I think unmast goes back to the days of the sailing ships. In a sea battle, ships would use chain shot to try to break the mast on a ship, and to render it immobile, they could then board and capture it. Replacing the mast was a routine operation in those days, so they would get a load of prize money it they managed to capture the ship. Dismast has gradually repaced the word, and these days it is is used to refer to a yacht that has had a mast broken through bad seamanship, or a gale.