Thermidor was one of 18 s built for the French Navy () in the first decade of the 20th century.
Design and description The Pluviôse class were built as part of the French Navy's 1905 building program to a double-hull design by Maxime Laubeuf.Smigielski, p. 209 Thermidor differed from her sisters as she was built to test the hull shape planned for the following . She had an overall length of , a beam of and displaced on the surface and underwater. The submarines' crew numbered 2 officers and 23 enlisted men.Garier 1998, pp. 51, 54, 56, 65
For surface running, the boats were powered by two 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 electric motor.Garier 1998, pp. 56–59 On the surface they were designed to reach a maximum speed of and underwater. The submarines had a surface endurance of at and a submerged endurance of at .Garier 1998, p. 67
The first six boats completed were armed with a single internal bow torpedo tube, but this was deleted from the rest of the submarines after an accident with their sister 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.Garier 1998, pp. 59–60
Construction and career 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.Couhat, p. 140; Garier 1998, pp. 49–50
On 10 June 1913, Thermidor collided with the submarine 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 in length in her external port ballast tank that required a substantial period of time to repair.Sous-Marins Français Disparus & Accidents: Sous-Marin Watt (in French) Accessed 29 April 2023
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