Walther Forstmann was one of the most successful and highly decorated U-boat commanders in the Kaiserliche Marine during World War I. He also served in the Kriegsmarine during World War II in different staff positions. In his time as commander of and in World War I, he conducted 47 patrols and succeeded in sinking 146 ships for a total tonnage of. As such he is the second most successful submarine commander ever, after de la Perière.
World War I
On August 12, 1916 Kapitänleutnant Forstmann of U-39 was awarded the Pour le Mérite for his achievements so far in the First World War. In 1917 Forstmann, in command of U-39, sank five valuable steamers within only two days in the Straits of Gibraltar with over, the , , , and the Japanese steamer . 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.
Interbellum
Forstmann wrote his memoirs after the war in the book Hunting in the Mediterranean. 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. Between 1933 and 1950 he was a member of the board of four large housing cooperatives of the Ruhr steel plants.
World War II
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.
Postwar career
1951 - 1955 He became one of the advisors of the housing company - Rheini dwellings A.G. in Duisburg which was responsible for the movement of several villages to enable development of open castcoal mining. In 1954 he became a member of the supervisory board of this company. 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 :de:Verein#Eingetragener Verein|e. V.
