 Built to meet the need for small <unk> submarines able to operate in the narrow , shallow seas off Flanders , the vessels were intended to be quickly constructed , then shipped by rail and assembled at their port of operation . The design effort began in mid @-@ August 1914 and by mid @-@ October the first 15 boats were ordered from two German shipyards . The German Imperial Navy subsequently ordered an additional pair of boats to replace two sold to Austria @-@ Hungary , who ordered a further three boats in April 1915 . A total of 20 <unk> Is were built . Construction of the first boats for Germany began in early November 1914 ; all 20 were completed by October 1915 . Several of the first boats underwent trials in German home waters , but the rest were assembled and tested at either Antwerp or Pola . The German boats operated primarily in the Flanders , Baltic , and Constantinople <unk> . The boats were about 28 metres ( 92 ft ) long and displaced 127 tonnes ( 125 long tons ) when surfaced and 142 tonnes ( 140 long tons ) while submerged . All had two bow torpedo tubes and two torpedoes , and were equipped with a deck @-@ mounted machine gun . 
