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