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DREDGING 531' sure being 95 ft per square in. This apparatus is capable of to the fore and aft movements of the pipes. When the vessel excavating sticky blue clayey mud, and will deliver the material arrived at East London on the 18th July 1897 there was a depth at 500 to 650 yds. distance. The best results are obtained when of 14 ft. on the bar at high water. On 10th October, scarcely the mixture of mud and water is as 1 to 6 '5. The average quantity three months afterwards, there was a depth of 20 ft. on the bar at excavated per diem by the apparatus is 1300 cubic yds., the low water. Working 22 days in rough weather during the month maximum quantity being 2500 cubic yds. of November 1898, the Kate raised and deposited 2£ miles at sea Kennard’s sand pump is entirely different from the pumps 60,000 tons of dredgings. Her best day’s work (12 hours) was on already described, and is a direct application of the ordinary lift the 7th November, when she dredged and deposited 6440 tons. pump. A wrought-iron box has a suction pipe fitted at the A large quantity of sand-pump dredging has been carried out at bottom rising about half-way up the inside of the box ; on the top Boulogne and Calais by steam hopper pump dredgers, workable of the box is fitted the actual pump and the flat valves. The when the head waves are not more than 3 ft. high and the cross apparatus was lowered by chains, and the pump worked from waves not more than 1£ ft. high. The dredgings are taken 2 above. As soon as the box was filled with sand it was raised, the miles to sea, and the price for dredging and depositing from catches holding up the bottom were released, and the contents 800,000 to 900,000 cubic metres in 5 or 6 years was 7‘25d. per discharged into a punt. cubic yard. The contractor offered to do the work at 4'625d. per Sand-pump dredgers, designed and arranged by Mr Darnton cubic yard on condition of being allowed to work either at Calais Hatton, were extensively used on the Amsterdam Ship Canal. A or Boulogne, as the weather might permit. Sand-pump dredging centrifugal pump with a fan 4 ft. in diameter was employed, the has also been extensively carried out at the mouth of the ports of suction and delivery pipes, each 18 in. in diameter, being at- Amsterdam, Rotterdam, and on the north coast of France by sand tached to an open wrought-iron framework. The machine was dredgers constructed by Messrs L. Smit & Son and G. & K. Smit. suspended between guides fixed to the end of the vessel, which The largest dredger, the Amsterdam, is 141 ft. by 27 ft. by 10 ft. was fitted with tackle for raising, lowering, and adjusting the 8 in., and. has engines of 190 I.H.P. The hopper capacity is machine. The vessel was fitted with a steam-engine and boiler 10,600 cubic ft., and the vessel can carry 600 tons of dredgings. for working and manipulating the pumps and the heavy side The pump fan is 6 ft. 3 in. in diameter by 10 in. wide, the plates chains for the guidance of the dredger. The engine was 70 horse- being of wrought-iron, and makes 130 revolutions a minute. The power, and the total cost of one dredger was £8000. The number pump can raise 230 cubic ft. a minute from a depth of 33 ft., of hands required for working this sand-pump dredger was one which, taking the proportion of 1 of sand to 7 of water, gives a captain, one engineer, one stoker, and four sailors. Each machine delivery of 29 cubic ft. of sand per minute. The hopper, conwas capable of raising about 1300 tons of material per day, the taining 10,600 cubic ft., was under favourable circumstances engines working at 60 and the pump at 180 revolutions per minute. filled in 40 minutes. The vessels are excellent sea boats. The sand was delivered into barges alongside the dredger. The Combined Bucket-Ladder and Sand -Pump Dredgers.— cost of raising the material and depositing it in barges was about Id. per ton when the sand pumps were working, but upon the year’s Recently bucket ladders and sand pumps have both been work the cost was 2‘4d. per cubic yd. for working expenses and fitted to the same dredger. A successful example of this repairs, and l-24d. per cubic yd. for interest and depreciation at practice is furnished by the hopper dredger Percy 10 per cent, upon the cost of the plant, making a total cost' for dredging of 3 -64d. per cubic yd. The cost for transport was 3 -588d. Sanderson (see Fig. 5), constructed under the direction per cubic yd., making a total cost for dredging and transport of °f. Sir Charles L. Hartley, engineer of the Danube Com7-234d. per cubic yd. Dredging and transport on the same works mission for the deepening of the river Danube and the by an ordinary bucket dredger and barges cost 8'328d. per cubic yd. Sulina bar. This dredger is 220 ft. by 40 ft. by 17 ft. Two of the largest and most successful instances of sand-pump dredgers are the Branker and the G. B. Crow, belonging to the 2 in., and has a hopper capacity for 1250 tons of dredgings. Mersey Dock and Harbour Board. Mr A. G. Lyster gave par- The buckets have each a capacity of 25 cubic ft., and are ticulars of the work done by these dredgers in a paper read before able to raise 1000 tons of ordinary material per hour. the Engineering Congress in 1899. They are each 320 ft. long, The suction pump, which is driven by an independent set 47 ft. wide, and 20-5 ft. deep, the draught loaded being 16 ft. They are fitted with 2 centrifugal pumps, each 6 ft. in diameter, of triple-expansion engines, is capable of raising 700 tons of with 36 in. suction and delivery pipes, united into a 45-in. sand per hour, and of dredging to a depth of 35 ft. below diameter pipe, hung by a ball and socket joint in a trunnion, so the water-line. The lower end of the suction pipe is conas to work safely in a sea-way when the waves are 10 ft. high. The suction pipe is 76 ft. long, and will dredge in 53 ft. of water. trolled by special steam appliances by which the pipe can The 8 hoppers hold 3000 tons, equivalent when solid to 2000 be brought entirely inboard. The Percy Sanderson raises cubic yds. ; they can be filled in three-quarters of an hour and and deposits on an average 5000 tons of material per day. discharged in five minutes. Mr Lyster stated that up to May Hopper Barges.—Where dredging operations on a large 1899 the quantity removed from bar and main-channel shoals scale are being carried out, steam hopper barges are amounted to 41,240,360 tons, giving a width of channel of 1500 It. through the bar, with a minimum depth of 27 ft. The cost generally employed. Good examples of these vessels are of dredging on the bar by the G. B. Crow during 1898, when the two steam hopper barges built for the Conservators 4,309,350 tons of material were removed, was 0‘61d. per ton for of the river Thames in 1898. Their dimensions are: wages, supplies, and repairs. These figures include all direct length 190 ft., breadth 30 ft., depth 15 ft. 3 in., hopper working costs and a proportion of the charge for actual superintendence, but no allowance for interest on capital cost or for depreci- capacity 900 tons. They are propelled by a set of tripleation. On an average, 20 per cent, of the sand and mud that are expansion engines of 1200 I.H.P., with two return tubular raised escapes over the side of the vessel. Mr Lyster has, however, boilers having a working pressure of 160 lb. Special to a considerable extent overcome this difficulty by a special arrange- appliances are provided to work the hopper doors by steam ment added to the hoppers. (See Proc. Inst. C. E. vol. cxxxviii.) power from independent engines placed at the forward end Another powerful and successful sand-pump dredger is the twimscrew dredger Kate (see Fig. 4), built in 1897 by Messrs of the hopper. A steam windlass is provided forward and William Simons & Co. Limited for the East London Harbour a steam capstan aft. The vessels are fitted with cabins for Board, South Africa. Its dimensions are: length 200 ft., the officers and crew. On their trial trip, the hoppers breadth 39 ft., depth 14 ft. 6 in., hopper capacity 1000 tons. having their full load, a speed of 11 knots was attained, the * The pumping arrangements for filling the hopper with sand or discharging overboard consist of two centrifugal pumps, each coal consumption being D44 lb per I.H.P. driven from one of the propelling engines. The suction pipes are Grabs. Various kinds of apparatus have been designed in each 27 in. in diameter, and are so arranged that they may be the shape of grabs or buckets for dredging purposes. These used for pumping either forward or aft, as the state of the are usually worked by a steam crane, which lets the open weather may require. Four steam cranes are provided for manipulating the suction pipes. Owing to the exceptional weather with grab down to the surface of the ground to be excavated which the vessel has to contend, special precautions were taken in and then closes it by a chain which forces the tines designing the attachments of the suction pipes to the vessel. The into the ground; the grab is then raised by the crane, attachment is above deck, and consists of a series of joints, which give a perfectly free and universal movement to the upper ends of which deposits the contents either into the hopper of the the pipes. The joints, on each side of the vessel, are attached to vessel upon which the crane is fixed or into another barge. a carriage, which is traversed laterally by hydraulic gear. By Priestman grab has perhaps been more extensively used than this means the pipes are pushed out well clear of the vessel’s sides anyThe apparatus of this sort. It is very useful for excavating when pumping, and brought inboard when not at work. Hydraulic mud,other gravel, and soft sand, but is less effective with hard sand or cushioning cylinders are provided to give any required resistance stiff clay, which is a general defect in this class of dredger. It is