 The German Imperial Navy stipulated that the submarines must be transportable by rail , which imposed a maximum diameter of 3 @.@ 15 metres ( 10 ft 4 in ) . The rushed planning effort — which had been assigned the name " Project 34 " — resulted in the Type UB I design , created specifically for operation from Flanders . The boats were to be about 28 metres ( 92 ft ) long and to displace about 125 tonnes ( 123 long tons ) with two bow torpedo tubes . 
