A Treatise On The Principles And Practice Of Dock Engineering

THE DESIGN OF GRAVING DOCKS. 475 on the draw-chain amounted to 22'88 tons. The power absorbed in lifting was lår ’ = 13-44 tons, leaving 9-44 tons for the power absorbed in over- 18 coming friction. This is équivalent to 3*9 per cent, of the weight lifted. In another experiment, made at a slipway on the River Hooghly, with a gradient of 1 in 24, the weight of the vessel and cradle amounted to 602 tons, and the effective haulage to 45-2 tons. The power absorbed in liftina being ^ = 25-1 tons, this left 20'1 tons as the power absorbed by friction, or 3*33 per cent, of the weight lifted. At Palermo the friction of a sliding slip on a gradient of 1 in 13-3 has been determined to be about 7^ per cent., and the power required, 20 per cent, of the whole load. In Messrs. Lightfoot and Thomson’s system, a ram for the return stroke has to be pushed home simultaneously with the lifting of the cradle. Indicating the pressure on this ram by the letter s, the inventors have deduced the following empirical formula from a number of actual experi- ments, and it has been found to answer with fair accuracy for slipways of about 1 in 20— p=s+!îù^ï±^. . . . (133) 8 A great deal depends upon the efficiency and condition of the ways. Unless kept clean, silt and other accumulations will cause a large increase- in the amount of resistance to movement. The fact also must not be overlooked that some additional force will be required to overcome the initial inertia of the load. /<7^ % ^ wü«/ ^2^114. tø ^8u(! Wtd I« The Design of Graving Docks. The principles affecting the design of graving docks do not materially differ from those enunciated in Ohap. vi. for the design of entrance locks. The one exception is in regard to the floor. Locks, although the water they contain is constantly undergoing changes of level, rarely have their floors uncovered, and then only for purposes of repair. On the other band, the very fonction of a graving dock demands that, for the greater portion of its useful time, it should be entirely free from water. With a natural foundation of hard and impervious rock, this fact entails no difference in the construction of the two chambers, but where the substrata are water- bearing, it is obvious that the floor of a graving dock must be made- sufficiently strong to resist a hydrostatic pressure on the underside, équivalent to the greatest head of water in the immediate neighbourhood. At first sight it may appear that, under these conditions, the graving dock floor is a beam, supported at each end by the side walls and loaded uniformly. That such is not the case is evident from the fact that few docks (if any) in the world would be capable of sustaining the estimated