CHAPTER XVI.

PLACING A SHIP IN DRY DOCK.

The operation of safely placing a ship in dry dock, safely supporting her with blocks and shores, and afterwards floating her out of dock, is so common that the care and experience necessary to ensure success in this operation are not generally understood, yet it is possible that very serious damage may occur during the operation, if intelligent supervision be neglected.

Every ship should carry a docking plan which shows: the length on the load water line; the length over all; the location of all the under water valves; the locations of the water-tight bulkheads, the engines, the boilers, the turrets (if any), and such other weights and fittings as are peculiar to any particular ship; the length of straight keel, together with dimensions locating accurately the cut-up (if any) of the dead wood aft, together with any peculiarities of the stern post and rudder; also such dimensions as will show the curvature of the forefoot, especial care being taken to locate the exact point where this curvature departs from the straight line of the keel. The docking plan of a battleship is shown on Plate 124. The docking plan should also contain information as to cross-sections amidships and elsewhere, showing the beam at or near the water-line, the shape and location of the keel, the docking keels and bilge keels, the struts, the propellers and all other objects below the water-line; in other words, the docking plan must furnish all necessary information concerning the under-water hull and its accessories, also dimensions as to projections above the water-line which increase the nominal beam of the vessel: the latter information is frequently of extreme value in foreign ports whose docks have their dimensions tabulated with reference to merchant vessels only.

In our navy yards, blue-prints of such plans are usually in the possession of the Naval Constructor-elsewhere it is necessary to furnish such plans to the responsible authorities of public or private docks-without them, the efficiency and safety of the docking are absolutely dependent upon the skill and experience, of those having control of the docks.

The dock master of any particular dock being given the docking plan of a ship to be placed therein, proceeds as follows: knowing the ship's draught, the maximum depth over the sill, together with the current and tidal variations in the vicinity, he decides upon the time the vessel should enter the dock, and so informs the commanding officer, who thereupon makes the necessary arrangements to ensure that the vessel at the time specified shall be absolutely upright, without any list either to starboard or port. The entrance to a dry dock may be closed by hinged gates, a floating caisson, or a sliding caisson; the first and last methods are often used in foreign docks, but in home docks, a floating caisson is most commonly found; it usually has a ship-shape form with sufficient stability to safely float upright when empty; to sink it, valves are opened which admit water to its interior, and to raise it, all outboard valves are closed and the water it contains is pumped overboard.

The dock floor carries along its center a line of blocks, called "keel blocks." These are usually of wood and are secured to the dock floor in various ways. Their distance apart varies in different docks, but it is customary to place these keel blocks much closer together under turrets and other heavy local weights on war ships, than is done with ordinary vessels. Those ships having straight keels, but whose fore-foot is cut away, are supported forward by building up the corresponding keel blocks to suit the contour shown on the docking plan. With ships having docking keels, a double line of keel blocks is provided, running parallel to the center line and at the proper distance therefrom, given on the docking plan. At intervals along the bottom of the dock and at right angles to the center line, are the bilge ways, along which slide the bilge blocks which can be moved towards or away from the center line by the hauling lines which are manipulated from the dock coping. Each bilge block is built up of a proper height and level, as determined from the docking plan, so that after the vessel's keel rests upon the keel blocks, the bilge blocks can be hauled and accurately fit against the bottom, thus thoroughly supporting the ship before the water has been pumped out of the dock. Care is taken that the bilge blocks are not hauled so that they will bear against an under-water valve or other accessory which would be injured by heavy local presIn foreign docks, bilge blocks are rarely used, shores being fitted to sustain the bottom after the dock is empty.

sure.

To maintain the vessel upright after she has grounded on the keel blocks, and before the bilge blocks are hauled, wale shores are used, one end resting against the ship's side, the other against the dock's side, wedges being used to set them taut. These shores are prepared of the desired length and placed in the vicinity of their final location by means of information obtained from the docking plan. Certain marks are also made on the coping which will accurately locate the ship's position in the dock, in order to ensure that as the water is pumped out, the under-water hull shall exactly coincide at the proper time with the various blocks and shores which have been made ready to receive it. Plate 124 shows the plan which would be prepared for a particular dock after receiving the docking plan of the vessel which was to be placed therein.

These preparations being completed, water is admitted, the caisson is floated and removed, and the dock is then ready to receive the ship. After the ship's bow has safely entered the mouth of the dock, the responsibility for her safety rests upon the dock master; the methods of securing this safe entrance are considered elsewhere. The dock master then hauls the ship into the dock until certain definite objects near her bow and stern coincide with the marks which he has laid out upon the dock coping in accordance with the docking plan. The caisson is then placed in position, the pumps which empty the dock are started; their operation between this time and the time of the complete emptying of the dock being controlled by the judgment of the dock

master.

In the meantime the necessary arrangements have been made to ensure the ship being safely centered in the fore and aft and athwartship directions, and the wale shores have been floated and placed approximately in their proper positions: during this period, by the use of sighting battens or other means, the variations of the ship from the upright are finally determined, and the necessary measures are taken to correct any listing to starboard or to port.

Under ordinary circumstances, the ship's keel first touches on the keel blocks aft, and with a ship having a large amount of drag, special precautions are necessary to prevent listing, because, under these circumstances, stability is lessened very rapidly. The grounding of the keel aft upon the keel blocks is indicated in various ways, but before this occurs, the dock master has ar