[Historical] A (very) brief look at ironclads

[Vespers War]

This past weekend was the anniversary of the Battle of Hampton Roads, where the USS Monitor and CSS Virginia faced off. Notoriously, both were unable to inflict serious damage on the other, and I decided to look at whether that was reasonable when statting the two ships using Fire, Fusion & Steel.

The CSS Virginia had a casemate of 4" of iron over 24" of wood, angled at 36 degrees. This works out to an approximate armor value of 41. Its hull was much weaker, with 2.5" of iron over the 24" of wood, which is only armor 16.

For armament, the ship carried a pair of 7" double-banded Brooke Rifles, a pair of 6.4" Brooke rifles, and six 9" Dahlgren guns. The Dahlgrens get maximum penetration firing explosive shell, at 16C, while the Brookes are better with solid shot, at 15/13/11/7 for the 7" and 12/10/9/6 for the 6.4". The maximum any gun can achieve is 28, if a Dahlgren rolls double sixes for penetration. While Brooke did work on a bolt specifically for penetrating armor, it wasn't available at Hampton Roads because he expected only wooden ships to be in the blockade and thus emphasized explosive shell (11C for the 7" and 9C for the 6.4"). At Drewry Bluff later in the war, Brooke rifles nearly penetrated the turret armor of the Monitor

The USS Monitor uses only iron for its turret, with 8 slabs of 1" armor constituting an armor value of 30. The hull has less iron, only 5", but it sits over 30" of wood, which means the hull is actually better-armored at 34. With the Virginia unable to achieve a roll over 28, the Monitor is effectively invulnerable.

However, the cheesebox on a raft has just a pair of 11" Dahlgren smoothbores. Like the 9" on its opponent, they function best with explosive shell, at 21C, meaning the maximum penetration is 33. It can't penetrate the Virginia's casemate, but it can penetrate the hull, although only by enough to cause minor damage (maximum is +17, which is 2 minor damage rolls). The design of both ships, minimizing the hull's visibility and dropping the main armor belt below the waterline, prevented the Monitor from striking the more vulnerable portions of the Virginia.

The What-If in this scenario is what happens if the Monitor had been equipped with the originally-planned 15" Dahlgrens. While their shot was actually no more powerful than the 11" due to a short barrel, their shell works out to a 31C, so a very lucky hit (11 or 12 on 2d6) could have damaged the casemate of the Virginia and it could easily cause major damage to the hull if it struck there. Conversely, penetrating the Monitor's turret armor would have required an 11" or 15" Dahlgren or a 150-pdr (8") Parrott Rifle, none of which were commonly available to the Confederacy. Only the 15" Dahlgren stood a chance of penetrating its hull.

[swaghauler]

This past weekend was the anniversary of the Battle of Hampton Roads, where the USS Monitor and CSS Virginia faced off. Notoriously, both were unable to inflict serious damage on the other, and I decided to look at whether that was reasonable when statting the two ships using Fire, Fusion & Steel.

The CSS Virginia had a casemate of 4" of iron over 24" of wood, angled at 36 degrees. This works out to an approximate armor value of 41. Its hull was much weaker, with 2.5" of iron over the 24" of wood, which is only armor 16.

For armament, the ship carried a pair of 7" double-banded Brooke Rifles, a pair of 6.4" Brooke rifles, and six 9" Dahlgren guns. The Dahlgrens get maximum penetration firing explosive shell, at 16C, while the Brookes are better with solid shot, at 15/13/11/7 for the 7" and 12/10/9/6 for the 6.4". The maximum any gun can achieve is 28, if a Dahlgren rolls double sixes for penetration. While Brooke did work on a bolt specifically for penetrating armor, it wasn't available at Hampton Roads because he expected only wooden ships to be in the blockade and thus emphasized explosive shell (11C for the 7" and 9C for the 6.4"). At Drewry Bluff later in the war, Brooke rifles nearly penetrated the turret armor of the Monitor

The USS Monitor uses only iron for its turret, with 8 slabs of 1" armor constituting an armor value of 30. The hull has less iron, only 5", but it sits over 30" of wood, which means the hull is actually better-armored at 34. With the Virginia unable to achieve a roll over 28, the Monitor is effectively invulnerable.

However, the cheesebox on a raft has just a pair of 11" Dahlgren smoothbores. Like the 9" on its opponent, they function best with explosive shell, at 21C, meaning the maximum penetration is 33. It can't penetrate the Virginia's casemate, but it can penetrate the hull, although only by enough to cause minor damage (maximum is +17, which is 2 minor damage rolls). The design of both ships, minimizing the hull's visibility and dropping the main armor belt below the waterline, prevented the Monitor from striking the more vulnerable portions of the Virginia.

The What-If in this scenario is what happens if the Monitor had been equipped with the originally-planned 15" Dahlgrens. While their shot was actually no more powerful than the 11" due to a short barrel, their shell works out to a 31C, so a very lucky hit (11 or 12 on 2d6) could have damaged the casemate of the Virginia and it could easily cause major damage to the hull if it struck there. Conversely, penetrating the Monitor's turret armor would have required an 11" or 15" Dahlgren or a 150-pdr (8") Parrott Rifle, none of which were commonly available to the Confederacy. Only the 15" Dahlgren stood a chance of penetrating its hull.

An added "what if" is... "What if the Captain had taken the Gun manufacturer's advice?" The Navy was told that those Dahlgrens were made with a newer process and could withstand loading a "double charge" of powder to push the projectile faster. The Navy erred on the side of caution and never "double charged" the cannon during firing. A higher velocity shot might have penetrated the CSS Virginia's hull.

[Vespers War]

An added "what if" is... "What if the Captain had taken the Gun manufacturer's advice?" The Navy was told that those Dahlgrens were made with a newer process and could withstand loading a "double charge" of powder to push the projectile faster. The Navy erred on the side of caution and never "double charged" the cannon during firing. A higher velocity shot might have penetrated the CSS Virginia's hull.

That is a valid what-if, but one where I don't think it would have made a difference. Solid shot from a Dahlgren is surprisingly ineffective, largely because they're short (less than 12 calibers in length except for the 8" at 12.54 calibers). For any caliber from 9" to 15", Pen with a kinetic energy shot is 9/8/7/4. With Virginia's 41 armor on the casemate, you'd need at least Pen 30 to inflict minor damage on a 12, and I don't think doubling powder will lead to a 333% increase in Pen to get close range shots up to 30. With the cannon of the time, the only things with a realistic chance to damage the casemate are the 15" Dahlgren and the 15" or 20" Rodman (which were shore artillery only AFAIK), all firing shell. Black powder and iron guns just couldn't get the velocity for that kind of armor-piercing capability.

[pmulcahy11b]

An added "what if" is... "What if the Captain had taken the Gun manufacturer's advice?" The Navy was told that those Dahlgrens were made with a newer process and could withstand loading a "double charge" of powder to push the projectile faster. The Navy erred on the side of caution and never "double charged" the cannon during firing. A higher velocity shot might have penetrated the CSS Virginia's hull.

Don't forget the so-called "Fuzzy Wuzzy Fallacy" (there's a story behind that name; if you want to know, let me know). A double charge would not have led to twice the velocity, speed, penetration, etc. -- It would have given the double charge sqroot2 times the firepower. (Works for most military firepower calculations as well.)

[swaghauler]

Except that you guys are forgetting that Ericsson had information on the armor thickness of the CS Virginia (provided by a Union spy) and actually tested several cannon with both normal loads and higher pressure loads to determine IF they could penetrate Virginia's armor. The 11" guns were selected because they were tested with the higher pressure loading and actually penetrated the mockup of Virginia's armor (while surviving the test). The Navy (wisely in my opinion) chose NOT to use a double charge of powder as Ericsson suggested because the three successful tests were a very small "sample size," and a couple of the guns that were being tested blew up (injuring some of the testers) during the experiment.

[Vespers War]

Except that you guys are forgetting that Ericsson had information on the armor thickness of the CS Virginia (provided by a Union spy) and actually tested several cannon with both normal loads and higher pressure loads to determine IF they could penetrate Virginia's armor. The 11" guns were selected because they were tested with the higher pressure loading and actually penetrated the mockup of Virginia's armor (while surviving the test). The Navy (wisely in my opinion) chose NOT to use a double charge of powder as Ericsson suggested because the three successful tests were a very small "sample size," and a couple of the guns that were being tested blew up (injuring some of the testers) during the experiment.

And you're forgetting that Ericsson designed Monitor to carry 15" guns, but they weren't ready in time.

Edit: Also, Dahlgren's mockup of the Virginia only sloped the armor at 15 degrees (4% increase in effective thickness) when the actual casemate was sloped at 36 degrees (24% increase in effective thickness).