K7 was the only one of the disastrous K class to engage with an enemy; on 16 June 1917 she fired a salvo of torpedoes at the U-boat U-95 and scored a direct hit. However, the torpedo failed to explode with what has been described as typical "K" luck; K-7 escaped retaliation by steaming away at speed.[1]
K7 was involved in an accident with the 4th Light Cruiser Squadron. She was also involved in the catastrophic series of accidents during a night exercise that came to be known sarcastically as the Battle of May Island; K7 was damaged by running over the sinking . K7 was sold on 9 September 1921 at Sunderland.
K7 displaced 1800LT when at the surface and 2600LT while submerged.[2] It had a total length of 338feet, a beam of 26feet, and a draught of 20feet. The submarine was powered by two oil-fired Yarrow Shipbuilders boilers supplying one geared Brown-Curtis or Parsons steam turbine that developed 10,500 ship horsepower (7,800 kW) to drive two 7inchesft6inchesin (ftin) screws. Submerged power came from four electric motors each producing 350to. It also had an 800hp diesel engine to be used when steam was being raised, or instead of raising steam.[3]
The submarine had a maximum surface speed of 24kn and a submerged speed of 9to.[4] It could operate at depths of 150feet at 2kn for 80nmi.[2] K7 was armed with ten 18inches torpedo tubes, two 4inches deck guns, and a 3inches anti-aircraft gun. Its torpedo tubes were fitted to the bows, the midship section, and two were mounted on the deck.[2] Its complement was fifty-nine crew members.[4]