A Treatise On The Principles And Practice Of Dock Engineering

KEEL-BLOCKS. 483 entirely disconnected from each other and the sides. Usually one pontoon is dealt with at a time, and the operation consists in raising it above the rest. The pontoon is separated from the sides by an open space of 2 feet, to which water has free access. Attachment is made by means of “ fish- plate” joints, consisting of steel lugs, secured together by steel taper pins. The drainage junction-pipes between the pontoon chambers, and the pumps in the side walls, are first of all disconnected. Then the taper pins are withdrawn. To complète this step, it is necessary to tilt the dock slightly, as in fig. 471, even trim being afterwards restored (fig. 472). One pontoon is now floating clear of the remainder of the structure. The dock is next sunk by the re-admission of water into its compartments until the relative positions are as shown in fig. 473. At this stage the floating pontoon is re-connected to the sides, at a higher level, by means of similar lugs and pins. The dock is raised bodily by pumping, the single pontoon leaves the water, and the operation is complète (fig. 474). Docks of L section have their component pontoons berthed upon one another, in the ordinary manner of docking a vessel. Equipment of Repairing Docks. The various items for the equipment of a repairing dock include keel- blocks, bilge-blocks, side-shores, lifting cranes, capstans, snatch blocks, bollards, hooks, &c. Fig. 475. —Keel-block, Belfast. 1. Keel-blocks.—These are for the purpose of affording a uniform and level base for a ship’s keel, and in order to give ready access thereto, they stand a few feet above the dock floor. The height usually ranges between 2 feet 6 inches and 4 feet. The greater height involves a corresponding additional depth of dock to accommodate the same class of vessel, but owing to the headroom it affords, the cost of repairs is reduced. The best material for keel-blocks is a matter of dispute. Cast iron was very largely employed, with timber caps, until the accident to the “ Fulda” threw cast-iron blocks into disrepute j most unjustifiably, because accidents have occurred with