The late Dr. Froude investigated this matter some years ago, and showed that such results were quite possible, independently of any fining of the lines, owing to the effect on the ship of the waves set up when in motion. One very curious illustration of how such waves may seriously affect a vessel is in that of a yacht built many years ago by an eminent firm on the Clyde, which failed to come anywhere near the performances guaranteed owing to the fact that as the speed increased the hollow following the wave formed at the bow increased and approached nearer and nearer to the paddle-wheels, until the water dropped below the floats and allowed the wheels to spin in the air; the propelling effect was thus entirely lost until the vessel slowed down sufficiently for the water to rise again to the level of the paddle-wheels. Such a thing could scarcely happen with a screw steamer; but the very bad steering qualities of certain naval ships is due to the fact that the inrush of water at the stern causes currents to flow _with_ the ship, and therefore to produce quite different results with the rudder from those which generally obtain.
[12] A nautical mile is 6,080 feet, the land mile being 5,280 feet. The knot is a measure of _rate_ of speed per hour. A vessel makes 20 knots when she is travelling at the rate of 20 nautical miles per hour.
[13] The dimensions, speed, etc., of the steamers here referred to, as well as other representative steamers from 1836 to 1890, are shown in the table on page 78.
[14] In the case of river steamers of moderate size there is not the same restriction on the position of the wheel, and as a matter of fact, as in the case of stern-wheelers, it is altogether at one end.
[15] It is now claimed for the twin-screw ship that she is not only capable of entering shallower harbors, but that she is in every way much safer, and it is most unfortunate that, owing to an act of carelessness, this was not conclusively shown in the recent accident to the City of Paris. But there is safety in the twin-screw beyond that which is rendered possible, as in the cases of the City of Paris and Majestic, by the division of the engine-rooms, viz., the fact that if one engine breaks down it is improbable that the other would do so at the same time, and that the vessel, although somewhat crippled in speed, would still be able to pursue her voyage; also, that in the event of accident to the steering apparatus the passage could be continued and the direction of the ship guided by regulating with one or both of the engines. Each of these features is pronounced, and the advantages have been proved on many occasions.