[Note 1] In eleven patrols UC-21 was credited with sinking 98 ships, either by torpedo or by mines laid.

They included the British hospital ship Donegal, which UC-21 torpedoed in the English Channel on 17 April 1917, killing 29 already wounded soldiers and 12 crew.

She had a dive time of 35 seconds and was capable of operating at a depth of 50 metres (160 ft).

When submerged, she could operate for 55 nautical miles (102 km; 63 mi) at 4 knots (7.4 km/h; 4.6 mph); when surfaced, she could travel 9,430 nautical miles (17,460 km; 10,850 mi) at 7 knots (13 km/h; 8.1 mph).

UC-21 was fitted with six 100 centimetres (39 in) mine tubes, eighteen UC 200 mines, three 50 centimetres (20 in) torpedo tubes (one on the stern and two on the bow), seven torpedoes, and one 8.8 cm (3.5 in) Uk L/30 deck gun.