Photo: Tapper-class Fast Patrol Boat, displacement of 62 tons, 22 meters (72′) in length (Credits: Swedish Armed Forces)
Naval News reports that the first of six Trapper class fast patrol boats has completed an upgrade that will allow these small vessels to hunt submarines. At 62 tons full load, these vessels are about 2/3s the size of the Coast Guard’s 87 foot Marine Protector class WPBs (91 tons).
Sweden has a history of suspected or known intrusions by submarines, midget submarines, and/or swimmer delivery vehicles, presumably from the Soviet Union/Russia.
What they seem to have done here is to use technology similar to the Sono-buoys used by airborne ASW units. While surface units do not have the speed of aircraft in getting to the scene, they are potentially more persistent, and because the buoys themselves do not have to fit within ejection tubes, they can be made larger with batteries that provide longer life.
Photo: Tapper-class enhanced ASW capabilities mainly rely on new sonobuoy integration (Credits: Swedish Armed Forces)
The post makes no mention of weapons or hull mounted sonars. When built in the 1990s, this class, originally of twelve vessels, based on a Swedish Coast Guard vessel design, had a searchlight sonar and small Anti-Submarine mortars that went by the designation RBS-12 or ASW600. The mortar projectiles were relatively small, only 100mm (3.95″) in diameter, weighing 4.2 kilograms (9 pounds 4 oz.), far smaller than the 65 pound (29.5 kilo) Hedgehog or Mousetrap weapons of WWII, but, unlike those systems, they did have a shaped charge. Apparently the weapon was removed at some point, but reportedly the weapon was reintroduced in 2018 on the Koster-class mine countermeasures vessels so it is possible it has been reintroduced here as well.
While the post seems to emphasize passive detection, the last paragraph suggest there is an active component.
“The Kongsberg Maritime sonar selected for this upgrade is being used for Anti-Submarine Warfare (ASW), Mine and Obstacle detection and Navigation (emphasis applied–Chuck), and is designed for use in shallow water.”