Still another safeguard becomes possible through the adoption of the twin screw. The propellers are worked by two complete and entirely independent sets of boilers and engines, and these are separated by a longitudinal bulkhead in addition to the transverse bulkheads already described. In a single-screw ship this longitudinal bulkhead is impossible, and the space in which her engine and boilers are situated is her most vulnerable point; if she is struck there with sufficient force to make a fissure large enough to admit any considerable quantity of water, nothing will save her from sinking. In the case of the twin-screw ship, however, we have had the best of evidence, within the past two years, that with one of her engine-rooms flooded and open to the sea, she will still float and be navigable.
For many years past the value of the twin screw has been debated by the builders, the managers, the captains, and the engineers of the great transatlantic lines, to whom it did not commend itself so readily as to the Admiralty. It was adopted for war-ships several years before any of the well-known passenger lines ventured to use it, and its first appearance in this service was in the City of New York, four years ago. Since then it has been adopted by the White Star and the Hamburg-American lines, and though the North German Lloyd has not yet applied it to the recent accessions to its fleet, its advantages over the single screw for passenger vessels, as well as for war-ships, are more generally conceded now than ever before. The Admiralty adopted it for the security it afforded, and for its superior capacity for rapid manœuvring. Another feature which recommends it is that, should one of the two sets of engines become disabled from the breaking of the shaft, or any other cause, the opposite engine would be equal to taking the ship into port; while a similar accident on a single-screw ship would compel her to make port under sail (a very difficult feat with the modern type of ocean steamers), or to wait for another steamer to take her in tow.
[Illustration: Off Fire Island, New York.]
[Illustration: Gedney’s Channel, outside New York Harbor, at Night.
Until quite recently, the breaking of the shaft was more frequent than any other kind of accident to the transatlantic steamers. When, perhaps, the ship was sailing along at full-speed, a jar would come and shake her from end to end, as though a rock or a submerged wreck had been struck. The engine would rattle and the sails flap loosely in the wind, and the familiar tremor of propulsion change to a softer heaving motion, like that of a sailing vessel. When the accident occurred in darkness and a gale, it was more alarming than in daylight and a calm sea. After a few minutes of uncertainty the news would fly that the shaft was broken, and that the captain and the chief engineer were consulting in the engine-room. Then would come days, and sometimes weeks, of drifting, with a corresponding and ever-increasing alarm on shore as the ship became overdue. Under favorable circumstances some headway could be made with sails, and occasionally the disabled vessel reached port without assistance. Oftener, however, she would drift helplessly in the vacant sea until she was sighted by another steamer powerful enough to tow her. Left to herself, she was in danger of falling into the trough of the sea and foundering, and near land she was exposed to the perils of a strong current and a lee-shore. Arriving in port, a claim for salvage was sure to be presented against her, and in some instances the amount awarded was as much as thirty thousand pounds.