The interior of the ships bridge. At 17.3 inches, the
conning tower armor is among the thickest in the ship.
Notice the viewing slit in the door.
|One of the main characteristics of a battleship is its ability to
withstand an attack. Few ships from the past and no modern ships can equal the
survivability of the Iowa Class Battleships. The decision of where to armor and how much
armor to use is a very complicated and sometimes frustrating process. Simply adding armor
can not be done since this greatly increases weight and reduces the top speed of the ship.
The process of protecting a battleship is an art that has been perfected over decades of
battleship design. Iowa Class Battleships are an excellent example of superior armor
protection and high top speed.
The armor systems of the Iowa
Class ships can be divided into two basic sections. First is the above water armor, which
is designed to protect the ship against gun fire and aerial bombing. The second is the
below water armor (side protective and triple bottom armor), which is designed to protect
the vessel from mines, near miss bombs and of course, torpedoes.
All the systems needed to keep these ship's combat
effective such as magazines, engineering spaces, steering, plotting rooms, command &
control, weapons, etc. are protected by heavy armor. The armor box, referred to as the
citadel, extends from just forward of Turret 1 to just aft of Turret III. The top, sides
and ends of the citadel are heavily armored, however the bottom is not ballistically
protected. Critical systems located outside the citadel such as the turrets, conning
tower, fire control, directors, etc. are armored extensions of the citadel.
Generally, a ship is armored to withstand hits from weapons equal to or smaller than its
own main guns. The Iowa's mount 16 inch main guns and are designed for optimal performance
at ranges between 19,000 and 30,000 yards. With this in mind, the deck and side armor is
designed to defeat the armor piercing shells fired by a ship mounting equal armament at
these same ranges.
One factor in the original building of the Iowa's was the availability of armor plating.
In 1939, when armor production began for the Iowa's, the United States could produce
75,000,000 tons of steel per a year. This may seem like plenty, however the United States
ability to produce armor, given the special manufacturing and testing procedures, meant
the steel mills could only produce 19,000 tons of armor per year.
In addition to the factors of weight and availability of armor, there was the matter of
urgency. Due to the onset of war in Europe and the Pacific, assembly time of different
armor types was taken into consideration. Another problem arose with the change from 2,240
pound. projectile to the new 2,700 pound. shell. First, the armor for BB61 and BB62 was
already on order and besides, the increase in armor to protect the ship from this new
shell would have drastically increased displacement (small changes were made to the armor
of BB63 and newer battleships).
the exception of its guns, the most awe inspiring aspect of the battleship is the huge
amount of armor employed to protect the ship. The overall design of the Iowa class
armor system is essentially the same as that of their predecessors, the four South
Dakota class battleships. Both feature an internal main belt which represents a
significant change from the previous two North Carolina class battleships and was
adopted only with reluctance. First of all, an internal belt is difficult and costly to
install and secondly, it is difficult to reach for repairs. The armor on the North
Carolinas was designed with an external belt designed to protect against a 14in shell.
The South Dakotas and later the Iowas were designed to have protection
against the 16in shell. To achieve this level of protection, the belt incline would have
to be increased to 19 degrees. An external belt inclined at the steeper angle would have
required a wider beam to maintain stability, but would have precluded passage through the
Panama Canal. Therefore, an internal belt was adopted.
Typical armor bolt used for securing the Class A upper bel
to its STS backing plate. One bolt per every 5 square feet
of armor plate.
BB61-64 Arrangement of protection for torpedo defense and
triple bottom systems. (Robert Sumrall)
The most notable
difference between modern warships and the Iowa Class battleships is the huge amount of
armor protection the Iowas employ. Modern warships are hardly armored at all, instead
relying on their ability to stop incoming threats before they can hit the ship. Newer
warships have only a few inches of armor plating and in an effort to save weight, have
even used aluminum in their superstructures. In contrast, the Iowas were built at a time
before missiles and since you could not shoot down or destroy an incoming projectile, the
ships were built to withstand the tremendous force of impact produced by naval gunfire.
Armor distribution on any warship
is a trade-off between protection and weight. If the armor is increased, the weight also
increases, which results in slower top end speed and maneuverability. The vertical side
armor consists of an upper and lower belt which is inclined to an angle of 19 degrees. The
total depth of the belt is 38 feet 6 inches and extends from just before turret 1 to just
aft of turret 3. The upper belt is Class A armor, 12.1 inches thick, while the lower belt
is Class B armor, 12.1 inches thick at the top and tapered to 1.62 inches at the bottom.
The deck consists of three parts,
the bomb deck, the main armor deck, and the splinter deck. The bomb deck is 1.5 inches STS
plate, the main armor deck is 4.75 inches Class B armor laid on 1.25 inches STS plate and
the splinter deck is 0.625 inches STS plate. The bomb deck is designed to detonate general
purpose bombs on contact and arm armor piercing bombs so they will explode between the
bomb deck and the main armor deck. Within the immune zone, the main armor deck is designed
to defeat plunging shells which may penetrate the bomb deck. The splinter deck is designed
to contain any fragments and pieces of armor which might be broken off from the main armor
Turret armor is constructed from a
combination of Class A and Class B armor and STS plate. The faces of the turrets are 17I
inches Class B armor over 2.5 inches STS plate. The side plates are 9.5 inches Class A
armor on .75 inch STS plate. The back plates are 12 inches Class A armor and the turret
roofs are 7.25 inches Class B armor.
The conning tower is constructed
from segments of Class B armor 17.3 inches thick. BB61 is three levels and BB62 on had 2
levels (the flag level was omitted). Roof plates are 7.25 inches Class B and the floor is
4 inches STS. The conning tower is connected to the citadel by a communications tube with
a wall thickness of 16 inches of Class B armor.
Construction of USS Kentucky in 1946. Construction
continued until she was about 75% complete. The Kentucky was never
completed, and she was scrapped in 1958.
Her machinery was carefully removed and used to power two
fast Combat Support Ships - the USS Sacramento (AOE1) and
USS Camden (AOE 2). (Iowa Class BBs / Sumrall)
The installation of heavy armor
plates was no easy task and the method of fastening them to the ship structure is worth
mentioning. A typical upper armor belt plate is 30 feet wide and 10 6" high.
The upper plate fits directly over three of the lower belt plates and is bolted to a
backing plate with specially designed watertight bolts. Since it is impossible to fit the
belt snugly against the backing plate, the bolts stand off the armor about 2 inches,
creating a small void between the belt and the plate. After the installation is complete,
the void space is filled with concrete, which provides support for the armor over the
entire surface. One bolt for every five square feet of surface area is used to secure the
plate. This method, along with welding and heavy rivets is used in the armor belt assembly
The side protection (torpedo defense)
and the triple bottom systems provide protection against underwater threats such as
torpedoes, mines and near-miss explosions. Both of these multi-layered systems are
intended to absorb the energy from an underwater explosion equivalent to a 700 pound
charge of TNT. The Navy derived at this amount of protection based on intelligence
information gathered in the 1930s. At that time, US Naval Intelligence was unaware
of the advances the Japanese had made in torpedo technology. One of these advances was the
Japanese 24 inch diameter "Long Lance" torpedo, which carried a charge
equivalent to 891 pounds of TNT. A Long Lance torpedo essentially defeated the USS
North Carolinas side protective system. The ship was hit by chance at its
narrowest, and therefore most vulnerable part of the side protection system. An Iowa Class
battleship would have taken lighter damage from the torpedo due to an improved torpedo
protection system over the North Carolina Class.
However, the Iowa Class
torpedo defense system is virtually the same as in the previous South Dakota Class
battleships. The side protection system consists of four tanks on the outboard side of the
hull extending from the 3rd deck to the bottom of the ship. The two outboard
tanks are liquid loaded with fuel oil or ballast and the two inboard tanks are kept void.
The liquid layers are intended to deform and absorb the shock from the explosion and
contain most of the shards from the damaged structure. The innermost void is expected to
contain any leakage into the interior ship spaces. The armor belt is designed to stop
fragments that penetrate the second torpedo bulkhead. This method should contain the
damage and protect the machinery and other vital spaces. Torpedo bulkheads #1, #2 and the
inner holding bulkhead are 5/8" thick steel. Bulkhead #3 is 12.1" thick armor
tapering to 1" thick at the bottom and is attached to a 1.5" special treated
steel (STS) plate. Additional tests in 1943 showed certain structural defects in the
system. Changes were made to BB65 and BB66 that would have improved system performance by
as much as 20 percent, but unfortunately, neither ship was ever completed.
The USS Missouri under construction. Her sleek external
provide no evidence of the massive armor belts that lie underneath.