L9 and its successors were enlarged to accommodate 21-inch (53.3 cm) torpedoes and more fuel. The submarine had a length of 238feet overall, a beam of 23feet and a mean draft of 13feet.[1] They displaced 914LT on the surface and 1089LT submerged. The L-class submarines had a crew of 38 officers and ratings.[2]
For surface running, the boats were powered by two 12-cylinder Vickers[3] 12000NaN0 diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 600hp electric motor.[1] They could reach 17kn on the surface and underwater. On the surface, the L class had a range of 3800nmi at .[4]
The boats were armed with four 21-inch torpedo tubes in the bow and two 18-inch (45 cm) in broadside mounts. They carried four reload torpedoes for the 21-inch tubes for a grand total of ten torpedoes of all sizes.[5] They were also armed with a 4adj=onNaNadj=on deck gun.[2] L12 was fitted with 16 vertical mine chutes in her saddle tanks and carried one mine per chute.[1]
HMS L12 was built by Vickers, Barrow-in-Furness. She was laid down on 22 January 1917 and was commissioned on 30 June 1918. The boat collided with the submarine off Milford Haven, Wales on 9 July 1929. She was able to surface and returned to Milford Haven; three sailors died. HMS L12 was sold to John Cashmore Ltd on 16 February 1932 for scrapping at Newport.