On 20 November, she sank the German armed trawler (formerly "Gauleiter Telshow") with torpedoes approximately northwest of Heligoland in position . The sinking of "V-209" was the first successful attack on an enemy ship by a British submarine during the Second World War. On 29 November, "Sturgeon" ended her fourth war patrol at Blyth. On 13 December 1939, "Sturgeon" departed Blyth for a patrol are off the British east coast. However, she was recalled two days later. On 17 December, she again departed Blyth for another war patrol, her sixth. She was ordered to patrol the area west of Denmark. On 20 December, "Sturgeon" was ordered to patrol the Skagerrak strait instead, but she was ordered to return to her original patrol area the next day. "Sturgeon" returned to Blyth on 30 December. After an uneventful seventh war patrol in the North Sea, "Sturgeon" underwent a refit at Wallsend until 14 April 1940. After shifting back to Blyth, she conducted an uneventful eighth patrol southwest of Stavanger, Norway from 30 April to 11 May 1940. From 22 May to 5 June, "Sturgeon" conducted another uneventful patrol northwest of Terschelling, Netherlands. On 9 June, she was ordered to patrol close to the British east coast because of fears that German warships would conduct a raid there. She was recalled the next day. Starting on 26 June, "Sturgeon" patrolled off Texel, Netherlands. On 30 June, she hit the deep bottom, sustaining minor damage. She ended her patrol on 11 July. On 27 July 1940, "Sturgeon" departed Blyth for her twelfth war patrol, off Texel, Netherlands. On 4 August, she sighted the German patrol boat , which was being towed by a tug after hitting a mine. Six torpedoes were fired, but all missed their target. "V-811"s escorts, the German auxiliary anti-aircraft ships "FL 21" and "FL 24", attacked "Sturgeon" for 30 minutes, dropping six depth charges. "Sturgeon" returned to Blyth on 9 August. "Sturgeon" departed Blyth again on 27 August, on her thirteenth patrol, in the Skagerrak strait. On 2 September, she torpedoed and sank the German troop transport north of Skagen, Denmark in position .