Like all post-E8 British E-class submarines, E42 had a displacement of 662LT at the surface and 807LT while submerged. She had a total length of 180feet[2] and a beam of 22feet. She was powered by two 800hp Vickers eight-cylinder two-stroke diesel engines and two 420hp electric motors.[3] [4] The submarine had a maximum surface speed of 16kn and a submerged speed of 10kn. British E-class submarines had fuel capacities of 50LT of diesel and ranges of 3255miles when travelling at 10kn.[2] E42 was capable of operating submerged for five hours when travelling at 5kn.
E42 was armed with a 12-pounder 76mm QF gun mounted forward of the conning tower. She had five 18-inch (450 mm) torpedo tubes, two in the bow, one either side amidships, and one in the stern; a total of 10 torpedoes were carried.[3]
E-Class submarines had wireless systems with 1kW power ratings; in some submarines, these were later upgraded to 3kW 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.[2]