French Submarine Le Surcouf showing off the 8" guns.

Surcouf was the largest French cruiser submarine. She served in both the French Navy and the Free French Naval Forces during the Second World War. She was lost during the night of 18/19 February 1942 in the Caribbean Sea, possibly after colliding with an American freighter. Surcouf was named after the French privateer Robert Surcouf. She was the largest submarine built until surpassed by the first Japanese I-400-class submarine in 1944.

Surcouf had a twin-gun turret with 203 mm (8-inch) guns, the same calibre as that of a heavy cruiser (the main reason of Surcouf being designated as croiseur sous-marin – "cruiser submarine") provisioned with 600 rounds.

Surcouf was designed as an "underwater heavy cruiser", intended to seek and engage in surface combat.^[1] For reconnaissance purposes, the boat carried a Besson MB.411 observation floatplane in a hangar built abaft of the conning tower. However, the floatplane was also mainly used for gun calibration purposes.

The boat was equipped with 10 torpedo tubes: four 550 mm (22 in) torpedo tubes in the bow, and two swiveling external launchers in the aft superstructure, each with one 550mm and two 400 mm (16 in) torpedo tubes. Eight 550mm and four 400mm reloads were carried.^[2] The 203mm/50 Modèle 1924 guns were in a pressure-tight turret forward of the conning tower. The guns had a 60-round magazine capacity and was controlled by a director with a 5 m (16 ft) rangefinder, mounted high enough to view an 11 km (5.9 nmi; 6.8 mi) horizon, and able to fire within three minutes after surfacing.^[3] Using the boat's periscopes to direct the fire of the main guns, Surcouf could increase this range to 16 km (8.6 nmi; 9.9 mi); originally an elevating platform was supposed to lift lookouts 15 m (49 ft) high, but this design was abandoned quickly due to the effect of roll.^[4] The Besson observation plane could be used to direct fire out to the guns' 26 mi (23 nmi; 42 km) maximum range. Anti-aircraft cannon and machine guns were mounted on the top of the hangar.

Surcouf also carried a 4.5 m (14 ft 9 in) motorboat, and contained a cargo compartment with fittings to restrain 40 prisoners or lodge 40 passengers. The submarine's fuel tanks were very large; enough fuel for a 10,000 nmi (19,000 km; 12,000 mi) range and supplies for 90-day patrols could be carried.

The test depth was 80 m (260 ft).

The first commanding officer was Frigate Captain (Capitaine de Frégate, a rank equivalent to Commander), Raymond de Belot.

The boat encountered several technical challenges, owing to the 203mm guns.^{[citation needed]}

• Because of the low height of the rangefinder above the water surface, the practical range of fire was 12,000 m (13,000 yd) with the rangefinder (16,000 m (17,000 yd) with sighting aided by periscope), well below the normal maximum of 26,000 m (28,000 yd).
• The duration between the surface order and the first firing round was 3 minutes and 35 seconds. This duration could have been longer in case the boat was going to fire broadside, which meant surfacing and training the turret in the desired direction.
• Firing had to occur at a precise moment of pitch and roll when the ship was level.
• Training the turret to either side was limited to when the ship rolled 8° or more.
• Surcouf was not equipped to fire at night, due to inability to observe the fall of shot in the dark
• The mounts were designed to fire 14 rounds from each gun before their magazines were reloaded.

To replace the floatplane whose functioning was initially constrained and limited in use, trials were conducted with an autogyro in 1938.