part's don't need credit.
copy past of an superstructure, mast, etc. from an another drawing, something that only that ship have and no one else: credit.
big copy and past: credit.
drawing where you just modify an ship: credit.
her you have an example
would you have credited the original artist in this drawing or:
yes credit is an two edged sword. but when an artist have used a lot of time getting references for an ship, then crediting him is very polite to do, it is a way to say thank you for an very good original drawing, and that I can use it. I don't have an problem that Gunship used my drawing as an base in this example... but is how did it, without asking. "It's always polite to ask before using large part's of a ship."