Immovable rod on a ship or air ship?

[Darth Credence]

Having consulted a fellow DM, he had a novel response: when you activate the Rod it becomes immovable in relation to whatever you place it in contact with. So if you are placing it against the ship it 'sticks' to it, but if you are just holding it in the air it defaults to the center of the planet/plane and 'moves'

I see a fundamental problem with this - it is always in contact with the person who sets it.