Why does English use definite articles before certain proper nouns, such as the names of ships?

Over on English Language Learners, a non-native speaker asked a question about adding "the" before movie titles.

I wanted to tell him or her that the rule in English is not to add a definite article before a proper noun, but to keep the article if it's part of the title, so you would have, for instance:

I went to see The Big Sleep.

but

I went to see Sleepless in Seattle.

But it occurred to me that there is an exception to this rule: boats, trains, and other means of transportation. My question is, why is it idiomatic to say:

Tomorrow morning we set sail on the Titanic.

or

The murder took place on the Orient Express.

when we would never use an article in that context with the name of a person, a city, or a country? Are there other categories of proper nouns that take definite articles, and if so, what if anything is the rule?

In the present-day style of the US Navy (and as far as I can tell also the Royal Navy, though I cannot find a suitable link), the definite article is inserted only when giving the type of vessel— never directly before the vessel's name itself. Thus, the publicity piece entitled “Nimitz Arrives Home” opens with

More than 3,000 Sailors on board the aircraft carrier USS Nimitz …

but subsequent references are bare:

While at sea, Nimitz completed approximately 374 launches and recoveries …

"I am very proud…" said Capt. Jeff Ruth, commanding officer of Nimitz.

In vernacular English the article is both prevalent and rather longstanding in use. Early in Shakespeare's Macbeth, the first witch tells a tale:

Her husband's to Aleppo gone, master o' the Tiger:

Or consider the innumerable examples in Purchas His Pilgrimes, a 1625 book "contayning a history of the world, in sea voyages & lande-travells by Englishmen & others," at least six of which lie in this excerpt:

Why this became conventional in English is difficult to say. Perhaps it became natural because the London would clearly refer to something named after bare London— meaning the city— General Slocum was a disaster, but nothing like the General Slocum. Many large or important physical objects and features idiomatically take the article, and a ship big or important enough to take a name might be expected to do so. But articles are highly idiosyncratic; we weekend on the Isle of Wight but on Isle Royale, we sail on the Great Bear Lake though on Lake Ontario, and even climb up the Matterhorn yet up Mont Cervin— the very same mountain, just known by several names.

Proper nouns are particularly twitchy, for not only are names themselves rarely logical (e.g. the people who call themselves Nederlanders we call the Dutch; the people who call themselves Deutsche we call Germans), but the entities they represent may have a preferred “house” style that differs from the styles preferred by other substantially similar entities. Elsewhere I provided the examples of

She is a professor at The Ohio State University. She received her Ph.D. from The George Washington University, and was prepared at The Lawrenceville School.

She is a professor at Kansas State University. She received her Ph.D. from George Mason University, and was prepared at Darrow School.

She is a professor at the University of Arizona. She received her Ph.D. from the College of Charleston, and was prepared at the Milton Hershey School.

Related questions include the following:

1. When to use a definite article in the name of a ship
2. Using the definite article before a country/state name
3. Why use “the” for oceans/seas/rivers etc. but not lakes?
4. Use of definite article before phrases like Heathrow Airport, Hyde Park, Waterloo Station, Edgware Road and Parliament Square
5. Why 'The' is used?
6. Should “the” ever be dropped from the beginning of a name/title?
7. Definite article with proper nouns, titles followed by a common noun
8. Document names and proper nouns/definite articles
9. Definite article before schools, colleges, and universities

I found this material that can probably help with your question:

Definite article:

The first mention of a ship should include type or prefix. For subsequent use the definite article before a ship’s name despite declining usage is always acceptable, except before a pronoun

For example:

leaving Gibraltar, the Victory led the fleet
leaving Gibraltar, Victory led the fleet

But not:

leaving Gibraltar, the HMS Victory led the fleet

Also, some ship names particularly in languages other than English contain the definite article. For example, consider L’Orient, La Splendide; these should never be preceded by the English definite article.