Losing the World’s Largest Submarine – The Unsolved Mystery of the Surcouf (1942)

Losing the World’s Largest Submarine – The Unsolved Mystery of the Surcouf

The Washington Times had a picture of an odd-looking French submarine on page A-3 of the January 20, 1942 edition. The caption read: Free French Operate World’s Largest Sub. I had previously written a story about a French submarine that escaped Germany’s clutches at the beginning of the war.


But I had never read the story about this particular submarine – the Surcouf.

The Surcouf was the largest submarine ever built right up until the Japanese produced the I-400-class subs. She was named after Robert Surcouf, who made a fortune by sailing the Indian Ocean as a privateer and merchant. Truthfully, Surcouf was more of a pirate than anything else so his name on a submarine was pretty appropriate for its time.

The submarine was intended to be part of the trio of boats and was given the pennant number N N 3 – although the remaining two were never built and Surcouf served her time as an experimental cruiser submarine.

An unusual boat

The Surcouf was built to act as an underwater cruiser and was meant to seek and engage surface ships. In order to carry out reconnaissance, she was equipped with a Besson MB.411 floatplane in a hangar that was specially built near the conning tower.

For combat purposes, she was equipped with 10 torpedo tubes. Six of them were 550mm (22 in) in diameter and four of them were 400mm (16 in). She also had two 203mm (8in) guns in a turret placed forward of the conning tower.

The 203mm was a medium naval gun that the French Navy employed and was characteristic of cruisers built because of the limitations of armaments imposed by the Washington Naval Treaty of 1922.

The history of the ship is also murky. In 1940 the ship was based in Cherbourg but was being refitted in Brest when the Germans invaded France.

Because the Allies didn’t want her taken, the crew was ordered to seek refuge across the channel in Plymouth. Surcouf ended up making the short journey across those dangerous seas with one engine inoperable.

On the 3rd of July came the first bloodshed the submarine would be involved in and it resulted in the needless and tragic death of three British men and one French sailor.

Because the Allies were concerned that French ships would be taken over by the German Navy, Operation Catapult was launched. This saw the Royal Navy blockade harbors that held French ships, who were given an ultimatum: rejoin the fight against the Axis powers, scuttle the ships or put themselves out of reach.

Back in Britain, the Surcouf was boarded to ensure that the ship would not fall into the wrong hands. The French took exception to his age-old foe forcing their way onto the jewel in the crown of the French Navy and a scuffle ensued.

After the dust had settled and the tempers calmed down, Cdr Denis Sprague, Lt Griffiths and LS Webb lay dead – while Yves Daniel was fatally wounded and would also pass away.

The British completed Surcouf’s refit by August 1940 and handed her over to the Free French Navy, who opposed the Vichy France regime. Tensions were still high at this point and both sides accused the other of working for the Germans via the puppet regime in France. One extraordinary British claim was that the sub was attacking British ships.

After being handed over the French resistance forces, Surcouf ended up in Quebec City in December 1941. It has got there via Halifax, Nova Scotia and a three-month refit at Portsmouth, New Hampshire, after being damaged by a German plane while escorting trans-Atlantic convoys.

In January 1942 the Free French sent the submarine to the Pacific and re-supplied in Bermuda. This change of direction sparked rumors that Martinique was going to be taken from the Vichy for Free France.

Then, all of a sudden, Surcouf was no more. After leaving on the 12th of February for the Panama Canal, communications suddenly ceased.

One theory is that she was sunk on the 18th of February 1942 after being ridden over by the American ship Thompson Lykes, which reported hitting and running down a partially submerged object.

Underwater submarine cruiser Surcouf (From the French Naval Web Site)

The submarine cruiser Surcouf was the result of squadron submarine projects that have been studied at length since the end of the First World War. The plans are drawn up by the Technical Service for Naval Construction and Weapons (STCAN) and signed by the Head of the Submarine Office Léon Roquebert.

The Q 5 project was adopted on 17 July 1926 by the Superior Council of the Navy, in the presence of Georges Leygues, Minister of the Navy. He ordered its construction in Cherbourg on 31 December 1926.

(The Washington Naval Treaty of 1922 had already been held and agreements signed by the time the project was started.)

Built in Cherbourg, in hold IV, the Surcouf was launched on 18 October 1929. Almost immediately its existence was threatened by the London Conference on Naval Disarmament in January 1930. But the firmness of the Minister of the Navy manages to save him. It entered service in May 1934.

It was the largest submarine of its time. It was equipped with an impressive artillery turret of 185 tons, waterproof, where are installed two guns of 203 mm. Three minutes after the “hunt everywhere” order, the pieces can fire shells from 120 kilos to 27,500 meters. In addition, a two-seater seaplane is housed in a sealed cylinder, on the hull, and allows to widen the field of exploration and ensure the direction of fire of the artillery.

At the declaration of war she escorted convoys from Halifax and the West Indies. In the refit at Brest during the German invasion, he joined Plymouth on his only electric motor. Rearmed by the Free French Naval Forces (FNFL), she then resumed escorts on the Atlantic.

After 3 months of refit at Killery in the United States, he participated in December 1941 in the rallying of Saint Pierre and Miquelon to Free France. When Japan entered the war, she was ordered to reach Sydney via Tahiti.


Size: Displacement:
Length x width x max draft: 110 x 9 x 7.18 meters; Displacement: 3,303 t (surface), 4,318 t (diving)
Energy / Propulsion
Propulsion: 2 Sulzer diesel engines (3,800 hp); 2 electric motors (1700 hp); 2 propellers
Power: 7,600 hp (surface), 3,400 hp (diving)  
Speed Radius of action
Speed: 18 nds (surface), 10 nds (diving) 12,000 miles at 10 knots (90 days of range)
2 turrets of 203 mm 4 x 550 mm torpedo tubes
2 x 37 mm AA guns 4 XA 13 mm machine guns
Crew Aviation
7 officers + 109 men 1 Observation seaplane Besson MB 411
Regarding the boarding of a seaplane, about thirty minutes was necessary to prepare the aircraft, it was first necessary to open the hydraulically locked door. Then, mounted on a handling cart equipped with a crane, the seaplane was taken out on deck. We then mounted the wings, then the propeller. It was still necessary to bolt the float and fill up with gas. Ultimate constraint: only a sea below force 2 allowed take-off…


The turrets of the Surcouf

The artillery turret contains two 203 mm guns. This turret weighing 200 tons is more spacious than those of heavy cruisers of 10,000 tons. It is waterproof during diving. Its capacity is 3 salvos per minute and 150 shots per piece. Behind it, a second small turret of 10 tons contains the telepointer. During the dive, the two turrets rest on rubber seals that ensure sealing. With the submarine back on the surface, hydraulic cylinders lift the turrets from a height of 20 mm, allowing them to rotate. The rate of fire is 3 to 4 shots per minute.

The lack of relative height on the water limits the effectiveness of the shot to 12,000 meters which can be increased to 16,000 meters using the periscope as a means of aiming. Beyond that, the use of the seaplane is essential (up to 26,000 meters). Originally a hydraulic telescopic watch mast 15 meters above the surface allows a watchman to observe the effectiveness of the shot. The roll movements of the ship led to the removal of this installation in 1934.

The story of the Surcouf submarine ends tragically, on the night of February 18, 1942, 75 miles from the Panama Canal in the Gulf of Mexico. The American freighter Thomson Lykes approaches and sinks it (officially accepted version). There are no survivors among the 130 crew members. Its commander was Commander Louis Blaison. The hull has been lying since then, 3000 meters deep northeast of Colon, at 10°40’N/79 32’W.

Surcouf vanished on the night of 18/19 February 1942, about 130 km (70 nmi) north of Cristóbal, Panama, while en route for Tahiti, via the Panama Canal.

An American report concluded the disappearance was due to an accidental collision with the American freighter Thompson Lykes, 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. Her lookouts heard people in the water but the freighter did not stop, thinking she had hit a U-boat, though cries for help were heard in English. A signal was sent to Panama describing the incident.

The loss resulted in 130 deaths (including 4 Royal Navy personnel), under the command of Frigate Captain Georges Louis Nicolas Blaison. The loss of Surcouf was announced by the Free French Headquarters in London on 18 April 1942, and was reported in The New York Times the next day. It was not reported Surcouf was sunk as the result of a collision with the Thompson Lykes until January 1945.

The investigation of the French commission concluded the disappearance was the consequence of misunderstanding. A Consolidated PBY, patrolling the same waters on the night of 18/19 February, could have attacked Surcouf believing her to be German or Japanese. This theory could have been backed by several elements:

  • The witness testimonies of cargo ship SS Thomson Lykes, which accidentally collided with a submarine, described a submarine smaller than Surcouf
  • The damage to the Thomson Lykes was too light for a collision with Surcouf
  • The position of Surcouf did not correspond to any position of German submarines at that moment
  • The Germans did not register any submarine loss in that sector during the war.
  • Inquiries into the incident were haphazard and late, while a later French inquiry supported the idea that the sinking had been due to “friendly fire”; this conclusion was supported by Rear Admiral Auphan in his book The French Navy in World War II. Charles de Gaulle stated in his memoirs that Surcouf “had sunk with all hands”.

As no one has officially dived or verified the wreck of Surcouf, its location is unknown. If one assumes the Thompson Lykes incident was indeed the event of Surcouf’s sinking, then the wreck would lie 3,000 m (9,800 ft) deep at 10°40′N 79°32′WCoordinates: 10°40′N 79°32′W.

A monument commemorates the loss in the port of Cherbourg in Normandy, France, and the loss is also commemorated by the Free French Memorial on Lyle Hill in Greenock, Scotland.

The evidence we know

As there is no conclusive confirmation that Thompson Lykes collided with Surcouf, and her wreck has yet to be discovered, there are alternative stories of her fate. James Rusbridger examined some of these theories in his book Who Sank Surcouf?, finding them all easily dismissed 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. Since no German submarine was lost in the area on that date, it could have been Surcouf. He suggested the collision had damaged Surcouf’s radio and the stricken boat limped towards Panama hoping for the best.

A conspiracy theory, based on no significant evidence, held that the Surcouf, during its stationing at New London in late 1941, had been caught treacherously supplying a German U–boat in Long Island Sound, pursued by the American training subs Marlin and Mackerel out of New London, and sunk. The rumor circulated into the early 21st century but is false since the Surcouf’s later movements south are well documented.

Until the hull is found and verified, the fate of the submarine on its final voyage will remain shrouded in mystery.

Mister Mac

Postscript: February 14, 1958, word of another new submarine class was announced.

These submarines would lead to larger submarines that would have a global reach in more ways than one.

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