Hull-down Position
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In sailing and warfare, to be hull down means that the upper part of a vessel or vehicle is visible, but the main, lower body (
hull Hull may refer to: Structures * The hull of an armored fighting vehicle, housing the chassis * Fuselage, of an aircraft * Hull (botany), the outer covering of seeds * Hull (watercraft), the body or frame of a sea-going craft * Submarine hull Ma ...
) is not; the term hull up means that all of the body is visible. The terms originated with sailing and
naval warfare Naval warfare is combat in and on the sea, the ocean, or any other battlespace involving a major body of water such as a large lake or wide river. The Military, armed forces branch designated for naval warfare is a navy. Naval operations can be ...
in which the curvature of the Earth causes an approaching vessel to be first visible "sails up". Beginning in the 20th century, ''hull down'' has also been used in
armoured warfare Armoured warfare or armored warfare (American English; American and British English spelling differences#-our, -or, see spelling differences), is the use of armoured fighting vehicles in modern warfare. It is a major component of modern Milita ...
. In modern armoured warfare, hull down is a position taken up by an armoured fighting vehicle (AFV) so 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. 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). The belly armour should not be exposed, because it is vulnerable to even modest antitank weapons.


Ships

When a ship moves away, due to the
curvature of the Earth Spherical Earth or Earth's curvature refers to the approximation of the figure of the Earth as a sphere. The earliest documented mention of the concept dates from around the 5th century BC, when it appears in the writings of Greek philosophers. ...
, the ship's hull will disappear under the
sightline The line of sight, also known as visual axis or sightline (also sight line), is an imaginary line between a viewer/observation, observer/wikt:spectator, spectator's eye(s) and a subject of interest, or their relative direction (geometry), relative ...
at a much smaller distance than its upper rigging. The geodetic visibility depends on the altitude of the observation site and the altitude of the object being viewed. For example, in clear air a lookout at the top of a 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 hull above the waterline of the other ship from only away. The discovery of the hull-down phenomenon in sailing was essential to disproving the
Flat Earth Flat Earth is an archaic and scientifically disproven conception of the Figure of the Earth, Earth's shape as a Plane (geometry), plane or Disk (mathematics), disk. Many ancient cultures, notably in the cosmology in the ancient Near East, anci ...
theory. 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 or armored warfare (American English; American and British English spelling differences#-our, -or, see spelling differences), is the use of armoured fighting vehicles in modern warfare. It is a major component of modern Milita ...
, 'hull down' is a position taken up by an
armoured fighting vehicle An armoured fighting vehicle (British English) or armored fighting vehicle (American English) (AFV) is an armed combat vehicle protected by vehicle armour, armour, generally combining operational mobility with Offensive (military), offensive a ...
(AFV) such that its hull (the main part of the vehicle) is behind a crest or other raised ground, but its
turret Turret may refer to: * Turret (architecture), a small tower that projects above the wall of a building * Gun turret, a mechanism of a projectile-firing weapon * Optical microscope#Objective turret (revolver or revolving nose piece), Objective turre ...
(or a superstructure or roof-mounted
weapon A weapon, arm, or armament is any implement or device that is used to deter, threaten, inflict physical damage, harm, or kill. Weapons are used to increase the efficacy and efficiency of activities such as hunting, crime (e.g., murder), law ...
) 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 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 weapon fire can be directed along its longest axis. A unit or position is "in de ...
''. 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 (), alternatively relief or topographical relief, is the dimension and shape of a given surface of land. In physical geography, terrain is the lay of the land. This is usually expressed in terms of the elevation, slope, and orientati ...
, 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 A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and battlefield mobility provided by tracks and a powerful engine; t ...
, below). In preparing
defensive Defense or defence may refer to: Tactical, martial, and political acts or groups * Defense (military), forces primarily intended for warfare * Civil defense, the organizing of civilians to deal with emergencies or enemy attacks * Defense indust ...
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 vehicle A military engineering vehicle is a vehicle built for construction work or for the transportation of combat engineering, combat engineers on the battlefield. These vehicles may be modified civilian equipment (such as the Armored bulldozer, armo ...
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 ridgeline is a dangerous manoeuvre for AFVs, as they are particularly exposed to enemy fire while silhouetted against the sky (''skylined''). 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 as part of a medieval castle or in early modern fortresses. They may be constructed of earth as a temporary structure or of stone in more permanent structure. More generally, a glaci ...
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 Anti-tank warfare refers to the military strategies, tactics, and weapon systems designed to counter and destroy enemy armored vehicles, particularly tanks. It originated during World War I following the first deployment of tanks in 1916, and ...
gunner has spotted the AFV, they may train their
sights A sight or sighting device is any device used to assist in precise visual alignment (i.e. ''aiming'') of weapons, surveying instruments, aircraft equipment, optical illumination equipment or larger optical instruments with the intended target. ...
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 find a route forward through the relative safety of hidden low ground (''dead ground''). If crossing a long crest is unavoidable, the vehicle can 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 A company, abbreviated as co., is a legal entity representing an association of legal people, whether natural, juridical or a mixture of both, with a specific objective. Company members share a common purpose and unite to achieve specifi ...
or
platoon A platoon is a Military organization, military unit typically composed of two to four squads, Section (military unit), sections, or patrols. Platoon organization varies depending on the country and the Military branch, branch, but a platoon can ...
s) 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 ''Overwatch'' (abbreviated as OW) is a multimedia franchise centered on a series of multiplayer first-person shooter (FPS) video games developed by Blizzard Entertainment. ''Overwatch (video game), Overwatch'' was released in 2016 with a success ...
). This is called mutual support, or ''fire and movement'', related to the infantry tactic of ''leap-frogging'', or, somewhat more loosely, the air combat tactic of flying with a
wingman A wingman (or wingmate) is the pilot of a secondary aircraft providing support or protection to a primary aircraft in a potentially dangerous situation, traditionally flying in formation to the side and slightly behind the primary craft. The t ...
. Co-ordination is accomplished by hand signals or radio messages. Lightly armed
reconnaissance In military operations, military reconnaissance () or scouting is the exploration of an area by military forces to obtain information about enemy forces, the terrain, and civil activities in the area of operations. In military jargon, reconnai ...
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

Tank A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and battlefield mobility provided by tracks and a powerful engine; ...
s and other fighting vehicles must be able to depress their
gun A gun is a device that Propulsion, propels a projectile using pressure or explosive force. The projectiles are typically solid, but can also be pressurized liquid (e.g. in water guns or water cannon, cannons), or gas (e.g. light-gas gun). So ...
to be able to take advantage of many hull-down positions, 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 The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...
and
Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...
n tanks after
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
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 may refer to: * Breech (firearms), the opening at the rear of a gun barrel where the cartridge is inserted in a breech-loading weapon * breech, the lower part of a pulley block * breech, the penetration of a boiler where exhaust gases leave ...
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, as Soviet tanks were designed to be used as an offensive weapon, fighting over flat terrain. Soviet tactics didn't neglect the defence, however. Newer Soviet tank models were equipped with an integral
dozer blade A bulldozer or dozer (also called a crawler) is a large tractor equipped with a metal blade at the front for pushing material (soil, sand, snow, rubble, or rock) during construction work. It travels most commonly on continuous tracks, though s ...
, so given time, they could improve a hull-down position. Soviet tactics also emphasize the use of tanks on the defence in the counterattack role, rather than engaging an enemy advance from prepared positions. The Swedish
Stridsvagn 103 The Stridsvagn 103 (Strv 103), also known as the Alternative S and S-tank, is a Sweden, Swedish Cold War, Cold War-era main battle tank, designed and manufactured in Sweden. "Strv" is the Swedish military abbreviation of ''stridsvagn'', Swedish f ...
, while resembling a
tank destroyer A tank destroyer, tank hunter or tank killer is a type of armoured fighting vehicle, predominantly intended for anti-tank duties. They are typically armed with a direct fire anti-tank gun, artillery gun, also known as a self-propelled anti-ta ...
, was actually a main battle tank. The turretless design was chosen to give it a low profile and thus increase protection, including in a hull-down position. It was however intended to be used in the offensive role, as the
armoured brigades Armour (Commonwealth English) or armor (American English; see spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, especially direct contact weapons or projectiles during combat ...
it served in were assault brigades intended for counter-offensive operations against enemy
beachhead A beachhead is a temporary line created when a military unit reaches a landing beach by sea and begins to defend the area as other reinforcements arrive. Once a large enough unit is assembled, the invading force can begin advancing inland. Th ...
s and airborne landings.


See also

*
Radar horizon The radar horizon is a critical area of performance for air traffic, aircraft detection systems, defined by the distance at which the radar beam rises enough above the Earth's surface to make detection of a target at the lowest level possible. I ...
*
Spherical Earth Spherical Earth or Earth's curvature refers to the approximation of the figure of the Earth as a sphere. The earliest documented mention of the concept dates from around the 5th century BC, when it appears in the writings of Ancient Greek philos ...


References


External links

{{Wiktionary, hull-down
Chapter 4: "Defensive Operations"
€”from US army manual ''FM 17-15 Tank Platoon''
How Boats "Vanish" Over The Horizon
Nautical terminology Naval warfare Land warfare Armoured warfare
Hull down In sailing and warfare, to be hull down means that the upper part of a vessel or vehicle is visible, but the main, lower body ( hull) is not; the term hull up means that all of the body is visible. The terms originated with sailing and naval war ...
Tank tactics