A Treatise On The Principles And Practice Of Dock Engineering

234 DOCK ENGINEERING. development of océan leviathans in recent years, the need of wide entrances is returning. In 1857 the Canada Lock was constructed at Liverpool, 100 feet wide. It was not until the year 1902 that another entrance of the same width was opened for traffic. During the interval the width considered requisite had fallen to 65 feet, from which it has gradually risen to its former dimension. No doubt a width of 100 feet is in excess of present- day requirements, the maximum breadth of a ship being as yet 70 feet, but another decade will probably see a large increase, so that the margin provided is no more than prudent foresight would warrant. Another factor involved in the determination of width is the ratio between the sectional area of the entrance and the cubic capacity of the dock, or, what is the same thing, between the width of the entrance and the area of the dock. If a dock entrance remains open for any length of time after high water, a gradually increasing current is generated owing to the fall of the tidal level outside, and the conséquent discharge of the water from within the dock through a narrow passage. If allowed to continue too long the current may become so rapid as to render the closing of the gates a hazardous proceeding. The limit of safety may be considered reached when the velocity is 3 feet per second. When the dock is of considerable area it may be necessary to provide two or more entrances, as much for facilities of traffic as for the reason given above. As regards depth, the sill of the old Canada Lock was such as to afford a depth of 26 feet 6 inches of water at high water of ordinary spring tides at Liverpool, and 19 feet 4 inches at high water of ordinary neaps. The latest entrances constructed at that port provide for 39 feet 2 inches and 32 feet respectively. The loaded draught of modern vessels, it is true, does not exceed about 32 feet as yet, but the greatest length consistent with that draught has now been reached, and an increment in length will necessitate a corresponding increase in depth. The obstacle to this development in depth has been the limited draught of water obtainable at the ports which the vessels frequent, and there can be no doubt that with increased depth of water there will corne increased depth of ships. The following abridged remarks of Dr. Francis Elgar,* made in 1893, are equally applicable at the- present date : — “ The deep draught of water is a most important element of speed at sea, and it is now strictly limited by the depth of water in the ports and docks used by the fast passenger steamships on both sides of the Atlantic. The result is that it is only a question of time, and not of a very long time with our present materials of construction and type of propulsive machinery, to find an absolute limit of speed imposed by the restriction of draught of water. The Atlantic trade is increasing at such a rapid rate that larger and swifter ships are certain to be soon called for ; but much deeper harbours and docks will be required if further great increases of speed at sea are to be obtained without excessive difficulty and cost.” * Elgar on “ Fast Ocean Steamships,” Min. Proc. Inst. N.A., 1893.