* Completed by Royal Dockyards.

These boats were also the survivors of a once numerous class built during the first world war. They were of Admiralty design and replaced the E class as the standard patrol submarine. These three boats although laid down during the first world war, were not completed until some years afterwards and then not by Vickers but in the Royal Dockyards. The 18" beam torpedo tubes included in the original design, were omitted and later a type 118 asdic set was fitted in their place. They were accepted in 1939 as operational but would have been replaced by new boats of the T-class within a year or so if the war had not broken out.

Dimensions	238 ½’ x 23 ½‘ x 16’ = 890 tons (surface) 1080 tons (submerged)
Armament	Four 21" bow torpedo tubes. Eight 21" Mark IV*S Torpedoes. One 4" QF Mk III gun on an S1 mounting. 90 rounds ammunition. One Lewis gun and five rifles.
Propulsion	Twin Diesel engines 2400 HP = 17.5 knots. 76 tons oil fuel = 2380 miles @ 16 kts or 4030 miles @ 8 kts. 1.7 tons per day on patrol. Main motors 1600 HP = 10 ½ kts submerged. Batteries. 336 cells of 135 tons. 1 ½ hrs at 9 knots submerged.
Hull	Saddle Tank type. Operational diving depth 150 feet.
Complement	4 Officers and 37 men.
Detecting Devices	Two 30 foot periscopes Type 118 asdic set.