or sixty miles. In order to recharge these batteries and
gain motive power for subsurface travel, the submarine
has to come to the surface. Yet the simple fact that the
submarine can accomplish its destructive work only when
submerged, and that it can avoid its enemy only by diving,
makes it plain that it must always hold itself in readiness
to submerge on a moment's notice and remain under water
the longest possible time. That is, its storage batteries
must always be kept at their highest efficiency; they must
not be wasted by unnecessary travelling under the water;
the submarine, in other words, must spend all its time
on the surface, except those brief periods when it is
attempting to attack a merchant ship or escape an enemy.
Almost the greatest tragedy in the life of a submarine is
to meet a surface enemy, such as a destroyer, when its
electric batteries are exhausted. It cannot submerge, for
it can stay submerged only when it is in motion, unless it
is in water shallow enough to permit it to rest on the
bottom. Even though it may have a little electricity,
and succeed in getting under water, it cannot stay there
long, for its electric power will soon be used up, and therefore it is soon faced with the alternative of coming to the
surface and surrendering, or of being destroyed. The
success of the submarine, indeed its very existence, depends
upon the vessel spending the largest possible part of its
time upon the surface, keeping its full supply of electric
power constantly in reserve, so that it may be able to
dive at a moment's notice and to remain under the water
for the maximum period.
This purely mechanical limitation explains why the German submarine was not a submarine in the popularly accepted meaning of that term. Yet the fact that this vessel remained for the greater part of its existence on the surface was no particular disadvantage, so long as it was called upon to contend only with surface vessels. Even with the larger part of its decks exposed the U-boat was a comparatively small object on the vast expanse of the sea. I have already made clear the great disadvantage under which destroyers and other patrolling vessels laboured in their attempts to "hunt" this type of enemy. A destroyer, small as it is, was an immensely larger object than the under-water boat, and the consequence was that the lookout on a submarine, proceeding along on the
