adverb (or adjective)
1. of a ship
a. : at such a distance that only the superstructure is visible
had cleared with a leading wind … and was hull down long before dark — Raymond McFarland
b. : with main deck awash
one ship … after another went hull down into the sucking tide — D.C.Peattie
2. of a tank or other armored vehicle : in a place of concealment but in position to observe the enemy and deliver fire
were standing hull down behind a hillock and on each of their turrets was a figure staring forward through field glasses — Peter Rainier