San Ildefonso was a ship of the Spanish Navy, built at Cartagena, Spain to a design by José Romero Fernández de Landa[1] and launched in 1785.

[1] Having fought the Royal Navy in various wars the Spanish admirals were concerned that their ships could not match equivalent British vessels for speed.

[3] However, unlike most other Spanish ships of the line (including all those present at Trafalgar), San Ildefonso did not carry any four-pounder anti-personnel obuses on her poop.

[3] San Ildefonso was designed by José Romero Fernández de Landa and built by him at the naval dockyard in Cartagena.

[1] San Ildefonso and her commander, Brigadier (Commodore) José de Vargas, were captured by the British third-rate HMS Defence.

[2] The Spanish vessel had already been damaged in the action and after a fierce fight lasting less than an hour surrendered to the British[2] Defence, having suffered casualties amounting to 34 dead and 126 wounded.

[6] The 145 m2 (1,560 sq ft) naval ensign that San Ildefonso flew at the Battle of Trafalgar was hung in St Paul's Cathedral at Admiral Nelson's funeral on 9 January 1806.