“I hereby name this ship ‘Titanic II’.”
“Propositions, we shall say, are the sharable objects of the attitudes and the primary bearers of truth and falsity. This stipulation rules out certain candidates for propositions, including thought- and utterance-tokens, which presumably are not sharable, and concrete events or facts, which presumably cannot be false.”
A proposition needs to make a claim about the world, about the way things are. This one aspect of a proposition, or truth claim, seems to be present in this sentence. You could rephrase, for instance; “The ship is named ‘Titanic II’.” (obviously a different sentence and a different claim) However, it needs also to assert its truth/falsity claim in such a way that argument can be be built upon it. It needs to be either agreed with or disagreed with, believed or disbelieved. As this sentence is an announcement or exclamation of an event, and alludes to a claim about the world (that the ship has a name), it cannot actually be argued with.
If you were to formulate the negation, you would have to write something like:
“I do not hereby name this ship ‘Titanic II’.”
It can be seen here that the truth value of the sentence depends upon the subject of the sentence, the “I”, rather than the predicate, “name this ship”. In a proposition it is the predicate that needs to be affirmed or denied, therefore it is the predicate upon which the truth value depends. In the negation the “do not” applies to the subject “I” rather than “name this ship”. It could be said therefore, that the proposition needs to be ‘grounded’ in the predicate of the sentence, rather than the subject.
This sentence is also a present tense verb, rather than the alternative past tense: “I named this ship ‘Titanic II’.” The past tense version of the sentence is a proposition as it is arguable. It can be believed or disbelieved as it is a claim about the world, about an event, and about an object, rather than a proclamation.