|Builder:||William Denny and Brothers, Dumbarton|
|Laid down:||October 1916|
|Commissioned:||27 May 1918|
|Fate:||Sold for scrapping, 30 June 1927|
|Class and type:||L-class submarine|
|Length:||238 ft 7 in (72.7 m)|
|Beam:||23 ft 6 in (7.2 m)|
|Draught:||13 ft 3 in (4.0 m)|
|Range:||3,800 nmi (7,000 km; 4,400 mi) at 10 kn (19 km/h; 12 mph) on the surface|
|Test depth:||150 feet (45.7 m)|
L9 and its successors were enlarged to accommodate 21-inch (53.3 cm) torpedoes and more fuel. The submarine had a length of 238 feet 7 inches (72.7 m) overall, a beam of 23 feet 6 inches (7.2 m) and a mean draft of 13 feet 3 inches (4.0 m). They displaced 914 long tons (929 t) on the surface and 1,089 long tons (1,106 t) submerged. The L-class submarines had a crew of 38 officers and ratings.
For surface running, the boats were powered by two 12-cylinder Vickers 1,200-brake-horsepower (895 kW) diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 600-horsepower (447 kW) electric motor. They could reach 17 knots (31 km/h; 20 mph) on the surface and 10.5 knots (19.4 km/h; 12.1 mph) underwater. On the surface, the L class had a range of 3,800 nautical miles (7,000 km; 4,400 mi) at 10 knots (19 km/h; 12 mph).
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. They were also armed with a 4-inch (102 mm) deck gun.
HMS L9 was laid down in October 1916 by William Denny and Brothers at their Dumbarton shipyard, launched on 29 January 1918, and completed on 27 May. She sailed with the Submarine Depot Ship HMS Ambrose (1903) to Hong Kong in 1919 as part of the 4th Submarine Flotilla. The boat was sunk during a typhoon in Hong Kong harbour on 18 August 1923. She was salvaged on 6 September 1923 and then recommissioned. L9 was sold in Hong Kong on 30 June 1927.