The construction of Fort Boyard

The construction of Fort Boyard

  • Aerial view of Fort Boyard, at low tide

  • Plan of the harbor of Île d'Aix by engineer Vanéchout [Organize the Force battery]

  • Fort Boyard at sea. Sections and elevations

  • Draft reorganization of Fort Boyard.

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Title: Aerial view of Fort Boyard, at low tide

Author :

Creation date : 1992

Date shown: 1992

Dimensions: Height 9 - Width 13

Technique and other indications: Photographic print

Storage location: Departmental Archives of Charente-Maritime website

Contact copyright: © Departmental archives of Charente-Maritime - Photo C. AYRAULT

Picture reference: 33 Fi

Aerial view of Fort Boyard, at low tide

© Departmental archives of Charente-Maritime - Photo C. AYRAULT

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Title: Plan of the harbor of Île d'Aix by engineer Vanéchout [Organize the Force battery]

Author :

Creation date : 1847

Date shown: 1847

Dimensions: Height 27 - Width 82.5

Technique and other indications: Full title: Plan of the harbor of Île d'Aix, showing the location of Fort Boyard with the crossing of the firing lines, established by Colonel Vanéchout Ink and color wash

Contact copyright: © Historical Service of the Army

Picture reference: Art.8. Sect. 1. Ile d'Aix. Cart. 9. F.5

Plan of the harbor of Île d'Aix by engineer Vanéchout [Organize the Force battery]

© Historical Service of the Army

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Title: Fort Boyard at sea. Sections and elevations

Author :

Creation date : 1878

Date shown: 1878

Dimensions: Height 57 - Width 85

Technique and other indications: Watercolor plan

Storage location: Departmental Archives of Charente-Maritime website

Contact copyright: © Departmental archives of Charente-Maritime

Picture reference: 12 J sup. 12 (C.R. 44)

Fort Boyard at sea. Sections and elevations

© Departmental archives of Charente-Maritime

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Title: Draft reorganization of Fort Boyard.

Author :

Creation date : 1891

Date shown: 1891

Dimensions: Height 32 - Width 43

Technique and other indications: Watercolor map with legend

Storage location: Departmental Archives of Charente-Maritime website

Contact copyright: © Departmental archives of Charente-Maritime

Picture reference: 12 J 23, n ° 636

Draft reorganization of Fort Boyard.

© Departmental archives of Charente-Maritime

Publication date: March 2016

Historical context

Building at sea in the 19th century

From its creation in 1665, the military arsenal of Rochefort provoked the incursions of the enemy maritime powers of France: Holland and England. However, despite its strategic position as a lock at the entrance to the Charente and the port of Rochefort, the island of Aix does not have a coherent defensive system.

In 1801, on the initiative of Bonaparte, First Consul, a commission studied the construction of two structures, one on the islet of Enet, the other on the Boyard bench: "The fires of this fort and those of the island of Aix corresponding to good range, the harbor will become inaccessible to the enemy. »To build Fort Boyard, the most difficult thing is to create a solid base in the open sea, 5 meters underwater, 2,400 meters from the island of Oléron and 2,800 meters from the island of Aix. In 1809, the English, determined to prevent any fortification of the site, destroyed the Rochefort squadron off the island of Aix using firebells (unmanned ships loaded with explosive or incendiary materials). Without fleet protection, work must be stopped.

Taken over under Louis-Philippe, but with new technical means, they lead in 1848 to the construction of a base which exceeds 2 meters above the level of the high sea. Then begins the construction of the fort itself by military engineers, operation completed in 1857 and completed by a breakwater and a landing stage in 1866. Depending on the period, construction required the presence of 300 to 500 workers for the installation of 88,860 m³ of riprap and 160,000 m³ of materials. The "Experience registers" kept by the genius of the Place d'Oléron record the experiments carried out on the materials used [1] and make it possible to measure the immense difficulties of the company.

Image Analysis

Like a warship

Fort Boyard, now a showcase of the historical heritage of Charente-Maritime, and world famous thanks to the game show that bears its name, stands out in the open sea with its elliptical architecture intended for the defense of Rochefort. The blocks shown in the aerial view, scattered around the fort, exposed by the low tide, come from the landing stages and breakwaters destroyed by the storms.

The wall is regularly pierced with firing embrasures for the 74 guns it was to include. The architecture incorporates the defensive architectural theories of the Marquis de Montalembert (1714-1800), based on the use of artillery towers allowing 360 ° fire. The fort, originally designed with a single level of casemates, was equipped during its construction, in the middle of the 19th century, with three levels of fire. Fortuitously, this architecture thus presents an analogy with the three bridges of warships from the end of the 18th century (often armed with 74 cannons). This stone vessel can fire from all sides has impressive dimensions: 68 meters long, 21 meters wide, 20 meters high at the ramparts, 27 meters high at the watchtower, 2.30 meters thick at the base of the walls. Its rounded heads face one south towards the entrance to the Charente, the other north facing the waves [2] lifted by the prevailing north-westerly winds. The fort was also oriented according to the currents and the possibility of strafing enemy vessels with the greatest intensity at the intersection of its fires with those of the Ile d'Aix.

The strategic role of Fort Boyard is clearly evident in the map of the harbor of Ile d'Aix, drawn up by the engineers in 1847, just before construction began.

The 1878 plan shows the three levels of passageways, the summit firing platform and its parapet, as well as the various vaults, described on the plan, to the right. The masonry structures could perhaps still withstand naval bombardment at that time: the vaulted ceilings were at least four feet thick. The sections show the fresh water tanks (in blue), the masonry of the breakwater on the north side and the landing stages to the south (lookout tower side). The level of "high seas of extraordinary spring tides" corresponds to the level of the equinox tides, often amplified by winds coming from the open sea, that of "low tide nays" with the lowest low tide coefficients.

The longitudinal section, south side, shows the hoist and freight elevator (in blue), one above the landing stage, the other located on the upper platform and serving the corridors of the inner enclosure.

The 1891 plan, intended to serve as a working base for the reorganization of the fort, shows the "logistics" area of ​​the building, grouped together in the lower parts: ground floor and bunkers. Accommodation is provided for 260 men in 66 casemates, the tanks being able to hold 300,000 liters of fresh water. The various stores and tanks guarantee an autonomy of two months.


Exceeded upon completion!

When it was completed under the Second Empire, the fort aroused pride: it was one of the most difficult and costly achievements of the century, and its model was presented at the Universal Exhibition of 1867. But, from that time onwards , advances in long-range artillery make it unnecessary. From now on, the forts of the Ile d'Aix and Ile d'Oléron can cross their shots and alone prevent access to Rochefort. Fort Boyard was used for a few years in prison, first for a few Prussians and then, in 1871, for more than 300 political prisoners convicted after the Commune, including journalist Henri Rochefort.

In 1872, the navy installed a new system of defense by torpedoes, of which Fort Boyard housed the command post. But at the end of the century, she gave up on redeveloping it in view of the progress of the artillery. The fort was finally decommissioned in 1913 and became the property of the Charente-Maritime department in 1998.

  • architecture
  • Bonaparte (Napoleon)
  • sea
  • perpendicular fortification
  • materials
  • Montalembert (Marc René, marquis of)
  • Commune
  • jail
  • Charente Maritime
  • Rochefort
  • engineer


Gérard CHAGNEAU "Stone vessel, creator monster, Fort Boyard", in The Notebooks of OléronSaint-Pierre d´Oléron, n ° 6, Remy DESQUENNES, René FAILLE, Nicolas FAUCHERRE and Philippe PROSTCharente-MaritimeChauray, Patrimoine et medias, coll. “Les fortifications du littoral”, 1993.


1. The use of sea water in the preparation of mortars “For the construction work of Fort Boyard, it was impossible to ensure a sufficient supply of fresh water for the extinction of the lime; and when the organization of the workshop no longer allowed mortar to be made on land, which also presented other drawbacks, we were forced to resort to the use of sea water […] Our mortars de Boyard treated with salt water have always seemed very satisfactory […] The consistency of the mortars thus crushed was favorable to the good execution of our masonry; they hardened quickly while retaining a slight humidity which was eminently beneficial to them, especially to the wind and the sun which they usually had to face […] "

Degradation of the facings “The limestone facings of Fort Boyard quickly deteriorated around the cement joints, although stones deposited on the Boyardville site had been used for 50 years. The weathering occurs several millimeters deep in the early years. However, at Fort Chapus, we do not notice that the facings built for 160 years with stones of the same origin present proportionate effects. Should we not attribute this difference in degradation to the difference in the cements used? "

Natural protection: oysters “Oysters, however, exhibit remarkable adhesion to stones, and it is possible that, if the hand of man did not contribute more than physical accidents to stopping their development at the level of the lowest seas, in the long run, the banks could take on the proportions of real rocks, and would replace more effectively than any artificial means the protection which one recognizes essential to provide to rubble riprap against the dynamic action of the sea […] "

Genius of Place d'Oléron, Register of experiences.

2. The sea attacking the fort “The commission responsible for examining the Boyardville riprap in 1837 concluded, from the displacement of several blocks and the immobility of several others, that the height of the blade of the summit to trough can reach 8 m. at most […] On several occasions blocks of 15 m³ were completely lifted before our eyes by the sea […] In November 1852, the sea knocked down a landing path from the south head, cubic about 280 m., arched against this strong […] This mass was not only overturned, but also rolled away […] It is nevertheless certain that the waves which rise around Fort Boyard are of a very great power. We have seen them, several times each year, pass through the fort in thick sheets, in the length of the northwest to the southeast, and it has been found that the entire mass of the fort, about 40,000 m³ of masonry, oscillates under the blows of these blades in the most violent storms. "

To cite this article

Luce-Marie ALBIGÈS and Bernard DELORY, "The construction of Fort Boyard"

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