Surcouf was a large French gun-armed cruiser submarine of the mid 20th century. She carried two 8" guns as well as anti-aircraft guns and a floatplane. Surcouf served in the French Navy and, later, the Free French Naval Forces during the Second World War.
Surcouf c. 1935
The Free French Memorial on Lyle Hill, Greenock: À la mémoire du Capitaine de frégate Blaison, des officiers et de l'équipage du sous-marin Surcouf perdu dans l'Atlantique Février 1942 ("To the memory of Frigate Captain Blaison, the officers[,] and the crew of the submarine Surcouf[,] lost in the Atlantic February 1942").
Model of Surcouf in Paris
A cruiser submarine was a very large submarine designed to remain at sea for extended periods in areas distant from base facilities. Their role was analogous to surface cruisers; 'cruising' distant waters, commerce raiding, and otherwise operating independently. When operating within a fleet, cruisers and cruiser submarines could be expected to scout for and screen the battle fleet. Cruiser submarines were successful for a brief period of World War I, but were less successful than smaller submarines during World War II. Large submarines remained vulnerable to damage from defensively equipped merchant ships (DEMS), were slow to dive if found by aircraft, offered a large sonar echo surface, and were less able to defensively maneuver during depth charge attacks.
Surcouf had the largest guns of any cruiser submarine.