How should I write a letter requesting that a very historic ship not be scrapped?

Address your letter to the relevant recipient, such as the head of the person in charge of demolishing the ship. Introduce yourself, offering just enough information to make your letter personal and ingratiate yourself to the recipient, but stay on topic: let them know you're concerned about the possible scrapping of a historic ship, and that you would appreciate if they would reconsider demolishing it. In your body paragraph, explain why you believe the ship should be preserved, addressing its significance not only to yourself, but its historical significance as well. Lean on both emotional persuasion (but avoiding becoming too passionate, as this may deter your reader) and on facts pertaining to the ship's history. This combination of personal and emotional persuasion as well as historical research is sure to affect your reader. Finally, close your letter by thanking the reader for their time, and respectfully signing off.