The best deterrent against submarine attack is robust defense—but as little as surface sailors like to discuss it, that defense has seldom been less assured.

Modern diesel-electric submarines (SSKs) are very hard to detect. It’s not that SSKs with air-independent propulsion (AIP) systems are much quieter, but they mitigate the SSK’s drawback: lack of speed and endurance on quiet electric power. When the Swedish AIP boat Gotland operated with the U.S. Navy out of San Diego in 2005-07, the Navy’s surface ships turned up all too often in a photo album acquired by the submarine’s mast. The sub was so quiet, that it consistently managed to get within easy torpedo range.

AIP submarines are a high priority in the budgets of nations such as Singapore, Korea and Japan. Russia has struggled with its Lada-class boats, but persisted, and is selling them to China. Sweden, whose Kockums yard developed the AIP technology for Japan’s big 4,100-ton Soryu-class subs, had trouble getting its A26 replacement submarine program started. In an indication of its importance, Saab will buy the Kockums yard back for Sweden from ThyssenKrupp Marine Systems.