1) The AP Mark 8 projectile was originally designed as the longer, heavier "big brother" to the AP Mark 5 projectiles used for the 16"/45 (40.6 cm) guns carried by the Colorado (BB-45) class battleships. Like the Mark 5, the Mark 8 projectiles were designed to be used in long-range gun actions against Japanese ships ("Plan Orange") and for that reason they were to be fired at relatively low muzzle velocities and high gun elevations. These conditions would result in a steeper angle of fall in order to enhance their deck armor penetration capabilities. In almost all respects the 1939 design of the Mark 8 Mod 0 was very similar to the Mark 5 except for length. However, around the end of 1944 the improved Mark 8 Mod 6 version came out with significantly better hardening techniques which gave a noticeable improvement in penetration at impact angles of 35 degrees or so against thick face-hardened armor. This performance was so much better that BuOrd put out a memo stating that battleships were to return all previous Mods of this projectile as quickly as the Mod 6 became available for loading aboard ship. The Mod 6 had an even blunter, rounded AP body nose (with no point) to further enhance penetration against deck armor at high obliquity. The Mod 8 had a heavier cap, blunter nose and harder body from improved heat treating techniques. The AP Mark 8 has a radius of ogive of 144 inches (366 cm) or 9crh and leaves the barrel rotating at about 4,000 RPM. This round creates overpressures exceeding 50 psi (3.5 kg/cm²) close to the muzzle and 7 psi (0.5 kg/cm²) at a distance of 50 feet (15 m) from the muzzle. The Mark 21 Base Detonating Fuze (BDF) had a delay of 0.033 seconds. Fuze activation required a resistance equal to 1.5 inches (3.8 cm) of armor at 0 degrees obliquity or 0.375 inches (1 cm) at 65 degrees obliquity. AP projectile bodies are painted black. The nose color indicates burster type, with yellow denoting Explosive D. A narrow colored band below the nose indicates the splash color.

Armament

With the fixed factors in mind, the first consideration was the main armament. BDAB had four options. Option number one was to match the Yamato (the biggest battleship facing the Americans) gun for gun with the Mk1 18-inch/45 caliber rifle. This monster gun, the largest ever assembled in America, could fire a two-ton shell several miles that could penetrate 16 inches of armor. There were several problems with using this weapon (option number one called for the use of nine Mk1s.) It would slow the ships down to a snail crawl of 22 knots, add six months to construction time, require that some of the armor be shed on the turrets, and most importantly, the beam of the ship would have to be increased to 131 feet in order to accommodate the sheer size of the guns.

Option number two was to have only dual mounted 18-inch guns. While it might have saved some space and displacement, BDAB quickly dismissed this option. It would have required additional design and test work, being that there was no dual mount available.

Options number three and four both considered the use of the smaller 16-inch rifles. There were two types available: the Mk6 16-inch/45 caliber rifle that was used on both North Carolina and South Dakota and the newer Mk7 16-inch/50 caliber rifle. While the Mk6 would make the designer's job easier because it was small and light, the Mk6 would not solve the firepower problem. Option 4, the Mk7 16-inch/50 caliber rifle, was a good compromise of firepower, size, weight, and cost.

The Mk7 had several other advantages over the other options. During a rapid-fire situation, the Mk7 could fire twice as fast as the 18-inch guns and would give the new class a further reach than the older Mk6 16-inch rifles. Additionally, the turrets for the Mk7 could be given sufficient armor to withstand an incoming 16-inch shell, unlike the Mk1 18-inch guns. Fully equipped, each of Wisconsin's triple mounted turrets weighed over 2,100 tons. At 6,300 tons, these three turrets alone made up about 12% of the ship's displacement.

The Naval Gun Factory (Building 76 and present site of the Navy Museum.) at the Washington Navy Yard located in Southeast Washington, DC designed and manufactured the guns. The installation of the nine Mk7 16-inch/50-caliber guns was among the most complicated procedures performed. With the skill of a surgeon, each gun had to be installed within a 1/4-inch of standards. Each gun, not including the gun breech, weighed around 120 tons (considered to be a "lightweight" design) and could fire a one ton shell 24 miles.

World War II battleships had a concern that their older Spanish-American War and World War I counterparts did not have to worry about: air attacks. Despite the modern battleship's classification as "dreadnought" battleships (which meant among other things that the vessel carried an all big gun arrangement), designers equipped their warships with several smaller caliber guns. From the start, these ships were to have 20 5-inch/38 caliber dual purpose guns mounted in 10 turrets. These guns could put up 22 shells per minute per gun in the anti-aircraft role. The Naval Gun Factory designed the smaller guns, but contracted their manufacture to other factories.

Designers initially gave the Iowa-class 20 Mk12 five-inch "dual purpose" guns installed in 10 gun mounts for secondary armament. Even though the keels had already been laid, designers continued to tweak the design. One of the things that made World War II different than most previous wars was that the combatants more quickly learned lessons of the battlefield and applied them to new weapon designs. The first instance was the British use of the antique Swordfish bi-plane in a torpedo bomber role. In 1940, Swordfish knocked out the Italian battlefleet and in 1941 is same bi-plane disabled Bismarck during her famous breakout sortie. The second event was the infamous Japanese air attack on Pearl Harbor on December 7, 1941. Further battle experience, such as the naval fights around Guadalcanal, gained by American battleships only reinforced the need for more secondary guns.

The very real threat of air attacks forced designers to place several smaller AA guns. Additional guns were to be placed to supplement the dual mounted five-inch guns. Twenty Mk2 quad mounted 40mm Bofers, 29 20mm single mounted guns, and several .50 caliber machine guns were added. The problem with adding such an impressive array of guns was that the ship would require an additional 1,900 sailors to an already cramped ship. Despite this problem, the risk of air attack was too great and the additional guns, and sailors, were ordered.


Armor

The second basic factor, after firepower, to be considered was Iowa class armor. The armor scheme was a copy of the armor used on North Carolina and South Dakota, only thicker. This armor could, in theory, stop a 16-inch shell coming in at a 45-degree angle. There was some idle talk about making the Iowa class armor tough enough to stop an 18-inch shell, but BDAB dropped the idea when it realized how much more weight and redesign work it would take.

Nickel-steel was used to manufacture the armor. This type of steel is a kind of stainless steel which has the added benefits that it does not corrode quickly, but bends easily. Nickel-steel was not a new material. From the start, armored warships like USS Indiana (BB-1) used this type of steel. One 17 1/2 inch belt of the nickel-steel ran from the deck to the below water line on both sides of the ship and covered the middle 2/3 of the ship. Eighteen inch plates were used in the turrets and 11 1/2 inch plates were placed on the decks.

It is interesting to note that much of the Iowa class's armor is just as thick as battleships built 50 years earlier. Wisconsin and her sisters, however, benefitted from advances in steel technology that allowed mills to forge the steel at higher temperatures and heat treatment, which in turn produced a much higher quality steel that was stronger and more elastic. Two plants, Bethehelm Steel's main mill in Bethehelm, PA and Luken Steel's Coatsville mill just ouside Phildadelphia, manufactured most of the armor plating. For the turret plate, however, a special forge was constructed just for the Iowa-class at the Charleston Ordnance Works in Charleston, WV.

In regards to Iowa's 38mm deck armor

Although true, it was not heavy enough to de-cap large caliber APC, and the inter-space was too small to ensure detonation of time delayed AP before reaching the main armour deck, it was still an essential element to the over all deck protection against large amour piercing artillery shells. In a June 1941 memo from the BoO to to Carngie Steel the true purpose of the 38mm upper deck is revealed. It was a yaw deck. By yawing large caliber APC it required greater velocity for the shells to penetrate the main armoured deck, or in other words by yawing the shells even slightly it increased the effective thickness of the main armoured deck. Yaw created by the spacing above of the upper deck 38mm armour was an essential feature of the US fast battleship deck protection and also had desirable properties against SAP bombs or non AP bombs.

Note: The above information is from "Battleships: United States Battleships 1935-1992" for a muzzle velocity of 2,500 fps (762 mps) and is based upon the USN Empirical Formula for Armor Penetration. These values are in substantial agreement with armor penetration curves published in 1942.

2) The AP Mark 20 probably had slightly less penetration, as its shorter, less aerodynamic ballistic cap would have resulted in somewhat lower striking velocities.

A Naval War College study performed during World War II estimated that an Iowa Class (BB-61) battleship firing with top spot against a target the size of the German battleship Bismarck would be expected to achieve the following hit percentages.