First patrol

U-123's first patrol began with her departure from Kiel on 21 September 1940. Her route took her across the North Sea, through the gap between the Faroe and Shetland Islands and into the Atlantic Ocean west of Ireland. She sank six ships in October, including Shekatika which was hit with no less than five torpedoes before she went to the bottom east southeast of Rockall. Nevertheless, her partial load of pit-props floated free before she went down.

The boat docked at Lorient in occupied France on 23 October.

Second patrol

U-123 returned to the same general area for her second patrol as for her first. She was also almost as successful, sending another five merchantmen to the bottom. The voyage was marred on 17 November 1940 when Mechanikergefreiter Fritz Pfeifer was lost overboard. After sinking the British convoy straggler, the ore-carrier SS Cree (torpedoed and sunk with the loss of all hands on 21 November) the boat was seriously damaged on 3 November by a collision with an unknown object ("probably a convoy vessel").

She returned to Lorient on 28 November.

Third patrol

Her score rose steadily, sinking another four ships. One, Grootekerk, was sunk after a nine-hour chase about 330 nmi (610 km; 380 mi) west of Rockall. There were no survivors.

Fourth patrol

Venturing further west of Ireland on her fourth sortie, the boat sank one ship, Venezuela on 17 April 1941. This was another vessel which required five torpedoes to ensure her destruction. There were also no survivors.

Having set-out from Lorient on 10 April, she returned to the same port on 11 May.

Fifth patrol

Patrol number five was conducted in the Atlantic, but in the vicinity of the Azores and the Canary Islands. Her first victim this time out was Ganda, a 4,333 GRT neutral registered in Portugal. She went down on 20 June 1941. Following her sinking with torpedoes and gunfire, it was realised what her status was. On her return to Lorient, U-123's war diary (KTB) was altered on the order of U-boat headquarters (BdU).^[3]

The U-boat sank four other ships between 27 June and 4 July, but was depth charged for 11 hours on 27 June and only escaped by diving to 199 m (654 ft). She was also unsuccessfully attacked by convoy escorts west of Portugal on 12 August, although she sustained moderate damage.

Sixth patrol

Despite criss-crossing the Atlantic, U-123 found the pickings rather thin, she did manage to damage the armed merchant cruiser (AMC) HMS Aurania on 21 October 1941 and take one crewman prisoner. The ship had been travelling behind Convoy SL-89 with five other AMCs. The vessel was hit by two torpedoes but empty drums in the holds kept her afloat. A 25 degree list was reduced to 15 degrees; men had abandoned ship prematurely – hence the PoW. The ship continued her voyage, albeit at reduced speed.

Seventh patrol

U-123 took part in the opening of Unternehmen Paukenschlag ("Operation Drumbeat"), also called the "Second Happy Time" in January 1942. She began by sinking the cargo ship Cyclops about 125 nmi (232 km; 144 mi) southeast of Cape Sable, Nova Scotia on the 12th. Moving down the coast, she sank Norness, Coimbra, Octavian , Norvana, City of Atlanta and the Latvian Ciltvaira. She approached New York's Lower Bay on the evening of 14th Jan and viewed an illuminated New York City. She was also credited with sinking San Jose on 17 January, although this ship was actually lost in a collision.^[4] The Malay was only damaged because Hardegen had under-estimated her size and chose to use the deck gun rather than a torpedo. In a reference to American unpreparedness, he commented after sinking Norvana: "These are some pretty buoys we are leaving for the Yankees in the harbor approaches as replacement for the lightships."

U-123 was attacked by an aircraft off New York City, but withdrew without any damage being sustained. She also had a lucky escape on 19 January when Kosmos II (headed by Captain Einar Gleditsch from Sandefjord Norway) tried to ram the boat off Oregon Inlet. At one point the ship was only 75 m (82 yd) away from the German submarine which had an inoperable diesel engine. The U-boat escaped when the recalcitrant power plant was restarted at the last minute and flares were fired at the larger vessel's bridge.

The Culebra and Pan Norway were also sunk off Bermuda. By now out of torpedoes and in the case of Pan Norway, the boat used the last of her deck gun ammunition and 37 mm AA weapon to destroy the Norwegian vessel. The U-boat then encountered a Greek ship, the Mount Aetna, under a Swiss charter, which was directed to the survivors.^[5]

Eighth patrol

The boat's second Paukenschlag mission was also successful – sinking Muskogee and Empire Steel on 22 and 23 March 1942 near Bermuda before moving closer to the US east coast.

She then attacked the USS Atik, a Q ship. This disguised merchantman was hit on the port side, the crew started to abandon ship on the starboard side. The U-boat moved closer, at which point Atik dropped her concealment and opened fire with all weapons. U-123 ran off, (one man died in the action), but she dived, returned and sank the American vessel with a torpedo. There were no survivors.

This was Captain Reinhard Hardegen's second patrol in charge and quoted from U123 K.T.B war patrol log the following:

Except for Holzer, no one was wounded and a low pressure test showed that the pressure hull was intact. We were incredibly lucky. Eight 20 mm hits were counted on the bridge the next day. After we were out of range I went below deck and saw that the condition of F.z.See Holzer was hopeless. A 20 mm round detonated in his right thigh, ripped open the flesh from the hip joint to the knee and partially removed it. One could not see if the bone had been shattered. The leg was only hanging on small flaps of skin. We bound off the leg. We could only wrap a towel around the big wound because we had not enough dressing material for such injuries. It was immediately clear that such an injury could not be treated even by a doctor under the circumstances aboard a U-boat. We were many days from the next neutral harbor, so I took the decision to make it as easy as possible for him and injected a substantial dose of morphine. Holzer acted bravely in an exemplary manner. For one hour he was conscious, without a single word of complaint.

F.z.See Holzer was buried at sea the following day.

The boat proceeded to sink or damage another eight ships; many of them resting on the sea bed in the shallow water with parts of their hulls above the surface. One such was Oklahoma which, although sent below in 40 ft (12 m) of water on 8 April, was re-floated, repaired and returned to service in December 1942. Another vessel, Gulfamerica was fatally struck about five miles from Jacksonville, Florida on 11 April. The ship had been on her maiden voyage from Philadelphia to Port Arthur, Texas, with 90,000 barrels (14,000 m³) of fuel oil. Nineteen crewmen were killed in the attack.^[6][7][8][9] She did not sink until 16 April.

Another victim was Alcoa Guide, engaged at the relatively close range of 400 m (440 yd) by the deck gun, (U-123 had run out of torpedoes), on 17 April.

The boat then returned to Lorient on 2 May and proceeded to steam to Bergen in Norway before carrying out a series of short journeys to Kristiansand, Aarhus, Kiel and Stettin.

Ninth patrol

For her ninth patrol, U-123 left Kiel on 5 December 1942 and returned to the Atlantic. She sank Baron Cochrane on the 29th after the ship had already been damaged by U-406 and missed by U-591. U-123 also damaged Empire Shackleton, a Catapult Armed Merchantman north of the Azores. (The wreck was sunk by U-435 on the same day).

The boat returned to Lorient on 6 February 1943.

Tenth patrol

U-123 sailed to the West African coast. She sank the Spanish-registered motor ship Castillo Montealegre on 8 April 1943 west of Conakry, French Guinea. As per maritime rules, the neutral ship had the Spanish flag painted in both sides. Commander Horst von Schroeter ordered the shooting of 3 torpedoes and she sunk in less than a minute. The submarine surfaced, the commander asked from the conning tower "What ship?" to the survivors. Although being confirmed he had just sunk a neutral ship, he left without giving any assistance to the 40 men adrift (five went down with the ship).

A few days later the Hill-class naval trawler HMS Inkpen rescued 29 survivors from a boat. 11 on a separated raft died. The affair was hushed-up by the government of Franco; indeed, the survivors were ordered to shut-up. The career of Commander Horst von Schroeter was unaffected by this affair and after the war he even became a NATO commander.^[10]

U-123 was also successful against a British submarine, HMS P-615 100 nmi (190 km; 120 mi) southwest of Freetown in Sierra Leone on 18 April. She sank Empire Bruce on the same day, also southwest of Sierra Leone. She sank three more ships off Monrovia on 29 April, 5 May and 9 May. This included the Holmbury, which was sunk on 5 May by two torpedoes and gunfire. The crew (minus 2 firemen who were killed by the first torpedo) survived, after sailing to the Liberian coast in the one remaining lifeboat. The captain, J B Lawson, was taken aboard U-123, where he was treated impeccably by Von Schroeter. Von Schroeter promised to send relevant photographs to Lawson a year after the war had ended – and did.^[11]

Eleventh patrol

U-123 was depth charged off Cape Finisterre (northwest Spain), by Allied escort vessels on approximately 25 August 1943. She was also attacked by a British De Havilland ('Tse Tse') Mosquito of No. 618 Squadron RAF on 7 November 1943; this aircraft, piloted by Flying Officer A.J.L. Bonnett of the Royal Canadian Air Force, was armed with a 6-pounder (57 mm) Molins gun and this was the first attack on a U-boat with one of these weapons. Bonnett fired eight rounds at U-123 and achieved several hits on the submarine's conning tower and hull.^[12] The boat was rendered unable to dive by a 18-by-6.5-centimetre (7.1 by 2.6 in) hole in the pressure hull. One crewman, Bootsmaat Günther Struve was killed and two others wounded.

Twelfth patrol

U-123's last patrol was her longest – 107 days, but after the incidents of the previous eleven, it was a bit of an anti-climax. She returned to Lorient unable to repeat her success, on 24 April 1944.