Redoutable-class submarine (1931)

The Redoutable class submarines were ocean-going submarines of the French Navy during 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.

Development
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.

General characteristics
The 1500s were built to a 92 metre 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, though in service depths of 120 metres were recorded. Diving time was 30 to 40 seconds, and their underwater range was 100 miles at 5 knots. Surface range was 10,000 miles at 10 knots, with a maximum of 19 to 20 knots, and a maximum submerged speed of 10 knots. Their armament was eleven 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 3.9 inch/100mm gun, and one to two 13.2mm 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 storage for foodstuffs.

In 1943 the five boats still in service were refitted in the USA. A second anti-aircraft gun was added, on a platform ahead of the conning tower, and the torpedo armament was rationalized; the two 400mm tubes were removed from the stern mount and replaced by a single 550 mm tube.

Sub-classes
Built over ten years by a number of different shipyards, the 1500s differed in details of design and construction. The first two (Redoutable and Vengeur) were equipped with a generator for recharging the batteries for the electric motors; the following vessels all  dispensed with this, recharging directly from the diesel engines. Nine vessels (Achille, Ajax, Archimede, Argo;, Le Centaure, Pasteur, Persee, Poncelet and Promethee), were equipped with Schneider diesels, the remainder with engines by Sulzer. There was also a progressive increase in motive power; the boats of the 1922 programme, (Redoutable and Vengeur) had engines rated at 4000 rpm (cite needed) those of the 1925, 1926 and 1927 programmes 6,000 hp. In the 1929 programme this was increased to 7,200 hp, and in the 1931 boats 8,600 hp. This in turn led to a progressive improvement in surface speed; the first series made 17 knots on the surface, the 1929 boats 19 knots, and the 1930 boats 20 knots.

These differences form the basis of the class division into sub-classes. A number of sources  make a three part division; the boats of the 1922, 1925, 1926 and 1927 programmes in the first series; those of the 1929 programme the second sub-class, or series, and the 1930 boats the third. Those sources that make a two class division regard Redoutable and Vengeur as one sub-class, (referred to as project M5) and the remaining 29 (project M6) as the other.

War service
The Redoutable class served with the Marine Nationale and with Vichy and Free French forces during 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 1500’s were with the French fleet, or at various overseas stations.

Five were lost in the months after the armistice; Three (Persee, Ajax and Poncelet) were sunk in the attack on Dakar, another (Sfax) was sunk in error by the German U-37, and Pegasee was decommissioned at Saigon in French Indochina.

In May 1942 three more (Bevezier, L'Héros and Monge) were sunk when Madagascar came under attack; in November another three (Actéon, Le Conquérant and Sidi Ferruch) were sunk resisting the Allied invasion of North Africa and a fourth (Le Tonnant) was scuttled. At the end of November, when the Germans occupied Vichy France, seven boats were scuttled at Toulon. The remaining five (Argo, Archimède, Le Centaure, Le Glorieux and Casabianca) still in French hands when she rejoined the Allies all survived until the end of the war, being decommissioned in 1952.