Defence by use of torpedoes was also possible, as Kvarven held one of only two land-based torpedo batteries in Norway at the time, and, after the alarm from detection of German attack at Oslo had gone out, a torpedo boat was maneuvering into position nearby. The other land-based torpedo battery, at Oscarsborg Fortress, defended Oslo against the simultaneous German invasion there and featured underwater release of its torpedoes. It was unknown to German military intelligence. Two obsolete torpedoes from the Oscarsborg Fortress fired at 4:34 am finished off gun-damaged German heavy cruiser "Blücher", which finally sank at 07:30 am, just a short while after the Karven fort surrendered. The Karven torpedo battery, on the other hand, was designed to launch its torpedoes by torpedo tube from above ground, and presumably the Germans could have known about it and planned countermeasures in some way, and in fact their landings on both sides of Korven were likely part of that. But according to a Norwegian military history website, no torpedoes in the battery could be fired, because all were missing their fuses and gyroscopes. Another nearby line of the torpedo defense was just then available: torpedo boat "Brand", with fully functioning torpedoes, had moved into an excellent position in the Gravdalsviken inlet partly defined by Kvarven's peninsula, just past, where it would have a 700-1000 meter firing shot opportunity on passing German ships, and it could likely have sunk one or more German ships. But the boat's commander inexplicably chose not to fire any torpedoes and to abandon the boat, instead. The "Brand" was used with success by the Germans, with some immunity from air attacks because of confusion on its identity, during the next few days, and also was useful for the duration of their occupation of Norway.