Page:The American Cyclopædia (1879) Volume VI.djvu/179

DOCK 171 which the vessel is lifted by pumping the water out of the side chambers and bottom tank ; and as the dock rises, the water around the ship in the middle chamber ebbs out, so that FIG. 6. Transverse Section of Eennie's Floating Dock. the quantity of water to be exhausted in rais- ing a vessel is in proportion to her weight and not to her bulk. Fig. 6 represents a cross sec- tion of an iron floating dock, built mainly upon the same principle as the New York floating dock, differing however in having air chambers in the upper part of the walls, by means of which the depth of sinking is controlled. It was constructed by Mr. G. B. Rennie for the port of Ferrol on the N". W. coast of Spain. It is 350 ft. long, 105 ft. wide, and 50 ft. high in extreme dimensions. The depth of the pon- toon is 12i ft., leaving a height of 37| ft. from the deck of the pontoon to the deck of the side walls, so that if the keel blocks occupy 5 ft. and the deck remains 4 ft. above water, there will be a clear depth of 28 ft. of water for the admission of ships. The total weight of the dock is about 5,000 tons, and the displacement by the pontoon is equal to 13,000 tons, leav- ing a lifting power of 8,000 tons. It is con- structed of plate, angle, and T iron, riveted to- gether in one structure. The pontoon is di- vided by a water-tight bulkhead running the whole length, each half being subdivided by 10 transverse bulkheads. The upper part of the side walls is composed of air-tight cham- bers having a capacity sufficient to prevent the dock from sinking below any desired level. The pontoon is strengthened by open lattice girders equal to it in depth, 34 of these being transverse, on either side. It is covered with three-inch teak plank, laid longitudinally, and upon this over every third transverse girder are transverse beams of teak 2 ft. square, for the support of the keel and bilge blocks. Keel blocks are also placed on the intermediate girders. Four large sluices, two on either side, communicate by pipes 18 in. in diameter with the several compartments, the pipes having cocks to regulate the quantity of water in each compartment. There are four pumps on either side, of 2 ft. 9 in. stroke and 26 in. diameter, worked by steam. Four capstans are placed at each end for moving and mooring. The New York sectional floating dock was patented by Phineas Burgess in 1838, and the first dock was begun by Burgess and Dodge in 1839. It consists of a number of floating pontoons, which form the dock by being placed side by side. At the end of each pontoon there is a frame- work of timber which supports machinery for pumping, being sufficiently high to remain out of water when the dock is submerged. This framework projects beyond the end of the pontoon, and its lower part contains what is called a balance tank, an air-tight chamber, which may be raised and lowered by means of a rack and pinion connected with the pumping machinery. These tanks have much the same use as the air chambers in the iron floating dock just described ; by raising them the depth of the deck of the pontoons will be increased, and by lowering them the pontoons will be raised, independently of the amount of air or water the latter may contain. Fig. V represents the largest of these docks owned by the com- pany. It consists of 10 sections, which when placed close together form a dock 350 ft. long and 110 ft. broad. These sections may be placed at some distance apart so as to increase the length of the dock when required, or if not FIG. 7. New York Sectional Dock. of sufficient buoyancy other sections may be added. The advantage claimed for this dock is, that the buoyancy of each section may be so regulated as to bear with equal pressure against all parts of the ship's bottom, so that if she has been warped there will be no strain. On the other hand, it is stated that irregularities of pressure caused by swells from steamboats or otherwise more or less reduce this advantage. Advantage it certainly has in the facility with which it may be repaired, by taking up any sec- tion and placing it upon the dock formed by the remainder. The arrangements for docking at the navy yard of Philadelphia consist of a float- ing sectional dry dock, and a permanent stone basin with two marine railways. The sectional dock consists of nine sections, which may be increased in number, each section being 105 ft. long and 32 ft. wide, and the pontoons 11 ft. deep. There are three pumps at each end of each section, moved by two engines of 20 and two of 12 horse power. The bottom of the basin is 350 ft. long and 226 ft. wide, and will hold 10 ft. 9 in. of water at mean high tide. The dry dock at San Francisco is a sectional dock similar to the one at Philadelphia, but is