San Ildefonso was a ship of the Spanish Navy launched in 1785. She was designed to be lighter than traditional Spanish vessels which had had difficulty matching the speed of ships of the Royal Navy. Though nominally a 74-gun ship the San Ildefonso actually carried 80 cannons and howitzers. She saw service against French and British vessels in the late 18th century, sailed twice to the Americas and was trapped in Cadiz by the British blockade. San Ildefonso was captured by the British third-rate HMS Defence at the Battle of Trafalgar and successfully weathered the storm afterwards to be taken into Royal Navy service as HMS Ildefonso.