Also, a train conductor would yell "all aboard" as a final call to anyone else who might want to get on the train because the train was about to leave the station. Aboard is an adverb or a preposition- very confusing. It is possible to say "I am aboard the ship." or "I am on board the ship." To me, neither one is incorrect. On-board (it's hyphenated in my dictionary) is an adjective.
"I am on-board." -- adjective modifying noun... situated on the ship; I don't see why this couldn't be considered a preposition as well though
"I am aboard." --preposition showing location of subject
"We will go aboard." --also a preposition showing location
"Welcome aboard, sir." -- adverb answering the question "where?", modifying the verb
This topic is very ambiguous. I truly don't think anyone would correct you if you used them interchangeably, except in the idioms "all aboard" and "welcome aboard". I can't even distinguish them clearly as different parts of speech! Maybe someone else has other ideas...?