Alma - class ironclad The " Alma " - class ironclads were a group of seven wooden - hulled , armored corvette s built for the French Navy in the mid to late 1860s . Three of the ships attempted to blockade Prussia n ports in the Baltic Sea in 1870 during the Franco - Prussian War . Three others patrolled the North Sea and the Atlantic , while the last ship was en route to Japan when the war began and blockaded two small Prussian ships in a Japanese harbor . Afterwards they alternated periods of reserve and active commissions , many of them abroad . Three of the ships participated in the French occupation of Tunisia in 1881 while another helped to intimidate the Vietnamese Government into accepting status as a French protectorate and played a small role in the Sino - French War of 1884 – 85 . Design and description . The " Alma " - class ironclad s were designed by Henri Dupuy de Lôme as improved versions of the armored corvette suitable for foreign deployments . Unlike their predecessor the ships were true central battery ironclads as they were fitted with armored transverse bulkheads . The original plan for these ships was to have a two - deck battery with four guns on the battery deck and four guns mounted above them on the upper deck , one gun at each corner of the battery . This design was changed to substitute four barbette s for the upper battery , but the addition of armored bulkheads proved to be very heavy and the rear pair of barbettes had to be deleted to save weight . In partial compensation the 164 - millimeter guns in the barbettes were replaced by an additional pair of 194 - millimeter guns . Like most ironclads of their era they were equipped with a metal - reinforced ram . The ships were built from the same general plan , but differed amongst themselves . They measured between perpendiculars , with a beam of . The ships had a mean draft of and displaced . Their crew numbered 316 officers and men . Propulsion . The " Alma " - class ships had a single horizontal return connecting - rod steam engine driving a single propeller . Their engine was powered by four oval boilers . On sea trials the engine produced between and the ships reached . Unlike the single funnels of the others , and had two funnels , mounted side - by - side . The ships carried of coal which allowed the ship to steam for at a speed of . They were barque - rigged with three masts and had a sail area between . Armament . The ships mounted four of their 194 - millimeter Modèle 1864 breech - loading guns in the central battery on the battery deck . The other two 194 - millimeter guns were mounted in barbettes on the upper deck , sponsoned out over the sides of the ship . The four guns were also mounted on the upper deck . is the only ship positively known to have exchanged her 194 mm guns for newer Modèle 1870 guns . The armor - piercing shell of the 20 - caliber Mle 1870 gun weighed while the gun itself weighed . The gun fired its shell at a muzzle velocity of and was credited with the ability to penetrate a nominal of wrought iron armour at the muzzle . The guns could fire both solid shot and explosive shells . Armor . The " Alma " - class ships had a complete wrought iron waterline belt , approximately high . The sides of the battery itself were armored with of wrought iron and the ends of the battery were closed by bulkheads of the same thickness . The barbette armor was thick , backed by of wood . The unarmored portions of their sides were protected by iron plates . Service . During the Franco - Prussian War of 1870 – 71 " Thétis " , " Jeanne d ' Arc " and " Armide " were assigned to the Northern Squadron that attempted to blockade Prussian ports on the Baltic until ordered to return to Cherbourg on 16 September 1870 . " Montcalm " , " Atalante " , and " Reine Blanche " cruised the North Sea and " Montcalm " later watched a Prussian corvette in Portuguese waters . " Alma " was en route to the Far East when the war began and she blockaded a pair of Prussian corvettes in Yokohama harbor once she arrived at Japan . After the end of the war many of the ships were placed in reserve or sent to foreign stations , often as the flagship . During the Third Carlist War of 1872 – 76 " Thétis " , " Reine Blanche " and " Jeanne d ' Arc " spent time in Spanish waters where they could protect French citizens and interests . In 1875 the latter ship rammed and sank the dispatch vessel " Forfait " . On 3 July 1877 " Thétis " rammed " Reine Blanche " who had to be run ashore to prevent her from sinking . Further abroad " Reine Blanche " and " Alma " bombarded the Tunisian port of Sfax in July 1881 as part of the French occupation of Tunisia . " Atalante " participated in the Battle of Thuan An in August 1883 . This was an attack by the French on the forts defending the mouth of the Perfume River , leading to the Vietnamese capital of Huế in an attempt to intimidate the Vietnamese government . During the Sino - French War of 1884 – 85 the ship was in Huế in early September 1884 , but she carried Admiral Amédée Courbet to Keelung , Taiwan on 23 September .