Zero.
They needed to turn the ship over to get back from the end of the world. It's basically how they "switched" worlds. They went over a waterfall to get there, but you can't go back up a waterfall, so you have to flip over to get back to the top in a sense.
I don't think anyone is supposed to look into the logic of it, especially since the world isn't supposed to have an end to it anyway. It's not supposed to be logical.