Hull-down

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The term hull-down describes the situation where the upper part of a vessel or vehicle is visible, but the main, lower body (hull) is not; the opposite term hull-up describes the situation where all of the body is visible.
The terms originated with sailing and naval warfare, but beginning in the 20th century, hull-down has also been used in reference to armoured warfare.

Sailing

Due to the convexity of the earth, a ship's upper rigging will be visible at a much greater distance than its lower hull: for example, in clear air a lookout at the top of mast 130 feet (40 m) above the water will be able to see the top of another 130 ft mast from over 24 nautical miles (44 km) away, but will be able to see the full hull of the other ship from only 12 nautical miles (22 km) away.^[1]
With a clear horizon, whether a vessel is hull-down or hull-up gives some idea of its distance from the observer, using the line-of-sight formula.

Tactical considerations

In naval warfare, while the upper rigging (of a sailing vessel) or radio mast and stacks (of a steam ship) may give some idea of its type, it is impossible to tell the true nature of a ship when it is hull-down and its armament and size are not visible. Especially during the age of sail, a naval vessel that chose to pursue a possible enemy vessel spotted hull-down ran the risk of unknowingly closing on a more powerful opponent — depending on the wind and other conditions, it might not be possible to flee once the other vessel was clearly visible hull-up. Hull Down was also used to describe a commercial sailing vessel being under sail and loaded sailing briskly to windward. Ideal sailing conditions permitting the vessel to make good time, while sailing to the optimum