four torpedo tubes, 42 mines, and a
larger number of torpedoes. The earlier _U-boats_ could keep the sea for
about five weeks without returning to a base or a supply ship; the later
_U-boats_ had much greater sea endurance.

The smaller _U.B.-boats_ were single-hulled, and about 100 feet long,
had a surface speed of 7 to 9 knots and a submerged speed of about 5
knots, and carried one 22-pounder gun, two torpedo tubes and four
torpedoes. These boats could keep the sea for about two weeks without
returning to a base or supply ship. A later class were double-hulled,
180 feet long, with greater endurance (8,000 miles at 6 knots), a
surface speed of 13 knots and a submerged speed of 8 knots; they carried
one 4.1-inch gun, five tubes and 10 torpedoes.

The earliest _U.C.-boats_ were 111 feet long, with a surface
displacement of 175 tons, a surface speed of 6-1/2 knots, and a submerged
speed of 5 knots. They carried 12 mines, but no torpedo tubes, and as
they had a fuel endurance of only 800 miles at 5-1/2 knots, they 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