A Treatise On The Principles And Practice Of Dock Engineering

94 DOCK ENGINEERING. but these did not eut the clay very weil. A new cutter was then built, with narrow spiral knives, and proved to be more efficient than the first ; but even with this cutter the quantity turned out per hour was never more than 60 per cent, of that contracted for. The clay, which it discharged behind the walls was in the form of nodules, varying in size between that of an egg and that of a Dutch cheese.” Notwithstanding some disappointing experiences, such as the foregoing, the clay-cutting gear has very strong partisans. Mr. A. W. Robinson* clainis for a dredger, the “J. Israel Tarte,” designed by himself, and working in blue clay in the channel of the river St. Lawrence below Montreal, “a world’s record for output, measured by the output, of any dredger under any conditions.”! And Mr. C. W. Darley, in his description of “ Dredging in New South Wales,”! speaks of them as valuable for cutting new channels through “tough or hard clay formations.” Any definite pronouncement on the value of the cutter dredger must therefore remain in abeyance, pendin« the completion of more extensive trials and the determination of the best form of cutting apparatus. The illustration (figs. 52 and 53) is one of a dredger on the Bates system ■constructed for the Russian Government. The cutters, of which there are four, are shown at the stern. The forward end is in connection with a discharge pipe. Ladder Dredgers, or bucket-ladder dredgers (figs. 54 to 58), consist, in principle, of an endless chain connecting a series of buckets which traverse in succession an inclined orbit, approximately elliptical, about two pivots or tumblers, excavating material at the lower tumbler and discharging it into a shoot while passing over the upper tumbler. Bucket dredgers of this type have either one or two ladders______“ladder” being the name applied to the frame, with its roller bearings, on which the buckets travel. In single-ladder dredgers the ladder coincides with the longitudinal axis of the vessel. The ladders of double dredgers are situated at each side of the vessel. A single-ladder dredger of the same capacity as a double dredger has the advantage of fewer nioving parts and, consequently, of less working friction. Ihe central position of the ladder also admits of a more convenient outline for the vessel, from the point of view of propulsion, and affords greater steadiness in a sea way. The broad beam of double-ladder dredgers renders it impossible for them to pass through narrow locks, though this difficulty has been overcome, in one case at least, bv construction - a one case at least, by constructing a dredger in detachable halves. On the other hand, a side-ladder dredger can work in greater proximity A. W. Robinson on “Modern Machinery for Excavating and Dredging” Ikngzneering Magazine, vol. xxv., No. 1, April, 1903. + This performance is stated to have consisted in the removal of 1,180,000 cubic yards of material during a period of two months, comprising 52 working davs’ î Eng. Conf., London, 1903. " '