 SM U @-@ 20 or U @-@ XX was the lead boat of the U @-@ 20 class of submarines or U @-@ boats built for and operated by the Austro @-@ Hungarian Navy ( German : Kaiserliche und Königliche Kriegsmarine or K.u.K. Kriegsmarine ) during the First World War . The design for U @-@ 20 was based on submarines of the Royal Danish Navy 's Havmanden class ( three of which had been built in Austria @-@ Hungary ) , and was largely obsolete by the beginning of the war . 
