They prefer to fire at 500 to 700 yards, which means that at this range the track or "wake" of a projectile would be discernible for, say, twenty-five to thirty seconds—not much time, indeed, for any ship to get out of the way. At 100 yards' range or less they do not care to fire unless compelled to, as the torpedo is nearly always discharged when the submarine is lying ahead of the object, i.e., to hit the ship coming up to it; it follows that a gun forward is more useful than one aft, the gun aft being of real service when a submarine starts shelling, which she will do for choice from aft the ship rather than from forward of her, where she would be in danger of being run over and rammed.
Owing to the big expense of wasted torpedoes it had been planned to build special larger U-boats, two-deckers carrying larger guns for shelling purposes—hence the sooner all merchantmen are well armed the better for them.
Talking one day of the number of submarines lost, the officers declared that the Allies' reports of their captures and sinkings were much exaggerated. Naturally they would say so, but the truth is probably best known at headquarters, where such figures are checked up. Once or twice when on the surface we were fired at at long range and quickly submerged, although the U crews recognize they are a difficult target to aim at, let alone hit, as I, too, knew from experience.
The periscope is about seven inches in diameter and moves up and down like a piston rod in a cylinder, accompanied by a buzzing kind of sound, which became familiar after a day or two. If required it can reach seventeen feet above the surface and may be used as a flagpost for signals. At its summit is a mirror, three to four inches in diameter, from which images are reflected to a larger mirror below used by the observer.
When a ship is first sighted the chief point is to determine her course, which may be easy if she is going in the same direction, but more difficult if not. The next point is to determine as near as possible the speed of the object ship, the size and general lines being considered, and deductions drawn. Next, the distance away of the approaching ship has to be noted.
Under ordinary circumstances on board ship the nearer one standing on deck is to the water the more limited will the horizon be, and likewise the higher from the water, as in the "crow's nest," the bigger the horizon. Standing on deck, seventeen feet from the surface of the water, the horizon, on a clear day, would, roughly speaking, be three miles. The same rule practically applies in periscope work, which, as it can extend seventeen feet in the air, will give a three-mile horizon vision. Therefore, according as is the position on the horizon rays, so will the distance away of the object ship be gauged.
The course, speed, and distance off being calculated, the U has then to manœuvre get into position for firing, taking care to avoid the actual track of the approaching ship and to get nearly "broadside on" to her. Hence there is nothing worries them and upsets their calculations more than when the ship alters her course frequently—this manœuvre being called zigzagging—and which accounts largely for the high percentage of misses registered. The more often the alteration of course takes place, the bigger and more uneven the zigzag, the better for the ship.
Daylight is most suitable for torpedo firing, but there is nothing to prevent its being accurate enough during moonlight, even so far as minor work is concerned. As the torpedo can be discharged from the surface as well as when submerged, the precaution of having every light on the ship extinguished is a necessary one. It is remarkable how few collisions at sea have taken place during the war, even though the ships are running without any navigation lights and even during fog no whistle is sounded. The periscope being in those days a "familiar object," it is believed that some of the later U-boats have another device in the shape of a small hatch opening upward which can be opened by the observer on the submarine, enabling him to look around the surface of the water.
Ordinary navigation on the U-boat is done on usual lines, i.e., sextant and compass and bearings from landmarks by day and observation by stars and of lighthouses by night, but the reckoning of distances by dead reckoning, that is, on the log, has to be dispensed with. The compass used is a gyroscopic one, so there is no deviation due to magnetic influence, and the courses are all therefore "true courses."
There were no signs of any soundings being taken while submerged, but of course they were frequently taken from the surface while approaching land. While on the surface, if the sea was at all rough, the U rolled considerably and the officers, in oilskins, were lashed to the nearest stanchion to prevent being washed overboard. On the for'ard deck on either side of the bow was a small anchor about the same size as used on a 500-ton ship.