Said of a vessel when it is so far away from the observer that the hull is invisible owing to the convexity of the earth's surface, while the masts are still seen. The opposite of hull up.

Is when a ship is so far off, that you can only see her masts.

said of a distant vessel when only her stack-tops and mast are visible above the horizon.

To take up a poistion in which on the turret of your tank is visible

said of a vessel far off on the horizon, which is visible only by its sails. "We spied a ship hull-down this morning, but she came no closer."