On the second patrol, Merten helped rescue the crews of the auxiliary cruiser Atlantis and the refuelling ship Python, which had been sunk by the Royal Navy.

[2] His younger brother Klaus, as a Feldwebel (staff sergeant) in a pioneer platoon, died in 1942 of wounds sustained on the Eastern Front.

[3] Aged thirteen, Merten joined the Königliches Preußisches Kadettenhaus (Royal Prussian Cadet House) in Köslin, present-day Koszalin, on 1 April 1918.

[Note 1] He underwent basic military training with the 5th company of the 2nd department of the standing ship division of the Baltic Sea on the Dänholm in Stralsund.

Following a 17-month stay on board the cruiser Emden (18 October 1926 – 24 March 1928), he advanced in rank to Fähnrich zur See (midshipman) on 1 April 1928.

The journey then continued to Japan and Alaska down the west coast of North and South America, around Cape Horn.

[Tr 14] In parallel to this assignment, he served as 2nd watch officer on the torpedo boat T-156 in the 2nd Minesweeper-Demi-Flotilla as well as Flag Lieutenant with the Commander of Scouting Forces (B.d.A.)

In back-to-back assignments, he was briefly transferred to the light cruiser Leipzig (8 March – 20 May 1937), serving as the anti-aircraft artillery officer, and then again on Karlsruhe (21 May – 11 June 1937).

Merten participated in the bombardment of the Polish base at Danzig's Westerplatte in the early morning hours of 1 September 1939.

On 30 November 1940, Merten was transferred to the 2nd U-boat Flotilla, joining the crew of Kapitänleutnant Heinrich Liebe's U-38 as a commander in training and watch officer.

U-68 was a Type IXC U-boat, designed as a large ocean-going submarine for sustained operations far from the home support facilities.

[13] During the first five days of this patrol, U-68 came under a depth charge attack and was almost rammed by a Royal Navy destroyer south of Iceland.

When the main bilge pump failed, which impeded the U-boats diving ability, Merten decided to abort the mission and headed for France.

On the journey to France, the starboard diesel engine failed and a member of the crew fell ill with pneumonia.

[16] On the second patrol (11 September – 25 December 1941), Merten headed for the middle and southern Atlantic Ocean, the Ascension Island, to Saint Helena and Cape Verde.

On 22 September 1941, Merten torpedoed his first ship the 5,302 GRT British Steamer SS Silverbelle sailing in convoy SL-87.

[20] On 23 November, U-68 received the message that Atlantis had been sunk by HMS Devonshire while resupplying U-126 under the command of Kapitänleutnant Ernst Bauer.

[26] On U-68 fifth patrol (20 August – 6 December 1942), Merten operated in the U-boat wolf pack Eisbär (Polar Bear Group), consisting of four submarines, U-68 (Merten), U-156 (Werner Hartenstein), U-172 (Carl Emmermann), U-504 (Hans-Georg Friedrich Poske) a fifth U-boat, U-159 (Helmut Witte) joined the group later, which in the course of a few weeks during September–October 1942, sank more than 100,000 GRT of shipping off South Africa.

[28][25] Cairo had been carrying 302 people, including 101 passengers, among them 29 women and 19 children (in addition to two stewardesses travelling as part of the crew).

[29] The ship sank in roughly 25 minutes, and when the second torpedo hit the liner, it capsized a lifeboat and smashed another, leaving women and children struggling in the water.

[29] Merten approached Boat 6, asking for the name, tonnage, and cargo of the ship, famously ending the conversation by saying, "Goodnight, sorry for sinking you".

On 30 January 1943, Dönitz awarded Merten the U-boat War Badge with Diamonds (U-Boot-Kriegsabzeichen mit Brillanten).

[30] On 31 January 1943, Merten, Dönitz and other Kriegsmarine officers traveled to the Wolf's Lair, Hitler's headquarters in Rastenburg (now Kętrzyn in Poland) for the Oak Leaves presentation.

In this meeting, Hitler appointed Dönitz as Oberbefehlshaber der Marine (Commander-in-Chief) of the Kriegsmarine following Raeder's resignation on 30 January 1943.

[13] During his tenure with the 24th U-boat Flotilla, Merten was in frequent conflict with the Gauleiter of East Prussia, Erich Koch.

In July 1944, Koch had ordered 6,000 untrained Hitler Youth boys to man the defensive positions around Memel, present-day Klaipėda, Lithuania, against the advancing Red Army.

[37] "We couldn't have been sunk by a nicer man"[38] On 14 September 1984, a reunion of the survivors of City of Cairo was celebrated aboard HMS Belfast.

The re-union was attended by 17 survivors and Merten and commemorated the publication of the book by Ralph Barker "Goodnight, Sorry for Sinking You".

[Note 2] This book criticizes Lothar-Günther Buchheim, especially his work Die U-Boot-Fahrer [U-Boat Sailors], for his anti-Dönitz demeanor.

[41] On 1 January 1969, Merten started working for the Ingenieur Kontor Lübeck (IKL), headed by Ulrich Gabler, as a military-tactical advisor.