 Because Bulgaria had joined the Central Powers , battleships of the Russian Black Sea Fleet , and aircraft from the seaplane carriers Almaz and Imperator Nikolai I began attacks on Varna and the Bulgarian coast on 25 October . UB @-@ 7 and UB @-@ 8 , both based out of Varna by this time , sortied to disrupt the bombardment . Off Varna on the 27th , UB @-@ 7 got in position to fire a torpedo at the Russian battleship Panteleimon ( most well @-@ known under her former name of Potemkin ) . Although UB @-@ 7 's crew heard what they thought was the torpedo explode , it did not hit Panteleimon . Despite the lack of success , the attempt did cause the Russians to break off their attacks and withdraw . 
