On 1 January 1921, Maertens was promoted to captain lieutenant (). On 17 March 1925, he was subordinated to the commander of the Torpedo and Mining Academy in Kiel where he stayed until 14 August 1928. From 15 August 1928 to 30 September 1934, he was the department head of the Naval Shipyard of the Naval Command (). In October 1934, Maertens was promoted to Frigate captain, (), which was the senior middle rank of the Kriegsmarine. From 1934 to 1936, he was Commander of the Naval Academy Mürwik. He was then posted to the Bureau of Inspection of the torpedo Service in Kiel as Director of Staff until 30 September 1937. During the same period he was ordered to be Acting Inspector of Torpedo Affairs until 17 April 1937. From October 1937 to April 1939, Maertens was Director of Staff of Inspections in Naval Signals and, order again, on a temporary basis, to be Acting Inspector of Naval Signals from May 1938 to March 1939. He then became leader and subsequent Commander of the Communication Test Institute of the OKM from 28 April 1939 to 18 November 1939. 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. On 1 July 1940, Maertens was promoted to rear admiral, Konteradmiral, and on 1 September 1942, promoted to Vizeadmiral. From 19 June 1941 to 5 May 1943, he was promoted to Group Director of the Naval Intelligence Service, ( III) of the Oberkommando der Marine (OKM/4 SKL III). From 6 May 1943 to June 1943 Maertens was Acting Shipyard Director of the Kriegsmarine shipyard in Kiel. From 28 November 1944 to 28 February 1945, he was placed at the disposal of Oskar Kummetz who was the Baltic Sea regional commander.
