A Treatise On The Principles And Practice Of Dock Engineering

SLIPWAY AT DOVER. 493 The amount of pressure obtained is 28 cwts. per circular inch, equal to about 4,000 Ibs. per square inch. From these pumps the water is discharged through wrought-iron pipes, j inch internal diameter and 1 inch external, and above 10,000 feet in length. “ The following is the manner of docking a vessel : —The girders witli the pontoon upon them are allowed to sink to the depth required for the particular vessel to be docked. She is then hauled over the pontoon and on to the blocks and shored, or rather wedged up by movable bilge blocks instead of breast shores. The pontoon and vessel are lifted out of the water and the water in the pontoon allowed to escape by valves. When empty the valves are closed, the girders lowered, and the pontoon left to bear the whole weight of the vessel and to be moved into any suitable position. To give greater accommodation Mr. Edwin Clark arranged a system of shallow docks, eight in number, communicating with a shallow basin of about 500 feet square, into one of which the pontoon has to be floated. The space occupied by the docks and basin is about 25 acres. Many vessels have been already lifted and repaired in this manner, the largest of which, the ‘ Calcutta,’ is of 1,800 tons burthen.” Slipway at Dover.* Originally constructed in 1849, the Dover slipway underwent an enlarge- ment in 1888, being lengthened to 556 feet, with a capacity for vessels up to 850 tons deadweight. The gradient is 1 in 18, and the width at quay level, 52 feet (see figs. 484 to 491). “ The upper part of the slipway is in made ground for a length of about 370 feet, and the remaining portion is upon the chalk. In the made ground, a layer of cement concrete, 3 feet 6 inches thick, with an additional foot under the centre pair of rails, is laid at the required inclination. Embedded in this are fir cross-sleepers, 12 inches by 6 inches, 32 feet long and about 11 feet pitch, carrying the longitudinals to which the cast-iron rails are trenailed. In the lower portion of the slip, the cross-sleepers are laid directly upon the chalk, with only enough concrete to bed them evenly, and are held down by six bars, about 2 feet long, driven into the rock. The upper part of the slip, for a length of about 260 feet above low water mark, is paved with Kentish rag, flush with the tops of the longitudinals, and the lower portions with bricks on end.” There are three pairs of cast-iron rails, in 10-foot lengths, and weighing 69 Ibs. per lineal yard, the centre pair and rack plate being in one. The single rails are bossed out at the ends and centre, and secured to the longitudinals by six trenails, tbe double rail being secured by trenails on each side of the rack plate. The cradle is in four sections—two of them, forming the main cradle, are together 133 feet long, the remaining two being auxiliary pieces, 25 feet * Beer on “ Ship Slipways,” Min. Proc. Inst. C.E., vol. cxviii.