A successful type, the single-hulled 'U' class boats were designed originally as unarmed targets to replace the elderly 'H' class boats, and were little larger. Three were laid down as such but, as the Royal Navy did not possess a modern 'coastal' submarine, it seemed advantageous to modify the bow to take torpedo tubes from the outset. The after hull had a sharp taper and the casing ended short of the stern, so all armament was set forward, four tubes in the pressure hull and, surprisingly (though a reflection on the doubtful accuracy of the torpedo salvoes of the day), the bow casing was also bulged to take two more. This was not a good feature as the restricted height of the design meant a shallow periscope depth, and the oversize bow casing made it both difficult to maintain constant depth and also caused a distinctive 'pressure hump' in the water above. With the outbreak of war a further group of 12 boats was ordered, 1.6m (5.25ft) longer to improve the lines and ease the cramped internals; most of these had only four tubes. Thirty-four more boats of this type followed, with improved lines and increased bunker space. Though extremely handy, the 'U' class boats were rather limited in diving depth and had a low surface speed.
Again, therefore, the design was updated. This time an extra midbody section was inserted to house uprated machinery, and the hull was rede signed to permit submergence to 91 m (300 ft) rather than the 60 m (200 ft) of the earlier boats, and to facilitate allwelded construction in modules that would produce faster building times. This later type was known as the 'V class, of which 33 were ordered but only 21 completed. It is a noteworthy fact that, except for two early units built in Chatham Dockyard, all 81 boats were built in the two Vickers Armstrong yards at Barrow and on Tyneside.
The 'U' and V classes were particularly suited to the shallow and confined waters of the North Sea and Mediterranean but, though successful, they suffered 19 losses. After the end of the Mediterranean war they had little use and many were either transferred or reverted to a training role.
Specification 'V class Displacement: 670 tons surfaced and 740 tons submerged Dimensions: length 62.79 m (206.0 ft); beam 4.88 m (16.0 ft); draught 4.72 m (15.5ft) Propulsion: two diesels delivering 596.6 kW (800 bhp) and two electric motors delivering 566.7 kW (760 hp) to two shafts Speed: 12,5 kts surfaced and 9 kts submerged Endurance: 8715 km (5,415 miles) at 10 kts surfaced and 113 km (70 miles) at 7 kts submerged Armament: one 76,2-mm (3-in) gun and four 533-mm (21 -in) torpedo tubes (all bow) for eight torpedoes Complement: 37