 Built to meet the need for small maneuverable 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 UB 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 Flotillas . 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 . 
 Built to meet the need for small maneuverable submarines able to operate in the narrow , shallow seas off Flanders , the vessels <a href='uninflect' title='1'><mark>were</mark></a> 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 <a href='uninflect' title='2'><mark>were</mark></a> ordered from two German shipyards . The German Imperial Navy subsequently ordered an additional pair of boats to replace two sold to Austria @-@ Hungary , <a href='null_relcl' title='3'><mark>who</mark></a>ordered a further three boats in April 1915 . A total of 20 UB Is <a href='uninflect' title='4'><mark>were</mark></a> built . Construction of the first boats for Germany began in early November 1914 ; all 20 <a href='uninflect' title='5'><mark>were</mark></a> completed by October 1915 . Several of the first boats underwent trials in German home waters , but the rest <a href='uninflect' title='6'><mark>were</mark></a> assembled and tested at either Antwerp or Pola . The German boats operated primarily in the Flanders , Baltic , and Constantinople Flotillas . The boats <a href='uninflect' title='7'><mark>were</mark></a> 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 <a href='uninflect' title='8'><mark>were</mark></a> equipped with a deck @-@ mounted machine gun . 
Built to meet the need fir small maneuverable submarines able to operate in the narrow , shallow seas off Flanders , da vessels was 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 tdafirst 15 boats was ordered from two German shipyards . The German Imperial Navy subsequently ordered an additional pair of boats to replace two sold to Austria @-@ Hungary , ordered a further three boats in April 1915 . A total of 20 UB Is was built . Construction of the first boats for Germany began in early November 1914 ; all 20 was completed by October 1915 . Several of the first boats underwent trials in German home waters , but the rest was assembled and tested at either Antwerp or Pola . The German boats operated primarily in the Flanders , Baltic , and Constantinople Flotillas . The boats was 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 was equipped w/ a deck @-@ mounted machine gun .