A Treatise On The Principles And Practice Of Dock Engineering

474 DOCK ENGINEERING. Sliding Slipways.—A distinct system of slipway from the foregoing is the- sliding slipway, in which a sledge takes the place of a cradle. The ways are necessarily weil greased, but in any case, the friction is greater and the wear of the structure much more considérable. The system is only adopted in isolated instances and under spécial circumstances, notably at Palermo,* where the configuration of the ground is precipitous. The space available was enough to admit of a slide, but not of a line of rails, the incline of which would have to be far less steep and therefore propor- tionately longer. The way is formed of a large number of cross sleepers on which four strong beams are placed longitudinally. Above the water level these are fixed, but the lower part is connected by hinges, and floats as soon as the weight is taken off. Broadside Slipway.—ARailwaydes transatlantiques at Lormont, Bordeaux, has the peculiarity of withdrawing vessels from the water broadside-on, as against the general practice of taking them end-on. The slipway is 400 feet l°ng, the length of cradle being 393 feet. Either one vessel of 410 feet length can be accommodated, or two single vessels, 203 and 180 feet long respectively. The lifting power is 3,000 tons. The extreme width is 46 feet. The slipways at the shipbuilding yard of the Imperial and Royal Danube Steam Navigation Co., at Alt-Ofen, in Hungary, have been constructed on the same plan. They have a riverside length of 650 feet, and a breadth inland of 280 feet, of which only 180 feet is permanent way. The largest vessels accommodated are 250 feet long and 460 tons light displacement. Stresses in Slipways.—The power required to raise a ship upon a slip- way is divisible into two portions—viz. (1) that for lifting the dead weight of the vessel and its cradle, and (2) that for overcoming friction. Theoretically, the force necessary to draw a given load, W, up a smooth ineline is something in excess of W sin 6, where 6 is the angle which the incline makes with the horizontal. But as élis very small in slipways, and tan tf is a much simpler quantity to deal with, the expression may be written W tan tf, without sensible error. Now, W is compounded of three items—the weight of the vessel (w^, the weight of the cradle (w2), and the weight of the hauling chain and rods (w3). Of these, at least two, and sometimes all three, contribute some frictional resistance to movement, in addition to their own weight. There is the friction of the cradle rollers, and possibly that of the rods, upon the ways; and furthermore, there will be a certain amount of friction in the hauling apparatus itself. Calling the former amount fv and the latter ^, and assuming a rigid base, we have the following general expression for the pull on the hauling chain :— P = (w + W>j + w2) tan tf + j^ + /2. . . (132) In an experiment made at the Dover slipway, where the gradient is 1 in 18, with a total load of 242 tons, it was found that the effective pull * Afin, Proc. Inst. C.E., vol. xlviii., p. 297.