Another point of the highest importance is that in a conflict of submarine against submarine the Allied boats had one great advantage over the German. Hans Rose and Valentiner and Moraht and other U-boat commanders, as already explained, had to spend most of their time on the surface in order to keep their batteries fully supplied with electricity, in readiness for the dives that would be necessary when the Allied destroyers approached. But the Allied submarine commander did not have to maintain this constant readiness; the reason, it is hardly necessary to say, is that the Allied submarine had no surface enemies, for there were no German surface craft operating on the high seas; the Grand Fleet at Scapa Flow was carefully attending to that very essential detail. Occasionally, indeed, our submarines were attacked by our own destroyers, but accidents of this kind, though uncomfortably frequent, were not numerous enough to interfere with the operation I have in mind. The statement seems almost like a contradiction in terms, yet it is entirely true, that, simply because the Allied submarines did not have to hold themselves constantly ready to submerge, they could in fact spend a considerable part of their time under the water, for they were not compelled to economize electric power so strictly. This gave them a great advantage in hunting the U-boats. British and American submarines could fully charge their batteries, drop under water and cruise around with enough speed to maintain a horizontal position at "periscope depth," that is, a depth just sufficient to enable them to project the periscope above the water whenever desired. This speed was so very slow--about one mile an hour--that it could be kept up an entire day without exhausting the electric batteries.
The net result was this: The German submarine necessarily sailed most of the time on the surface with its conning-tower and deck exposed, whereas the Allied submarine when on its hunting grounds, spent all of the daylight hours under water, with only the periscope visible from time to time for a few seconds. Just as the German U-boat could "spot" an Allied destroyer at a great distance without being itself seen, so could the periscope invariably see the German submarine on the surface long before this tiny object came within the view of a U-boat conning-tower. Our submarine commander could remain submerged, sweep the ocean with his periscope until he had picked up the German enemy; then, still under water, and almost invariably unseen, he could steal up to a position within range, and discharge a torpedo into its fragile side. The German submarine received that same treatment which it was itself administering to harmless merchantmen; it was torpedoed without warning; inasmuch, however, as it was itself a belligerent vessel, the proceeding violated no principle of international law.