 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.