Definitions for "Hull Down"
Keywords:  mast, horizon, far, poistion, turret
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.