Saturday, 27 June 2009
Wikipedia
I've decided that the wikipedia article for George McClellan is hopelessly biased and needs some updating. If anyone is able to provide a more balanced PoV please do. I'll start making modifications as I dig my books out.
Thursday, 18 June 2009
The effect of temperature on iron
This post covers a very odd, but utterly vital fact about iron armour. The source is Andrew Lambert's Warrior: Restoring the World's First Ironclad (London, 1987), a book a read many years ago and partially misremembered. However, while looking for a copy on the Handbook of Polymers in the Imperial College library the other day I realised it also housed the Science Museum Library, with a lot of source material on ironclads.
On page 70 of the aforementioned they discuss a series of tests conducted on a section of Warrior's hull removed when she was made into a jetty, but stored aboard. On testing something unexpected was observed, the armours resistance to impact is highly dependent on the temperature:
The observation that iron is less ductile and more brittle at lower temperatures is not a new one, but the degree to which protection changes is astounding. The armour offers twice the protection in the tropics than in the arctic. Thus Lambert concludes that the temperature, weather &c. all play a role, and this can explain the erratic results of armour tests.
My own observation is that it implies armour is probably more effective in a steaming warship (which is kicking out a lot of heat) than on the firing range.
In the ACW it implies that the ambient temperature of the armour is a major factor. The operating temperature in a Monitor turret was pretty high, so I guess the armour was near full strength. This bears further investigation.
On page 70 of the aforementioned they discuss a series of tests conducted on a section of Warrior's hull removed when she was made into a jetty, but stored aboard. On testing something unexpected was observed, the armours resistance to impact is highly dependent on the temperature:
Temp (degrees C): Absorbed Energy (j)
0: 11
15: 17
40: 24
100: 26
The observation that iron is less ductile and more brittle at lower temperatures is not a new one, but the degree to which protection changes is astounding. The armour offers twice the protection in the tropics than in the arctic. Thus Lambert concludes that the temperature, weather &c. all play a role, and this can explain the erratic results of armour tests.
My own observation is that it implies armour is probably more effective in a steaming warship (which is kicking out a lot of heat) than on the firing range.
In the ACW it implies that the ambient temperature of the armour is a major factor. The operating temperature in a Monitor turret was pretty high, so I guess the armour was near full strength. This bears further investigation.
Sunday, 14 June 2009
Monitors and other Ironclads
Monitor
The monitor type of warship is in some ways iconic of the American Civil War and its' "modernity" compared with Europeans, at least in a certain mindset. This legend of invulnerability came about after the Monitor herself met the Virginia at Hampton Roads and was largely unharmed. Monitor was hit 22 times, 2 to the pilothouse, 9 to the turret, 8 to the side armour and 3 times in the deck (ref), although another report says 23, and suggests far more serious damage than Stimers. It seems that Stimers was already exaggerating the prowess of the Monitor.
However, what Stimers &c. failed to appriciate was that Virginia had only steamed with 5 rounds of ball per gun, and so was firing shell, a much less penetrating projectile. Indeed Virginia's (which was pretty beaten up already, having lost 2 guns the previous day, see ORN) armament was only 2 7" (42pdr) Brooke RML, 2 6.4" (32pdr) Brooke RML and 6 9" Dahlgren shell guns. The 7" rifles were the chasers (ref, incidently, note the final correspondence concerning a proposed breakout of the Virginia and a naval attack on NY Harbor!). I have not seen any reference to which 2 guns were rendered unservicable the day before she confronted Monitor, I believe they were 9" Dahlgrens and have reference they were still fought even in their poor condition.
Tyson points this out in a chapter of The Battle of Hampton Roads. It seems Monitor had in fact been rendered hors de combat by the Virginia, which may explain why she refused to fight her on the 11th April or on 8th May, 1862. Such an action would likely have gone badly for the Monitor, as Virginia was fully repaired, rearmed and was carrying solid shot and wrought iron (armour piercing) bolts. Virginia continued her attack after Monitor retired for a while, but was afraid of approaching too close to Minnesota because of the shallows and Minnesota's much more powerful broadside (ref).
The next test of the Monitor came at Drewry's Bluff. Here Monitor could elevate her guns enough to fire on the forts and her main contribution was as a target. However, very little fire was directed at her, with only 3 hits reported, all 8" shot at extreme range which bent in the plates (1 on the turret, 2 on the side) (ref). With her was Galena, which was struck some 41 times with 13 penetrations (ref), but still managed to fire off her entire supply of ammunition against the fort and withdraw in good order (ref).
Monitor's capture of the small, grounded tugboat Teaser (4th July 1862) was her last action; she floundered off Cape Hatteras in December 1862.
Charleston
Looking to the combat experience of the monitors that followed the nameship, we come to the later battles. Of these, I wish to especially examine two examples, Charleston (7th April 1863) and Wassaw Sound. There are two other significant battles (aside from the continuing siege of Charleston), Mobile Bay and the attempted breakout of the James River in 1865. I shall not consider the inland rivers.
The attack at Charleston demonstrates just how vulnerable the monitors were. None approached closer than 900 yards from the nearest fort, and most stayed out beyond 1,000 yards from Fort Sumter, and were nearly a mile away from the supporting forts. European navies considered 800 yards "long range" and usually expected to close 200 yards before opening fire, and frequently got much closer (see the Battle of Lissa, this is the first wikipedia articles I wrote).
There were 69 guns engaged and ammunition expenditure of 2,209 rounds (ref). Most significant of these are obviously those of Fort Sumter, since it was both the closest, best armed and fired the most shots. It engaged with 2 7" Brooke RML (with wrought iron bolts), 4 8" and 4 10" Columbiads (by the report at least some Columbiads are old pattern shell guns), 6 42pdr rifles, 2 9" Dahlgrens, 8 32 pounders, 4 8" shell guns and 3 10" coastal mortars. These last weapons appear to have Martin Shells (see glossary here), indicating their ammunition was and least partially supplied by Britain. Most of this fire was at 1,300-1,400 yards, and only one ironclad ventured to close to 900 yards. New Ironsides stayed out at 1,700 yards. The Confederates had range markers out, so their estimates can be assumed to be accurate.
Fort Moultrie was around 1,400 yards from the enemy and engaged with 9 8" Columbiads, 5 32pdr rifles, 5 32pdrs and 2 10" mortars. At closest approach they were at 1,000 yards. At closest approach Battery Bee (5 10" and 1 8" Columbiads) was 1,400 yards and that of Battery Beauregard (3 32 pdr rifles and maybe some 8" Columbiads) was 1,600 yards at closest approach. The other batteries conducted at best a desultory fire and can be ignored.
A summary of the damage done to the ironclads is:
Passiac: 15 to the side (many near the waterline), 5 to the deck (one "very badly"), 10 to the turret (including a partial penetration, another which knocked out the 11" and another which jammed the turret), 2 to the pilot house. The commander reports the deck isn't strong enough to withstand the shot.
Weehawken: Hit 53+ times, including 36 rifle bolts embedded in the turret, an area near the waterline where all the armour had been blasted away exposing the wood, and a large hole in the deck which admitted water.
Montauk: 14 Hits, "no material damage". 4 hits on the side (1 "severe", having blasted off a section of the armour), 3 on turret, 1 on pilothouse, 3 on deck (none severe) and 3 on smokestack.
Patapsco: 47 hits, no further descriptions
Catskill: 20 hits, including one which hit the deck, smashed through both iron sheets, the backing and the iron stachion below, admitting water
Nantucket: 51 hits, 18 to turret (including one whch knocked out the 15"), the turret was jammed, 9 to side, 1 to pilothouse and 12 partial penetrations of the deck, amongst others
Nahant: 36 hits, turret jammed. 9 hits to turret, including a partial penetration, 6 to pilot house, others not mentioned
Keokuk: 90 hits
New Ironsides: over 95 hits, including multiple penetrations to her less well protected ends (ref)
None of this bodes well to stand 200 yards off a European ironclad or even a heavy wooden warship. At 200 yards from Warrior, for example, a Monitor is under fire from 6 7" Rifles and 13 68pdrs. As a comparitor*, lets check the powder charges of the guns above vs these:
Warrior
110 pdr Armstrong: 14 lb service charge
68 pdr: 16 lb service charge
Forts
10" Columbiad: 18 lbs
8" Columbiad: 10 lbs
7" Brooke Rifle: 10 lbs
42 pounder rifle: 8 lbs (ref)
32 pounder rifle: 7 lbs (ref)
32 pounder: 4.5-6 lbs, depending on model
It can thus be seen that the only guns mounted on the forts perhaps more powerful than those on Warrior were the 10" Columbiads, and potentially the 7" Brookes (some of which had wrought iron bolts). However, penetrating an extra 800 yards of air will have significantly slowed down the projectiles.
The forts rate of fire is slow, Warriors guns would fire every 2-3 minutes in action. At 200 yards the rate of hitting will be much greater than the forts 19%. Thus Warriors broadside represents much more firepower than the forts (perhaps x5 for the extra accuracy and ROF, and x2-3 for the number of heavy guns), perhaps an order of magnitude greater. If this figure were accepted then the number of hits would be equivalent standing off Warrior for 4.5 minutes, and at shorter range the hits would be far more devastating.
Wassaw Sound
The other example I wish to examine is Wassaw Sound. Here we have CSS Atlanta knocked out by just 3 shots (the 4th and 5th were fired after Atlanta struck), the vessel being grounded and unable to move or fight her own guns. The shots were 3x 15" cored shot fired with 30 lbs charge and 2x 11" solid shot (charge unknown). The 11" did not penetrate Altanta's 2 layers of 2" iron, indeed one struck a shutter but did not penetrate (but did break it). The 1st 15", fired at 3-400 yards, achieved a partial penetration, blasting spalling back into gundeck and wounding upto a dozen men. The 2nd 15" blasted the top off the pilothouse, wounding almost everyone within, whilst the 3rd 15" missed (ref)(ref).
There is the question of what quality Atlantas' iron was, it does appear to have been railroad iron (ref). This would make her protection much worse than Virginias', which had Tredegar rolled iron. Making matters worse, her armour was not the full 4", put in places was only 2" (ref). It seems that the Atlanta's iron armour bent in, and the wooden backing (16" thick) splintered and caused the casualties.
Why then did the Atlanta strike? Essentially because she was helpless, she had, by grounding and then by the destruction of the helm, became completely unable to maneouvre and couldn't aim her guns properly (none of her 7 shots hit).
Warrior and Wooden Vessels
Now to turn this against a Warrior target. In this case the Warrior is a much better protected vessel, her 4.5" of single thickness high quality rolled iron, backed by 18" of teak on a 5/8ths" iron hull was found to resist solid shot from a 15" Rodman fired with 50 lbs British powder** at 200 yards, but did penetrate at 100 yards. If an armour piercing shot was used (a palliser bolt?) then the penetration range increased to 500 yards.
Thus in a matchup between the Warrior and a monitor with 15" guns the Warrior is going to win. Her fire, even if not smashing through and destroying the ship in one blow, will render the monitor unfightable in a matter of minutes. The monitor meanwhile is likely to over get 1 or 2 shots from it's 15", and even if they penetrate the worst they will do is knock out a single gun. It should be considered that the iron will be broken with multiple hits and penetrations will eventually occur if the fight continues long enough (ref).
Now, the question that begs my mind is just how well would a monitor stand up to a wooden battleship or frigate. At least the monitor can fire shell, which is likely to be more effective. However, the concentrated firepower of a 2 decker or a frigate is likely to quickly make a monitor unfightable, even if her vitals are intact. Worse, the relatively plunging fire is likely to smash into the thin deck armour of the monitor. Monitor's decks were thin enough even 32 pounders are likely to start making holes and letting in water.
The effect of shellfire is well known, the RN (plus the French and Ottomans) had taken their wooden steamships under the fire of several Russian forts in the previous decade and sustained hundreds of hits from shells and hotshot without losing a ship. However, an argument has been made that 15" shell would be much more effective than smaller shells. However, the bursting charges of Dahlgren guns are:
15": 13 lbs
11": 6 lbs
10": 4 lbs
9": 3 lbs
8": 1.85 lbs
So the 15" only has just over 4 times the bursting charge of a 9" shell. Explosive power will attenuate with the root of the charge so:
15": 2.1
11": 1.4
10": 1.2
9": 1 (by definition)
8": 0.8
So the shellfire of a monitor with a 15" and 11" is about as destructive as 3.5 9" guns (the normal US broadside piece). Since wooden warships could stand prolonged periods under such fire I would doubt the ability of a monitor to sink a wooden warship quickly. Consider just how long it took Virginia to do her work; Cumberland was sunk by ramming, then Congress ran aground and was only hit twice by the Virginia before striking (ref). Unable to take her a prize, the confederates put several hotshot in her. It was the hotshot that was truly destructive, rather than shell (a munition monitors can't use) (ref). The report of the Minnesota (which received the lions share of Virginia's fire) show that the exploding shells were not really that devastating at all (ref).
Conclusions
My general conclusion is that the ironclads weren't nearly as invulnerable as some think, nor where wooden ships as vulnerable as some think. After all, the 30" of white oak covering the sides of the steam battleship are, in fact, armour; they are simply a different and less effective form of armour.
The monitors were gunboats, and as such lacked the firepower to meet heavy warships. It should be noted that most of their combats were against other gunboats, and hence there is perhaps some degree of comparison. However, against a heavy warship their large shells are not particularly more effective than the smaller shells of broadside vessels (and counting the lowered rate of fire for the turret, they are actually putting less bursting charge per gun per minute into the enemy). The hail of fire from a broadside vessel will rapidly jam the turret, knock in the shutters, destroy the smokestack and cause water to enter via damage to the weaker deck and sides even if the turret remains unpenetrated.
Thus, in my opinion, the monitor class of vessels is not even a match for a steam frigate, let alone a Warrior.
Notes
* A better comparitor is charge divided by circumference of the round (in the following I shall not divide through by pi, but just charge/d). In this both the British guns score 2, the 10" Columbiad scores 1.8, the 7" Brooke 1.4, the 42 pdr rifle 1.1, the 32 pdr rifle 1.0, and the 32 pdr smoothbore 0.9. However, this certainly overstates the Armstrong (which had a leaky breach) and the Columbiad (which was known to belch forth unburnt powder at higher charges). The 15" at Wassaw Sound beat the British guns by this measure (2.3), whilst the 11" are much less powerful (1.4).
For a long time I did not understand why the modellers of the time used circumference, but I now know it is due to modelling the penetration was a sheering event around the circumference of the penetrating round.
** Due to the higher burning rate, 50 lbs of British powder was reckoned to be equivalent to 60 lbs of Mammoth powder. Since Mammoth powder was notorious for not being all burnt this is probably correct, only when weapons of 35 calibres were adopted in the late 1870's did Mammoth powder actually increase gunpower.
The monitor type of warship is in some ways iconic of the American Civil War and its' "modernity" compared with Europeans, at least in a certain mindset. This legend of invulnerability came about after the Monitor herself met the Virginia at Hampton Roads and was largely unharmed. Monitor was hit 22 times, 2 to the pilothouse, 9 to the turret, 8 to the side armour and 3 times in the deck (ref), although another report says 23, and suggests far more serious damage than Stimers. It seems that Stimers was already exaggerating the prowess of the Monitor.
However, what Stimers &c. failed to appriciate was that Virginia had only steamed with 5 rounds of ball per gun, and so was firing shell, a much less penetrating projectile. Indeed Virginia's (which was pretty beaten up already, having lost 2 guns the previous day, see ORN) armament was only 2 7" (42pdr) Brooke RML, 2 6.4" (32pdr) Brooke RML and 6 9" Dahlgren shell guns. The 7" rifles were the chasers (ref, incidently, note the final correspondence concerning a proposed breakout of the Virginia and a naval attack on NY Harbor!). I have not seen any reference to which 2 guns were rendered unservicable the day before she confronted Monitor, I believe they were 9" Dahlgrens and have reference they were still fought even in their poor condition.
Tyson points this out in a chapter of The Battle of Hampton Roads. It seems Monitor had in fact been rendered hors de combat by the Virginia, which may explain why she refused to fight her on the 11th April or on 8th May, 1862. Such an action would likely have gone badly for the Monitor, as Virginia was fully repaired, rearmed and was carrying solid shot and wrought iron (armour piercing) bolts. Virginia continued her attack after Monitor retired for a while, but was afraid of approaching too close to Minnesota because of the shallows and Minnesota's much more powerful broadside (ref).
The next test of the Monitor came at Drewry's Bluff. Here Monitor could elevate her guns enough to fire on the forts and her main contribution was as a target. However, very little fire was directed at her, with only 3 hits reported, all 8" shot at extreme range which bent in the plates (1 on the turret, 2 on the side) (ref). With her was Galena, which was struck some 41 times with 13 penetrations (ref), but still managed to fire off her entire supply of ammunition against the fort and withdraw in good order (ref).
Monitor's capture of the small, grounded tugboat Teaser (4th July 1862) was her last action; she floundered off Cape Hatteras in December 1862.
Charleston
Looking to the combat experience of the monitors that followed the nameship, we come to the later battles. Of these, I wish to especially examine two examples, Charleston (7th April 1863) and Wassaw Sound. There are two other significant battles (aside from the continuing siege of Charleston), Mobile Bay and the attempted breakout of the James River in 1865. I shall not consider the inland rivers.
The attack at Charleston demonstrates just how vulnerable the monitors were. None approached closer than 900 yards from the nearest fort, and most stayed out beyond 1,000 yards from Fort Sumter, and were nearly a mile away from the supporting forts. European navies considered 800 yards "long range" and usually expected to close 200 yards before opening fire, and frequently got much closer (see the Battle of Lissa, this is the first wikipedia articles I wrote).
There were 69 guns engaged and ammunition expenditure of 2,209 rounds (ref). Most significant of these are obviously those of Fort Sumter, since it was both the closest, best armed and fired the most shots. It engaged with 2 7" Brooke RML (with wrought iron bolts), 4 8" and 4 10" Columbiads (by the report at least some Columbiads are old pattern shell guns), 6 42pdr rifles, 2 9" Dahlgrens, 8 32 pounders, 4 8" shell guns and 3 10" coastal mortars. These last weapons appear to have Martin Shells (see glossary here), indicating their ammunition was and least partially supplied by Britain. Most of this fire was at 1,300-1,400 yards, and only one ironclad ventured to close to 900 yards. New Ironsides stayed out at 1,700 yards. The Confederates had range markers out, so their estimates can be assumed to be accurate.
Fort Moultrie was around 1,400 yards from the enemy and engaged with 9 8" Columbiads, 5 32pdr rifles, 5 32pdrs and 2 10" mortars. At closest approach they were at 1,000 yards. At closest approach Battery Bee (5 10" and 1 8" Columbiads) was 1,400 yards and that of Battery Beauregard (3 32 pdr rifles and maybe some 8" Columbiads) was 1,600 yards at closest approach. The other batteries conducted at best a desultory fire and can be ignored.
A summary of the damage done to the ironclads is:
Passiac: 15 to the side (many near the waterline), 5 to the deck (one "very badly"), 10 to the turret (including a partial penetration, another which knocked out the 11" and another which jammed the turret), 2 to the pilot house. The commander reports the deck isn't strong enough to withstand the shot.
Weehawken: Hit 53+ times, including 36 rifle bolts embedded in the turret, an area near the waterline where all the armour had been blasted away exposing the wood, and a large hole in the deck which admitted water.
Montauk: 14 Hits, "no material damage". 4 hits on the side (1 "severe", having blasted off a section of the armour), 3 on turret, 1 on pilothouse, 3 on deck (none severe) and 3 on smokestack.
Patapsco: 47 hits, no further descriptions
Catskill: 20 hits, including one which hit the deck, smashed through both iron sheets, the backing and the iron stachion below, admitting water
Nantucket: 51 hits, 18 to turret (including one whch knocked out the 15"), the turret was jammed, 9 to side, 1 to pilothouse and 12 partial penetrations of the deck, amongst others
Nahant: 36 hits, turret jammed. 9 hits to turret, including a partial penetration, 6 to pilot house, others not mentioned
Keokuk: 90 hits
New Ironsides: over 95 hits, including multiple penetrations to her less well protected ends (ref)
None of this bodes well to stand 200 yards off a European ironclad or even a heavy wooden warship. At 200 yards from Warrior, for example, a Monitor is under fire from 6 7" Rifles and 13 68pdrs. As a comparitor*, lets check the powder charges of the guns above vs these:
Warrior
110 pdr Armstrong: 14 lb service charge
68 pdr: 16 lb service charge
Forts
10" Columbiad: 18 lbs
8" Columbiad: 10 lbs
7" Brooke Rifle: 10 lbs
42 pounder rifle: 8 lbs (ref)
32 pounder rifle: 7 lbs (ref)
32 pounder: 4.5-6 lbs, depending on model
It can thus be seen that the only guns mounted on the forts perhaps more powerful than those on Warrior were the 10" Columbiads, and potentially the 7" Brookes (some of which had wrought iron bolts). However, penetrating an extra 800 yards of air will have significantly slowed down the projectiles.
The forts rate of fire is slow, Warriors guns would fire every 2-3 minutes in action. At 200 yards the rate of hitting will be much greater than the forts 19%. Thus Warriors broadside represents much more firepower than the forts (perhaps x5 for the extra accuracy and ROF, and x2-3 for the number of heavy guns), perhaps an order of magnitude greater. If this figure were accepted then the number of hits would be equivalent standing off Warrior for 4.5 minutes, and at shorter range the hits would be far more devastating.
Wassaw Sound
The other example I wish to examine is Wassaw Sound. Here we have CSS Atlanta knocked out by just 3 shots (the 4th and 5th were fired after Atlanta struck), the vessel being grounded and unable to move or fight her own guns. The shots were 3x 15" cored shot fired with 30 lbs charge and 2x 11" solid shot (charge unknown). The 11" did not penetrate Altanta's 2 layers of 2" iron, indeed one struck a shutter but did not penetrate (but did break it). The 1st 15", fired at 3-400 yards, achieved a partial penetration, blasting spalling back into gundeck and wounding upto a dozen men. The 2nd 15" blasted the top off the pilothouse, wounding almost everyone within, whilst the 3rd 15" missed (ref)(ref).
There is the question of what quality Atlantas' iron was, it does appear to have been railroad iron (ref). This would make her protection much worse than Virginias', which had Tredegar rolled iron. Making matters worse, her armour was not the full 4", put in places was only 2" (ref). It seems that the Atlanta's iron armour bent in, and the wooden backing (16" thick) splintered and caused the casualties.
Why then did the Atlanta strike? Essentially because she was helpless, she had, by grounding and then by the destruction of the helm, became completely unable to maneouvre and couldn't aim her guns properly (none of her 7 shots hit).
Warrior and Wooden Vessels
Now to turn this against a Warrior target. In this case the Warrior is a much better protected vessel, her 4.5" of single thickness high quality rolled iron, backed by 18" of teak on a 5/8ths" iron hull was found to resist solid shot from a 15" Rodman fired with 50 lbs British powder** at 200 yards, but did penetrate at 100 yards. If an armour piercing shot was used (a palliser bolt?) then the penetration range increased to 500 yards.
Thus in a matchup between the Warrior and a monitor with 15" guns the Warrior is going to win. Her fire, even if not smashing through and destroying the ship in one blow, will render the monitor unfightable in a matter of minutes. The monitor meanwhile is likely to over get 1 or 2 shots from it's 15", and even if they penetrate the worst they will do is knock out a single gun. It should be considered that the iron will be broken with multiple hits and penetrations will eventually occur if the fight continues long enough (ref).
Now, the question that begs my mind is just how well would a monitor stand up to a wooden battleship or frigate. At least the monitor can fire shell, which is likely to be more effective. However, the concentrated firepower of a 2 decker or a frigate is likely to quickly make a monitor unfightable, even if her vitals are intact. Worse, the relatively plunging fire is likely to smash into the thin deck armour of the monitor. Monitor's decks were thin enough even 32 pounders are likely to start making holes and letting in water.
The effect of shellfire is well known, the RN (plus the French and Ottomans) had taken their wooden steamships under the fire of several Russian forts in the previous decade and sustained hundreds of hits from shells and hotshot without losing a ship. However, an argument has been made that 15" shell would be much more effective than smaller shells. However, the bursting charges of Dahlgren guns are:
15": 13 lbs
11": 6 lbs
10": 4 lbs
9": 3 lbs
8": 1.85 lbs
So the 15" only has just over 4 times the bursting charge of a 9" shell. Explosive power will attenuate with the root of the charge so:
15": 2.1
11": 1.4
10": 1.2
9": 1 (by definition)
8": 0.8
So the shellfire of a monitor with a 15" and 11" is about as destructive as 3.5 9" guns (the normal US broadside piece). Since wooden warships could stand prolonged periods under such fire I would doubt the ability of a monitor to sink a wooden warship quickly. Consider just how long it took Virginia to do her work; Cumberland was sunk by ramming, then Congress ran aground and was only hit twice by the Virginia before striking (ref). Unable to take her a prize, the confederates put several hotshot in her. It was the hotshot that was truly destructive, rather than shell (a munition monitors can't use) (ref). The report of the Minnesota (which received the lions share of Virginia's fire) show that the exploding shells were not really that devastating at all (ref).
Conclusions
My general conclusion is that the ironclads weren't nearly as invulnerable as some think, nor where wooden ships as vulnerable as some think. After all, the 30" of white oak covering the sides of the steam battleship are, in fact, armour; they are simply a different and less effective form of armour.
The monitors were gunboats, and as such lacked the firepower to meet heavy warships. It should be noted that most of their combats were against other gunboats, and hence there is perhaps some degree of comparison. However, against a heavy warship their large shells are not particularly more effective than the smaller shells of broadside vessels (and counting the lowered rate of fire for the turret, they are actually putting less bursting charge per gun per minute into the enemy). The hail of fire from a broadside vessel will rapidly jam the turret, knock in the shutters, destroy the smokestack and cause water to enter via damage to the weaker deck and sides even if the turret remains unpenetrated.
Thus, in my opinion, the monitor class of vessels is not even a match for a steam frigate, let alone a Warrior.
Notes
* A better comparitor is charge divided by circumference of the round (in the following I shall not divide through by pi, but just charge/d). In this both the British guns score 2, the 10" Columbiad scores 1.8, the 7" Brooke 1.4, the 42 pdr rifle 1.1, the 32 pdr rifle 1.0, and the 32 pdr smoothbore 0.9. However, this certainly overstates the Armstrong (which had a leaky breach) and the Columbiad (which was known to belch forth unburnt powder at higher charges). The 15" at Wassaw Sound beat the British guns by this measure (2.3), whilst the 11" are much less powerful (1.4).
For a long time I did not understand why the modellers of the time used circumference, but I now know it is due to modelling the penetration was a sheering event around the circumference of the penetrating round.
** Due to the higher burning rate, 50 lbs of British powder was reckoned to be equivalent to 60 lbs of Mammoth powder. Since Mammoth powder was notorious for not being all burnt this is probably correct, only when weapons of 35 calibres were adopted in the late 1870's did Mammoth powder actually increase gunpower.
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