|Builder:||Cammell Laird, Birkenhead|
|Laid down:||23 October 1915|
|Fate:||Sold, 6 September 1922|
|Class and type:||E class submarine|
|Length:||181 ft (55 m)|
|Beam:||15 ft (4.6 m)|
HMS E42 was a British E class submarine built by Cammell Laird, Birkenhead. She was laid down on 23 October 1915 and commissioned in July 1916. She served during the First World War, making an unsuccessful attack on U-92 on 1 July 1918. E42 was sold for scrap at Poole on 6 September 1922.
Like all post-E8 British E-class submarines, E42 had a displacement of 662 tonnes (730 short tons) at the surface and 807 tonnes (890 short tons) while submerged. She had a total length of 180 feet (55 m) and a beam length of 22 feet 8.5 inches (6.922 m). She was powered by two 800 horsepower (600 kW) Vickers eight-cylinder two-stroke diesel engines and two 420 horsepower (310 kW) electric motors. The submarine had a maximum surface speed of 16 knots (30 km/h; 18 mph) and a submerged speed of 10 knots (19 km/h; 12 mph). British E-class submarines had fuel capacities of 50 tonnes (55 short tons) of diesel and ranges of 3,255 miles (5,238 km; 2,829 nmi) when travelling at 10 knots (19 km/h; 12 mph). E42 was capable of operating submerged for five hours when travelling at 5 knots (9.3 km/h; 5.8 mph).
E42 was armed with a 12-pounder 76 mm (3.0 in) 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.
E-Class submarines had wireless systems with 1 kilowatt (1.3 hp) power ratings; in some submarines, these were later upgraded to 3 kilowatts (4.0 hp) systems by removing a midship torpedo tube. Their maximum design depth was 100 feet (30 m) although in service some reached depths of below 200 feet (61 m). Some submarines contained Fessenden oscillator systems.
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- "E Class". Chatham Submarines. Retrieved 20 August 2015.
- Koerver, Hans Joachim. Room 40: German Naval Warfare 1914-1918. Vol II., The Fleet in Being. Steinbach, Germany: LIS Reinisch, 2009.
- Hutchinson, Robert (2001). Jane's Submarines: War Beneath the Waves from 1776 to the Present Day. London: HarperCollins. ISBN 978-0-00-710558-8. OCLC 53783010.