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.