Cruiser submarine

Their role was analogous to surface cruisers; 'cruising' distant waters, commerce raiding, and otherwise operating independently.

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.

Three German Type U 139 submarines and seven former merchant submarines, each armed with two 15-centimetre (5.9 in) 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.

These distant patrols enjoyed unique immunity to the defensive convoy measures which limited successful submarine attacks in the vicinity of the British Isles.

Japanese focus on the distances of their Pacific trade routes encouraged development of the widest variety of submarine cruisers, notably the types Junsen (巡潜, "Cruiser Submarine") including the types J, A, B, and C. Germany decided against building projected 3,140-ton type XI U-boats with an aircraft hangar and four 5-inch (13 cm) guns.