A Treatise On The Principles And Practice Of Dock Engineering

4^4 DOCK ENGINEERING. other kinds of Blocks. Many engineers prefer wooden Blocks—oak for preference, pitchpine often on account of its cheapness. In the latest and largest graving dock at Liverpool the Blocks are of cast iron surmounted By a 12-inch birch log, capped with 3 inches of soft wood. Similar blocks, capped with greenheart, are used at Belfast (fig. 475). On account of the flotation, wood blocks must be anchored. Curved cast-iron caps have been used at Amsterdam, the object being self-adjustment between block and ship. With the same object in view, hydraulic blocks have been proposed, but the consequent uniformity of pressure produced all the effects of rigidity, and the method was abandoned after trial. Steel, greenheart, elm, and teak have also been employed for blocks. The distance apart of the block centres varies from 2 to 5 feet, being governed by the load to be carried. Wide intervals, where possible, are convenient. On the other band, it is often necessary to support a large ship by inserting temporary intermediate blocks between the permanent ones. The large Atlantic liners are continuous-blocked in this fashion for a great portion of their lengths. As regards shape, wedge-shaped blocks have been found most convenient for adjustment. The wedges should be readily removable and portable. Wooden blocks, however, are generally rectangular and bound at the ends with iron bands. Bilge-blocks or Side Cradles are not so commonly employed in graving as in floating docks, though they form useful adjuncts to keel-blocks. Their drawback is that they rather interfere with freedom of movement, and consequently they are arranged at greater intervals—say, about 50 feet. Their upper surfaces have to be adjusted to the level of the ship’s bilge. Sometimes props under the bilge keels are substituted for them. Side-shores.—These form a series of lateral supports to a vessel upon the blocks. They are of wood, about 9 or 10 inches square at the centre, tapering slightly each way to the ends, which are bound with iron. They are lowered into position as the pumping proceeds, in somewhat primitive fashion by means of ropes, and are tightened up with wedges, so that one end bears firmly against the vessel’s side and the other against an altar course. It has been suggested that a series of horizontal steel shores, worked in and out of the sides of the dock by mechanical means, would be a great improvement. No doubt the method would be more scientific, but it has certain obvious difficulties attached to it in the way of regulating the level of the shores so as to suit ships of all sizes. Moreover, since pumping is a process involving some time, ample opportunity is afforded for setting shores by hand without causing extra or undue delay. Hooks, fixed to the quay at coping level with rope moorings, are some- times employed for securing a vessel in position, more especially when she is only shored on one side, as in the case of a graving dock capable of accommodating two ships side by side. The vessel is then given a slight list towards the nearer quay. Bollards and mooring posts serve the same purpose..