 When it became apparent to the Austro @-@ Hungarian Navy that the First World War would not be a short war , they moved to bolster their U @-@ boat fleet by seizing the plans for Denmark 's Havmanden class submarines , three of which had been built at Whitehead & Co. in Fiume . Although the Austro @-@ Hungarian Navy was not happy with the design , which was largely obsolete , it was the only design for which plans were available and which could be begun immediately in domestic shipyards . The Austro @-@ Hungarian Navy unenthusiastically placed orders for U @-@ 20 and her three sister boats on 27 March 1915 . 
