 While escorting SS Elysia 12 nmi ( 14 mi ; 22 km ) off Queenstown on 16 June , lookouts on O 'Brien sighted a periscope . Heading toward the submarine for an attack , a lookout in the foretop saw the submerged boat pass close along the starboard side . A depth charge was dropped but no immediate evidence of damage was found . Nearly three hours later , the British vessel HMS Jessamine reported a large patch of oil in approximately the same position . The next morning , Cushing also reported and confirmed Jessamine 's report . The British Admiralty believed the submarine was probably seriously damaged . However , later investigation reveals that German submarine U @-@ 16 , the submarine in question , continued to operate and completed her cruise . 
