic yards of thick clay in 15 hours, and when the clay was not hard it could deliver 150,000 cubic yards in a month for several consecutive months. A dredger (Fig. 3) constructed by Messrs Hunter & English for reclamation works on Lake Copais in Greece was fitted with delivery belts running on rollers in steel lattice frames on each side of the vessel, supported by masts and ropes. It could deliver 100 cubic metres per hour at 85 ft. from the centre of the dredger, at a cost of 1 ‘82d. per cubic metre for working expenses, with coal at 45s. per ton, including O'OSd. per cubic metre for renewal of belts, upon which the wear and tear was heavy. Another instance of the successful application of shore-delivery apparatus is that of a dredger for Lake Titicaca, Peru, which was fitted with long shoots on both sides, conveying the dredged material about 100 ft. from the centre of the dredger upon either side. The shoots were supported by shear-legs and ropes, and were supplied with water from a centrifugal pump in the engine-room. This dredger could excavate and deliver 120 cubic yards per hour at a cost of l‘725d. per cubic yard with coal costing 40s. per ton. If coal had been available at the ordinary rate in England of 20s. per ton, the cost of the dredging and delivery would have been 0 ed. per cubic yard for wages, coal, oil, &c., hut not including the salary of the superintendent. An interesting example of a shore-delivering dredger is a lightdraught dredger for the Lakes of Alberpera at the mouth of the Ebro in Spain. The conditions laid down for this dredger were that it should float in 18 in. of water and deliver the dredged material at 90 ft. from the centre of its own hull. In order to meet the latter requirement, the vessel was made of steel plates § in. thick, and longitudinal girders ran from end to end of the vessel, the upward strain of flotation being conveyed to them from the skin plating by transverse bulkheads at short intervals. The dredger was 94 ft. long, 25 ft. wide, and 3 ft. deep, and the height of the top tumbler above the water was 25 ft. When completed it drew 17 in. of water. The dredgings were delivered by the buckets upon an endless belt, driven from the main compound surface condensing engine, which ran over pulleys supported upon a steel lattice girder, the outer end of which rested upon an independent pontoon. This belt delivered the dredgings at 90 ft. from the centre of the dredger round an arc of 180 degrees. The dredger delivered 125 cubic yards per hour of compact clay at a cost of lT6d. per cubic yard or 0‘86d. per ton for wages, coal, and stores. Another method of delivering dredgings is that of pneumatic delivery, introduced by Mr F. E. Duckham, of the Millwall Dock Company, by which the dredgings are delivered into cylindrical tanks in the dredger, closed by air-tight doors, and are expelled by compressed air either into the sea or through long pipes on to the land. The Millwall Dock dredger is 113 ft. long, with a beam of 17 ft. and a depth of 12 ft. The draught loaded is 8 ft. It contains two cylindrical tanks, having a combined capacity of 240 cubic yds., and is fitted with compound engines of about 200 I.H.P., with a 20-in. air-compressing cylinder. The discharge pipe is 15 in. diameter by 150 yds. long. The nozzles of the air-injection pipes must not he too small, otherwise the compressed air, instead of driving out the material, simply pierces holes through it and escapes through the discharging pipe, carrying with it all the liquid and thin material in the tanks. The cost of- working the Millwall Dock dredger is given by Mr Duckham at l 75d. per cubic yard of mud lifted, conveyed, and deposited on land 450 ft. from the water-side, for working expenses only. This dredger is believed to be the first one constructed with a traversing ladder, as suggested by the late Captain Gibson when dock-master of the Millbank Docks.

Sand-Pump Dredgers.—Perhaps the most important development which has taken place in dredging during recent years has been the employment of sand-pump dredgers, which are very useful for removing sandy bars where the particular object is to remove quickly a large quantity of sand or other soft material. They are, however, apt to make large holes, and are therefore not fitted for positions where it is necessary to finish off the dredging work to a uniform flat bottom, for which purpose bucket dredgers are better adapted. Pump dredgers are, however, admirable and economical machines for carrying out the work for which they are specially suited. In a discussion upon Mr J. J. Webster’s paper upon “Dredging Appliances” at the Institution of Civil Engineers in 1886, Sir John Coode stated that he had first seen sand-pump dredgers at the mouth of the Maas in Holland. The centrifugal pump was placed against the bulkheads in the after part of the vessel, and the sand and water were delivered into a horizontal hreeches-piece leading into two pipes running along the full length of the hopper. The difficulty in preventing the sand from running overboard was entirely obviated by its being propelled by the pump through these pipes, the bottoms of which were perforated by a series of holes. In addition, there were a few small flap-doors fixed at intervals, by means of which the men were able to regulate the discharge. On being tested, the craft pumped into this hopper 400 tons of sand in 22 minutes. The coamings round the well of the hoppers were constructed with a dip, and when the hopper was full the water ran over in a steady stream on either side. The proportion of sand delivered into the hopper was about 20 per cent, of the total capacity of the pump. The dredger was constructed by Messrs Smit of Kinderdijk, near Eotterdam. In the same discussion Mr A. A. Langley, then engineer to the Great Eastern Railway Company, gave particulars of a sand pump upon the Bazin system, which had been used successfully at Lowestoft. The boat was 60 ft. long by 20 ft. wide, and the pump was 2 ft. in diameter, with a two-bladed disc. The discharge pipe was 12 in. in diameter. The pump raised 400 tons of sand, gravel, and stones per hour as a maximum quantity, the average quantity being about 200 tons per hour. The depth dredged was from 7 ft. to 25 ft. The pump was driven by a double-cylinder engine, having cylinders of 9 in. diameter by 10 in. stroke, and making 120 revolutions per minute. An important addition was introduced by Mr Ball in fitting the working faces of the pump with indiarubber, which was very successful and largely reduced the wear and tear. The cost of the dredging at Lowestoft was given by Mr Langley at 2d. per ton, including delivery two miles out at sea. The quantity dredged was about 200,000 tons per annum. One of the earliest applications of pumps for dredging, made by Mr Woodford, consisted of a horizontal disc with two or more arms working in a case somewhat similar to the ordinary centrifugal pump. The disc was keyed to a vertical shaft which was driven from above by means of belts or other gear coupled to an ordinary portable engine. The pump within rested on the ground; the suction pipe was so arranged that water was drawn in with the sand or mud, the proportions being regulated to suit the quality of the material. The discharge pipe was rectangular and carried a vertical shaft, the whole apparatus being adjustable to suit different depths of water. This arrangement was very effective, and has been used on many works. Burt and Freeman’s sand pump, a modification of the Woodford pump, was used in the construction of the Amsterdam Ship Canal, for which it was designed. The excavations from the canal had to be deposited on the banks some distance away from the dredgers, and after being raised by the ordinary bucket dredger, instead of being discharged into barges, they were led into a vertical chamber on the top side of the pump, suitable arrangements being made for regulating the delivery. The pump was 3J ft. in diameter, and made about 230 revolutions per minute. It drew up the water on the bottom side which mixed with the descending mud on the top side, and the two were discharged into a pipe 15 in. in diameter. The discharge pipe was a special feature of this work, and consisted of a series of wooden pipes jointed together with leathern hinges and floated on buoys from the dredger to the bank. In some cases this pipe was 300 yds. long, and discharged the material 8 ft. above the water-level. Each dredger and pump was capable of discharging an average of 1500 cubic yds. per day of 12 hours. Schmidt’s sand pump is claimed to be an improvement on the Burt and Freeman pump. It consists of a revolving wheel 6 ft. in diameter, with cutters revolving under a hood which just allows the water to pass underneath. To the top side of the hood a 20in. suction pipe of an ordinary centrifugal pump is attached. The pump is driven by two 16-in. by 20-in. cylinders^ at 134 revolutions per minute, and the cutter by two 12-in. by 12-in. cylinders, working at 120 revolutions per minute, the boiler pres-