French submarine Thermidor

The Pluviôse class were built as part of the French Navy's 1905 building program to a double-hull design by Maxime Laubeuf.[1] Thermidor differed from her sisters as she was built to test the hull shape planned for the following Brumaire class. She had an overall length of 52.15 m (171 ft 1 in), a beam of 5.42 m (17 ft 9 in) and displaced 396 t (390 long tons) on the surface and 551 t (542 long tons) underwater. The submarines' crew numbered 2 officers and 23 enlisted men.[2]

For surface running, the boats were powered by two 350-metric-horsepower (345 bhp; 257 kW) triple-expansion steam engines, each driving one propeller shaft using steam provided by two Du Temple boilers. When submerged each propeller was driven by a 230-metric-horsepower (227 bhp; 169 kW) electric motor.[3] On the surface they were designed to reach a maximum speed of 12 knots (22 km/h; 14 mph) and 8 knots (15 km/h; 9.2 mph) underwater.[1] The submarines had a surface endurance of 865 nautical miles (1,602 km; 995 mi) at 11.6 knots (21.5 km/h; 13.3 mph) and a submerged endurance of 70 nmi (130 km; 81 mi) at 2.8 knots (5.2 km/h; 3.2 mph).[4]

The first six boats completed were armed with a single 450-millimeter (17.7 in) internal bow torpedo tube, but this was deleted from the rest of the submarines after an accident with their sister Fresnel in 1909. All of the boats were fitted with six 450 mm external torpedo launchers; the pair firing forward were fixed outwards at an angle of seven degrees and the rear pair had an angle of five degrees. Following a ministerial order on 22 February 1910, the aft tubes were reversed so they too fired forward, but at an angle of eight degrees. The other launchers were a rotating pair of Drzewiecki drop collars in a single mount positioned on top of the hull at the stern. They could traverse 150 degrees to each side of the boat. The Pluviôse-class submarines carried eight torpedoes.[5]

Thermidor, named after the second month of summer in the French Republican Calendar, was ordered on 26 August 1905 from the Arsenal de Cherbourg. The submarine was laid down in 1906, launched on 3 July 1909 and commissioned on 13 July 1910.[6]

On 10 June 1913, Thermidor collided with the submarine Watt while Watt was moored at a wharf at the Brest Arsenal in Brest, France. Both submarines suffered damage, particularly Watt, which sustained a gash over 40 centimetres (16 in) in length in her external port ballast tank that required a substantial period of time to repair.[7]

• Couhat, Jean Labayle (1974). French Warships of World War I. London: Ian Allan. ISBN 0-7110-0445-5.
• Gardiner, Robert & Gray, Randal (1985). Conway's All The World's Fighting Ships 1906–1921. London: Conway Maritime Press. ISBN 0-85177-245-5.
• Garier, Gérard (2002). A l'épreuve de la Grande Guerre. L'odyssée technique et humaine du sous-marin en France (in French). Vol. 3–2. Bourg-en-Bresse, France: Marines édition. ISBN 2-909675-81-5.
• Garier, Gérard (1998). Des Émeraude (1905-1906) au Charles Brun (1908–1933). L'odyssée technique et humaine du sous-marin en France (in French). Vol. 2. Bourg-en-Bresse, France: Marines édition. ISBN 2-909675-34-3.