Here's What You Need to Know: Quiet submarines are difficult to hunt.
The Gotland-class of submarines are the pride of the Swedish Navy. They’re really quiet—and are getting even quieter.
The Gotland-class has a number of features that are optimized for a minimal acoustic signature. A lot of the work was done by Saab, which fills quite a few defense tenders for the Swedish military. According to the Saab website, the Gotland-class uses a unique air-independent power Stirling engine for increased efficiency and a reduced acoustic signature. Though the system is a bit complicated, Saab explained how a Stirling engine works.
“In a Stirling engine, the necessary heat is produced in a separate combustion chamber and transferred to the engine’s working gas, operating in a completely closed system. The working gas forces the pistons in the engine to move, thus producing mechanical energy.”
Saab’s Stirling engines have a very low acoustic signature and burn a mixture of liquified oxygen and diesel—which can also be used to power the onboard diesel engines, simplifying logistics. Saab says that the diesel engines are only used for “long-distance transit at medium speed in either surfaced or snorting condition.”
AIP systems supply the sub’s motor with electricity, without having to use stored electrical power in the batteries, and therefore allow non-nuclear submarines to run for longer without having to surface. Without an AIP system, diesel-electric submarines need to surface for access to the atmosphere, which on-board diesel generators require in order to charge the submarine’s batteries.
The Gotland-class is undergoing some modifications that will make it even quieter. Both the HMS Gotland and the HMS Upland recently underwent modification. According to a Swedish armed forces statement, the modifications to the Gotland-class include “a new ship surveillance system, a new combat and fire management system, diving locks and more modern Stirling engines.”
These modifications were helped in part by cutting open the submarines and extending them. “The submarines have also been extended by two meters by cutting the pressure hull in the middle and welding it together again. The new two-meter section allows, among other things, new systems for energy optimization inboard.” The statement also explained the implications of this upgrade, saying that, “the submarines are even quieter, making it harder to hunt and fight.”
In the mid-2000s, the U.S. Navy leased the HMS Gotland for one year, in order to improve the Navy’s anti-submarine warfare. The Navy chose the Gotland due to its advanced AIP technology, which was leveraged to improve American anti-submarine techniques.
During training, the Gotland was able to pierce Carrier Strike Group 7’s defenses, and even managed to take a photo of the USS Ronald Regan off the coast of San Diego. This qualified as a sinking. Due in part to the Gotland’s success, the initial one-year lease was extended for a second year.
Saab says that all of Sweden’s submarines use Saab’s Sterling AIP engine technology. The U.S. Navy made a good decision in evaluating the Swede’s AIP—and hopefully learned from it.
Caleb Larson holds a Master of Public Policy degree from the Willy Brandt School of Public Policy. He lives in Berlin and writes on U.S. and Russian foreign and defense policy, German politics, and culture.
This article first appeared in May 2020.
Image: U.S. Navy photo by Mate 2nd Class Patricia R. Totemeier