Regular trans-Atlantic steam service was not inaugurated until 1838, but for many decades steamers were equipped with sails to assist them when the wind was favorable.
The most notable of early steamships was the Great Eastern, a combined screw and paddle-wheel ship, 692 feet long, built in 1858. She held the record for size until 1899 when the Oceanic, 704 feet long, was put into service. At present the Leviathan, formerly the Vaterland, holds the record with a length of 920 feet. It is difficult to judge of the size of a vessel out on the open water. If the Leviathan were placed in Broadway, New York, she would span nearly four blocks. Because of her 100-foot beam she would be too wide to be wedged in between the skyscrapers that border lower Broadway. If she were set up on end she would tower 158 feet above the pinnacle of the Woolworth Building. Her power plant consists of four turbines which total 90,000 horsepower and the huge vessel is driven at a speed of 25.8 knots or nearly thirty miles per hour.
The wonder of these huge floating structures lies not merely in their gigantic proportions but in the fact that they are able to weather the terrific wrenching strains of heavy ocean storms. A skyscraper is built to withstand only the steady and direct pull of gravity and the variable thrust of the wind which, except in western cyclones, seldom amounts to thirty pounds per square foot. Bridge building is more difficult because of the leverage of the parts overhanging the foundations. Wind pressures must be calculated and also the live load of objects moving over the structure. In naval architecture enter the problems of building construction combined with those of bridge building, complicated by the fact that there is no fixed foundation for the structure to rest upon. At one moment a ship may be spanning a trough in the seas and at the next it may be seesawing over the crest of the wave. Of course the bottom of the boat is seldom if ever out of the water and a certain amount of support is provided throughout the length of the vessel, but the ship is subjected to the strains of a cantilever bridge when she is passing over a wave, and to the strains of a truss bridge when spanning a wave trough. These strains are increased by the fact that the structure is in constant motion. A certain degree of flexibility is demanded of the materials which go into the structure and of the joints between the frame members.
BOATS OF ARTIFICIAL STONE
Originally wood was used for the hulls of ships; then between 1845 and 1855 iron supplanted wood, Between 1875 and 1885 steel supplanted iron and to-day efforts are being made to supplant steel with concrete. The advantages offered by concrete are cheapness and speed of construction. The first large vessel built of this material was the Faith, an 8,000-ton ship. This boat stood up very well in heavy weather despite the rigidity of her structure. It is doubtful, however, that a large boat comparable in size to the Leviathan could weather a severe ocean storm.
The proposal to build ships of cement created almost as much of a popular sensation as did the first iron boat. Although the public had accepted iron and then steel as a perfectly proper material for shipbuilding, concrete seemed too much like stone and it did not seem possible that artificial stone could be made to float. They did not realize that a cubic foot of steel weighs four times as much as the same volume of concrete. Of course concrete does not begin to have the tensile strength of steel and consequently the walls of a concrete ship must be made relatively thick. For this reason a concrete vessel is heavier than a steel vessel. She draws more water and requires a larger power plant, and because of her greater mass she is not so readily maneuvered.
SUBMARINE NAVIGATION
As was explained in Chapter VI, a body will float only so long as it is lighter than the volume of water it displaces. It is almost impossible to keep a body suspended in water unless some portion of it is exposed above the surface. If it starts sinking it will keep on going down until it reaches the bottom of the sea. There is a popular notion that at great depths water becomes dense enough to float solid iron, but water is practically incompressible and its density at a depth of five miles is only slightly greater than that at the surface. An object must therefore either float on the surface or sink to the bottom, unless its weight is exactly equal to the difference between the upward pressure of the water under it and the downward pressure of the water above it. Such an ideal balance it is practically impossible to obtain unless the object itself is compressible.
How then can a submarine navigate under water without sinking to the bottom?