Besides thoroughly reconnoitering a harbor, the submarine can lay a few contact mines, as we explained in a chapter that has gone before, in positions of which the enemy fleet is in entire ignorance and which will be more than likely to result in the destruction of at least a part of the fleet.
Further, the submarine is used to destroy fields of mines which have been laid by the enemy in a harbor. This is done by mines thrown from the torpedo tubes among those that have been planted and which explode by the concussion.
The Submarine as a Blockader.—The most important use to which the submarine has ever been put and one that was never thought of seriously until the present conflict is that of a blockader.
Ever since the beginning of this war Germany has realized the tremendous need of keeping the neutral[34] countries from supplying the Allies with munitions and food supplies, and she has prepared for years a blockade of a new and very effective kind, and this is by destroying merchantmen by submarines.
England and the other Allies have done the same thing with the Central Powers—which is not a very hard thing to do because Germany’s fleet of warships is cooped up in her various ports and dare not venture forth, and so the task is left entirely to her submarine flotilla.
And what makes it still harder for the German submarines is that the Allies keep on the constant watch for these enemy undersea craft, and this they do with their submarine destroyers, and the United States is after them with her submarine chasers, to say nothing of England’s aircraft attacks.
Altogether it is very hard for the enemy submarines to keep in touch with their respective bases or to receive orders as to their courses of action. When on blockade duty, then, the captain of a submarine is in very truth, the commander of his craft and it is strictly up to him to determine what her tactics shall be.
As long as he does his work well, which means that he sinks a fair number of all the ships that enter his zone, his superior officer, wherever he is, will have no quarrel with him as to when or how he does the work.
So you see the tactics used by the captain of a submarine while doing this kind of work depend entirely on the conditions he encounters at the moment, and on the quick decision and judgment of the captain depends the success or failure of the attack.
How a Submarine Attacks a Merchantman.—As a general thing submarines travel alone when merchantmen are to be torpedoed.