After UB @-@ 8 was transferred to the Bulgarian Navy in May 1916 , she was renamed Podvodnik No. 18 ( in Cyrillic : Пoдвoдник No. 18 ) . She was Bulgaria 's first submarine , and was engaged primarily in coastal defense duties off Bulgaria 's main Black Sea port of Varna . Podvodnik No. 18 survived the war and was ceded to France after the Treaty of Neuilly @-@ sur @-@ Seine .