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.
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 above the water will be able to see the top of another 130 ft mast from over away, but will be able to see the full hull of the other ship from only away.
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.
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 vessle being under sail and loaded sailing briskly to windward. Ideal sailing conditions permitting the vessle to make good time, while sailing to the optimum
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 relatively flat or gently rolling terrain, a hull-down position may be difficult to find. The actual protecting rise of ground may be hundreds of metres long. In steep or abrupt terrain cover is usually 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.
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, he may train his 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 will reverse away from a crest, and try to find a route forward through the relative safety of hidden low ground (dead ground).
If crossing a long crest is unavoidable, the vehicle will first back down and jockey at least 50 metres across the covered back of the slope, before advancing over the crest at high speed. An enemy gunner will have little time to locate the target, train his sights on it, and take the shot. If the terrain is hilly enough, the AFV can quickly enter low ground, then advance through it to another hull-down position.
Small armoured units (companies or platoons) make use of these tactics in co-ordinated fashion, when contact with the enemy is expected. Since firing while moving was until recently impossible or relatively ineffective, elements of a unit (platoons, patrols, or individual vehicles) take turns moving and supporting each other from the halt (see overwatch). This is called mutual support, or fire and movement, related to the infantry tactic of leap-frogging. Co-ordination is accomplished by hand signals or radio messages.
Lightly-armed reconnaissance elements make much use of covered movement and stealth, while offensive units such as tanks move much more aggressively. When speed is paramount, modern tanks (which can fire effectively while moving) may dispense with fire and movement, and move all at once.
Tanks and other fighting vehicles must be able to depress their gun to be able to take advantage of a hull-down position, since a vehicle's hull is usually tilted upwards when it is behind a crest. A vehicle with a relatively small range of gun depression may have to drive up onto an exposed crest or forward slope to be able to fire on lower ground to the front.
Notably, Soviet and Russian tanks after World War II have very low profiles, but pay for this advantage by having a poor range of gun depression. Their low turret roof stops the rising gun breech when the muzzle is depressed. Thus, Soviet tank crews would have a hard time finding a hull-down position from which they could cover much of the terrain by fire. The typical Soviet tank had a range of elevation of -5 to +15 degrees, about two thirds that of Western tanks with a range of about -10 to +20 degrees.
This disadvantage was deemed acceptable, since Soviet armoured doctrine emphasized the massed attack with local superiority in numbers. Soviet mechanized forces were supposed to spend more time advancing under fire than in defensive hull-down positions. In contrast, NATO tanks would have been at an advantage in ridge-top hull-down positions, attacking top and side armour of advancing Soviet tanks in killing zones where they couldn't fire back directly.
Soviet doctrine didn't neglect the defence, however. Newer Soviet tank models were equipped with an integral dozer blade, so given time, they could improve a hull-down position. Soviet tactics also emphasize the use of tanks on the defense in the mobile counter attack role, rather than engaging an enemy advance from relatively fixed positions where they could be more vulnerable to air attack.
The polar opposite of the Soviet tanks is the defensively-designed Swedish Stridsvagn 103. Built like a high-tech tank destroyer, but serving the role of a tank, this turretless vehicle is made to nearly disappear in a hull-down position. It has an even lower profile than Soviet MBTs, with a fixed, auto-loaded gun that is nearly flat against the roof. Its design gets around the Soviet tanks' limitations by tilting the entire tank hull to elevate (and depress) the gun. The Stridsvagn 103 also has an integral dozer blade, and a second set of rear-facing driver controls for the radio operator, allowing it to withdraw from a hull-down position at full speed. This radical, specialized design was created for its hypothetical defensive role on roads in a forested country.