The guns had a 60-round magazine capacity and were 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.

[4] Using the boat's periscopes to direct the fire of the main guns, Surcouf could increase the visible 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.

The boat encountered several technical challenges: To replace the floatplane, whose functioning was initially constrained and limited in use, trials were conducted with an autogyro in 1938.

Surcouf, which would have exceeded these limits, was specially exempt from the rules at the insistence of Navy Minister Georges Leygues,[4] but other 'big-gun' submarines of this boat's class could no longer be built.

Under command of Frigate Captain Martin, unable to dive and with only one engine functioning and a jammed rudder, she limped across the English Channel and sought refuge in Plymouth.

The Royal Navy blockaded the harbours where French warships were anchored, and delivered an ultimatum: rejoin the fight against Germany, be put out of reach of the Germans, or scuttle.

French ships lying at ports in Britain and Canada were also boarded by armed marines, sailors and soldiers, but the only serious incident took place at Plymouth aboard Surcouf on 3 July, when two Royal Navy submarine officers, Commander Denis 'Lofty' Sprague, captain of HMS Thames, and Lieutenant Patrick Griffiths of HMS Rorqual,[6][7] and French warrant officer mechanic Yves Daniel[8] were fatally wounded, and a British seaman, Albert Webb,[6] was shot dead by the submarine's doctor.

While the Admiral was in Ottawa, conferring with the Canadian government, Surcouf's captain was approached by The New York Times reporter Ira Wolfert and questioned about the rumours the submarine would liberate Saint-Pierre and Miquelon for Free France.

Ira Wolfert's stories – very favourable to the Free French (and bearing no sign of kidnapping or other duress) – helped swing American popular opinion away from Vichy.

The Axis Powers' declaration of war on the United States in December 1941 negated the agreement, but the U.S. did not sever diplomatic ties with the Vichy Government until November 1942.

In January 1942, the Free French leadership decided to send Surcouf to the Pacific theatre, after she had been re-supplied at the Royal Naval Dockyard in Bermuda.

Steaming alone from Guantanamo Bay on what was a very dark night, the freighter reported hitting and running down a partially submerged object which scraped along her side and keel.

[10][11] The loss resulted in 130 deaths (including 4 Royal Navy personnel), under the command of Frigate Captain Georges Louis Nicolas Blaison.

James Rusbridger examined some of these theories in his book Who Sank Surcouf?, finding them all easily dismissible except one: the records of the 6th Heavy Bomber Group operating out of Panama show them sinking a large submarine the morning of 19 February.

[20] The Surcouf is the subject of an underwater search by the fictional organization NUMA and international terrorists in the Clive Cussler novel "The Corsican Shadow", published in 2023.

The sinking is even attributed to Surcouf's radio antenna being damaged in the collision with the Thompson Lykes, and then finished off by the reported attack of an A-17 bomber the next morning.