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Concept
Hull down
Summary
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 warfare 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.
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.
When a ship moves away, due to the curvature of the Earth, the ship's hull will disappear under the sightline 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 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.
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.
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