Summary and Key Points: Sweden's Gotland-class submarines, known for their advanced diesel-electric and Air-Independent Propulsion (AIP) systems, are now part of NATO's arsenal following Sweden's NATO membership.

These submarines are ideally suited for the shallow waters of the Baltic Sea, a region challenging for nuclear-powered submarines.

Their stealth capabilities were proven in 2005 when a Gotland-class submarine "sank" the U.S. aircraft carrier USS Ronald Reagan during exercises.

With their low cost and high effectiveness, the Gotland-class submarines enhance NATO's naval strength, especially against potential Russian threats in the Baltic region.

As Sweden has now officially joined NATO, the alliance will have yet another potent tool in its arsenal. In addition to the F-35 fighters that are in service with multiple alliance members, the U.S. bomber fleet, and notably the nearly dozen nuclear-powered aircraft carriers, NATO will have one of the most capable diesel-electric submarines in service in the world.

Stockholm's three Gotland-class submarines are ideally suited to the waters of the Baltic Sea, which is fittingly known as a "flooded meadow" due to its average depth of around 60 meters. That makes it too shallow for the nuclear-powered submarines that comprise the bulk of Russia's submarine fleet and all of the U.S. Navy's subs.

"The Swedish submarine fleet is well prepared for this environment and will add greatly to NATO's overall submarine capabilities in the Baltic," a NATO official told Reuters last summer.

The Gotland-class boats, which many military analysts have described as the most advanced conventionally powered submarines ever built, could be vital to keeping the waters navigable in a NATO conflict with Russia. They ran so quietly that they successfully "sank" a U.S. carrier during drills in 2005.

The crazy part: These subs only cost $100,000,000--and they 'sank' a $4,500,000,000 aircraft carrier in such wargames. 

Gotland-Class Submarine: Silent Running

As the UK-based Forces.net explained, the Gotland-class were the world's first diesel submarines to use an Air-Independent Propulsion (AIP) system based on the Stirling engine – where heating and cooling gases in a closed circuit can be harnessed for motion.

The AIP system, which is usually exclusive to nuclear-powered submarines, allowed Stockholm's vessels to stay submerged for weeks. Diesel engine submarines typically need to surface every few days or draw in air from the snorkel to recharge the batteries. Instead, with the Gotland-class, the batteries are saved for times when the submarines need speed but otherwise cruise on the AIP. The subs can reach speeds of eleven knots on the surface and twenty knots submerged.

The Gotland-class submarine uses liquid oxygen and diesel to heat the engine and cold seawater for cooling, which expands or contracts gases in the closed engine system which pushes a piston to drive the sub's systems.

Another advantage of the AIP system is that the submarines cost far less than a nuclear-powered submarine, but remain deadly quiet while cruising underwater at low speeds, even for weeks at a time.

However, it isn't the AIP that makes the boats so difficult to detect. Each of the Swedish subs are equipped with 27 electromagnets, strategically positioned to counteract magnetic signatures that could reveal their presence to enemy sensors. That technology is particularly useful for countering magnetic influence mines, making the Gotland-class submarines less susceptible to this type of threat, Interesting Engineering also reported.

Moreover, to reduce acoustic detection, the hulls of the Gotland-class submarines are coated with an innovative sonar-resistant material that can absorb and scatter sound waves, making the submarines more challenging to detect with sonar systems. Finally, a radar-absorbent tower further reduces the radar cross-section, minimizing the chances of being detected by airborne surveillance systems, such as maritime patrol aircraft or shipborne radar.

A Trio of Boats

The Swedish Navy currently operates three of the diesel-electric submarines, which were designed and built by Kockums shipyard. Developed as a multirole submarine and built between 1990 and 1997, the Gotland-class can be employed in anti-surface warfare (ASuW), anti-submarine warfare (ASW), collection of intelligence including communications intelligence (COMINT), and electronic signals intelligence (ELINT), as well as forward surveillance, special operations, and mine-laying – the submarines have proven to be well-suited to each.

In 2022, the Swedish Defense Material Administration (FMV) announced that it had awarded a mid-life upgrade (MLU) contract to Saab for the Gotland-class submarine HMS Halland. Under the upgrade project, which is reportedly valued at $116.75 million, Saab will carry out modernization and modification work on the submarine – during which time the boat's existing combat systems will also undergo an overhaul. The first two submarines of the class have already gone through the MLU program, and as part of the contract, Saab will install more than 50 new systems on HMS Halland.

HSMS Gotland vs. Aircraft Carrier USS Ronald Reagan

The AIP technology and other stealthy characteristics proved especially successful when the United States Navy leased the HSMS Gotland (A-19) for use in anti-submarine warfare (ASW) exercises in 2005. During the brief service with the U.S. Navy, the submarine was actually able to "sink" the U.S. Navy's Nimitz-class aircraft carrier USS Ronald Reagan (CVN-76).

The outcome was replicated multiple times and the stealthy Swedish sub was able to "run rings" around the then newly constructed $6.2 billion aircraft carrier, and its strike group.

As Stavros Atlamazoglou wrote for The National Interest, "Despite having an entire carrier strike group, including destroyers, helicopters, and planes hunting for it, the HSMS Gotland managed to sneak by the formidable anti-submarine defensive net around the USS Ronald Reagan and score several simulated torpedoes 'hits.'"

The U.S. Navy held multiple exercises with the Gotland over a year, and each time the submarine successfully and silently maneuvered around destroyers and nuclear attack submarines. U.S. officials were so impressed (and likely dismayed) that they leased the Swedish boat for another year to understand how it went undetected so successfully.

A Few Considerations

Though numerous reports have suggested that the Gotland-Class Swedish submarine could have sunk that U.S. carrier, it was still only a drill. As Atlamazoglou further noted, the USS Ronald Reagan still has a top speed of more than 30 knots, about 35 miles per hour, which is most impressive for a ship weighing 100,000 plus tons.

Moreover, even if the HSMS Gotland had been a real enemy, the U.S. aircraft carrier would have picked up the torpedoes and initiated evasive maneuvers to avoid them.

Then there is the fact that supercarriers like the USS Ronald Reagan are extremely well-armored and could probably withstand several hits from the small Swedish sub.

"Although the HSMS Gotland might not have sunk the American aircraft carrier, it would have most likely caused it significant damage and forced it to retire for repairs," wrote Atlamazoglou.

With that in mind, it is still a very good thing that the Gotland and her sisters will soon be on NATO's side, should the alliance find itself in a war with Russia.

Author Experience and Expertise

Peter Suciu is a Michigan-based writer. He has contributed to more than four dozen magazines, newspapers, and websites with over 3,200 published pieces over a twenty-year career in journalism. He regularly writes about military hardware, firearms history, cybersecurity, politics, and international affairs. Peter is also a Contributing Writer for Forbes and Clearance Jobs. You can follow him on Twitter: @PeterSuciu.