The sea is mapped out into zones, as certain areas or parts of the sea are called, and each zone, or part, is usually assigned to a single craft; the submarine patrols this zone constantly, and the captain and his officers keep their weather eye open for passing merchantmen or vessels which might in any way aid or carry supplies to the enemy.
Courtesy of Leslie’s Weekly
A GERMAN U-BOAT “BREAKING WATER” PREPARATORY TO EXAMINING THE CARGO OF AN ENEMY SHIP
To do this the captain of the submarine stops every ship that comes his way and has her papers and cargo examined, and in this way finds out whether the ship is what she seems to be or if she carries contraband—that is arms, ammunition, and war supplies of any kind—or not.
The way in which this interesting procedure is done is as follows:
The submarine, let us say, has sighted a ship, and seeing that it is to all intents an unarmed merchantman she rises to the surface and trains her rapid-fire guns on the craft. Next she signals the ship to stand by and at the same time she runs toward her.
Now the captain of the ship has three courses open to him: (1) to put on full speed and try to get away, trusting to luck to prevent his craft from being shot full of holes or torpedoed; (2) to take a still greater chance and try to ram the submarine with the sharp, steel-shod bow of his ship and so either disable or sink her, and (3) to comply gracefully to the request and heave to.
If the latter is done, the collapsible boat of the submarine is rigged up and a couple of the officers and crew row over to the ship, when they are hauled aboard and go through her. Should they find nothing of a suspicious or contraband nature, the boat returns to the submarine and the ship is allowed to go her way.
But, on the other hand, if contraband is found on board, the captain of the submarine will do one of these two things: (1) he will warn the crew of the enemy ship that he is either going to open fire on her with his guns or torpedo her, and to take to the life-boats, or (2) he will sink her without warning if the whim so seizes him. Often the captain of the destroyed craft is taken aboard the submarine and held as a hostage.