Despite the problems , the " tin tadpoles " , as the Germans referred to them , were in active service from March 1915 through the end of the war , with half of the 20 boats lost during the war . Boats of the class served in three navies : the German Imperial Navy , the Austro @-@ Hungarian Navy , and the Bulgarian Navy . In German service , they served primarily in the Flanders Flotilla , the Baltic Flotilla , and the Constantinople Flotilla .