Type M is the name of a class of British diesel-electric submarines from the First World War and the post-war period. The first unit of this type was launched in 1917, and the last - two years later. In total, only three units of this type were built. The Type M ship was 90-92 m long, 7.55 m wide, and had a displacement of about 1,950 tons. The maximum underwater speed is around 8-9 knots. The ships of this type had a single 305 mm gun, a single 75 mm gun, one 7.62 mm machine gun, and four 450 mm torpedo tubes.
The Type M ships were submarines with the most powerful artillery armaments ever commissioned. They were developed and put into production on the initiative of the first Lord of the British Admiralty, John Fisher, when he held this office for the second time in 1914-1915. It was assumed that ships of this type would approach enemy ships underwater and then, after surfacing, would start exchanging fire with them by surprise. However, these assumptions turned out to be problematic, and ships of this type did not take an active part in World War I. After 1918, new types of weapons were tested on them, such as mines or new types of seaplanes. Vessels of this type also turned out to be very unlucky, as two of the three sank in tragic accidents in 1925 and 1932. The last ship in the series, HMS M3, was decommissioned in 1932 and scrapped a year later.