A Treatise On The Principles And Practice Of Dock Engineering

468 DOCK ENGINEERING. displacement of a vessel of this size would be at least 25,000 tons, and,, according to Herr Howaldt and Messrs. Clark and Standfield alike, the cost of a floating dock to receive her would lie between £225,000 and £250,000. Or, looking at the matter another way, the proportion of deadweight of a floating dock to lifting power, from a number of examples, averaging about 45 to 100, the deadweight of a floating dock as above would be 11,250 tons, which at, say, £20 per ton (to include all fittings and pumping machinery), cornes to £225,000 as before. This is without taking into consideration any ancillary works, such as shore connections, site dredging, &c. So that, as regards the cost of the largest docks, the balance inclines in favour of the graving dock. This opinion receives confirmation in the report ofthe engineer (Mr. Wm. Ferguson) to the Port of Wellington, N.Z., who after a tour of inspection of the repairing docks in Great Britain, Australia, and the United States, recommends the adoption of a concrete graving dock for that port as less expensive than a floating dock of similar capacity.* 6. Maintenance and Repairs.—The structures of slipways and graving docks, if solidly built in the first instance, require very little attention afterwards, whereas owing to the destructive action of salt water on ironwork, floating docks call for regular inspection and frequent painting. In slipways the cradle wheels occasionally get broken, but this item should equitably be included in repairs to machinery, which are common to all three types, though possibly a more pronounced item in floating docks. The structural repairs of a concrete or masonry graving dock, with greenheart gates, are infinitesimal. If iron gates are used, they will necessitate some expense of upkeep, as against a reduction in their cost of construction compared with wooden gates. No doubt, the timber graving docks prevalent in the United States require extensive repairs from time to time, but in this case also, the capitalised amount is balanced by a corresponding economy in initial expenditure, and they represent, moreover, a very limited class. According to some statistics, supplied by Messrs. Clark and Standfield, the average annual cost of upkeep of iron floating docks ranges between •75 and 1'5 per cent, of the invested capital. The former figure represents exceptional care in primary preparation, the outside surfaces being par- ticularly well painted and “the whole of the mill-scale having sweated off before launching, so that the paint was fairly on the iron.” 7. Working Expenses.—In this respect the floating dock exhibits an economy far beyond that of the graving dock, because, in the former case, the quantity of water to be removed by pumping is little more than the actual displacement of the vessel which is being docked, while in the latter case, unless any assistance can be rendered by a falling tide, the volume of water to be pumped out is the cubic contents of the graving dock, less the displacement of the ship. Furthermore, while for a graving dock the * Report on Docking Facilities for the Port of Wellington, 1901.