Sweden’s Famously Stealthy Submarine Is Now Even Quieter

When Sweden loaned one of its AIP (Air Independent Power) submarines to the U.S. Navy, it made waves. HMS Gotland operated out of San Diego for over 2 years in the late 2000s, reputably ‘sinking’ the carrier USS Ronald Reagan (CVN-76) in an exercise. It wasn’t fast, but it was quiet which is a valuable quality in underwater warfare. Now that submarine has been upgraded with an even stealthier propulsion system.

A cornerstone of Gotland’s effectiveness in exercises with the U.S. Navy would have been its AIP propulsion. Indeed it was a major factor in why the Navy wanted to exercise against this specific boat. AIP meant that it could stay submerged for much longer than other non-nuclear boats.

Today the AIP is a generation newer than when the U.S. Navy faced her. Gotland was upgraded by Saab in 2018 and a second of the class, HMS Uppland, is nearly ready for delivery.

The upgrade to adds 6.5 ft to its overall length. The extra space accommodates what the Swedish Navy describes as “new systems for energy optimization” (Press release in Swedish). This may refer to cooling systems which are part of the new AIP system which will also be fitted to the new A-26 Blekinge Class submarines. Two of these are currently being built for the Swedish Navy.

AIP stands for Air Independent Power (also often described as Air Independent Propulsion). This provides electric power to the submarine’s motor while it is submerged, so that it does not have to use its batteries. Whereas normal non-nuclear submarines have to rely on batteries underwater. Once these have depleted, every few days, the submarine has to put a snorkel above the water to get air to run its diesel engines to recharge them. Snorkeling is comparatively noisy and makes the submarine more vulnerable to detection.

AIP submarines keep the batteries for when they need speed, but can cruise on the AIP. It is a popular myth that they use the AIP to recharge the batteries, that would be inefficient.

Sweden was the first country to build submarines with the modern concept of AIP. Their first boat with the system was HMS Näcken which was modified in 1987-88. The Swedish boats use closed-cycle Stirling generators as their AIP source. There are other types of AIP such as the fuel cells used on German submarines.

The submarine also go a new combat management system and new sensors for intelligence gathering. The latest Swedish Torped-62 heavyweight torpedoes will be compatible and the next generation Torped-47 lightweight torpedo will be added to the armory soon. There is also a new diver lock-out chamber for Special Forces operations.

Sweden’s AIP submarines will continue to be stealthy operators. They are routinely upgraded and frequently exercised, keeping pace with developments. What remains to be seen is whether the new generation of Swedish Stirling AIP will find itself on to other country’s boats. They might.