Armor is actually a system of components that contribute to
the overall physical protection of the ship.
Since this blog abhors uninformed opinions, let’s examine one aspect of
ship armor and see if we can’t educate ourselves.
We’ve already discussed the general purpose of armor (see,
“Armor for Dummies”) and noted that its purpose is not to provide total
immunity to every weapon past, present, or future. While armor often is designed to provide a
degree of immunity to a specific set of weapons under a specific set of
conditions, the larger purpose is to mitigate damage. That missile/shell that hit your ship doesn’t
have to mean a sunk ship or instant mission kill if you can mitigate the
damage and that’s what armor does: it mitigates damage.
I am not an armor expert, by any means, but it is important
to grasp the basic concepts. To that
end, let’s take a look at the use and function of transverse armored
bulkheads. Transverse bulkheads are
simply ‘walls’ that span the ship from one side to the other. They go across the ship (transverse) as
opposed to running the length of the ship (longitudinal).
The term ‘armored’, in this case, refers to the totality of
the thickness of the bulkhead, the type of steel used, the structural design of
the bulkhead, and the conceptual armoring design of the bulkhead.
It should also be noted that commercial and naval transverse
bulkhead construction and governing rules may not be the same although there is
a great deal of overlap. Commercial
bulkhead standards are readily available on the Internet, for those interested.
Friedman’s book on U.S. cruiser design provides an excellent
description of the development of armor within the Des Moines class cruiser (see, “Des Moines Class Cruiser”) and,
specifically, the use of armored bulkheads:
There
could be no hope of stopping very heavy guided bombs with deck armor, but if
the ship were divided with heavy transverse armored bulkheads within the
armored box, the fore-and-aft extent of damage would be limited. The available tonnage would buy four 2.5-in
STS [Special Treatment Steel] bulkheads, which would stop even the heavy nose
fragments of a 16-in shell and would certainly limit damage by even a 3,000-lb
armor-piercing bomb. Bulkheads within
the five armored zones so defined would probably be shattered by a large bomb,
but the ship would remain afloat, since her floodable length would exceed the
size of the compartment thus breached.
The
heavy bulkheads were set between the no. 1 and 2 turrets; the no. 2 turret and
the forward main machinery group; the two main machinery groups; and the after
main machinery group and the no. 3 turret.
As a consequence, the ship could be expected to survive one major
armor-piercing bomb hit without losing more than on turret or one main
machinery group (boilers and turbines), unless she suffered the mass detonation
of a magazine. That was unlikely,
because in the two relevant cases – the Boise, which suffered a shell hit, and
the Savannah, a bomb hit – severe fires in the magazines were extinguished by
flooding through holes in the shell plating before powder fires had built up sufficient
pressure to cause an explosion. The same
heavy bulkheads would localize the effect of a torpedo hit; they represented a
major advance in U.S. cruiser protection. (1)
We see, then, that the multiple transverse bulkheads
separate the ship into multiple ‘boxes’ assembled end to end. One can easily imagine that a single ‘box’,
having been breached by an explosion, may well flood but the adjoining boxes
will remain intact, thus preserving the ship’s overall integrity and buoyancy.
It should be noted that such transverse bulkheads were
generally not penetrated by doors, cable runs, etc. below the armor deck. Thus, they had no weak points for leakage in
the event of flooding beyond whatever holes might be physically torn in them by
an explosion. You’ll recall that it was
exactly this weakness and failure which lead to the recent sinking of the
Norwegian frigate whose compartments had multiple penetrations which turned out
not to be watertight.
The passages above vividly describe the main function of
armor which is to mitigate damage. Too
many observers mistakenly believe that if armor cannot provide total immunity
to every known or future weapon then armor has no value. As we see from the description, the value of
armor lies as much, or more, in mitigating damage, ensuring ship survival, and
keeping the ship in the fight as it does in providing immunity to attacking
weapons.
Of course, the armor does provide immunity to a certain
range of weapons, depending on the design and weight of the armor. Generally and loosely, ship armor was
intended to provide immunity to weapons equivalent to its own under a defined
set of conditions: for example, a
battleship was designed to be immune to an enemy battleship’s guns, a cruiser
was designed to be immune to an enemy cruiser’s guns, and so on. The immunity was not total but depended on
range, angle of impact, location, etc.
Again, generally, an immune zone was established in the ‘center’ of the
ship where the critical magazines, guns, and machinery resided. Outlying areas, like the bow and stern, were
much less armored since they contained less critical equipment and functions.
The point of this post is not to debate the exact thickness
of some bulkhead or the exact location or number of welds in a bulkhead. In fact, there are many variations on the
general scheme. The point is to
understand that armor is more than just a giant piece of steel bolted onto the
side of a ship, as so many people believe.
Ship’s armor was a system of various components – the side armor belt
being one of them – that functioned together to prevent or mitigate
damage. Understanding the various
components and what each contributes to the overall armor protection scheme
allows us to see the purpose behind armor and why modern ships should also be
armored. Nothing has fundamentally
changed, over the years, about combat, battle damage, explosive weapons, or
ship survivability and yet we’ve completely abandoned armor. Does that make sense? We’re now sending mutli-billion dollar ships
into combat where a single hit will likely sink the ship and almost certainly
render it a mission kill, at least. Is
this really wise, given the cost and time required to replace a ship lost in
combat?
We should also note that transverse bulkheads are just one
component of the overall armor scheme.
Other components include bomb deck armor, main deck armor, void spaces,
belt armor, citadels, collapsible sections, etc. The overall system of armor is what made WWII
warships so resilient to battle damage.
It is long past time to re-incorporate armor into all of our
ship designs.
(1)“U.S. Cruisers, An Illustrated Design History”, Norman
Friedman, Naval Institute Press, 1984, ISBN 0-87021-718-6, p.360