An important armament variant was the submarine , which was adapted to be able to lay mines on covert missions off enemy ports. The Germans designed several specialized submarines with vertical mine tubes in the hull. Some submarines carried 48 mines in addition to torpedoes. The wartime development of the submarine concept for anti-submarine warfare was also notable. British submarines sank 17 German submarines during the conflict. Early success in anti-submarine combat led the British to develop the R-class submarines specialized for this role. These were relatively small boats, 163 feet long, with a surface displacement of 410 tons, and only one propeller (most submarines at the time had two). The diesel engines could propel them at nine knots on the surface, but once submerged, large batteries allowed the electric motors to power them at a speed of 15 knots underwater for two hours. (Until after World War II, a submarine speed of 10 knots was common.) They ...