A Treatise On The Principles And Practice Of Dock Engineering

544 DOCK ENGINEERING. A double set of cylinders and rams is the system adopted by Messrs. Hayward, Tyler & Co., and the apparatus is so arranged that one set is in forward motion while the second is returning. By attaching the links alter- nately to each set, the cradle is maintained in almost continuous motion. The hauling gear of Messrs. Day and Summers consists of a wire rope, 12 inches in circumference, used either in single tension or with multiply- ing sheaves, coiled upon a drum, some 9 or 10 feet in diameter, whicli is actuated by steam or other convenient power. The smoothness and regularity of the hydraulic ram commend it for the purpose of slipway haulage, particularly in dealing with vessels of large size. Steel wire rope, on the other hand, is light and flexible. Its durability has been contested, but appears to be satisfactory. Pumping Machinery.—Permanent pumping power, as distinguished from that of a temporary nature, dealt with in a previous chapter, is required in connection with docks for two important objects :—(1) Por emptying graving docks, and (2) for artificially raising the level of the water in wet docks. This latter expedient is adopted in cases where, greater draught being required for vessels, the deepening of a dock is deemed inadvisable on constructive or economical grounds. The use of pumping plant in connection with hydraulic accumulators .is, of course, obviously necessary where such power is adopted. The type of pump most commonly employed for the first named objects is that known as the centrifugal, in which the rapid rotation of a series of blades or fans causes the water within the pump chamber to be whirled round and propelled in an upward direction. Valvular pumps are unsuit- able for dealing with dock water, on account of the great quantity of refuse matter to be found in it ; corks, straw, chips, and ship scrapings are a few examples only of the multitudinous small objects which suflice to obstruct the action of valves. Centrifugal pumps themselves have to be protected by entrance gratings from the risk of entanglement with ropes and canvas, to say nothing of more serious damage by log-ends, pieces of planking, and wedges. It is no uncommon experience for a pump to have its intake pipe choked by eels and small fish, and the writer knows of one instance in which the pump blades were smashed by a piece of timber which had mysteriously intruded itself into the well. The following incident, narrated by Mr. John Hayes, is likewise instructive : — Two large centrifugal pumps and engines, at Demerara, had been fitted up and set to work in connection with drainage operations on a somewhat extensive scale. One day, after they had been some considerable time in operation, the Resident Engineer observed that the engine and pump suddenly pulled up and then went on again immediately afterwards. For a long time the cause was undiscovered, but eventually the remains of an alligator, 14 feet long, were found in the outlet of the pump. The reptile had passed through the pump, and had been cut into three pieces, which the Resident Engineer caused to be stuffed, as a specimen of what centrifugal