 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 (by tonnage sunk), after de la Perière.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.