A Treatise On The Principles And Practice Of Dock Engineering

24 DOCK ENGINEERING. faet, is so beset with possibilities and contingencies as to admit of no definite solution. Experience alone will demonstrate the adequacy or otherwise of the quay space appropriated to any particular vessel or class of good.s. Ratio of Periphery to Surface.—The proper proportion between the surface of a dock and its periphery is largely dependent upon the combination of length and breadth which is most suitable for the twofold purpose in view—viz., the provision of sufficient space for manæuvring ships and of sufficient quayage for berthing them. Of the two dimensions which produce the area, the length will either be the greatest available, or that, at least, which is judged adequate for present and future requirements. In assigning a breadth to a dock, it must be borne in mind that a steamship will not infrequently coal from hulks alongside during the same period in which she is receiving and discharging cargo. She may also have several lighters in attendance for goods destined to be forwarded by river or canal. In faet, employing a concrète example, it will be well to make allowance for a row of barges, 20 feet in width, to lie between the vessel and the quay and for two rows of similar craft on the other side of the vessel. Taking the beam of the latter at between 60 and 70 feet, it is evident that the berth must extend to some 120 or 130 feet in width. Doubling this for two sides, and allowing a central margin of 100 feet for the passage of ships in and out of the berths, it is clear that 350 feet is no excessive width for a dock. Indeed, an examination of Table v. will show that this dimension is frequently exceeded. At the same time, it must be observed that in cases of extreme width the dock will generally be found intersected by projecting arms or jetties. It has already been remarked that the square form is not economical from the point of view of obtaining the greatest amount of quayage from a given area. This discrepancy is most marked in docks of large size. If the side of the dock be s, the ratio of surface to periphery is s2 to 4«, or s to 4, so that the disparity increases with the length of the side. In a rectangle of length, 1, and breadth, b, the ratio is Ib to 21 + 2b, or if b = nl where n is any proper fraction: nl2 to 2Z (1 + n), i.e.— z . 2 (1 ^) n By giving n the values, in succession, of |, J, j, |, &c., we get the following ratios :— ? : 6, 8, 10, 12. When n = 1, the figure is a square and the ratio becomes Z : 4 as before. Grouped Docks.—The growth of trade being graduai, docks inerease in number as circumstances at each port demand. Where a series of docks are thus brought into existence they will generally be placed in intercom- munication by means of passages. Grouping can be effected systematically in various ways, as will be evident from a considération of what may be called the “chain ” system at Buenos Ayres (fig. 8), the “ comb” system at