Austro-Hungarian Navy
When it became apparent to the Austro-Hungarian Navy that the First World War would not be a short one, they moved to bolster their U-boat fleet by seizing the plans for the Danish "Havmanden" class submarines, three of which had been built at Whitehead &amp; 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-21" and her three sister boats on 27 March 1915.