A Treatise On The Principles And Practice Of Dock Engineering

CAPACITY. 465 the open. Hence gridirons, slipways, and lifts have a certain advantage over docks, and, of the latter, the floating dock is more open than the graving dock. 3. Light.—Artificial light can, of course, be provided, but natural light is always better and more economical. The same comparison holds good as in the case of ventilation. Apart from these general requirements, there are various points of view from which the advantages of the systems may be estimated, and, accordingly, we will deal with these in order. Setting aside the gridiron as too primitive and the hydraulic lift as now superseded by its development, the floating dock, we may usefully confine our comparison to the remaining three types. 4. Capacity.—Although no apparent limitation attaches to the size of slipways, yet it will be found that they have only been constructed for a comparatively small class of vessel—those with lengths not exceeding 350 feet and dead weights of not more than 5,000 tons. This arises from three causes: first, the excessive length of slipway, both above and below water, required for the reception of larger ships ; secondly, the liability of sucli ships to undergo strain during the process of getting them on to the cradle ; and, thirdly, the difliculty of keeping a very large slipway remunera- tively engaged. Theoretically, there is no reason why a ship of any length and weight should not be supported npon a slipway of sufficient size and stability, and to economical reasons alone must be attributed the main objection to its more extended utility. Judging from existing examples, the size of graving and floating docks is restricted by no such consideration, and their maximum capacity has yet to be determined. Every succeeding year witnesses an increase in dimensions. As regards their relative capacities there is some difliculty in instituting a comparison, for that of a graving dock is based upon its linear dimensions, the weight of any incoming vessel not entering into account, while a floating dock, open at each end, is gauged by the weight which it can lift, and is practically independent of size. The largest vessels, designed or in existence at the present time, are nearing or have reached a length of 760 feet, a beam of 78 feet, a loaded draught of 36 feet, and a displacement of upwards of 38,000 tons. The largest graving docks have lengths of over 850 feet, entrances more than 85 feet wide, and a draught of water on sill at high water of ordinary spring tides somewhat exceeding 32 feet. While the superficial area of such graving docks is largely in excess of all present requirements, it will be noticed that there is an apparent insufficiency in draught, and this fact is often alleged as a disqualification. But in the great majority of cases, a vessel will discharge the whole or the larger part of her cargo before entering the dock and so reduce her draught by several feet. At the same time, it must be admitted that the margin thus obtained is by no means a large one, and it frequently disappears at neap tides, while there is always the remote contingency of a seriously damaged vessel having to be docked fully loaded immediately upon its arrival at a port. It is an 30