The later _submarine cruisers_ were double-hulled, 275-320 feet long, had a surface speed of 16-18 knots, and a submerged speed of about 7 to 8 knots. They carried either one or two 5.9-inch guns, six torpedo tubes, and about 10 torpedoes. They had a very large radius of action, viz., from 12,000 to 20,000 miles, at a speed of 6 knots. A large number (some 30 to 40) of these boats were under construction at the time of the Armistice, but very few had been completed. There were two or three types of _U-boats_. The earlier vessels were 210 to 220 feet long, double-hulled, with a surface displacement of about 750 tons, a surface speed of 15 to 16 knots, and a submerged speed of about 8 knots. They carried one or two 4.1-inch guns, four to six torpedo tubes, and about 10 torpedoes. Later vessels of the class were 230 to 240 feet long, and of 800 to 820 tons surface displacement, and carried six torpedo tubes and 16 torpedoes. Some of them, fitted as minelayers, carried 36 mines, and two torpedo tubes, but only two torpedoes. A later and much larger class of minelayers carried a 5.9-inch gun, 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 6-½ 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 5-½ 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.