On 29 April 1940, Seal left Immingham laden with 50 mines. On entering the Skagerrak, she met HMS Narwhal just leaving the area after having stirred up German defences by scoring six hits with six torpedoes. Seal was running at shallow depth to maintain speed and conserve energy, when she was spotted by a German Heinkel He 115 on 4 May at about 02:30.