In 1917 Forstmann, in command of U-39, sank five valuable steamers within only two days in the Straits of Gibraltar with over , the  (3,862 tonnes),  (3,847 tonnes),  (4,385 tonnes),  (4,702 tonnes) and the Japanese steamer  (3,555 tonnes).
In 1917, Forstmann, commanding U-39, sank five significant steamers within two days in the Straits of Gibraltar with more than , the (3,862 tonnes), the (3,847 tonnes), the (4,385 tonnes), the (4,702 tonnes) and the Japanese steamer (3,555 tonnes).