Most of the major maritime nations at sometime or other experimented with the idea of the cruiser submarine. All were larger than usual, with an exceptional surface armament and good endurance. Some carried an aircraft to increase their effective search radius. The only design to combine, reasonably successfully, all these features in one hull was the Surcouf Ordered under the 1926 programme as the first in a class of three, she was destined to be the only unit of the 'Surcouf class, and the largest submarine in the world in terms of displacement, though shorter than both the American 'Narwhal' and the Japanese 'A' boats.
At the time of the Washington Treaty the British Ml to M3 had 304,8-mm (12in) guns and, to prevent further escalation in this direction (though even these were overlarge and totally unwieldy) the treaty limited future submarines to 203.2-mm (8-in) weapons. Only the French ever fitted the latter, and these to the Surcouf, paired in a complex pressure-tight turret. This structure was faired into a pressuretight 'hangar' abaft it and containing a specially-designed Besson M.B.411 floatplane. This had to be taken out and the wings attached before it could be lowered into the water, a timeconsuming and highly risky business which, while acceptable in 1926, was certainly not in 1939-45. Only the French could ever have specified the torpedo tube fit. This comprised four 550-mm (21,65-in) tubes set in an orthodox bow arrangement, with six reloads; one quadruple 550-mm trainable mounting in the casing threequarters aft; and a quadruple 400-mm (15.75-in) tramable mounting in the casing right aft, with four reloads.
The suggested mode of operation of submarines such as these was always rather woolly and the Surcouf, like the rest of her kind, was never to find a proper role. Seized in Plymouth in July 1940, she was operated by a Free French crew on several Atlantic patrols, In December 1941 she participated with three French corvettes in the seizure of the Vichy islands of St Pierre and Miquelon, in the St Lawrence estuary. In February 1942 she sank in the Caribbean after a collision.
Specification 'Surcouf class Displacement: 3,270 tons surfaced and 4,250 tons submerged Dimensions: length 110.00 m (360.89 ft); beam 9.00 m (29,53 ft); draught 9,07 m (29.76 ft) Propulsion: two diesels delivering 5667.3 kW (7,600 bhp) and two electric motors delivering 2535.4 kW (3,400 hp) to two shafts Speed: 18 kts surfaced and 8.5 kts submerged Endurance: 18531 km (11,515 miles) at 10 kts surfaced and 111 km (69 miles) at 5 kts submerged Armament: two 203,2-mm (8-in) guns, two 37-mm guns, eight 550-mm (21.65in) torpedo tubes (four bow and four in a trainable mounting), and four 400mm (15,75-in) torpedo tubes (ma trainable mounting aft) Complement: 118