In 1926 the French Navy ordered a huge submarine armed with medium caliber guns and capable of carrying an observation seaplane on board. Construction lasted until 1934, when the Surcouf entered service. At that time, this was the longest and heaviest submarine in the world, and also the best armed. In addition to the two barbette mounted 203mm/50 Modèle 1924 naval guns, the same type as those installed on the heavy cruisers of the time, it carried 10 torpedo tubes, 4 of 550mm in the bow and two external mounts installed behind the conning tower, each with a 550mm torpedo tube and two 400mm torpedo tubes. It carried four 400mm torpedoes plus eight 550mm for reloads and six hundred 203mm shells. The guns had a rangefinder and were controlled by a director and had a maximum range of 31 km, although the effective range was around 16 km with the observation devices on board. To make fire at a greater distance, the observation seaplane was used to direct the fire.The spectacular armament was completed by a Besson MB.411 seaplane, two 37mm anti-aircraft guns and four 13.2mm machine guns. The submarine had a cargo hold that could accommodate up to 40 passengers and had enough fuel and supplies to be on patrol for three consecutive months. The Surcouf suffered from problems and limitations regarding the use of its guns, which prevented it from being effective in combat. She had no equipment to fire at night and could not use the guns in rough seas. In addition, their turn could not be done if the boat rolled more than 8º. Surcouf took no less than three and a half minutes from when the order was given to surface until the first shot was fired, as long as the conditions were favorable and the turret did not have to be rotated.Her military career was not very successful and was plagued with bad luck. Although in June 1940 she was able to escape from Brest during the German invasion, she went to take refuge in Plymouth, where she was captured by the British after an unfortunate confrontation. After finishing a conditioning started in France, the Surcouf was handed over to the Free French Navy (FNFL), although due to a lack of crew and due to a certain mistrust of the Royal Navy, the boat ended up in the service of the British, who sent it to the Canadian base at Halifax.In 1941 she was escorting trans-Atlantic convoys. In 1942 the Free French Navy decided to send the Surcouf to the Pacific, where it was thought that it could be effective against the Japanese merchant fleet. During the crossing from Halifax to Sydney, the submarine disappeared with all hands on the night of February 18-19 on its way to Tahiti. Apparently, the Surcouf accidentally collided with the American freighter Thompson Lykes, which despite hearing shouts in English after the impact, did not stop to help the crew, although it notified Panama of the incident. However, the unfortunate end of the Surcouf is not at all clear and there are still several open hypotheses about what really happened.
