These carriers had hangars for storing and maintaining the aircraft, but no flight deck as in a true aircraft carrier. Instead, they used cranes to lower the aircraft into the sea for takeoff and to recover them after landing. The ships were normally converted merchant vessels rather than specially constructed for the task. As aircraft improved, the problems of using seaplanes became more of a handicap. The aircraft could only be operated in a smooth sea and the ship had to stop for launching or recovery, both of which took around 20 minutes. The tender was often stationed or so in front of the main battle fleet with the cruiser screen so that it would not fall hopelessly behind when it launched its aircraft. Seaplanes also had poorer performance than other aircraft because of the drag and weight of the floats. Seaplane tenders had largely been superseded by aircraft carriers in the battle fleet by the end of the First World War, although aircraft were still of minor importance compared to the firepower of naval artillery.