|Builder:||Armstrong Whitworth, Newcastle Upon Tyne|
|Laid down:||September 1917|
|Commissioned:||1 May 1919|
|Fate:||Sunk in collision 23 March 1922|
|Class & type:||H class submarine|
|Length:||171 ft 0 in (52.12 m)|
|Beam:||15 ft 4 in (4.67 m)|
On 23 March 1922, H42 was practising torpedo attacks against British destroyers steaming off Europa Point, Gibraltar, when she surfaced unexpectedly only 30 or 120 yards (27 or 110 metres) – sources differ – ahead of the destroyer HMS Versatile. Versatile, making 20 knots, went to full speed astern on her engines and put her helm over hard to port, but had not yet begun to answer her helm when she rammed H42 abaft the conning tower, almost slicing the submarine in half. H42 sank with the loss of all hands. An investigation found H42 at fault for surfacing where she did against instructions.
Like all post-H20 British H-class submarines, H42 had a displacement of 440 tonnes (490 short tons) at the surface and 500 tonnes (550 short tons) while submerged. It had a total length of 171 feet (52 m), a beam length of 15 feet 4 inches (4.67 m), and a draught length of 12 metres (39 ft). It contained a diesel engines providing a total power of 480 horsepower (360 kW) and two electric motors each providing 320 horsepower (240 kW) power. The use of its electric motors made the submarine travel at 11 knots (20 km/h; 13 mph). It would normally carry 16.4 tonnes (18.1 short tons) of fuel and had a maximum capacity of 18 tonnes (20 short tons).
The submarine had a maximum surface speed of 13 knots (24 km/h; 15 mph) and a submerged speed of 10.5 knots (19.4 km/h; 12.1 mph). Post-H20 British H-class submarines had ranges of 2,985 nautical miles (5,528 km; 3,435 mi) at speeds of 7.5 knots (13.9 km/h; 8.6 mph) when surfaced. H42 was fitted with an anti-aircraft gun and four 21 inches (530 mm) torpedo tubes. Its torpedo tubes were fitted to the bows and the submarine was loaded with eight 21 inches (530 mm) torpedoes. It is a Holland 602 type submarine but was designed to meet Royal Navy specifications. Its complement was twenty-two crew members.
- Submariners Association, Barrow-in-Furness Branch: Boat Database H42
- Richardson and Hurd 1923, p.31.
- "H-class". Battleships-Cruisers, Cranston Fine Arts. Retrieved 20 August 2015.
- Derek Walters (2004). The History of the British 'U' Class Submarine. Casemate Publishers. pp. 2–. ISBN 978-1-84415-131-8.
- Colledge, J. J.; Warlow, Ben (2006) . Ships of the Royal Navy: The Complete Record of all Fighting Ships of the Royal Navy (Rev. ed.). London: Chatham Publishing. ISBN 978-1-86176-281-8. OCLC 67375475. Retrieved from Naval-History on 20 August 2015.
- J. D. Perkins (1999). "Building History and Technical Details for Canadian CC-Boats and the Original H-CLASS". Electric Boat Company Holland Patent Submarines. Retrieved 20 August 2015.
- Hutchinson, Robert. Submarines, War Beneath The Waves, From 1776 To The Present Day.
- Richardson, Alexander and Archibald Hurd. (editors). Brassey's Naval and Shipping Annual 1923. London, William Clowes, 1923.