Zone of immunity

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The zone of immunity around a warship is an area from which both plunging and direct enemy fire is relatively ineffective.

Warships traditionally have vertical, or near vertical armour, which protects against missiles travelling horizontally, and deck armour, which protects against plunging shell fire. A shell fired at short range will travel at a high velocity on a more or less horizontal trajectory; a shell fired at long range will travel on a parabolic trajectory, arriving at its target at a steep angle of descent but at a velocity markedly less than its original muzzle velocity.

If a ship is too close to an adversary, shells fired horizontally may pierce inadequate vertical armour; beyond a certain range, determined by the kinetic energy of incoming projectiles, plunging shells will penetrate deck armour. The distance between these two situations, which is determined by the energy of incoming ordnance and the thickness of the armour of the target vessel, is known as the zone of immunity.

Reference

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