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).
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.