A Treatise On The Principles And Practice Of Dock Engineering

DURABILITY. 469 amount of pumping increases with a decrease in the size of the vessel, for a floating dock the reverse is the case, since it need only be sunk to a depth sufficient to take the vessel’s keel. There is only one point which slightly reduces the overwhelming advantage of the floating dock, and that is in reference to the head pumped against. In the case of the graving dock, the head varies from zero to the depth of the floor below free water level, and the mean head may be approximately stated at one-half this depth. In the case of the floating dock, the initial head is the draught of the vessel plus the depth of the floor pontoon, and the flnal head is the latter of these two amounts. Hence, supposing two vessels of equal draught taken, the one on to a floating dock and the other into a graving dock, the depth of water in the docks being likewise the same, then the mean head of pumping in the former instance would exceed that in the latter by one-half the depth of the floor pontoons. But this advantage is more apparent than real, for it only occurs in the isolated case of a vessel of maximum draught using the graving dock. In the majority of cases the clearance between keel and floor is much greater than the semi-depth of a floating pontoon. From a specific comparison between two docks of equal capacity, it has been found that the pumping power required for the graving dock was nearly four times that required for the floating dock, the duration of pumping being the same in both cases. If the power had been equalised by differentiating the time, the excess consumption of fuel and oil would still have been retained. Again, apart from the primary emptying of a graving dock, an auxiliary drainage pump is required to deal with leakages. In a floating dock there is no leakage, and, therefore, no necessity for a drainage pump. On the other hand, it must not be overlooked that, the main pumps being only intermittently employed, it is quite feasible for a single pumping station to serve two or more graving docks, whereas each floating dock requires its own pumping plant, and this is often subdivided and dis- tributed throughout the dock. Again, on account of the necessity of maintaining equilibrium in the floating dock, great care has to be exercised and attention paid to numerous valves. This entails a large working staff. 8. Durability.—Here the balance of merit reverts to the granite, brick- work, or concrete graving dock, which is practically indestructible. The life of an iron or steel floating dock depends naturally on the care which is devoted to its maintenance, and upon the locality in which it is placed. In Chap. viii., it has been stated that a pair of iron gates, under average conditions, may be expected to last thirty years, but as overhauling and repairing can be carried out much more effectively, and with greater facility in the case of a self-docking floating dock, these more favourable conditions warrant the expectation of somewhat greater longevity—say forty or forty-five years. The Bermuda Dock, launched in 1868, was found to have suffered