A Treatise On The Principles And Practice Of Dock Engineering

470 DOCK ENGINEERING. considerably by the end of the century, and other docks of the non-self- docking type have undergone equally rapid deterioration ; but, on the other hand, the Cartagena Dock, built in 1859, is still in good repair, as also are the pontoons of the Victoria Dock, constructed in 1857. A timber graving dock must necessarily be very liable to decay owing to its alternate exposure to the wet and the dry condition. lt has been pertinently pointed out that a dock may outlast its period of usefulness ; that, with the rapid increase in size and alteration in shape of modern ships, a repairing dock ultimately becomes incapable of receiving any but those which are obsolete. This may be true to some extent, but it is no less true that both graving and floating docks are capable of being altered within certain limits, so as to adapt themselves to new conditions. They have been lengthened in more than one instance. Any incrément in width and depth, however, can only be obtained at practically prohibitive expense, and the author is only aware of a very few instances in which such alterations have been carried out. The cutting away, in some cases, of the lowermost altar-courses of masonry docks has produced an additional few feet of bottom-width at a moderate cost. 9. General Adaptability.—-There are several detached points of practical importance which may be grouped under the above heading. (1) A floating dock has the advantage of mobility. It may be towed to another port. Per contra it may founder or suffer shipwreck. (2) A floating dock may conceivably be trimmed by water ballasting to take a ship with a list so pronounced that it could not pass through the vertical profile of a graving dock entrance. Practically, such a step would be attended with serious risk of capsizing. (3) Accidents are more rare in graving docks. Floating docks have sunk under ships of heavy tonnage, though not, it must be admitted, in recent times or with docks of the latest type. (4) A floating dock takes comparatively little time to construct—say, from seven to nine months with expedition. An average graving dock could hardly, under the most favourable circumstances, be built in less than two years. (5) Where land is dear, or the site restricted, a floating dock either renders its purchase needless or allows of its allocation to other purposes. Design and Construction of Slipways. The essential parts of a slipway are: —(1) The foundation, (2) the- permanent way, (3) the cradle, and (4) the hauling machinery. The Foundation should, if possible, be absolutely incompressible; but, failing that ideal, a very slight settlement is permissible, provided it be uniform throughout. Any transition from an elastic to a rigid base, or vice versâ, throws considérable local strain upon the cradle, often resulting in broken rollers. The intensity of pressure on slipways is not great, the