Tonnant was a coastal defense ship built for the French Navy (Marine Nationale) that served during the last two decades of the nineteenth century.

However, in 1889, Tonnant formed part of the force that successfully defended Cherbourg in a naval exercise against the more powerful ironclads of the Channel Squadron (Escadre du Canal).

From the 1870s, the French Navy (Marine Nationale) developed a smaller ironclad to complement seagoing battleships but more suited to coastal defence, having a lower speed and less endurance.

[1] Following the French defeat in the Franco-Prussian War and the emergence of the Jeune École doctrine, these matured as the Tempête class of ironclad coastal defense ships.

[2] Originally ordered in January 1875 as the third member of the class, Tonnant was designed by the naval architects Louis de Bussy and Émile Marchegay.

This was submitted to the Board of Construction (Conseil de travaux) on 31 August 1878 and approved by the Minister of the Navy on 24 September.

[3] Tonnant was built as a barbette ship with a low-freeboard hull that was turtle-backed forward and had a built-up superstructure.

[4][5] The design had features of the Bélier class, including the low stern of a monitor turret ship.

While undertaking sea trials, Tonnant reached a speed of 11.561 knots (21.4 km/h; 13.3 mph) from 1,994.04 ihp (1,486.96 kW).

[8] Tonnant carried a main battery of two 340 mm 18 caliber Modèle 1875 guns in two single-gun barbette turrets, one forward of the superstructure and the other aft.

Initially commissioned for trials on 1 January 1885, Tonnant sailed to Cherbourg between 15 and 17 March, escorted by the aviso Travailleur.

[13] On 22 and 23 July, Tonnant accompanied her fellow coastal defense ship Tempête, along with more torpedo boats, sailed from Brest to defend Cherbourg once more.