 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.The cruiser submarine concept originated during the unrestricted submarine warfare campaign of 1917.Three German Type U 139 submarines and seven former merchant submarines, each armed with two guns, patrolled areas distant from their North Sea bases to sink Allied merchant shipping as part of an effort to end World War I by starving the United Kingdom of Great Britain and Ireland.