Engineering Wonders of the World
Volume I

DOCKS. the water is pumped out of the dock. As she rests on the blocks men prop her up with tim- ber shores, whose lower ends are supported by the steps or 11 altars ” in the side-walls. When the repairs, etc., are completed, the water is let into the dock again, and the vessel is floated out at full tide. In some cases there is no need to wait for the flood in order to dock or undock a ship. If a dry dock is access- ible from a wet dock, the work can be done at any time. The emptying of a large dry dock looks a much more serious operation than it really is. The big graving dock at Govan holds 13,762,500 gallons, yet at the very first trial it was pumped dry in an hour and forty minutes. On the second trial the work was done in an hour and a half—about 700 tons being discharged per minute. The dock is divided into two sections. If there had been a ship displacing 10,000 tons in the larger of these sections, the time would have been re- duced to less than 35 minutes. The machinery used consists of two 60-inch centrifugal pumps, each driven by a pair of vertical direct-acting steam engines with cylinders of 28 inches in diameter and a stroke of 24 inches. The floor of a graving dock slopes down on each side from the centre, to allow the water to find its way by gravitation to the main culverts underneath, whence it is pumped out. The capacity of a dry dock is determined both by the width of the entrance to it and the depth of water over the sill against which the gates close. Although the largest of the dry docks at Tilbury is 875 feet long, and has a depth over the sills of 31J feet at high water of neap tides, it is inaccessible to a Dreadnought or a Lusitania because of its entrance being not more Large Dry Docks. than 70 feet wide. Cases of the same kind are to be found in other ports, because in the past ten years ships have developed in beam to an unexpected extent. The majority of these long but inadequately wide docks have been subdivided by placing other gates at some convenient point between the entrance and tho end of the dock. In one of the docks at Belfast there are two pairs of extra gates and three sub-divisions. Where this practice is followed, the docking of two or three vessels in the one dock independently of each other becomes possible, with obvious limitations. A vessel or vessels in the inner berth or berths could not be undocked with- out undocking the vessel or vessels in the outer berth or berths. The following table gives particulars of some of the most important dry docks :— Dock. Length. Width at Entrance. Depth over Sills. Canada Dock, Liverpool 925| ft. 94 ft. 29 ft. Tranmere, on the Mersey 900 ft. 95 ft. 33 ft. Brocklebank, Liverpool 804 ft. 96 ft. 31 ft. 10 in. Trafalgar, Southampton 860 ft. 90 ft. 33 ft. No. 10, Devonport (double)....459&250ft, . 95 ft. 47| ft- Kiel 570 ft. 94 ft. — Kaiser, Bremerhaven 741| ft. 98| ft. 35| ft. Wilhelmshaven (No. 4) 580 ft. 101 ft. — Havre 787 ft. t 98| ft. 29| ft. Spezia G87 ft. 105| ft. 33 ft. Nagasaki 714 ft. 96| ft. 34J ft. Charlestown, Boston, U.S.A. 718| ft. 112 ft. 30 ft. These figures indicate the magnitude of the (Photo, S. Cribb.) THE TRAFALGAR DOCK, SOUTHAMPTON.