A Treatise On The Principles And Practice Of Dock Engineering

HYDRAULIC LIFT. 463 to float the vessel into position and leave her suitably moored ; the tide does the rest. This system, though simple and effective in its way, has many defects. In the first place, it is only practicable in localities where there is sufficient range of tide for the purpose. Then repairing operations are intermittent and have to be suspended with each recurring period of high water, occasion- ing delay and the repetition of manœuvres. And lastly, the floor, in the absence of any means of adjustment to the keel of the superimposed vessel, does not lend itself to any but the rudest kind of support. The Carthaginians seem to have discovered an improved method of dealing with the problem by the introduction of Artificial Shpways, in which smooth timber slides formed a less frictional surface for the haulage of ships than the rough and irregular contour of a natural beach. Furthermore, they had the decided advantage of being utilisable in almost any situation. This was the origin of the modern slipway and slip-dock. The design has naturally undergone many modifications and improvements since the days of triremes and galleys, and it now exists in several distinct forms, but it is still essentially the same design. It would, of course, be superfluous to trace the various stages of its development, and we need only concern ourselves with the features displayed by its representative of the present day. Long timber ways, carrying iron rails, are laid at a uniform slope, ranging in different cases from about 1 in 15 to about 1 in 25, from some distance under water to a point at which the longest vessel to be accommodated is com- pletely out of the range of the tide. A cradle or travelling frame is passed down the ways and under the oncoming vessel’s keel. The latter takes a bearing upon the cradle, which is then drawn up to the highest point by suitable hauling gear. Despite its advantages, the drawbacks to the system are sufficient to prevent its general adoption. The length of a slipway is necessarily great, on account of its prolongation under water to a depth equal to the draught of vessels using it. This entails the appropriation of valuable water space and offers obstruction to navigation. To obviate these ill effects to some degree, the cradle has been made telescopic or collapsible, so that it consists of sections attached to one another by sliding bars. These sections, compressed at the foot of the slipway, are drawn out to their full extent by the hauling apparatus as each portion receives its propor- tionate load. The percentage of length, however, saved by this device is small. The appropriation of land space in congested districts is also an expensive matter, and récognition of this fact has led to the introduction of side walls and a pair of watertight gates at low-water level. The ship has then only to be withdrawn within the gates which shut out the tide. In this respect the slipway trenches upon the province of the graving dock and becomes a Slip-dock. To do away with the excessive length of a slipway, the Hydraulic Lift was devised, towards the middle of last century, by the late Edwin Clark. In some respects it is akin to the gridiron, consisting of a horizontal platform