HMS Impregnable was a 104-gun first rate three-decker ship of the line of the Royal Navy, launched on 1 August 1810 at Chatham.

In the attack on Algiers, Impregnable, isolated from the other ships was a large and tempting target, attracting attention from the Algerian gunners who raked her fore and aft, she was severely damaged.

The Impregnable saw little further action, apart from a short commission in the Mediterranean, and in 1819 she was placed in the Reserve Fleet at Devonport.

From May 1839 to October 1841 she had relieved HMS Royal Adelaide[citation needed] as the Commander-in-Chief's flagship moored at the entrance to the Hamoaze.

Three years later on 22 September 1891, she was once again re-named, this time HMS Caledonia, and became a Scottish boys training / school ship moored at Queensferry in the Firth of Forth.

The heavy oak beams of the cloister of St Conan’s Kirk were made from Caledonia and HMS Duke of Wellington.