The U class were originally developed as unarmed training boats to replace the WWI H-class. They were fitted with torpedo tubes to allow them to be used operationally. Small and manoeuvrable (630 tons) they were well suited to the confined waters of the North Sea and Mediterranean. They were succeeded by the very similar but partially welded V-class. Cheap, reliable and quickly produced, these were the workhorses of the wartime submarine service.

The specifications as given above are for the first group.

Further specifications on the later groups;
2nd Group;
Same as the first group except;
Complement: 31 men
Length: same as for the first group except: Umpire, Una, Unbeaten ,Undaunted, Union and Urchin 196.75 feet
Armament:
1 3" gun
3 MG AA
As designed: 6 21" torpedo tubes (4 bow, 2 bow external) except Umpire, Una, Unbeaten ,Undaunted, Union and Urchin 4 21" torpedo tubes (4 bow)
As designed: 10 torpedoes except Umpire, Una, Unbeaten ,Undaunted, Union and Urchin 8 torpedoes

It appears that all submarines of the 2nd group were not equipped with the external torpedo tubes although the ships not listed in the above text were fitted with a different type of bow which could accomodate the external tubes.

3th group;
Same as the first group except;
Displacement: 545 / 740 BRT
Complement: 31 men
Length: 196.75 feet
Armament:
1 3" gun
3 MG AA
4 21" torpedo tubes (4 bow)
8 torpedoes
Speed:14.25 / 9 knots

Undine, Unity and Ursula are often listed as a special Unity class of Small Patrol Submarines. When that is the case the Second group below becomes the first and the Third becomes Group II.

Submarines of this class that have detailed patrol info added are shown in bold in the list below.