Career
In his time as commander of U-12 and U-39 in, he conducted 47 patrols and succeeded in sinking 146 ships for a total tonnage of 384,304 gross register tons (GRT). As such he is the second most successful submarine commander ever (by tonnage sunk), after de la Perière. In 1917 Forstmann, in command of U-39, sank five valuable steamers within only two days in the Straits of Gibraltar with over 20,000 GRT, the Steamship Normanton (3,862 tonnes), Steamship Mersario (3,847 tonnes), Steamship Almora (4,385 tonnes), Steamship Nuceria (4,702 tonnes) and the Japanese steamer Steamship Sitosan Maru (3,555 tonnes).
The ships carried a total load of 31,500 tonnes of coal, of which more than 26,000 tonnes were meant for Italy for use in the winter.
Forstmann wrote his memoirs after the war in the book Hunting in the Mediterranean (German: Auf Jagd im Mittelmeer). In 1921 he qualified as an attorney and in 1924 he became social departmental head and director with the August Thyssen coal company in Duisburg.
From 1929 to 1933 he was a city delegate and parliamentary group leader of the German People"s Party in Duisburg. I In the Second World War Forstmann served as chief of different commands in Osnabrück and Copenhagen.
In Copenhagen, he was the commander of the Wehrwirtschaftsstab Denmark, Germany"s chief industrial purchasing entity in occupied Denmark.
He was promoted to Kapitän zur See on July 1, 1942. He retired from the navy in 1945. 1951 - 1955 He became one of the advisors of the housing company - Rheini dwellings Attorney - General in Duisburg which was responsible for the movement of several villages to enable development of open cast coal mining.
In 1956 he was involved in the design structure of the 30 Pestalozzi villages and became vice-president of the German settler federation, chairmen of the executive committee of youth centre place work est