In regard to their general outward appearance, submarines of various types to-day vary comparatively little. In many respects they resemble closely in shape, torpedo boats--the earlier submarines particularly. In size, of course, they differ in accordance with the purposes for which they have been designed. As compared with earlier submarines the most notable difference is that modern submarines possess more of a superstructure. Almost all of them are built now with double hulls. The space between the outer and the inner hull is utilized primarily for ballast tanks by means of which submergence is accomplished and stability maintained and regulated. Some of these tanks, however, are not used to carry water ballast, but serve as reservoirs for the fuel needed by the engines. The stability of the submarine and the facility with which it can submerge also depend greatly on the distribution of weight of its various parts. This problem has been worked out in such a way that to-day there is little room for improvement. Its details, however, are of too technical a nature to permit discussion in this place.
Hydroplanes both fore and aft are now generally used to assist in regulating and controlling stability in the submerged state. The motive power of the modern submarine is invariably of a two-fold type. For travelling on the surface internal combustion engines are used. The gasoline engine of former years has been displaced by Diesel motors or adaptations of them. Although these represent a wonderful advance over the engines used in the past there is still a great deal of room for improvement. The opinions of engineers in this respect vary greatly, American opinion being generally unfavourable to the Diesel type, and whether the final solution of this problem will lie in the direction of a more highly developed motor of Diesel type, of an improved gasoline engine, or of some other engine not yet developed, only the future can tell. Simplicity of construction and reliability of operation are the two essential features which must be possessed by every part of the power plant of a submarine. For underwater travel electric motors and storage batteries are employed exclusively. These vary, of course, in detail. In principle, however, they are very much alike. Although this combination of electric and oil power is largely responsible for having made the submarine what it is to-day, it is far from perfect. Mechanical complications of many kinds and difficulties of varying degrees result from it. Up to comparatively recently these were considered insurmountable obstacles. But engineers all over the world are giving their most serious attention to the problem of devising a way to remove these obstacles and continuous progress is made by them.
As an immediate result of the development of motive power in the submarine its speed both on and below the surface of the water as well as its radius of action has been materially increased. To-day submarines travel on the water with a speed which even a few years ago would have been thought quite respectable for the most powerful battleships or the swiftest passenger liners. And even under water, submarines attain a velocity which is far superior to that of which earlier submarines were capable on the surface of the water. How immensely extended the radius of action of the submarine has become in recent years, has impressed itself on the world especially in the last few years. Both English and French submarines have travelled without making any stops from their home ports to the Dardanelles and back again. And used to, and satiated as we are with mechanical wonders of all kinds the whole world was amazed when in 1916 German submarines made successful trips from their home ports to ports in the United States and returned with equal success. This meant a minimum radius of action of 3500 miles. In the case of the German U-boat which in 1916 appeared at Newport for a few hours, then attacked and sank some merchantmen off the United States coast and later was reported as having arrived safely in a German port, it has never been established whether the boat renewed its supplies of food and fuel on the way or carried enough to make the trip of some 7000 miles.
