Maertens was known for underestimating British intelligence, and specifically, overrating the security of the Naval Enigma cipher machine.
In the second enquiry, ordered by the Commander-in-Chief of the Navy (German: Oberbefehlshaber der Kriegsmarine) Karl Dönitz, in May 1943, he investigated a number of areas, which exculpated Enigma security in the end, for the second time, incorrectly blaming British 9.7 centimetre centimetric radar for the massive losses in U-boats by mid 1943.
[3] On 1 April 1910, Maertens joined the Imperial German Navy as a Seekadett and had basic training on the heavy cruiser SMS Hansa until 31 March 1911.
He completed his submarine training on 26 February 1916, promoted on 22 March 1916 to Oberleutnant zur See, and was posted to U-boat U-47 as the watch officer the day after.
He was again promoted to Director of the Technical Signals Affairs in the Naval Weapons Office in the Oberkommando der Marine from 19 November 1939 to 15 June 1941.
Maertens was a career navy signals officer who was promoted to the Director of the Naval Intelligence Command (German: Marinenachrichtendienst) in June 1941.
Vice admiral Karl Dönitz requested confirmation that the sinking of the submarine effected the change in movement of a convoy that was targeted, and was specifically asking for assurances of Enigma M's security.
Maertens was ordered to lead a formal enquiry into the "control and investigation of own processes" following the August 1941 capture of the U-boat U-570 (later renamed HMS Graph by the Royal Navy), a potential leak of German secure communication details.
[5] One of these investigations had been conducted by Kurt Fricke, Chief of Naval War Command, on another incident, the sinking of the German battleship Bismarck on 27 May 1941, caused great consternation in the Kriegsmarine, which resulted in a number of changes to Enigma cipher processes.
On 18 October 1941, Maertens completed his analysis of the security consequences by stating in his report that "a current reading of our messages is not possible."
On the next page, however, he conceded that if the enemy had found the Enigma cipher machine untouched with all key documents, then a current reading was possible.
The U-boats routinely carried between two and three months worth of daily key settings, so the enemy could have used U-570's material to read enciphered messages until November 1941.
On 24 October 1941, Maertens overall conclusion was stated in a letter to Donitz: The acute disquiet about the compromise of our Secret Operation cannot be justified.
[9]In fact, the Naval Intelligence Division had solved a message intercepted from U-111, and the Admiralty had dispatched the submarine HMS Clyde to destroy the U-boats in the Bay of Tarrafal on the island of Santo Antão.
The discovery of centimetric radar, on a downed British bomber in Rotterdam, which operated on a wavelength of 9.7 cm, supported his assumptions.
[12] One reason for Maertens and the Kriegsmarine's very high confidence in the Enigma cipher machine was the incorporation of a secret procedure that they believed would thwart any possibility of cracking the code.