Submarines had to take to cover when these chasers were about. _Herr Kommandant_ did not even dare to take a look around through his periscope, because the streak of foam that trailed in its wake would betray him and immediately the speedy motor-boats would take up the chase; and they had a disagreeable way of dropping bombs which, even if they did not sink the submarine, might produce such a concussion as to spring its seams. His foes had discovered one of his most serious defects. He was blind under-water and they were making the most of this handicap.
Forward End of a U-boat. Note the Four Torpedo Tubes Behind the Officer]
Groping along under-water by dead-reckoning was not any too safe a procedure near land, because he was liable at any moment to crash into an uncharted rock or maybe into the wreck of some submarine victim. He could not correct his bearings without coming to the surface, and, in the black depths of the sea, a slight miscalculation might send him to his doom. As was explained in the previous chapter, he had to keep moving, because he could not remain suspended under water.
[Illustration: (C) Press Illustrating Service
A Depth-bomb Mortar and a Set of "Ash Cans" at the Stern of an American Destroyer]
He was more helpless than a ship sailing in the densest of fogs. A ship can stop and listen to sound-signals, or even to the beating of the surf on the shore, or it can take soundings to locate its position; and yet it is no uncommon occurrence for a ship to run ashore in a fog. How much easier it is for a submarine to lose its bearings when obliged to travel by dead-reckoning, particularly in the disconcerting excitement of the chase! To avoid the danger of collision with surface vessels, the commander chose to run at a depth of sixty-five feet. That was the upper limit of his safety-zone. A depth of over two hundred feet was his lower limit, because, as stated before, the water-pressure at that depth would crush in his hull or at least start its seams. If the bottom were smooth and sandy, and not too deep, he could settle gently upon it and wait for darkness, to make his escape.
But while he lay on a sandy bottom, he was still in danger. Trawlers were sweeping the bottom with nets. He might be discovered; and then if he did not come up and surrender, a bomb would let in the sea upon him.