Like all post-E8 British E-class submarines, E9 had a displacement of at the surface and while submerged. She had a total length of and a beam length of. She was powered by two Vickers eight-cylinder two-stroke diesel engines and two electric motors. The submarine had a maximum surface speed of and a submerged speed of. British E-class submarines had fuel capacities of of diesel and ranges of when travelling at. E9 was capable of operating submerged for five hours when travelling at. E9 was not fitted with a deck gun during construction, and it is not known whether one was fitted later, as was the case with many boats up to E19. She was the first of her class to be constructed with five 18 inch torpedo tubes, two in the bow, one either side amidships, and one in the stern; a total of 10 torpedoes were carried. E-Class submarines had wireless systems with power ratings; in some submarines, these were later upgraded to systems by removing a midship torpedo tube. Their maximum design depth was although in service some reached depths of below. Some submarines contained Fessenden oscillator systems.
At dawn on 13 September 1914, the submarine, commanded by Lieutenant-CommanderHorton, torpedoed the German light cruiser six miles southwest of Heligoland. Hela was hit amidships with the two torpedoes, fired from a range of 600 yards. All but two of her crew were rescued by the German submarine U-18 and another German ship. Although pursued most of the day by German naval forces, E9 managed to reach Harwich safely. Three weeks later, Horton sank the German destroyer off the mouth of the River Ems. For sinking the cruiser and the destroyer, Horton was awarded the Distinguished Service Order. E9 was scuttled outside Helsinki off Grohara Light in the Gulf of Finland on 3 April 1918 to avoid seizure by advancing German forces. HMS E9 was salvaged for breaking in Finland in August 1953.
While in command of the E9, Horton initiated the tradition of submarines flying the Jolly Roger upon returning from successful combat patrols. Remembering comments by First Sea Lord Admiral Sir Arthur Wilson, who complained that submarines were "underhanded, unfair, and damned un-English" and that personnel should be hanged as pirates, Horton few the flag when the E9 returned to port following the sinking of the Hela. They flew additional flags after each successful patrol, but when there was no room for more flags, they began adding symbols, each indicating a certain achievement, to a single large flag. This practice was imitated by other British submarines during World War I, and it was renewed again during World War II. The Admiralty initially disapproved of the practice, but was unable to stop it. The Jolly Roger has since been adopted as the logo of the Royal Navy Submarine Service.
