A topic that often arises in discussing battleship designs and tactics is that of the "Immunity Zone," or IZ. The IZ is a theoretical range band between which a ship's protective scheme, or some portion of it, is proof against a particular gun's shells. The IZ is different for every interaction of a protective scheme with a different gun. If the ship's protective scheme contains significant variations (such as lesser protection for machinery than magazines) there are separate IZs for both. The lower of the two numbers in an IZ represents the minimum range at which the protective armor on vertical surfaces (side belt, turret faces, conning tower sides, boiler uptakes) will defeat a shell striking them. The upper of the two numbers in an IZ represents the maximum range at which protective armor on horizontal surfaces (decks over machinery, magazines and casemates, turret tops, conning tower roofs) will defeat a shell striking them. Between these two ranges, a ship could theoretically be fought without having its armor protection penetrated by a particular variety of gun. This assumes the ship actually has an IZ. If the range at which vertical armor can be penetrated exceeds that at which horizontal armor can be penetrated, the ship enjoys no IZ against that particular gun and shell combination.
Despite the absolutist nature of its name, the Immunity Zone concept does not imply any sort of invulnerability. Regardless of whether the armor holds up, a ship hit between the ranges of the IZ has still suffered an impact by an object weighing between roughly 1000 and 3000 pounds, travelling at somewhere between 1.5 and 3.0 Mach. Such an impact tends to leave an impression. Even a deflected shell could displace armor and smash unprotected things along its path until it exited the hull or came to rest. Undeflected shells might be broken up and fail to penetrate or detonate, but they could still bend, crack or dismount armor plates, buckle framework supporting the plates, cause leaks around them, spall them and perforate unarmored areas around the armor with fragments. A ship fighting within its IZ still suffers when hit, but it is theoretically proof against singularly catastrophic hits to the magazines or machinery spaces.
The IZ concept developed out of a recognition, whether initially conscious or subconscious, that weaponry was overtaking armor's ability to resist. As naval guns grew more powerful, ship commanders naturally sought safety from their most destructive effects in range. In response, fire control systems were improved to permit guns to hit successfully at long ranges, and larger weapons were developed to both shoot farther, and hit harder. The see-saw of warship and weapon development dictated that ships either move farther yet apart, or carry more armor. Given the cost of increased armor in both monetary and design terms, infinite quantities could not be shipped. As the gap between destructive and resistive capabilities continued to grow, ship captains sought recourse in tactics, fighting their ships at the optimum ranges to take advantage of their weaponry against the enemy while exploiting their own protection against the enemy's weapons. As the tactic of the IZ was developed, designers realized its value, as did naval program planners. Design needs for gun ships came to be expressed in terms of a specified IZ against a specified gun from about 1929 on (in the U.S.), permitting a savings in armor weight which could be used to enhance other design features such as armament or propulsion, or deleted altogether to keep size and cost growth down.
Pitfalls in the IZ concept included the mistaken belief by some that it afforded invulnerability, a propensity to forget that the gun faced was not the same weapon the accepted IZ was calculated for, a failure to resolve the question of how one obtained and maintained the optimum range versus the enemy, how to prioritize IZs when faced with multiple types of gun and how to resolve the situation when one ship's IZ matched that of the enemy ship. The actual conditions of encounter battles also made the idea largely moot. The IZ concept worked best in daylight, during an action begun at horizon range. The ranges encountered during night actions, or encounters in poor weather were often so short that no practical scale of armor could protect any ship. Still the IZ was and is a useful concept in gauging the relative vulnerability of a ship's vitals. Given the influence it exercised over the design process of ships built in the era of long-range gunfire, it is also essential to understand IZ to properly evaluate the design decisions made as a result of such calculations.
- "Battleships and Battle Cruisers, 1905-1970" by Breyer
- "Battleships: United States Battleships in World War II," by Dulin and Garzke
- "Battleships: Allied Battleships in World War II," by Dulin and Garzke
- "Battleships: Axis and Neutral Battleships in World War II," by Dulin and Garzke
- "U.S. Battleships: An Illustrated Design History" by Friedman
- 1 May 2000