Transverse Bulkheads

A sizable group of naval observers and commentators, as well as the Navy, have little or no concept of what ship armor is or does and yet they have very definite, usually negative, opinions about it.  Most people believe that armor is simply a giant piece of steel that is bolted onto the side of a ship and which has the incidental effect of slowing the ship down, if not sinking it under the weight of the armor.  In fact, I’ve seen comments stating that the presence of armor on a ship will just make the ship sink faster due to the weight!  Nothing could be further from the truth. 

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.

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(1)“U.S. Cruisers, An Illustrated Design History”, Norman Friedman, Naval Institute Press, 1984, ISBN 0-87021-718-6, p.360