When launched, the Surcouf was the largest submarine in the world, only surpassed in the Second World War by the Japanese I-400-class submarine.
Her intended role was that of commerce raider and her design was intended to maximise this mission. She had a range of 10,000 miles, could sail for 90 days and was equipped with a Besson MN-411 floatplane in a hanger aft of the tower to scout for victims and spot for her main battery.
Her torpedo outfit included four forward internal tubes, a quad trainable mount aft and a second quad trainable mount aft equipped with lightweight torpedoes. She even had a prison hold for up to 40 captives. However, the most striking aspect of her design was the inclusion of a special, watertight turret forward of the tower which carried two eight-inch naval guns.
Surcouf was a very complex design, plagued with mechanical troubles. She was difficult, slow to dive and rolled badly on the surface in rough seas. She was also so low to the horizon that the effective range of her 8-inch guns was greatly reduced. When war broke out, Surcouf was in the French Antilles and by the time she returned to Brest was in need of repairs. When France fell, Surcouf escaped to England, where she was seized by British sailors. She was subsequently turned over to the Free French who regarded her with pride while the British Admiralty considered her, perhaps unfairly, something of a joke.
—description from the publisher's website