I think the sentence 1 is correct:

Sentence 1: The **vessel Queen Elisabeth** weighed anchor today at 19:00 at the port of Malta.

The word vessel is a noun and "Queen Elisabeth" is the noun used to name the specific vessel.

I wonder whether it would also be possible to follow the same structure in this other context:
A mathematics teacher tells his students:

Sentence 2: I am going to teach you today the **theorem Isosceles Triangle**.
The word theorem is a noun and the words "Isosceles Triangle" is the noun specifying the name of the theorem the teacher is going to teach.

With reference to written American English, I would like to know the following. If sentence 2 is not idiomatic and instead "Isosceles Triangle Theorem" should be used, I would like to know why sentence 2 is not idiomatic whereas sentence 1 is idiomatic, considering they both seem to have the same grammar structure.

Thanks