Training

U-804 began training exercises with the 4th U-boat Flotilla on 4 December 1943, and finished her sea trials on 30 June 1944. On 16 June 1944, U-804 was attacked by a Norwegian Mosquito aircraft from No. 333 Squadron RAF, but succeeded in shooting down its attacker at the cost of eight crew members wounded; only minor damage was inflicted on the submarine. The two-man crew of the Mosquito was picked up by U-1000 on 18 June, and taken to occupied Norway.^[1]

First patrol

U-804 began her first war patrol on 19 June 1944 (while still undergoing training) with the 10th U-boat Flotilla. She left Bergen and headed into the North Sea, passing north of the British Isles into the North Atlantic, where she remained for 116 days. On 2 August, during a special hunt for several submarines known to be transmitting weather information from stations in the central and north Atlantic (of which effort U-804 was a part), two American destroyer escorts, USS Fiske and USS Douglas L. Howard, were detached from the task group to investigate the whereabouts of U-804, which both had made contact with. Upon sighting the destroyers, the U-boat quickly dived, but the two escorts detected her on their sonar and began their attack approach. Suddenly, USS Fiske was torpedoed on her starboard side by U-804, and within 10 minutes, she broke in two. Thirty-three of her men were killed and 50 were wounded, but all the survivors were rescued by USS Farquhar. Amidst the confusion following the sinking of USS Fiske, U-804 slipped away and returned to her patrol.^[3][4]

Second patrol & loss

On 12 October 1944, U-804 returned to the port of Flensburg after 116 days at sea. Five days later she left Flensburg for Kiel, where she remained until 4 April 1945 before leaving for occupied Norway. While en route in company with another of the flotilla's boats, U-1065, the two submarines were detected and attacked in the Skagerrak strait on 9 April 1945 by over 30 Mosquito aircraft from three Royal Air Force squadrons based at Banff. U-1065 succeeded in shooting down one of the attacking aircraft before being hit by several rockets fired by 10 Mosquitos from 143 and 235 Squadrons; she exploded and sank with the loss of her crew of 45 men.

U-804 suffered the same fate - after being hit by rockets from the attacking Mosquitos she also exploded and sank at 57°58′N 11°15′E, with no survivors from her crew of 55 men.^[1][3][5]