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Hey could operate only in southern waters. The later _U.C.-boats_ were
170 to 180 feet long, double-hulled, had a surface speed of 11 to 12
knots and a submerged speed of about 7 knots, carried 18 mines, three
torpedo tubes, five torpedoes, and one 22-pounder gun, and their fuel
endurance was 8,000 to 10,000 miles at a speed of 7 to 8 knots. At the
end of February, 1917, it was estimated that the enemy had a total of
about 130 submarines of all types available for use in home waters, and
about 20 in the Mediterranean. Of this total an average of between
one-half and one-third was usually at sea. During the year about eight
submarines, on the average, were added monthly to this total. Of this
number some 50 per cent, were vessels of the mine-laying type. All the
German submarines were capable of prolonged endurance submerged. The
U-boats could travel under water at the slowest speed for some 48 hours,
at about 4 knots for 20 hours, at 5 knots for about 12 hours, and at 8
knots for about 2 hours. They were tested to depths of at least 180
feet, but many submerged to depths exceeding 250 feet without injury.
They did not usually lie on the bottom at depths greatly exceeding 20
fathoms (120 feet). All German submarines, except possibly the _cruiser
class_, could dive from diving trim in from 30 seconds to one minute.
The _U.B. class_ had particularly rapid diving qualities, and were very
popular boats with the German submarine officers. Perhaps the most
noticeable features of the German submarines as a whole were their
excellent engines and their great strength of construction. Prior to the
month of February, 1917, it was the usual practice of the enemy
submarine in the warfare against merchant ships to give some warning
before deli