HMS Thunderer was a two-deck 84-gun second rate ship of the line, a modified version of the Canopus/Formidable-class launched on 22 September 1831 at Woolwich Dockyard. She was constructed with diagonal framing and improved underwater lines on the principles of Sir William Symonds, Surveyor of the Navy. In 1840, HMS Thunderer fought in the Syria campaign, taking part in the battle of Sidon, which was the last fleet action conducted purely by wooden ships of the line under sail. In the same year she acted as flagship at the bombardment and capture of the fortress at St. Jean d'Acre, which was the first action at which steam vessels were present, albeit as support vessels rather than fighting ships. She was fitted with iron-clad plate in 1863 for trials of new armour-piercing guns.[citation