the head waves are not more than 3 ft. high and the cross waves not more than 112 ft. high. The dredgings are taken 2 m. to sea, and the price for dredging and depositing from 800,000 to 900,000 cub. metres in 5 or 6 years was 7·25d. per cub. yd. The contractor offered to do the work at 4·625d. per cub. yd. on condition of being allowed to work either at Calais or Boulogne, as the weather might permit. Sand-pump dredging has also been extensively carried out at the mouth of the ports of Amsterdam, Rotterdam and on the north coast of France by sand dredgers constructed by Messrs L. Smit & Son and G. & K. Smit. The largest dredger, the “Amsterdam,” is 141 ft. by 27 ft. by 10 ft. 8 in., and has engines of 190 i.h.p. The hopper capacity is 10,600 cub. ft., and the vessel can carry 600 tons of dredgings. The pump fan is 6 ft. 3 in. in diameter by 10 in. wide, the plates being of wrought iron, and makes 130 revolutions a minute. The pump can raise 230 cub. ft. a minute from a depth of 33 ft., which, taking the proportion of 1 of sand to 7 of water, gives a delivery of 29 cub. ft. of sand per minute. The hopper containing 10,600 cub. ft. was under favourable circumstances filled in 40 minutes. The vessels are excellent sea boats.
Combined Bucket-Ladder and Sand-Pump Dredgers.—Bucket ladders and sand pumps have also been fitted to the same dredger. A successful example of this practice is furnished by the hopper dredger “Percy Sanderson” (Plate I. fig. 8), constructed under the direction of Sir C. A. Hartley, engineer of the Danube Commission for the deepening of the river Danube and the Sulina bar. This dredger is 220 ft. by 40 ft. by 17 ft. 2 in., and has a hopper capacity for 1250 tons of dredgings. The buckets have each a capacity of 25 cub. ft., and are able to raise 1000 tons of ordinary material per hour. The suction pump, which is driven by an independent set of triple expansion engines, is capable of raising 700 tons of sand per hour, and of dredging to a depth of 35 ft. below the water-line. The lower end of the suction pipe is controlled by special steam appliances by which the pipe can be brought entirely inboard. The “Percy Sanderson” raises and deposits on an average 5000 tons of material per day.
Grab Dredgers.—The grab dredger was stated by Sir Benjamin Baker (Proc. Inst. C.E. vol. 113, p. 38) to have been invented by Gouffé in 1703, and was worked by two ropes and a bar. Various kinds of apparatus have been designed in the shape of grabs or buckets for dredging purposes. These are usually worked by a steam crane, which lets the open grab down to the surface of the ground to be excavated and then closes it by a chain which forces the tines into the ground; the grab is then raised by the crane, which deposits the contents either into the hopper of the vessel upon which the crane is fixed or into another barge.
The Priestman grab has perhaps been more extensively used than any other apparatus of this sort. It is very useful for excavating mud, gravel and soft sand, but is less effective with hard sand or stiff clay—a general defect in this class of dredger. It is also capable of lifting large loose pieces of rock weighing from 1 to 2 tons. A dredger of this type, with grab holding 1 ton of mud, dredged during six days, in 19 ft. of water, an average of 5212 tons and a maximum of 6812 tons per hour, and during 12 days, in 16 ft. of water, an average of 48 tons and a maximum of 58 tons per hour, at a cost of 1·63d. per ton, excluding interest on the capital and depreciation. The largest dredger to which this apparatus has been applied is the grab bucket hopper dredger “Miles K. Burton” (Plate I. fig. 9), belonging to the Mersey Docks and Harbour Board. It is equipped with 5 grabs on Morgan’s patent system, which is a modification of Priestman’s, the grabs being worked by 5 hydraulic cranes. It raised and deposited, 12 to 15 m. at sea, 11 loads of about 1450 tons each with a double shift of hands, at a cost of about 1s. 5d. per cub. yd. of spoil, including the working expenses for wages of crew, fuel and stores. Mr R. A. Marillier of Hull has stated that “the efficiency of these grabs is not at all dependent upon the force of the blow in falling for the penetration and grip in the material, as they do their work very satisfactorily even when lowered quite gently on to the material to be cut out, the jaws being so framed as to draw down and penetrate the material as soon as the upward strain is put on the lifting chain. Even in hard material the jaws penetrate so thoroughly as to cause the bucket to be well filled. The grab is found to work successfully in excavating hard clay from its natural bed on dry land.” It is claimed on behalf of grabs that they lift a smaller proportion of water than any other class of dredger.
Since the beginning of the 20th century considerable advance has been made in the use of Priestman grabs, not only for dredging and excavating (for which work they were originally designed), but also in discharging bulk cargo. The first quadruple dredger used by the Liverpool Docks Board had grabs of a capacity of 30 cub. ft., but subsequently second and third quadruple dredgers were put to work in the Liverpool Docks, with grabs having a capacity of 70 and 100 cub. ft. respectively. In discharging coal at Southampton, Havre, Erith, as well as at the coaling station at Purfleet on the Thames, grabs having a capacity of about 80 cub. ft. are in constant use. Perhaps the most difficult kind of bulk cargo to lift is “Narvick” iron ore, which sets into a semi-solid body in the holds of the vessels, and for this purpose one of the largest grabs, having about 150 cub. ft. capacity and weighing about 8 tons, has been adopted. This grab was designed as a result of experiments extending over a long period in lifting iron ore. It is fitted with long, forged, interlocked steel teeth for penetrating the compact material, which is very costly to remove by hand labour. The Priestman grab is made to work with either one or two chains or wire ropes. Grabs worked with two chains or ropes have many advantages, and are therefore adopted for large undertakings.
Wild’s single chain half-tine grab works entirely with a single chain, and has been found very useful in excavating the cylinders in Castries harbour. Upon experimenting with an ordinary grab a rather curious condition of things was observed with respect to sinking. On penetrating the soil to a certain depth the ground was found as it were nested, and nothing would induce the grab to sink lower. Sir W. Matthews suggested that a further set of external tines might possibly get over this difficulty. A new grab having been made with this modification, and also with a large increase of weight—all the parts being of steel—it descended to any required depth with ease, the outside tines loosening the ground effectually whilst the inside bucket or tines picked up the material.
Miscellaneous Appliances.—There are several machines or appliances which perhaps can hardly be called dredgers, although they are used for cleansing and deepening rivers and harbours.
Kingfoot’s dredger, used for cleansing the river Stour, consisted of a boat with a broad rake fitted to the bow, capable of adjustment to different depths. At the sides of the boat were hinged two wings of the same depth as the rake and in a line with it. When the rake was dropped to the bottom of the river and the wings extended to the side, they formed a sort of temporary dam, and the water began to rise gradually. As soon as a sufficient head was raised, varying from 6 to 12 in., the whole machine was driven forward by the pressure, and the rake carried the mud with it. Progress at the rate of about 3 m. an hour was made in this manner, and to prevent the accumulation of the dredgings, operations were begun at the mouth of the river and carried on backwards. The apparatus was very effective and the river was cleansed thoroughly, but the distance travelled by the dredger must have been great.
In 1876 J. J. Rietschoten designed a “propeller dredger” for removing the shoals of the river Maas. It consisted of an old gunboat fitted with a pair of trussed beams, one at each side, each of which carried a steel shaft and was capable of being lowered or raised by means of a crab. An ordinary propeller 3 ft. 6 in. in diameter was fixed to the lower end of the shaft, and driven by bevel gear from a cross shaft which derived its motion by belting from the fly-wheel of a 12 h.p. portable engine. The propellers were lowered until they nearly reached the shoals, and were then worked at 150 revolutions per minute. This operation scoured away the shoal effectively, for in about 40 minutes it had been lowered about 3 ft. for a space of 150 yds. long by 8 yds. wide.
A. Lavalley in 1877 designed an arrangement for the harbour of Dunkirk to overcome the difficulty of working an ordinary bucket-ladder dredger when there is even a small swell. A pump injects water into the sand down a pipe terminating in three nozzles to stir up the sand, and another centrifugal pump draws up the mixed sand and water and discharges it into a hopper, the pumps and all machinery being on board the hopper. To allow for the rising and falling of the vessel—either by the action of the tide or by the swell—the ends of the pipes are made flexible. The hopper has a capacity of 190 cub. yds., and is propelled and the pumps worked by an engine of 150 i.h.p. From 50 to 80 cub. yds. per hour can be raised by this dredger.
The “Aquamotrice,” designed by Popie, and used on the Garonne at Agen, appears to be a modification of the old bag and spoon arrangement. A flat-bottomed boat 5112 ft. long by 612 ft. wide was fitted at the bow with paddles, which were actuated by the tide. Connected with the paddles was a long chain, passing over a pulley on uprights and under a roller, and a beam was attached to the chain 14 ft. 8 in. long, passing through a hole in the deck. At the end of the beam was an iron scoop 2 ft. wide and 2 ft. 6 in. deep. When the tide was strong enough it drew the scoop along by means of the paddles and chains, and the scoop when filled was opened by a lever and discharged. About 65 cub. yds. of gravel could be raised by the apparatus in 12 hours. When the tide failed the apparatus was worked by men.
The Danube Steam Navigation Co. removed the shingle in the shallow parts of the river by means of a triangular rake with wrought-iron sides 18 ft. long, and fitted with 34 teeth of chilled cast iron 12 in. deep. This rake was hung from the bow of a steamer 180 ft. long by 21 ft. beam, and dragged across the shallows, increasing the depth of water in one instance from 5 ft. 6 in. to 9 ft., after passing over the bank 355 times.
A combination of a harrow and high pressure water jets, arranged by B. Tydeman, was found very efficacious in removing a large