Now take for another comparison a slow-speed merchantman of the tramp type making ten knots, which is about the economical speed for this class of ship. Her smoke might be the first thing to betray her approach, but for purposes of comparison take her smoke-stack also, which is the first solid portion of the ship to appear. The smoke-stacks of this class of vessel would probably not be over forty feet in height above water level, therefore, if she were making the same course as the high-speed ship, it will be observed by referring to diagram, position No. 4, and the distance and speeds mentioned thereon, that the submarine at a speed of ten knots has more time to get nearer the course of the approaching ship and can have more time to calculate the enemy's speed of approach and direct course, and thus launch his torpedo with more certainty of making a hit. But assume that this approaching slow-speed ship had no solid opaque portion extending over fifteen feet above the surface of the water, as is the case in a cargo submarine as shown in position No. 3 on the diagram of the earth's surface. One now sees that she would pass the waiting submarine below the horizon, and the intervening round of the sea's surface would prevent the submarine from seeing her; thus she would pass by unseen and in safety.
The above series of diagrams will show the percentage of safety of ships of different characteristics when coming within the range of visibility of a submarine lying on the ocean highway waiting for passing ships; the submarine is assumed to have a submerged speed of ten knots in each instance. From an analysis of these diagrams it cannot be denied that practically one hundred per cent. safety could be secured could these cargo-carrying submarines cross the ocean from one friendly port to another and remain invisible during the entire journey, but at the present time this is impossible, because there is no known means of supplying sufficient power for long under-water voyages without drawing on the upper air in large quantities to assist combustion in either prime or secondary power-generating machinery.
The diagram plainly shows that a cargo-carrying submarine running awash, with her periscope and air intakes only above the water line, may approach within about five and three-quarters miles of any waiting military submarine without danger of being seen, as her "wake" would be below the horizon. Such cargo-carrying submarines can be built and can cross the Atlantic Ocean in this condition at a speed of about ten knots, and by maintaining a sharp lookout would have as much chance of seeing a military submarine as the military submarine would have of seeing them; and by the application of certain tried devices which I do not feel it proper to disclose at this time, but which are within the knowledge of our government authorities, the range of visibility can, I believe, be reduced to less than one mile. This type of vessel can almost instantly become entirely invisible by submerging at the least intimation of danger.
Such a type of vessel travelling with a freeboard of five feet would become visible to a submarine lying in ambush when she approached within eight miles. This increases the area of danger from one hundred and three square miles, as shown in diagram, position No. 1, in the first instance to two hundred and one square miles, as per diagram corresponding to position No. 2, but in comparison with the usual type of surface cargo-carrying ship of the so-called tramp type she is comparatively safe, as she has the ability to submerge in less than two minutes; and it is hardly likely that she would be attacked without warning, for fear she might be a friendly military submarine. Any communication in the way of wireless, sound, or other signals would, if she were a merchant ship, give her warning, and she would at once submerge, as her only business would be to deliver her cargo and not communicate with or expose herself to either friend or foe. When far from land she might take a chance and navigate entirely on the surface with a freeboard of fifteen feet, in which condition she can make a speed of eleven knots, as her position No. 3, on the surface of the ocean. This increases the danger area to about three hundred and thirty square miles, as on diagram, position No. 3, about three times the danger area shown on position No. 1, but as the area to be covered by the military submarine on the high seas, far from land, is also much greater, the real danger would be proportionately less than that with the lower visibility in a more thickly infested zone.
TORPEDO BEING FIRED FROM THE DECK TUBES OF THE SUBMARINE "SEAL"
This vessel was fitted with two double-barrel torpedo guns, housed in a superimposed superstructure. These four torpedoes could be fired to either broadside. The above photograph shows a torpedo in the act of leaving one of these tubes above water. They may be discharged either above or below the surface.
High Speed.—Speed is better than no defence, but no one would consider building twenty-five-knot freighters. The cost would be far out of proportion to the service. So long as U-boats do not betray their presence, a fast vessel is almost as liable as a slower one of less freeboard or lower top hamper. One can never tell where the submarine may be lurking, and her capacity to harm is determined by her ability to locate her prey. There are three means available to her to locate her target: first, her own sight; second, her sound-detecting devices; third, by wireless directions given to her by others who may advise her of the vessel's position. Her own sight is the best and usual means for locating her target. The above diagrams show that the largest and fastest ships can be located at much greater distances than the low visibility ships, and that the area of visibility becomes the area of danger, which is practically ten times greater in an expensive, large, high-speed liner over that of the comparatively low-cost cargo-carrying submarine.
BRITISH SUBMARINE NO. 3 PASSING NELSON'S OLD FLAGSHIP "VICTORY"