Helmut Rosenbaum began his naval career with the Reichsmarine on 15 August 1932 as a late for the year member of "Crew 32" (the incoming class of 1932).

He underwent basic military training in the 2nd department of the standing ship division of the Baltic Sea in Stralsund (15 August 1932 – 7 October 1932).

During this assignment Rosenbaum received his officer's commission holding the rank Leutnant zur See (Second Lieutenant) as of 1 January 1936.

His U-boat training began on 1 February 1937 ending with his assignment as watch officer on U-35 in the Saltzwedel Flotilla on 3 April 1937.

[3] On his second patrol (25 March 1941 – 24 April 1941) on U-73 Rosenbaum departed from Lorient and attacked and sank five ships, returning to St. Nazaire after four weeks at sea.

On 11 August 1942 he made contact with the convoy, which included HMS Eagle and fired four torpedoes at the aircraft carrier sinking it.