|Ordered:||7 August 1939|
|Builder:||Bremer Vulkan, Bremen-Vegesack|
|Laid down:||16 September 1940|
|Launched:||17 May 1941|
|Commissioned:||16 August 1941|
|Fate:||Sunk, 15 July 1943|
|Class and type:||Type VIIC submarine|
|Height:||9.60 m (31 ft 6 in)|
|Draught:||4.72 m (15 ft 6 in)|
|Complement:||4 officers, 40-56 enlisted|
She was laid down at the Vulkan-Vegesackerwerft in Bremen on 16 September 1940 as yard number 14, launched on 12 June 1941 and commissioned on 16 August with Oberleutnant zur See Friederich-Hermann Praetorius in command.
German Type VIIC submarines were preceded by the shorter Type VIIB submarines. U-135 had a displacement of 769 tonnes (757 long tons) when at the surface and 871 tonnes (857 long tons) while submerged. She had a total length of 67.10 m (220 ft 2 in), a pressure hull length of 50.50 m (165 ft 8 in), a beam of 6.20 m (20 ft 4 in), a height of 9.60 m (31 ft 6 in), and a draught of 4.74 m (15 ft 7 in). The submarine was powered by two MAN M 6 V 40/46 four-stroke, six-cylinder supercharged diesel engines producing a total of 2,800 to 3,200 metric horsepower (2,060 to 2,350 kW; 2,760 to 3,160 shp) for use while surfaced, two Brown, Boveri & Cie GG UB 720/8 double-acting electric motors producing a total of 750 metric horsepower (550 kW; 740 shp) for use while submerged. She had two shafts and two 1.23 m (4 ft) propellers. The boat was capable of operating at depths of up to 230 metres (750 ft).
The submarine had a maximum surface speed of 17.7 knots (32.8 km/h; 20.4 mph) and a maximum submerged speed of 7.6 knots (14.1 km/h; 8.7 mph). When submerged, the boat could operate for 80 nautical miles (150 km; 92 mi) at 4 knots (7.4 km/h; 4.6 mph); when surfaced, she could travel 8,500 nautical miles (15,700 km; 9,800 mi) at 10 knots (19 km/h; 12 mph). U-135 was fitted with five 53.3 cm (21 in) torpedo tubes (four fitted at the bow and one at the stern), fourteen torpedoes, one 8.8 cm (3.46 in) SK C/35 naval gun, 220 rounds, and a 2 cm (0.79 in) C/30 anti-aircraft gun. The boat had a complement of between forty-four and sixty.
The boat sank three ships totalling 21,302 gross register tons (GRT) and damaged a fourth of 4,762 GRT.
The submarine's first patrol began with her departure from Kiel on 24 December 1941. Her route took her across the North Sea and into the Atlantic Ocean via the passage between the Orkney and Shetland Islands. As part of wolfpack 'Ziethen', she sank Gandia on 22 January 1942 420 nmi (780 km; 480 mi) east of Cape Race, (Newfoundland). She arrived at St. Nazaire in occupied France on the 31st.
Her second foray was northeast of Iceland, but she returned to another port, Brest, on 3 April 1942.
Her third sortie was her longest, at 71 days. Having departed Brest on 26 April 1942, she sank Fort Qu Appelle on 17 May north of Bermuda. She also sank Pleasantville on 8 June northwest of Bermuda before returning to St. Nazaire on 5 June.
U-135 discovered and shadowed convoy ON 122 during her fourth patrol, and was able to remain on patrol following heavy damage received when attacked with depth charges and Hedgehog by HNoMS Potentilla and HMS Viscount. The boat was later attacked by a Czech-crewed Vickers Wellington aircraft of No. 311 Squadron RAF on 3 October 1942 in the Bay of Biscay. Only minor damage was sustained, but one man was killed and another died of his wounds.
Her fifth sortie took her across the Atlantic, almost to the coast of Newfoundland.
U-135s sixth patrol was toward Greenland; she was attacked by a British B-24 Liberator of 120 Squadron northeast of Ireland on 8 February 1943. Some damage was repaired, but the boat was forced to return to Lorient on 10 March when further leaks were discovered.
For her last patrol, she left Lorient on 7 June 1943. Northeast of the West Indies, she attacked and damaged Twickenham on the 15th. She then moved to the east Atlantic where she was attacked by the sloop HMS Rochester, the corvettes HMS Migonette and Balsam and a US PBY Catalina flying boat of VP-92. U-135 was sunk east of the Canary Islands on 15 July. Five men died, there were 41 survivors.
U-135 took part in ten wolfpacks, namely.
|22 January 1942||Gandia||Belgium||9,626||Sunk|
|17 May 1942||Fort Qu Appelle||United Kingdom||7,127||Sunk|
|18 June 1942||Pleassantville||Norway||4,549||Sunk|
|15 July 1943||Twickenham||United Kingdom||4,762||Damaged|