 On the morning of 28 August , Mainz was at anchor in the mouth of the Ems ; her sister Cöln , the flagship of Rear Admiral Leberecht Maass was re @-@ coaling in Wilhelmshaven , Ariadne lay in the entrance to the Weser . These three cruisers were assigned to support the cruisers Stettin and Frauenlob , and the aviso Hela , which were stationed on the patrol line that morning . At 07 : 57 , the Harwich Force encountered the outer German torpedo boats , which fled back to the German cruisers . In the ensuing Battle of Heligoland Bight , Stettin engaged the British force first , and was quickly reinforced by Frauenlob . At 09 : 47 , Mainz was ordered to steam out behind the British to cut off their line of retreat . She got under way by 10 : 00 , and operated in conjunction with a floatplane used for reconnaissance . 
