A Treatise On The Principles And Practice Of Dock Engineering

LADDER DREDGERS. 97 to the face of a dock or quay wall than is feasible in the case of a central ladder. But, under these circumstances, the discharge of dredged material has to take place across the whole width of the vessel (unless it be a hopper dredger, which is unlikely, from its unsuitable form for navigation), and either the cross shoot will be too flat to be thoroughly effective, or else the lift of the buckets is excessively high for ordinary purposes. It will generally be found necessary to employ an auxiliary pump to flush the shoot. A central ladder dredger can discharge indifferently to either side, but again, if any mishap occur to a link or bucket, the whole dredger is placed out of action, whereas in a double ladder dredger one ladder may be quite disabled without interfering with the work of the other. In cases where very powerful machines are required, double dredgers have the recommendation of providing greater lifting capacity with buckets of a less unwieldy size. The bucket dredger is eminently suitable for steady continuous work in hard material. It is the only form of dredger which will excavate rock, and it has proved capable of raising boulders much larger than its own buckets. In stiff clay it is much superior to dredgers of any other type. Altogether, it is an excellent machine, but it cannot be worked in a swell nor in very shallow water. It is not an economical machine in the matter of power. Owing to the necessity of discharging through a shoot, in cases where an attendant hopper is employed to receive the dredged material, lifting has to be performed by the machinery to the extent of 25 or 30 feet (the writer knows of a case of 35 feet) above the water line, representing a corre- sponding waste of energy. The dilficulty of dealing with shoals and banks has been solved by a special form of dredger, devised by Messrs. Wm. Simons & Co., of Renfrew, called the traversing bucket dredger. The ladder is supported upon a horizontal longitudinal framing, by means of which it can be projected in advance of the dredger, and thus enabled to cut the flotation of the latter through shallow places. By the same arrangement the ladder can be entirely removed from the water, and less obstruction is, in conséquence, offered to its passage, when acting as a carrier hopper or otherwise. Central ladder dredgers are themselves susceptible of subdivision into two classes, according as the well is situated at the bow or the stern of the vessel. The former is the more general type for simple dredgers, but a stern well hopper dredger derives the advantage of increased speed from a normal stem, with improved manæuvring qualities and a better shaped hull for encountering heavy seas. The following are points of practical importance in connection with the utility of bucket dredgers. Buckets.—No object is gained by bringing the lip of the bucket too far forward. The limit of filling will generally be the horizontal line through the inner edge when in the inclined position; hence the bucket is equaliy 7