Submarines are getting quieter, stealthier, and better armed. And that could mean major trouble for the U.S. Navy and its aging fleet of sub-hunters. The tactical balance between the surface warship and the submarine has strategic impact. The submarine is not made for a show of force. Its principal weapon is designed not to damage a ship, but to sink it—rapidly and probably with much loss of life. It’s a sure way to shift the trajectory of any conflict in a more violent direction.

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.

AIP—which uses stored liquid oxygen and fuel to generate power underwater—seems to be here to stay, whether it uses the Swedish-developed Stirling-cycle engine (a 19th-century curiosity, but very efficient) or fuel cells, favored by ThyssenKrupp’s German yards and Russia. Lithium-ion batteries will further increase underwater performance. Kockums advertises another step in invisibility called Ghost (genuine holistic stealth) which, like stealth technology on an airplane, involves the careful blending of hull shapes and rubber-like coatings to make the submarine into a weak sonar target.

Other improvements are making the submarine more elusive and lethal. Masts with high-definition cameras are as clear as direct-vision optics—so the mast needs only to break the surface and make a single sweep to provide a full horizon view. Finmeccanica’s WASS division and Atlas Electronik offer modern all-electric torpedoes with multiple guidance modes, from fiber-optic to wake-homing, and back-breaking influence fuzes that work too often for comfort.

Antisubmarine warfare (ASW) has not stagnated, but it shows signs of disarray. After the end of the Cold War stopped the Soviet Union’s push for quieter submarines, the U.S. scrapped improvements to the P-3 sub-hunting plane and the P-3’s replacement. The carrier-based S-3 Viking went the same way, and the U.K., more recently, retired the Nimrod and canceled its deeply flawed MRA4 replacement sub-hunters. ASW assets and crews have been diverted to reconnaissance missions in overland and littoral wars. The Navy’s strategy for the new Boeing P-8A Poseidon is to get the airframes first, because P-3s are wearing out.

The U.S. Navy’s ASW future hinges on two new technologies: multistatic, active, coherent (MAC) acoustic systems, or sonar, and automated radar detection of periscopes. Today, airplanes mainly hunt submarines by para-dropping a pattern of sonobuoys, most of which are passive listening devices. “Active” search nodes depend on noise sources that can be as simple as an explosive squib. Planned for later P-8A models, MAC uses buoys that can transmit tones and sophisticated waveforms that, when they bounce off the sub and are picked up by the other buoys in the network, can accurately pin down its position. MAC is likely to be quite costly to operate—the P-8A carries many more buoys than a P-3, and the buoys are more complex. Testing so far has not been a disaster, but it has been limited. One series of tests last year was truncated so that the test aircraft and crew could go and chase drug-runners. Picking real targets from false targets and clutter is still down to operators.

Better ways to detect periscopes—with the radar cross-section of a floating Coke can—have been under study since the early 1990s, but the Navy has vacillated on deployment plans. The new Automatic Radar Periscope Detection and Discrimination (ARPDD) technology—which uses very fast scanning and a lot of signal processing to tell a slow-moving scope from drifting debris—was to be used on upgraded P-3 radars. But in 2005—after the Gotland tests started, which may not have been a coincidence—the plans changed to stress close-in defense of the aircraft carrier, with ARPDD used first on MH-60R helicopters and on a radar mounted on the carrier itself. ARPDD disappeared from the P-8 radar requirement, then returned. More recently, the carrier-mounted radar has been discontinued and surface combatants will have ARPDD.

But the key to telling the periscope and the Coke can apart is that one of them is moving purposefully, and an electronic mast that surfaces intermittently makes an even less obvious track than a direct-view periscope that has to stay up to function. That change was not in sight when ARPDD was conceived.

Surface warfare may be heading for a strategic dilemma. The surface combatant is vital for many missions—but its utility could be drastically limited if a submarine threat imposes a no-go area. And as more new AIP subs enter service, denying the problem is less and less of an option.

This column also appears in the May 12 issue of Aviation Week & Space Technology. For more of Sweetman’s columns, see: aviationweek.com/columns/commanders-intent