Initially U-81 to U-83 had one 10.5 cm gun with 140-240 rounds. U-84 - U-86 on the other hand had two 8.8 cm guns. In 1917 U-84 - U-86 were refitted with a single 10.5 cm gun (240 rounds)
Type U 81 submarines were preceded by the shorter Type UE I submarines. U-84 had a displacement of 808t when at the surface and 946t while submerged. She had a total length of 70.06m (229.86feet), a pressure hull length of 55.55m (182.25feet), a beam of 6.3m (20.7feet), a height of 8m (26feet), and a draught of 4.02m (13.19feet). The submarine was powered by two 2400PS engines for use while surfaced, and two 1200PS engines for use while submerged. She had two propeller shafts. She was capable of operating at depths of up to .
The submarine had a maximum surface speed of and a maximum submerged speed of . When submerged, she could operate for at ; when surfaced, she could travel at . U-84 was fitted with four torpedo tubes (two at the bow and two at the stern), twelve to sixteen torpedoes, and one 10.5sp=usNaNsp=us SK L/45 deck gun. She had a complement of thirty-five (thirty-one crew members and four officers).
| Date | Name | Nationality | Tonnage[1] | Fate[2] | |
|---|---|---|---|---|---|
| 14 December 1916 | Aamot |  Norway | 1,362 | Captured as prize | |
| 18 December 1916 | Malcolm |  Sweden | 2,100 | Captured as prize | |
| 9 January 1917 | Alexandrian |  United Kingdom | 4,467 | Damaged | |
| 10 January 1917 | Bergenhus | Norway | 3,606 | Sunk | |
| 12 January 1917 | Auchencrag | United Kingdom | 3,916 | Sunk | |
| 15 January 1917 | Kinpurney | United Kingdom | 1,944 | Sunk | |
| 15 January 1917 | Omsk |  Denmark | 1,574 | Sunk | |
| 20 January 1917 | Bulgarian | United Kingdom | 2,515 | Sunk | |
| 20 January 1917 | Neuquen | United Kingdom | 3,583 | Sunk | |
| 17 February 1917 | Bayonne |  France | 2,589 | Sunk | |
| 17 February 1917 | Romsdalen | United Kingdom | 2,548 | Sunk | |
| 18 February 1917 | United Kingdom | 11,137 | Damaged | ||
| 18 February 1917 | Hunsworth | United Kingdom | 2,991 | Damaged | |
| 18 February 1917 | Juno | Norway | 2,416 | Sunk | |
| 18 February 1917 | Valdes | United Kingdom | 2,233 | Sunk | |
| 21 February 1917 | Dukat | Norway | 1,408 | Sunk | |
| 22 February 1917 | Invercauld | United Kingdom | 1,416 | Sunk | |
| 13 April 1917 | Argyll | United Kingdom | 3,547 | Sunk | |
| 13 April 1917 | Lime Branch | United Kingdom | 5,379 | Damaged | |
| 18 April 1917 | Cragoswald | United Kingdom | 3,235 | Sunk | |
| 18 April 1917 | Rowena | United Kingdom | 3,017 | Sunk | |
| 19 April 1917 | Elswick Manor | United Kingdom | 3,943 | Sunk | |
| 20 April 1917 | Malakand | United Kingdom | 7,653 | Sunk | |
| 1 July 1917 | Bachi |  Spain | 2,184 | Sunk | |
| 1 July 1917 | Demerara | United Kingdom | 11,484 | Damaged | |
| 4 July 1917 | Goathland | United Kingdom | 3,044 | Sunk | |
| 7 July 1917 | Condesa | United Kingdom | 8,557 | Sunk | |
| 7 July 1917 | Oxø | Norway | 831 | Sunk | |
| 12 August 1917 | Ursus Minor | Norway | 623 | Sunk | |
| 13 August 1917 | 1,290 | Sunk | |||
| 24 November 1917 | Actaeon |  United States | 4,999 | Sunk | |
| 1 December 1917 | Antonios Stathatos |  Greece | 2,743 | Sunk | |
| 2 December 1917 | Birchgrove | United Kingdom | 2,821 | Sunk | |
| 9 January 1918 | Bayvoe | United Kingdom | 2,979 | Sunk | |
| 10 January 1918 | Cardiff | United Kingdom | 2,808 | Damaged | |
| 11 January 1918 | Mereddio | United Kingdom | 3,069 | Sunk | |
| 12 January 1918 | Chateau Laffite | France | 1,913 | Sunk | |
| 17 January 1918 | Messidor | United Kingdom | 3,883 | Damaged |