and the time taken up in sea trips may be usefully employed in overhauling the buckets and pins and in effecting any necessary repairs. A possible demur to this last contention on the ground that both machinery and crew would be too fully occupied with purely navigatory functions to admit of such extraneous duties, may be met by the explanation that repairs would be limited in each voyage to those buckets which were actually accessible, and that the presence of one or two additional hands in order to attend to them would be fully compensated for by the saving in time. In undertakings of considerable magnitude, where time and interest on capital are factors of the highest importance, it will, on the whole, be found expedient to adopt the separate system with a large fleet of hopper barges in constant attendance upon the dredgers; for, though the outlay may be greater, the increased rapidity of execution will fully compensate for it. Apart from the foregoing classification, dredgers are capable of inclusion in a great variety of divisions, according to the very varied manner in which they individually discharge their functions. Indeed, the subject is one of such wide scope and importance as to claim a special treatise, if anything of the nature of an adequate dissertation were to be attempted. In the limited space at our disposal we can only afford to deal in a general way with the relative merits of the more important types, and to give a brief description of their salient features. For this purpose we will adopt the following succinct classification : : Suction dredgers. Ladder dredgers. Dipper dredgers. Grab dredgers. Suction dredgers, hydraulic dredgers, or sand pump dredgers, as they are very commonly called, consist essentially of a continuous pipe or tube through which, by means of suitable machinery, sand or other light material is sucked up from the bottom (see fig. 50). The sand is naturally accompanied by a very large volume of water which is delivered with it into the hopper, and this fact, combined with the disposition of the water to escape over the sides of the hopper with the sand still in suspension, causes a great deal of unremunerative pumping, the loss in sand amounting to as much as 20 per cent. of the quantity actually raised. Considerable diminution of this waste has been effected by a device introduced by Mr. A. G. Lyster, the engineer to the Mersey Docks and Harbour Board* (fig. 51). The hopper is entirely covered over with the exception of a narrow central portion, 4 feet wide, provided with adjustable coamings, raised to a height of 5 feet. The sand is delivered near the sides of the hopper, and having a considerable distance to travel before it can reach the top of the central opening, the greater portion settles en route and the effluent is comparatively clear. * Lyster on "Sand Pump Dredgers," Min. Proc. Inst. C.E., vol. cxxxviii. should not be overlooked, however, that this arrangement, whilst extremely effective for its particular purpose, somewhat reduces the useful capacity of the hopper for solid material, by adding to the gross load carried. The suction pump dredger would also be applicable to silt and mud, were it not that the lower specific gravity of such material renders it practically impossible to secure its deposition within the limits of the receiving hopper. Silt will take nearly as many hours to settle as sand takes minutes. It is sometimes, however, an advantage to bring a suction pump to bear on mud in situations otherwise inaccessible, such as gate platforms and recesses. The mud thus disturbed settles in more open positions, where it can conveniently be removed by other appliances. The discharge of the muddy effluent of a suction pump into a tidal or other current is a simple but efficacious means of maintaining a waterway, provided that the deposit be light and the current sufficiently powerful to retain it in suspension until it reaches a place where its settlement will do no harm. Adjustable Coaming: Discharge Adjustable Coaming Discharge Fig. 51.-Section of Hopper fitted with Adjustable Coamings. Suction pumps possess very great advantages in exposed situations, where the incessant motion of the waves materially interferes with the working of other forms of dredging apparatus. Equipped with telescopic pipes and flexible joints, they can adjust themselves to the rise and fall of the vessel and be quite independent of variations of level, either momentary or prolonged. The manifest convenience and safety attaching to dredgers of this class has led to repeated attempts to adapt them to the removal of material other than sand. With this object in view the lower end of the suction pipes has been fitted with a number of cutting blades, the revolution of which, by suitable gearing, is intended to disintegrate clay, marl, and other compact material to such a degree as will admit of their being drawn up the suction pipe. This is the basis of the Bates, the von Schmidt, and other systems of dredger. The cutters, generally speaking, are cylindrical, hollow, straight, or spiral blade milling cutters, mounted around and concentrically with the end of the suction pipe. They consist of a number of knives (from 10 to 15) united by suitable discs, or rings, at one or both ends. The whole cutter may be secured to the end of the suction pipe and rotary motion imparted to them together, or the cutter shaft may be journalled in a suitable bearing provided in the end of the suction pipe, which is then made stationary. The use of cutters is only practicable in fairly smooth water; in situations where there is much swell, other means must be found for loosening and disintegrating the material to be removed. One alternative expedient is the application of numerous water jets through a series of orifices, specially provided for the purpose in the bars which traverse the mouth of the drag-piece, and communicating by means of suitable ports with a pipe running along the front of the mouthpiece. This system of nozzles is supplied with water under pressure through a flexible pipe. The result is much inferior to that attained by the action of cutters, and, in order to obtain the best effect, it is necessary to concentrate the pressure of the jets upon a small surface, and to direct the stream towards the intake pipe. The value of the cutter appliance in dealing with beds of hard sand has been abundantly demonstrated on the Mississippi, the Scheldt, and the Volga But after witnessing a number of trials of a similar type of dredger upon stiff clay, the writer is inclined to doubt the efficacy of the system in dealing with material of an argillaceous character, though he is prepared to admit that much may depend upon the precise form of cutter adopted. In this view he is confirmed by some remarks made by Mr. J. H. Apjohn at a recent engineering conference, which, indeed, are worth quoting as demonstrating the scope existing for experimental investigation.* "The author's experience of rotary cutters has been with a dredger designed for the purpose of excavating clay for dock extension. The clay being silty, it was thought it would be easily broken up by the cutter, but this was not the case. The cutter had fourteen straight knives, set at an angle of 26° to the tangent of the circle round which they were placed and overlapping each other to a slight extent. The dredger was first operated at a small depth where the soil was brittle and the cutter proved efficient, but when the clay was reached at a greater depth, the openings between the blades of the cutter clogged with the tenacious plastic clay, with the result that the proportion of clay found in the water discharged through the pipe-line was extremely small. The cutter was then unshipped, and a width of some inches was cut off the inner edge of each blade, so that the overlap was done away with, and at the same time the circular opening at the bottom of the cutter was reduced in area. When again tried the cutter worked better, there being but little clogging between its blades, * Apjohn on "Dredging with special reference to Rotary Cutters," Proc. Eng. Conf., London, 1903. |