A Treatise On The Principles And Practice Of Dock Engineering

DREDGERS AND DREDGING PLANT. 89 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 any- thing 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 accom- panied 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. It * Lyster on “ Sand Pump Dredgers,” Min. Proc. Inst. C.E., vol. cxxxviii.