It is now becoming apparent why the great battle fleet, which always sailed behind a protecting screen of such destroyers, was practically immune from torpedo attack. In order to assail these battleships the submarine was always compelled to do the one thing which, above all others, it was determined to avoid--to get within depth-charge radius of the surface craft. In discharging the torpedo, distance, as already intimated, is the all-important consideration. The U-boat carries a torpedo which has a much shorter range than that of the destroyer; it was seldom effective if fired at more than 2,000 yards, and beyond that distance its chances of hitting became very slight. Indeed, a much shorter distance than that was desirable if the torpedo was to accomplish its most destructive purpose. So valuable were these missiles and so necessary was it that every one should be used to good advantage, that the U-boat's captain had instructions to shoot at no greater distance than 300 yards, unless the conditions were particularly favourable. In the early days, the torpedoes which were fired at a greater distance would often hit the ships on the bow or the stern, and do comparatively little damage; such vessels could be brought in, repaired in a short time, and again put to sea. The German Admiralty discovered that in firing from a comparatively long distance it was wasting its torpedoes; it therefore ordered its men to get so near the prey that it could strike the vessel in a vital spot, preferably in the engine-room; and to do this it was necessary to creep up within 300 yards. But to get as close as that to the destroyers which screened the battleships meant almost certain destruction. Thus the one method of attack which was left to the U-boat was to dive under the destroyer screen and come up in the midst of the battle fleet itself. A few minutes after its presence should become known, however, a large number of destroyers would be dropping depth charges in its neighbourhood, and its chances of escaping destruction would be almost nil, to say nothing of its chances of destroying ships.