A Treatise On The Principles And Practice Of Dock Engineering

20 DOCK ENGINEERING. most to be recommended, combining, as it does, the advantages of both the previous cases without any of their drawbacks in an acute form. But, in order to fulfil the ideal conditions, the estuary must be broad and well sheltered, free from shoals and from a shallow bar. Shape.—The outline of a dock or basin may be that of any geometrical figure, or of several figures in combination. Figures approaching the curvature of the circle, unless, indeed, the radius be extremely great, are obviously unsuitable for enclosures destined to accommodate long straight vessels in contact with their sides. Curves are undoubtedly employed to advantage in many cases, in connecting outlying arms and branches, and in training ships through changes of direction, but their effective use is limited and otherwise to be deprecated. The most suitable forms are rectilinear, and those generally available for the purpose are the triangular, the square, the rectangular, the diamond (or lozenge), the machicolated, and the digital. The triangular form is rarely used, not so much, perhaps, on the ground of any inherent defect, as that the quay arrangements are not always con- formable to a plan of that cliaracter. It has possible advantages for an entrance basin acting as a vestibule to a group of docks, as exemplified in the basin leading to the Albion and Island Docks at Rotherhithe (fig. 18). This example, however, be it noted, is somewhat defective, though not radically so. Other triangular outlines, more or less complète, are to be found in the Prince of Wales Dock at Swansea (fig. 20), the Morpeth Branch Dock at Birkenhead (fig. 6), and the Manchester Dock at Liverpool (fig. 5). The square dock offers the advantage of plenty of space for the turning of the vessels it accommodâtes. In the majority of instances a vessel leaves, and should leave, a dock stem first. As she generally makes her entry in the same manner, it behoves that sufficient room be provided for turning her within the dock. This proviso is of most importance in exposed situations with narrow entrance channels. With a wide open fairway, sufficiently sheltered, it is a matter of indifference whether the turning takes place within or without the dock. Many ships will take advantage of an outer basin in order to make their entry stern first, so as to be ready for direct departure. The disadvantage attaching to the square dock is the excessive proportion of its water area to the amount of quayage, which renders it unsuitable for the accommodation of large vessels. It is doubtful whether any existing dock is absolutely square, but the Albert and Colling- wood Docks, at Liverpool (fig. 5), are sufficiently close approximations for the purpose of illustration. The rectangular dock is a modification of the square dock, designed to overcome the defect just mentioned. By proper manipulation the length and breadth may be arranged so as to give the maximum amount of quay frontage consistent with the water space absolutely required for manceuvring purposes. This ratio will be discussed later. The rectangular form is common. A few instances of its adoption may