U-18 began its first sortie (7th in total) in its new area of operations on 26 May 1943, however no success was achieved. A torpedo was fired at a steamer on 30 May, but it missed. Pursued by a minesweeper, the U-18 did not get another opportunity to shoot. The next day the submarine was fortunate to escape an attack by a Soviet SB bomber just as it rendezvoused with U-9. The Russian aircraft circled twice, but it apparently identified the U-19 as a Russian boat because of the red star on its conning tower. The bomber instead attacked U-9. The only “sinking” achieved on this patrol was a drift mine, which was exploded by machine-gun fire on 7 June. The U-18 achieved success on its 8th patrol (16 June — 22 July 1943). While it missed an opportunity to attack a Soviet submarine on 18 June, according to its war diary it subsequently sank two steamers and a lighter. Available records do not confirm the sinking of either steamer. The Leningrad (1,783 GRT), claimed sunk on 23 June 1943, bad been damaged by a German aircraft in October 1941 and during the period in question was in drydock in Batumi undergoing repair. The second ship claimed by U-18, the steamer Vorosbilov (3.906 GRT), had also been damaged by a German aircraft in May 1942, and in June 1943 was in dock in Suchimi.
On 25 April 1944 during its 12th patrol, U-18's red star was almost its undoing. After attacking a Soviet submarine, a BV 138 flying boat opened fire on the “suspected Russian” despite the 18 recognition flares fired by U-18. U-18 was bit by gunfire and finally dove to safety. In the eyes of the flying boat crew the red star had too clearly identified the submarine as Russian!
Earlier on this patrol on 7 April the U-18 had sunk a small cargo ship with its deck gun. Two more operations followed in the summer of 1944 off the coast of the Caucasus. Both produced no results. On 18 July 1944 Oblt.z.S Fleige was decorated with the Knight's Cross for his success as commander of the U-18. He was the only Black Sea submarine commander to be so decorated.
The Red Army's advance and Rumania’s declaration of war on Germany ended German submarine operations in the Black Sea. The bases had to be evacuated. This also meant the end of U-18. Heavily damaged in an air raid on Constanza immediately after its 14th patrol, on 25 August 1944 it was scuttled by its crew off Constanza at position 43°47’ N/28°45 E.
U-Boot im Focus - ediiton no.2 (2007)