A cruiser submarine was a very large submarine designed to remain at sea for extended periods in areas distant from base facilities. Cruiser submarines were successful for a brief period of World War I, but were less successful than smaller submarines during World War II. Large submarines remained vulnerable to damage from Defensively equipped merchant ships (DEMS), were slow to dive if found by aircraft, offered a large sonar echo surface, and were less able to defensively maneuver during depth charge attacks.[1]