III
German submarines have sunk over 7,250,000 tons of the Allied shipping. In December, 1916, it was stated in the British Parliament that the merchant marine of Great Britain had at that time over 20,000,000 tons. Within the first three months of the unrestricted submarine warfare, 1,100,000 tons of British shipping went to the bottom. At this rate, England would lose 25 per cent of her merchant marine per annum. It is for this reason that the attention of the entire world is concentrated upon the vital problem of the submarine menace. On land, the Central Powers are still holding their ground, but there is a continuous increase of the forces of the Allies which should lead finally to such a preponderance of power as will overwhelm the forces opposed to them. The Allied armies, however, depend for their sustenance and supplies upon the freedom of the seas. The trade routes of the world constitute the arteries which feed the muscles of these armies. Germany is endeavoring to cut these arteries by the submarine. Should she even appreciably limit the supplies that cross the ocean to the Allies, she will bring about a condition that will make it impossible to augment their armies. In this way there will inevitably be a deadlock, which, from the German standpoint, would be a highly desirable consummation.
Obviously, the first method of handling the submarine problem would be to bottle the German undersea craft in their bases. There has been a number of proposals as to how best to accomplish this. It has been stated that the English Navy has planted mines in channels leading from Zeebrugge and other submarine bases; but it is necessary only to recall the exploits of the E-11 and the E-14 of the British Navy at the Dardanelles, to see that it would not be impossible for the Germans to pass in their U-boats through these mine-fields into the open sea. It will be remembered that the E-11 and the E-14 passed through five or more mine-fields, thence through the Dardanelles into the Sea of Marmora, and even into the Bosphorus under seemingly impossible conditions. Yet, in spite of the tremendous risks that they ran, these boats continued their operations for some time, passing up as far as Constantinople, actually shelling the city, sinking transports, and accomplishing other feats which have been graphically described in the stories of Rudyard Kipling. And again, if the mine-fields were placed in close proximity to their bases, it would be comparatively easy for German submersibles of the Lake type, possessing appliances to enable divers to pass outboard when the vessel is submerged, to go out and cut away the mines and thus render them ineffective.
Nets are also used to hinder the outward passage of the submarine. These nets can likewise be attacked and easily cut by devices with which modern U-boats are equipped. The problem of placing these obstacles is a difficult one, in view of the fact that the ships so engaged are harassed by German destroyers and other enemy craft. Outside of Zeebrugge, shallow water extends to a distance of about five miles from the coast, and it has been suggested that a large number of aircraft, carrying bombs and torpedoes, should be used to patrol systematically the channel leading from that port to deep water, with the intent of attacking the submersibles as they emerge from this base. It is ridiculous to suppose that the Germans would not be able to concentrate an equally large number of aircraft, to be supported also by anti-aircraft guns on the decks of destroyers and by the coast defenses. We have not yet won the supremacy of the air, and it must inevitably be misleading to base any proposition on the assumption that we are masters of that element.
The problem of bottling up the submersibles is enormously difficult, because it necessitates operations in the enemy's territory, where he would possess the superiority of power. I believe that the question of operations against the submarine bases is not a naval but a military one, and one which would be best solved by the advance of the Western left flank of the Allied armies.
The second method is to attack the submarines with every appliance that science can produce. In order to attack the submarine directly with any weapon, it is necessary first to locate it. This is a problem presenting the greatest difficulty, for it is by their elusiveness that the submarines have gained such importance in their war on trade. They attack the more or less helpless merchant ships, and vanish before the armed patrols appear on the scene.
Almost every suitable appliance known to physics has been proposed for the solution of the problem of submarine location and detection. As the submarine is a huge vessel built of metal, it might be supposed that such a contrivance as the Hughes induction balance might be employed to locate it. The Hughes balance is a device which is extremely sensitive to the presence of minute metallic masses in relatively close proximity to certain parts of the apparatus. Unfortunately, on account of the presence of the saline sea-water, the submersible is practically shielded by a conducting medium in which are set up eddy currents. Although the sea-water may lack somewhat in conductivity, it compensates for this by its volume. For this reason, the induction balance has proved a failure.
But another method of detecting the position of a metallic mass is by the use of the magnetometer. This device operates on the principle of magnetic attraction, and in laboratories on stable foundations it is extremely sensitive. But the instability of the ship on which it would be necessary to carry this instrument would render it impossible to obtain a sufficient degree of sensitiveness in the apparatus to give it any value. The fact that the submersible is propelled under water by powerful electric motors begets the idea that the electrical disturbances therein might be detected by highly sensitive detectors of feeble electrical oscillations. The sea-water, in this case, will be found to absorb to a tremendous extent the effects of the electrical disturbance. Moreover, the metallic hull of the submersible forms in itself an almost ideal shield to screen the outgoing effect of these motors.
Considerable and important development has been made in the creation of sensitive sound-receiving devices, to hear the propeller vibrations and the mechanical vibrations that are present in a submersible, both of which are transmitted through the water. There are three principal obstacles to the successful use of such a device: when the submersible is submerged, she employs rotary and not reciprocating prime-movers, being in consequence relatively quiet when running under water, and inaudible at any considerable distance; the noises of the vessel carrying the listening devices are difficult to exclude, as are also the noises of the sea, which are multitudinous; finally, the sound-receiving instruments are not highly directive, hence are not of great assistance in determining the position of the object from which they are receiving sounds.[3]
To locate the submersible, aërial observation has been found useful. It is particularly so when the waters are clear enough to observe the vessel when submerged to some depth, but its value is less than might be supposed in the waters about the British Isles and Northern Europe, where there is a great deal of matter in suspension which makes the sea unusually opaque. The submersible, however, when running along the surface with only its periscope showing, is more easily detected by aircraft than by a surface vessel. Behind the periscope, there is a characteristic small wake, which is distinguishable from above, but practically invisible from a low level of observation. Many sea-planes are operating on the other side for the purpose of locating enemy submersibles and reporting their presence to the surface patrol craft. In order to overcome the disadvantages of creating the periscope wake which I have mentioned, it is reported that the Germans have developed special means to allow the U-boats, when raiding, to submerge to a fixed depth without moving. To maintain any body in a fluid medium in a static position is a difficult matter, as is shown in the instability of aircraft. One of the great problems of the submersible has been to master the difficulties of its control while maintaining a desired depth. The modern submersible usually forces itself under water, while still in a slightly buoyant condition, by its propellers and by the action of two sets of rudders, or hydroplanes, which are arranged along its superstructure and which tend to force it below the surface when they are given a certain inclination; but should the engines stop, the diving rudders, or hydroplanes, would become ineffective, and, because of the reserve buoyancy in the hull, the vessel would come to the surface.
In order to maintain the vessel in a state of suspension under water without moving, it would be necessary to hold an extremely delicate balance between the weight of the submarine and that of the water which it displaces. Variations in weights are so important to the submersible that, as fuel is used, water is allowed to enter certain tanks to compensate exactly for the loss of the weight of the fuel. To obtain such an equilibrium, an automatic device controlled by the pressure of the water, which, of course, varies with the depth, is used. This device controls the pumps which fill or empty the ballast-tanks, so as to keep the relation of the submersible to the water which it displaces constant, under which condition the vessel maintains a fixed depth. The principle of this mechanism is, of course, old, and was first embodied in the Whitehead torpedo, which has a device that can be set so as to maintain the depth at which it will run practically constant. With the addition of a telescopic periscope, which can be shortened or extended at will, it will be possible for the U-boat to lie motionless with only the minute surface of the periscope revealing her position.