A Treatise On The Principles And Practice Of Dock Engineering

464 DOCK ENGINEERING. npon which a vessel can be floated. Here, however, the resemblance ceases, for in this case the platform is formed of pontoons, the whole of which are raised by hydraulic pressure until the vessel is entirely above water. The operation, in fact, produces the effect of a falling tide and avoids the incon- venience of a rising one. The whole structure remains afloat until the time cornes for the vessel to be re-launched. We now corne to the Dry or Graving Dock, the principle of which is the reverse of those already described ; instead of withdrawing the ship from the water, the water is withdrawn from the ship. In its earlier stages, it is but the natural and logical development of the beaching process. Tinding the inconveniences of only having access to their vessels during short periods at low water, the obvious advantage of enclosing them within temporary mounds or banks of earth would suggest itself to enterprising shipwrights of ancient times. Then, in order to reduce the labour of constructing a continuous dam, the selection of a natural creek or inlet would occur, involving a dam across only one end. From a natural creek to an artificial chamber is but a single step, though, no doubt, some time would intervene between the two stages. A modern graving dock is an excavated chamber, three sides and the floor of which are lined, either naturally or artificially, with watertight material. The fourth side, or end, is the entrance, and is provided with a pair of gates or a caisson. After the entry of a ship, the entrance is closed and the water is pumped from within the dock, though in certain cases the operation may be partially effected or, at any rate, assisted by the fall of the tide. Lastly, we have the Floating Dock—a hollow structure, formerly of wood but now universally of iron or steel, generally similar to a graving dock in outline, but gradually diverging therefrom in process of evolution, and entirely dissimilar in action, in that it reverts to the former principle of withdrawing the vessel from the water. It is, in fact, an outcome of the hydraulic lift. To receive its charge the floatingdock is sunk to the requisite depth by allowing its air chambers to fill with water, which is afterwards removed by pumping when the vessel has been berthed. This process causes the dock to rise bodily and, in so doing, to lift the vessel above the water line. Thus far we have very briefly reviewed the rise and progress of various repairing systems. We will now proceed to consider them more closely with reference to their construction and equipment. But, before doing so, it will be well to lay down three general essential requirements of any system :— 1. Accessibility.—All parts of a vessel’s keel and under side must be readily accessible. Beaching is deficient in this respect, unless the position of the vessel be changed, and this is not always feasible. 2. Ventilation.—If a vessel has to be painted, it is essential that her sides should dry as quickly as possible, and this result is best achieved in