A Treatise On The Principles And Practice Of Dock Engineering

462 CHAPTER XI. GRAVING AND REPAIRING DOCKS. VaRIOUS MetHODS OF EFFECTINO REPAIES TO ShIPS—CaRBENINO—BeACHINO—THE Gridiron—The Slipway — The Hydraulic Lift — The Graving Dock — The Floating Dock—Essential Requirembnts of a Repairing Depôt—Comparison OF THE VAKIOUS Types IN REGARD TO ACCESSIBILITY, VENTILATION, LIGHT, Capacity, Initial Cost, Maintenance and Repairs, Working Expenses, Dura- BILITY AND GENERAL ADAPTABILITY—DESIGN AND CONSTRUCTION OF SLIPWAYS— Foundation—Permanent Way—Cradle—Sliding Slipways—Broadside Slip- ways—Stresses in Slipways—Design and Construction of Graving Docks- Types of Floating Docks—Procbss of Overhauling—Equipment of Repair- ing Docks Distribution of Pressure on Keel Blocks — Description of Gridirons at Liverpool, Hydraulic Lift at London, Slipway at Dover, Graving Docks at Bremerhaven, Liverpool, Glasgow, Barry, and London, and Floating Docks at Cartagena and Bermuda. The iiecessity of providing at every port sites, suitable in situation and Equipment, where vessels can from time to time undergo examination, painting, and repair, is self-evident. There would be danger, to say nothing of loss of time and inconvenience, in transferring a disabled vessel from one port to another, however short the distance might be ; and, apart from this, any lack of facilities for repair must inevitably react upon the prestige of a port and prejudice its development. But, if the desirability of such a site be generally admitted, opinion upon the form it should take is not so unanimous. There are strong advocates for several very different types of repairing depôt. When we have examined the claims put forward in favour of each of these, we may possibly be able to assign some order to their respective merits. Apart from the operation of careening, in which a water-borne vessel was temporarily given a pronounced list, the earliest means of obtaining access to the under side of a ship was that of dragging it by hand out of the water on to some moderately sloping strand of firm sand or gravel. If too heavy for manual haulage, the vessel was caused to take ground at high water, so that the receding tide left her high and dry. Such was the method of beaching as practised by the Phænicians, the Egyptians, and other nations during the infancy of the mercantile marine. For light vessels of shallow draught the method is, no doubt, quite satisfactory and sufficient and, despite its primitive nature, it is still in use at the commencement of the 20th century. Its modern prototype is the Gridiron, located in a tidal basin, and consisting of an extended series of parallel beams or logs laid at regular intervals upon a firm masonry foundation. The operation is simply