The Redoutable-class submarines were ocean-going submarines of the French Navy in the Second World War. They were officially called "Long patrol submarines" (Fr:Sous-marins de grande patrouille), or "Type 1 submarines". They were also referred to as the 1500 Series, and regarded as being in three sub-classes (Though French Navy recognize only two) The Redoubtable class were generally regarded as successful, being reliable and seaworthy, with a good range and armament. A total of 31 were built, the largest class of submarines built by the French Navy, and comprising one-third of its total submarine force.
The Redoutable class were built for service in the Atlantic, operating as scouts, or as commerce raiders, and for colonial service. They were built to conform to the interwar naval treaties arising from the 1922 Washington and 1930 London conferences, which placed restrictions on the number and size of warships of various types that nations could build. The Redoutable class were designed and built as successors to the Requin class, France's first post-war Type 1 design. Orders were placed in 1924 for the first two boats, Redoutable and Vengeur, followed by orders for seven more (1925), then five (1926), and a further five (1927). In 1929 a further order was placed for six boats of an improved design with more powerful engines, followed in 1930 by another six, again with improved engines and speed
The 1500s were built to a 92-metre (301 ft 10 in) double-hulled design, with an emphasis on surface speed and a long range. They had a surface displacement just above 1,500 tons, the upper limit by treaty. They were rated for a dive depth of 80 metres (260 ft), though in service depths of 120 metres (390 ft) were recorded. Diving time was 30–40 seconds, and their underwater range was 100 miles (160 km) at 5 knots (9.3 km/h; 5.8 mph). Surface range was 10,000 miles (16,000 km) at 10 knots (19 km/h; 12 mph), with a maximum speed of 19 to 20 knots (35 to 37 km/h; 22 to 23 mph), and a maximum submerged speed of 10 knots.
Their armament was 11 torpedo tubes (4 forward, 3 midships, and 3-4 aft) with an outfit of 13 torpedoes. As with other French submarines of this period, the 1500s had torpedo tubes fitted externally in trainable mounts; in this case they were midships and astern. The stern mount also had tubes of two different sizes, for different targets. They had a single 100 mm (3.9 in) gun, and one to two 13.2 mm machine guns. and were manned by crews of 61 men.
One drawback suffered by these vessels was their engines, which, though reliable, were noisy, both diesel and electric, a disadvantage when operating with stealth. They were also criticized for their habitability, with inadequate ventilation and food storage.
In 1943 the five boats still in service were refitted in the United States. A second anti-aircraft gun was added, on a platform ahead of the conning tower, and the torpedo armament was rationalized; the two 400 mm (15.7 in) tubes were removed from the stern mount and replaced by a single 550 mm (21.7 in) tube.
The Redoutable class served with the Marine Nationale and with Vichy and Free French Forces in World War II in a full range of front-line duties and missions. Of the 29 boats that served in World War II (two were lost in the pre-war period), 24 were lost.
At the outbreak of war there were 29 vessels of the 1500 class; two had been lost accidentally in the 1930s. At the Fall of France four boats undergoing repair at Brest (Pasteur, Achille, Agosta and Ouessant) were scuttled; another (Protée) was interned at Alexandria. The remaining 1500s were with the French fleet, or at various overseas stations.
Poncelet was scuttled at 18 November 1940, off Port Gentil, Gabon after being depth charged by HMS Milford