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Four M-class submarines replaced the order for the last four K class, K17-K21. Although they were similar in size, the M-class was an entirely different design from the K-class although it's possible that some material ordered for the K-boats went into them. In any event, the end of the First World War meant that only three were completed. M2 was laid down at Vickers shipyard at Barrow in Furness in 1916, and launched in 1919. Like the other members of her class she was armed with a single gun as well as torpedo tubes. The Mark IX gun was taken from spares held for the Formidable class of battleships. The British M class submarines were very large for the time at long. They were designed to operate as part of the main fleet of battleships and so needed to be fast. They displaced 1,600 tonnes on the surface and 1,950 when submerged. Two twelve-cylinder diesel engines powered them on the surface producing, underwater they were driven by electric motors producing . The maximum speed on the surface was, which proved to be slower than the newer battleships and the concept of fleet submarines was soon scrapped, with the River class of submarines being the last. After the accidental sinking of HMS M1 in 1925, M2 and her sister M3 were taken out of service and reassigned for experimental use. She had her gun removed because of the limit in submarine gun calibre of 8 inches imposed by the Washington Naval Treaty, and it was replaced by a small aircraft hangar, the work being completed in 1928. This could carry a small Parnall Peto seaplane, specially designed for the M2 and which could be launched by hydraulic catapult within a few minutes of surfacing. The aircraft would land alongside the submarine on completion of its sortie and be winched aboard using a crane. The submarine was to operate ahead of the battle fleet in a reconnaissance role, flying off her seaplane as a scout. The accidentM2 left her base at Portland on 26 January 1932, for an exercise in West Bay carrying Parnall Peto serial N255. Her last communication was a radio message at 10:11 to her Submarine depot ship, HMS Titania to announce that she'd dive at 10:30 am. The captain of a passing merchant ship, the Newcastle coaster Tynsider, mentioned that he'd seen a large submarine dive stern first at around 11:15. Unaware of the significance of this, he only reported it in passing once he reached port.Her crew of 60 were all killed in the accident. The submarine was found on 3 February, eight days after her loss. Ernest Cox, the salvage expert that had raised the German battleships at Scapa Flow, was hired to salvage the M2. In an operation lasting nearly a year and 1,500 dives, on 8 December 1932, she was lifted to within six metres of the surface before a gale sprang up, sending her down to her final resting place. The hangar door was found open and the aircraft still in it. The accident was believed to be due to water entering the submarine through the hangar door, which had been opened to launch the aircraft shortly after surfacing. This is a similar reason to the loss of the RO-RO cross channel ferry Herald of Free Enterprise in 1987, which capsized when the sea entered the ship through the large car-deck door which was close to the waterline. Two explanations have been advanced. The first is that since the crew were always trying to beat their record time for launching the aircraft, they'd simply opened the hangar door on surfacing whilst the deck was still awash. The other theory is that the flooding of the hangar was due to failure of the stern hydroplanes. High pressure air tanks were used to bring the boat to the surface in an awash condition but to conserve this limited resource, compressors were then started to completely clear the ballast tanks of water by blowing air into them. This could take as long as 15 minutes to complete. The normal procedure for launching the aircraft was therefore to hold the boat on the surface using the hydroplanes whilst the hangar door was opened and the aircraft launched. Failure of the rear hydroplanes would have sent the stern down as observed by the merchant officers and water would have eventually entered the hangar. The aftermathThe submarine currently lies upright on the sea bed at OSGB36. Her keel is about 32 metres below the surface at low tide, and her highest point at the top of the conning tower at around 20 metres. She is a popular dive for scuba divers.With the loss of M2, the Royal Navy abandoned submarine-launched aircraft, although most other navies also experimented with the concept in the interwar years. Possibly the last and the most impressive aircraft carrying submarines were the Japanese Sen Toku class which were three times the size of M2. The largest submarines of World War II, each carried three seaplanes and were intended to launch air attacks on the Panama Canal and American West Coast cities but the war ended before they saw action. Further Information
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