SHIP STRUCTURES UNDER SAIL AND UNDER GUNFIRE The ships of the three nations that fought at Trafalgar were serving in their navies for years before the battle. Their ages ranged from few months to over forty years. Their hulls and masts suffered from high seas and from ferocious combats as most of those ships sailed across the Atlantic and the Mediterranean. Many of these ships were both old comrades and well-fought adversaries that joined in ports and met at sea in other encounters before Trafalgar. They were engineering masterpieces that sailed swiftly before and against the wind, with powerful wooden walls meant to give protection against the heaviest cannon balls. Their designers and builders include the top creators of wooden ships of 18th century: Slade and Hunt designed 16 of the British ships; Sané and Rolland built 11 of the best French ships; and Gautier, Romero Landa and Retamosa constructed the best 13 Spanish ships. The structures of those hulls are here studied as living creatures that suffered scratches, illnesses and even gaping wounds, to find treatment and healing at the arsenals. The actual structures of significant ships of the three nations are analyzed and compared along their life cycles, with respect to their response to sea loads; and representative hull details are studied with analytical and experimental tools to show the response of the wooden walls to waves and gunfire.