A Treatise On The Principles And Practice Of Dock Engineering

520 DOCK ENGINEERING. are open to the objections that the pressure is by no means constant, that the storage is generally insufficient, and that, in sonie instances, there is loss arising from the absorption of air by the water, which has to be replaced by an auxiliary feed-pump. There are situations, however, such as on board ship, where accumulators are inadmissible and where air vessels have the advantage of lightness. Fluctuations in Pressure.—Hydraulic power in application to dock-work is liable to extreme changes in amount. The constantly varying number of machines under action, while the area of the supply main is always the same, causes the intensity of pressure to fluctuate considerably. It frequently falls much below the nominal value, and sometimes, under the influence of surging, it may rise above it. The following readings, recently taken in connection with the working of the entrance gates to a dock at Liverpool, illustrate this irregularity very forcibly : — TABLE XLIL Time. Locality. Draught of Water on Sill. Maximum Pressure Prior to Movement of Kam. Working Pressure fairly Constant throughout Stroke. A.M. Feet Ins. Lbs. Lbs. 6.50 80 feet entrance, 30 0 730 680 6.58 40 25 0 730 730 7.0 100 29 0 730 530 9.0 80 21 0 730 690 9.10 40 16 3 750 350 9.15 100 20 6 760 560 11.0 80 15 6 750 720 11.5 40 11 0 750 300 11.10 100 15 0 760 600 P.M. 1.0 80 16 6 740 720 1.10 40 12 0 760 260 1.20 100 17 6 760 580 The normal pressure was 750 Ibs. at the accumulator. The areas of the ranis were as follows:—80-feet entrance, 227 square inehes ; 40-feet entrance, 113 square inches ; 100-feet entrance, 283'5 square inches. Electrical Distribution of Energy.—Electricity, as a practical science, is much the junior of hydraulics, and, in reference to dockwork, it has only been adopted to any noticeable extent within the last decade. Hamburg and the German ports of the Baltic introduced it about the year 1892. It was speedily taken up by Rotterdam, Amsterdam, Bordeaux, Havre, and Copenhagen. Southampton is apparently the first port at which it appeared in this country, but the use of electricity is now rapidly becoming general and, where the question is not complicated by the prior existence of a hydraulic installation, its claims for selection are admittedly pre-eminent. The electric current is either continuous or alternating, and this latter case either single or multiphase.