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. Armoured warfare In modern armoured warfare, hull-down is a position taken up by an armoured fighting vehicle (AFV) such that its hull (the main part of the vehicle) is behind a crest or other raised ground, but its turret (or a superstructure or roof-mounted weapon) is exposed. This allows it to observe and fire upon the ground ahead, while the hull is protected from enemy fire behind hard cover. A hull-down AFV is said to be in defilade. Taking advantage of hull-down positions is an element of tactical movement. Turret-down is the position in which the vehicle's crew can observe forward from roof hatches, but the vehicle is completely hidden (usually a few metres further back from a hull-down position). This can also apply to vehicles without turrets. In flat or gently rolling terrain, a hull-down position is difficult to find. The actual protecting rise of ground may be hundreds of metres long. In steep or abrupt terrain cover is plentiful, but it may be difficult to find covered positions from which the vehicle's main gun can fire upon terrain ahead (see tank design, below). In preparing defensive works, a hull-down position can be created or improved by digging shallow "tank scrapes". Tank units usually have one or two tanks with 'dozer' blades attached for this purpose, and some tank models have a built-in blade. Combat engineering vehicles often accompany armoured vehicles as they manoeuvre to dig tank scrapes, as they can accomplish the task more quickly. Tactical movement Crossing a crest or ridge-line is a dangerous manoeuvre for AFVs, as they are particularly exposed to enemy fire while silhouetted against the sky (sky-lined). While cresting a steep slope, the thin armour on the front bottom of a tank's hull (below the thick glacis plate) can be exposed to fire. After cresting, the thin top armour may be exposed while it moves down the forward slope. If an antitank gunner has spotted the AFV, they may train their sights on it and wait for an easy shot while it moves forward. After observing from a hull-down or turret-down position, an armoured vehicle will try to advance while minimizing these risks. If possible, it may reverse away from a crest, and try to