British E-class submarines had fuel capacities of 50 long tons (51 t) of diesel and ranges of 3,255 miles (5,238 km; 2,829 nmi) when travelling at 10 knots (19 km/h; 12 mph).

E20 was fitted, possibly uniquely within her class, with a 6-inch howitzer deck gun, forward of the conning tower.

[1] Operating in the eastern Mediterranean, E20 was scheduled to rendez-vous with the French submarine Turquoise on 6 November 1915.

However, on 30 October, Turkish forces sank the Turquoise off Nagara Point in the Dardanelles, refloating her shortly afterwards, her confidential papers retrieved intact.

The Imperial German Navy submarine UB-14, which was at Constantinople, was duly sent to intercept E20,[4] reportedly going so far as to radio messages in the latest British code.