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The Océan-class ironclads were a class of three wooden-hulled armored frigates built for the French Navy in the mid to late 1860s. FRENCH IRONCLAD Océan attempted to blockade Prussian ports in the Baltic Sea in 1870 during the Franco-Prussian War and FRENCH IRONCLAD Marengo participated in the French conquest of Tunisia in 1881. FRENCH IRONCLAD Suffren was often used as the flagship for the Cherbourg Division, the Channel Division, Mediterranean Squadron and the Northern Squadron during her career. The ships were discarded during the 1890s.

An innovation in the Océan-class was that part of the main armament was mounted in barbettes. French officers who commanded these ships were very pleased with the barbette guns, and spoke "particularly of the very large offensive power that it gave them; that it not only gave them a large arc of fire, but... they dwelt upon the possibility of seeing the enemy at the the points of your sights the whole way round, even when you had an obstruction in the shape of rigging which prevented firing, instead of having been at one moment hidden from you by the superstructure forward, and then next moment suddenly appearing in sight, through a narrow port, where your eye is almost blinded with light, and where you are not in a position to recognise the presence of the object till it has been for some seconds within your range of view."

Design and description
The Océan-class ironclads were designed by Henri Dupuy de Lôme as an improved version of the Provence-class ironclads. The ships were central battery ironclads with the armament concentrated amidships.

For the first time in a French ironclad three watertight iron bulkheads were fitted in the hull, but they probably of little value in a wooden-hulled ship. "[W]ooden hulls lacked the structural rigidity of iron and could not, therefore, have effective internal compartmentalization."

Like most ironclads of their era they were equipped with a metal-reinforced ram.

The ships measured 87.73 m overall, with a beam of 17.52 m. They had a maximum draft of 9.09 m and displaced 7749 t. Their crew numbered between 750 and 778 officers and men. The metacentric height of the ships was very low, between 1.7 - 2.2 ft. The ships were over-weight as completed; their draught so exceeded that designed for them that an increase of stability by ballast was impossible. The Océan-class were reported to be able to carry all sail safely, were good sea-boats, steady and well-behaved, but lacking in stiffness (resistance to heeling).

Weights
According to Nathaniel Barnaby (Chief Constructor of the Royal Navy) in 1872, the Océan-class ships had 1,280 LT of armour.

Naval architects expect the weight of machinery to be proportional to the power for similar compound-engines. It is difficult to understand why ratio of power to machinery weight was so much lower for the Marengo than for contemporary British ironclads. Perhaps the French figures included weights that were not included in the British figures.

Propulsion
The Océan-class ships had a horizontal-return, connecting-rod, compound steam engine, driving a single propeller using steam provided by eight oval boilers. Indret made the engines for Océan and Marengo; Schneider made the engines for Suffren. On sea trials the engines produced between 3600 - 4100 ihp and the ships reached 13.5 - 14.3 kn. They carried 650 MT of coal which allowed them to steam for approximately 3000 nmi at a speed of 10 kn.

The Océan-class ships were barque or barquentine-rigged with three masts and had a sail area around 2000 sqm. The rigging of the foremast and mizzenmast were brought down to plates, tied under the main deck beams. Later in life, the rig was reduced and fighting tops were added to the main and mizzen masts.

Armament
The initial design was to have a main armament of four 19 cm guns and four 16 cm guns on the main deck in an armoured central battery, and four 24 cm guns in open-topped armoured barbettes on the spar deck. In 1869, the armament was changed to four 24 cm guns in barbettes and either six or eight 24 cm guns in the battery, which was then changed to four 24 cm guns in barbettes and four 27.4 cm cm guns in the battery,   The ship's sides were not recessed, so the main deck guns could not fire fore or aft. But the barbettes were slightly sponsoned out over the sides of the hull. If the barbette guns were fired at angles smaller than 45 degrees from the keel-line, the crew had to be withdrawn from the extremities of the ship; for practical purposes, the arc of fire of each barbette was about 100 degrees. The main deck battery was 3.9 m above the waterline and the spar deck and barbette guns were 8.3 m above the waterline. The barbettes had steam-powered turntables. The depression of the guns was not sufficient for plunging fire on the deck of an enemy ship at very close quarters. Elevation of the barbette guns was considered very good, and allowed the Océan at engage targets at 5,400 yd, with Model 1864-67 guns whose muzzle velocity was only 1,115 ft/s.

They also had a secondary armament mounted on the broadside on the spar deck of six 14 cm or 12 cm guns, the rear pair could be moved to the stern to fire aft.

At some point the ships received a dozen 37 mm Hotchkiss 5-barrel revolving guns. They fired a shell weighing about 500 g with a muzzle velocity of about 610 m/s to a range of about 3200 m. They had a rate of fire of about 30 rounds per minute. Late in the ships' careers four above-water 356 mm torpedo tubes were added.



French Model 1864 and 1870 naval guns were breechloaders made of cast iron, lined with steel as far as the trunnions, reinforced externally from the breech to the trunnions by rings of puddled steel shrunk onto the body. The breech mechanism was an interrupted screw. The construction of Model 1870 differed from Model 1864, and allowed greater pressures, and thus higher muzzle velocity.

The Model 1864 came into service in the French Navy in 1867.

Manufacture of Model 1870 guns began after the Franco-Prussian War ended in 1871. Model 1870 guns had polygroove rifling, which by the standards of 1870s naval guns was very advanced. They also had enlarged chamber for the propellant, which again was advanced for the 1870s. The endurance of Model 1870 guns was said to be very great. These guns were less expensive than contemporary British naval guns; for example the 24 cm Model 1870 cost about £580, whereas the nearest equivalent British naval gun cost about £740.


 * 27 cm Model 1870: weight 23 LT, overall length 212 in, bore length 194 in 18 calibres long, rifling 28 grooves:
 * Chilled iron or steel shell: projectile 475 lb, propellant 92.4 lb, muzzle velocity 1,417 ft/s, muzzle energy 20.03 MJ (6,596 foot tons), penetration of wrought iron armour 12.5 in
 * Common shell (cast iron): projectile 396 lb, bursting charge 24 lb, propellant 92.4 lb, muzzle velocity 1,542 ft/s, muzzle energy 19.76 MJ (6,506 foot tons)
 * Canister: projectile 317 lb
 * 24 cm Model 1870: weight 15.5 LT, overall length 195 in, bore length 179 in 19 calibres long, rifling 24 grooves:
 * Chilled iron or steel shell: projectile 317 lb, propellant 61.6 lb, muzzle velocity 1,443 ft/s, muzzle energy 13.95 MJ (4,561 foot tons), penetration of wrought iron armour 11.1 in
 * Common shell (cast iron): projectile 264 lb, bursting charge 17 lb, propellant 61.6 lb, muzzle velocity 1,555 ft/s, muzzle energy 13.41 MJ (4,414 foot tons)
 * Canister: projectile 220 lb
 * 27 cm Model 1864: weight 20 LT, overall length 184 in, bore length 167 in 15 calibres long, rifling 5 grooves:
 * Chilled iron shell: projectile 475 lb, propellant 79.2 lb, muzzle velocity 1,086 ft/s, muzzle energy 11.76 MJ (3,871 foot tons), penetration of wrought iron armour 12.5 in
 * Common shell (cast iron): projectile 317 lb, bursting charge 14.6 lb, propellant 52.8 lb, muzzle velocity 1,188 ft/s, muzzle energy 9.38 MJ (3,088 foot tons)
 * Canister: projectile 321 lb
 * 24 cm Model 1864: weight 14 LT, overall length 180 in, bore length 165 in 17 calibres long, rifling 5 grooves:
 * Chilled iron shell: projectile 317 lb, propellant 52.8 lb, muzzle velocity 1,115 ft/s, muzzle energy 8.57 MJ (2,821 foot tons), penetration of wrought iron armour 11.1 in
 * Common shell (cast iron): projectile 220 lb, bursting charge 10.3 lb, propellant 35.2 lb, muzzle velocity 1,188 ft/s, muzzle energy 6.51 MJ (2,144 foot tons)
 * Canister: projectile 211 lb or 220 lb
 * 19 cm Model 1864: weight 7.75 LT, overall length 150 in, bore length 138 in 18 calibres long, rifling 5 grooves:
 * Chilled iron shell: projectile 165 lb, propellant 27.5 lb, muzzle velocity 1,128 ft/s, muzzle energy 4.41 MJ (1,451 foot tons), penetration of wrought iron armour 7.0 in
 * Common shell (cast iron): projectile 115 lb, bursting charge 4.8 lb, propellant 17.6 lb, muzzle velocity 1,168 ft/s, muzzle energy 3.28 MJ (1,081 foot tons)
 * Canister: projectile 105 lb
 * 16 cm Model 1864: weight 4.75 LT, overall length 133 in, bore length 124 in 19 calibres long, rifling 3 grooves:
 * Chilled iron shell: projectile 99 lb, propellant 16.5 lb, muzzle velocity 1,132 ft/s, muzzle energy 2.66 MJ (876 foot tons), penetration of wrought iron armour 5.87 in
 * Common shell (cast iron): projectile 69 lb, bursting charge 3 lb, propellant 11 lb, muzzle velocity 1,197 ft/s, muzzle energy 2.09 MJ (687 foot tons)
 * Canister: projectile 66 lb or 220 lb

The chilled iron shells were similar to British Palliser shells. The French started to manufacture cast steel armour-piercing shells in the mid-1870s. These were followed by forged steel armour-piercing shells, and in 1886 Holtzer produced the first chromium-steel armour-piercing shells.

In the 1870s, the propellant used by the French Navy was Wetteren Powder, which was made up of 75% saltpetre, 10% sulphur, and 15% charcoal. During manufacture, the propellant was mixed, then made into a cake, which was cut into a standard grain size that varied with the calibre of gun:
 * 27 cm: 57 grains per kg (26 per lb)
 * 24 cm: 110 grains per kg (50 per lb)
 * 19 cm: 227 grains per kg (103 per lb)
 * 16 cm: 362 grains per kg (164 per lb)
 * 12 cm: 710 grains per kg (322 per lb)

Armor
The Ocean-class ships had a complete 178 - 203 mm waterline belt of wrought iron. The sides of the battery itself were armored with 160 mm of wrought iron. The barbette armor was 150 mm thick. The unarmored portions of their sides were protected by 15 mm iron plates. Gardiner and Gibbons say that the barbette armor was later removed to improve their stability, but this is not confirmed by any other source.

Initial cost
The American, Chief Engineer James Wilson King, gave the cost of each ship as $1,302,000 for the hull and machinery excluding armament, rigging and first outfit of stores, and $260,680 for machinery alone. Thomas Brassey gave the cost of the Marengo as £280,000, and the Suffren as £260,400.

Service
During the Franco-Prussian War of 1870–71 Océan was assigned to the Northern Squadron that attempted to blockade German ports. When part of the French fleet went through the Kattegat and the Great Belt to get into the Baltic in August 1870, the Océan's draft of 28 feet meant that at times she only had 1 1/2 feet of water under her keel. On 17 August, the French squadron tried to engage a group of German gunboats (the Grille, Drache, Blitz and Salamander) that were anchored 5,400 yd from the French squadron at the head of the Bay of Wittow (a bay in the island of Rügen); only the Océan's barbette guns had sufficient range to open fire. After six shots from the Océan, the German gunboats retreated through the Seegat and reached safety near Wittow Posthaus. Eventually Vice-Admiral Édouard Bouët-Willaumez decided that the Océan had too deep a draft for blocade duties in the Baltic and sent her back to the North Sea. On 16 September 1870, the Northern Squadron was ordered to return to Cherbourg. Afterward she was assigned to the Evolutionary Squadron until 1875 when she was placed in reserve. Océan was recommissioned in 1879 for service with the Mediterranean Squadron. She had a lengthy refit in 1884–85 and was assigned to the Northern Squadron after it was completed. Around 1888 the ship was transferred back to the Mediterranean Squadron until she was reduced to reserve around 1891. Océan was assigned to the Gunnery School that same year and later became a training ship for naval apprentices before being condemned in 1894.

Marengo was running her sea trials when the Franco-Prussian War began and was immediately put in reserve. She was recommissioned in 1872 for service with the Mediterranean Squadron until 1876 when she was again placed in reserve. On 2 October 1880 the ship was recommissioned and assigned to the Mediterranean Squadron. Marengo was transferred to the Levant Squadron (Division Navale du Levant) on 13 February 1881 and bombarded the Tunisian port of Sfax in July 1881 as part of the French conquest of Tunisia. She remained in the Mediterranean until she was assigned to the Reserve Squadron in 1886. In 1888 Marengo became the flagship of the Northern Squadron and led the squadron during its port visit to Kronstadt in 1891. She was reduced to reserve the following year and sold in 1896.

Suffren was placed into reserve after she completed her sea trials and was not commissioned until 1 March 1876 when she became flagship of the Cherbourg Division. Throughout her career the ship was often used as a flagship because of her spacious admiral's quarters. On 1 September 1880 the ship was assigned to the division that participated in the international naval demonstration at Ragusa later that month under the command of Vice Admiral Seymour of the Royal Navy in an attempt to force the Ottoman Empire to comply with the terms of the Treaty of Berlin and turn over the town of Ulcinj to Montenegro. Suffren was reduced to reserve in 1881 and not recommissioned until 23 August 1884 when she was assigned to the Northern Squadron. The ship was transferred to the Mediterranean Squadron about 1888 and remained there until paid off in 1895 and condemned in 1897.