calibre guns. M2 was refitted with a seaplane hangar forward of the conning tower and a catapult to launch a small Parnall Peto seaplane. This conversion was a success, and M2 could surface from periscope depth, open the hangar door, catapult the plane, close the door and dive again within five minutes. She was subsequently lost during exercises in the English Channel in 1932 when her hangar door was left open.
In 1927, M3 was converted to an experimental minelayer, stowing her 100 mines on rails inside a large free-flooding casing outside the hull. The mines were laid over her stern by means of a chain-conveyor belt. M3 was finally taken out of service in April 1932, and scrapped in 1933.
Although the M Class may be considered unsuccessful because
