The term hull-down describes the situation where the upper part of a vessel or vehicle is visible, but the main, lower body (hull
Hull (watercraft)
A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...

) 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
Naval warfare
Naval warfare is combat in and on seas, oceans, or any other major bodies of water such as large lakes and wide rivers.-History:Mankind has fought battles on the sea for more than 3,000 years. Land warfare would seem, initially, to be irrelevant and entirely removed from warfare on the open ocean,...

, but beginning in the 20th century, hull-down has also been used in reference to armoured warfare
Armoured warfare
Armoured warfare or tank warfare is the use of armoured fighting vehicles in modern warfare. It is a major component of modern methods of war....



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.

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.

Armoured warfare

In modern armoured warfare
Armoured warfare
Armoured warfare or tank warfare is the use of armoured fighting vehicles in modern warfare. It is a major component of modern methods of war....

, hull-down is a position taken up by an armoured fighting vehicle
Armoured fighting vehicle
An armoured fighting vehicle is a combat vehicle, protected by strong armour and armed with weapons. AFVs can be wheeled or tracked....

 (AFV) such that its hull (the main part of the vehicle) is behind a crest or other raised ground, but its turret
Gun turret
A gun turret is a weapon mount that protects the crew or mechanism of a projectile-firing weapon and at the same time lets the weapon be aimed and fired in many directions.The turret is also a rotating weapon platform...

 (or a superstructure or roof-mounted weapon
A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

) is exposed. This allows it to observe and fire upon the ground ahead, while the hull is protected from enemy fire behind hard cover
Cover (military)
In military combat, the concept of cover refers to anything which is capable of physically protecting an individual from enemy fire. This differentiates it from the similar concept of concealment, in that an object or area of concealment only affords the benefit of stealth, not actual protection...

. A hull-down AFV is said to be in defilade
Enfilade and defilade
Enfilade and defilade are concepts in military tactics used to describe a military formation's exposure to enemy fire. A formation or position is "in enfilade" if weapons fire can be directed along its longest axis. A unit or position is "in defilade" if it uses natural or artificial obstacles to...

. 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
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...

, 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
A bulldozer is a crawler equipped with a substantial metal plate used to push large quantities of soil, sand, rubble, etc., during construction work and typically equipped at the rear with a claw-like device to loosen densely-compacted materials.Bulldozers can be found on a wide range of sites,...

' blades attached for this purpose, and some tank models have a built-in blade. Combat engineering vehicle
Combat engineering vehicle
Military engineering vehicles are vehicles built for the construction work or for the transportation of combat engineers on the battlefield. These vehicles can range from civilian equipment to purpose built military vehicles....

s 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
A glacis in military engineering is an artificial slope of earth used in late European fortresses so constructed as to keep any potential assailant under the fire of the defenders until the last possible moment...

 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
Telescopic sight
A telescopic sight, commonly called a scope, is a sighting device that is based on an optical refracting telescope. They are equipped with some form of graphic image pattern mounted in an optically appropriate position in their optical system to give an accurate aiming point...

 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.

Mutual support

Small armoured units (companies
Company (military unit)
A company is a military unit, typically consisting of 80–225 soldiers and usually commanded by a Captain, Major or Commandant. Most companies are formed of three to five platoons although the exact number may vary by country, unit type, and structure...

 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 ineffective, elements of a unit (platoons, patrols, or individual vehicles) take turns moving and supporting each other from the halt (see overwatch
In modern warfare, overwatch is the state of one small unit or military vehicle supporting another unit, while they are executing fire and movement tactics. An overwatching, or supporting unit has taken a position where it can observe the terrain ahead, especially likely enemy positions. This...

). 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
Reconnaissance is the military term for exploring beyond the area occupied by friendly forces to gain information about enemy forces or features of the environment....

 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.

Tank design

A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility, tactical offensive, and defensive capabilities...

s and other fighting vehicles must be able to depress their gun
Tank gun
A tank gun is the main armament of a tank. Modern tank guns are large-caliber high-velocity guns, capable of firing kinetic energy penetrators, high explosive anti-tank rounds, and in some cases guided missiles. Anti-aircraft guns can also be mounted to tanks.-Overview:Tank guns are a specific...

 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 Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

n tanks after World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 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
Breech-loading weapon
A breech-loading weapon is a firearm in which the cartridge or shell is inserted or loaded into a chamber integral to the rear portion of a barrel....

 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
Armoured warfare
Armoured warfare or tank warfare is the use of armoured fighting vehicles in modern warfare. It is a major component of modern methods of war....

Military doctrine
Military doctrine is the concise expression of how military forces contribute to campaigns, major operations, battles, and engagements.It is a guide to action, not hard and fast rules. Doctrine provides a common frame of reference across the military...

 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 prepared positions where they could be more vulnerable to air attack.

The polar opposite of the Soviet tanks is the defensively-designed Swedish Stridsvagn 103
Stridsvagn 103
The Stridsvagn 103 , or S-Tank , was a Swedish post-war main battle tank. It was known for its unconventional turret-less design, with a fixed gun traversed by engaging the tracks and elevated by adjusting the hull suspension...

. Built like a high-tech tank destroyer
Tank destroyer
A tank destroyer is a type of armored fighting vehicle armed with a gun or missile launcher, and is designed specifically to engage enemy armored vehicles...

, 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.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.