Can anyone tell me the reason vessel names, boats or spaceships for example, always appear in italics?
Depending on the style guide the author or company uses, they can appear either italicized or not. Most newspapers use AP Style, but companies like the NY Times have their own style manual, which is respectively similar to the AP.
My guess, however, would be that names of vessels teeter between a name and a title. Titles of books are italicized. But then these aren't books. Their names, however, are rare and owners of ships and boats very much try to avoid using the name of some other boat. They don't fit the names of car makes and models like say a Subaru BRZ(dream car) because the name itself isn't a model.
They are not always italic--newspapers do not typically use italic type. This is a convention that dates back to the use of metallic type, when that font was "not easily accessible," according to the NYT, though the AP justifies its own lack of italics with the statement that the typeface "cannot be sent through AP computers." So, in the NYT, you will see this: "Of the eight battleships hit by torpedoes and bombs that Sunday in 1941, only the Oklahoma and the battleship Arizona were damaged beyond repair."
This breaks another common convention, however. Ship names are names, not titles, and the use of "the" before "Oklahoma" is therefore discouraged by many style guides, such as the Shipping Law Blog. More than just advice on how to style a ship's name, this blog offers the following reason for why ship names must be set in a distinctive way:
In the absence of any grammatical or punctuational emphasis, vessel names would be hard to distinguish in written prose, because they are often named after people ("Mary Rose"), animals ("The Red Fox"), places ("Arendal") or other things ("Time Bandit").
For this reason there are four generally accepted ways to identify a vessel in writing (n.b. as proper nouns ship's names should always begin with a capital letter): i) Place the name in double quote marks - "Leopard 1" (recommended; as used in the Law Reports) ii) Capitalise the name - LEOPARD 1 (also common in the industry; if used, quote marks are unnecessary) iii) Italicise the name - Leopard 1 (the norm outside the industry; used by novelists, newspapers etc.) iv) Underline the name - Leopard 1 (also non-industry; some publishers prefer underlining to italics)
Strangely, the NYT uses quotation marks around the names of movies and books, but has decided against using quotation marks for ships, instead choosing to use "the" before a ship name. I can offer no explanation for that decision, but at the very least it seems to reinforce the notion that ship names do require some special treatment to aid readability and avoid being confused with the names of places and living persons.
Please note as well that exactly what vessels should have their names italicized is sometimes a gray area. From the Chicago Manual:
Q. I know that ship and vessel names are italicized, but what is your criterion for determining what is a ship or vessel? I thought the idea was that the thing could carry people, but I must be wrong, because you set the Phoenix Mars lander in italics in your example. Are artificial satellites such as Sputnik set in italics? How about things like the International Space Station or the James Webb Telescope?
A. For every guideline in CMOS, at some point we have to stop narrowing our criteria and examples and just trust readers to use their own best judgment and record their decisions in their style sheets. Depending on the context, you might want to style gray-area terms consistently with other terms that you confidently styled per CMOS.