A Treatise On The Principles And Practice Of Dock Engineering

98 DOCK ENGINEERING. effective with a short face as with a long one, and the former outline is better adapted for discharging. The mouthpieces, or lips, should be of hard steel rivetted to the face of the buckets which, together with the links and pins, are also of steel of special quality. A hole or two in the front is useful for the escape of water. Large buckets free themselves better than small buckets from adhesive material. Shoots. — The least inclination for the unassisted discharge of mis- cellaneous material is somewhere about 1 in 4j but this is not always obtainable. With the assistance of continuous and ample flushing, together with some manual appliance, such as a pricker, the limit may be raised to 1 in 10 for mud, 1 in 15 for clay, and 1 in 20 for sand. Tumblers. —The top tumbler actuates the rotary motion of the buckets and should be as small as possible, in order to reduce the amount of Inter- mediate gearing. The ideal form would be the circular, but with straight links and flat backed buckets, a square or pentagonal section must be adopted. The latter is préférable, as it brings all faces of the tumbler equally in contact with the buckets. To achieve this condition with a square tumbler, an additional, or “ hunting,” link would have to be inserted at some point in the chain. The bottorn tumbler does not transmit power and should be made of large diameter to diminish friction, say, with six or more sides. It is suspended from a cross beam on the dredger, and has to be readily adjustable to the depth of water in which the dredger may be working. For the guidance of the buckets, the lower tumbler should be provided with large flanges. Power.—Mr. J. J. Webster,* from observation of a large nurnber of indicator diagrams, submits the following empirical formulæ for determining the indicated horse-power required to dredge different qualities of material under varying conditions of lift. If II be the height of the upper tumbler shaft from the surface of the ground to be dredged, and W the number of tons per hour to be dredged, then the indicated horse-power required is approximately —• 04 W ^/II for very stiff clay or mud. •034 W ^/H for hard clay and indurated mud. ■026 W ^/H for soft mud and light sand. The illustrations (figs. 54-61) are of the dredger “ Cairndhu” and one of her attendant hopper barges, belonging to the Clyde Navigation. The Dipper Dredger, which is almost exclusively an American type, being much used in connection with the improvement and maintenance of river beds and channels in the United States, is so identical in principle and mode of action with the steam navvy (p. 81 ante), or land excavator, already described, that there is no necessity to make more than a very brief and passing reference to it. * Webster on “Dredging Operations and Appliances,” Min. Proc. Inst. C.E., vol. Ixxxix.