Assault and Battery

During chapter 8 part 2, we are told offhand that "Cochrane’s ironclads and mortar ships opened yet another bombardment of Portland’s battered forts" (on August 20)

The image this conveys is of the forts stoutly resisting bombardment that has been going on since June (when the landing took place) without being sufficiently reduced to allow the British into the harbour itself.
In fact, this would be very difficult indeed.




The defenders


We have exact details for the forts defending Portland, because TFSmith states them directly - they are Fort Preble and Fort Scammell, forts constructed decades prior and upgraded as far as possible, along with Fort Gorges (recently constructed); all three have been extensively armed since December (when they were all but unarmed). TFSmith states that they have been improved with "additional earthworks and batteries" but his list of guns present would not suffice to fill the existing forts so we can assume the forts are fundamentally the same as historical.

Forts Preble, Gorges and Scammell were fairly typical of US forts of the period. Preble was and is a Second System fort, a star fort predating the War of 1812; Scammell about the same age, but circular, and modernized to some extent by extending the walls in the Third System period. Fort Gorges was new construction, with vertical sides and constructed of masonry (granite) - no earthworks were planned, and earthworks were not considered standard for American forts of the time.
The armament is, in BROS:

By June, Preble had been fitted with 36 guns (16 8 inch, 17 32 pounder, 3 24 pounder, 2 12 pounder, 4 6 pounder, and 4 mortars), while Scammell had 59 pieces (14 8 inch guns, 14 8 inch howitzers, 23 32 pounders, 8 24 pounders, 5 12 pounders, and 2 6 pounders), and Fort Gorges had emplaced its first battery, of six brand new 8 inch rifles.

Forts Preble and Scammell cover the main (but not only) viable channel into Portland Harbor; one face of Fort Gorges does the same.
All the guns are in barbettes or uncovered first tier casemates, meaning none of them have overhead protection (making them vulnerable to mortar fire or other long range bombardment) - the same would also be true of any additional earthworks.

Discounting guns of the field gun size, the number of guns present for the Union is (16+14+14+6) = 50 heavy guns and (17+3+23+8) = 51 medium guns, with a few mortars.

There is a main channel which the big ironclads (Warrior and Defence) can follow, where - anchoring 600 yards away - they would be under the fire of the original round brick face of Fort Scammell, the south battery of Fort Preble, and the circular battery of Fort Preble's star fort.
There is also a secondary channel to the north which bypasses Preble and Scammell and would allow shallow draft gunboats or ironclads (such as Thunderbolt and Terror) to attack Fort Gorges directly - none of the faces that would stop ships coming in through this channel have six or more casemates, so the "first battery" at Fort Gorges cannot stop them. Fort Gorges is naked to ships coming through the north channel - but we will discount this.


The number of guns at Scammell which can bear on the main channel is 22; at Fort Preble perhaps 27 guns can bear, and the six in Gorges are presumably able to bear as well. This puts the number of guns able to bear on the assumed bombardment position at 55, of which perhaps half will be "heavy".



The attackers


TFSmith does not include the conventional wooden ships in his list of attackers, so nor shall we.

The two mortar frigates each have two 13 inch mortars, along with ten other guns; for now the mortars are the important part, and mainly because of the point they make about earthworks. Since guns on earthworks cannot be in casemates and must be en barbette, if any of the guns in Preble, Scammell or Gorges were in earthworks instead of the masonry of the main forts then they would be vulnerable to mortar fire. This is one reason for the assumption about masonry.

The ironclads present are the Warrior, Defence, Thunderbolt, and Terror.

Thunderbolt and Terror are ironclads built in the late 1850s, each with 16 guns which could be used to produce a broadside of ten to twelve guns with proper ballasting (as there were more gun ports than guns). Their armour was 4" rolled wrought iron, thinly backed, which resisted piercing but produced spalling when tested against high velocity 68 pounder smoothbore fire. They could perhaps be penetrated by the 8" Parrott rifles at Fort Gorges, but only at relatively close range.
Historically they carried 68 pounder high velocity smoothbores, but they could instead be armed with 110 pounder Armstrong rifles (which were somewhat lighter).

Defence and Warrior are ironclads built more recently, both armed with a mixture of 68 pounder smoothbores and 110 pounder rifles; both ships also had a few smaller 40 pounder rifles. Between them, the two ironclads had on each broadside eighteen 68 pounder smoothbores, nine 110 pounder rifles and four 40 pounder rifles.
Their armour was 4.5" rolled wrought iron, thickly backed, which resisted both piercing and spalling. None of the Union guns listed could penetrate them at any range greater than a few hundred yards (the 8" Parrott rifles could just about penetrate, and they were effectively immune to the rest).

The combined broadside aimed at the forts, assuming five rifles and five smoothbores per 1850s ironclad, would be roughly:
28 68 pounder smoothbore
19 110 pounder rifle
4 40 pounder rifle

for a total of 51 guns, 47 of them "heavy".


The calculation


Before we go on, it should be pointed out that in the Age of Sail there was a formula for the effectiveness of ships versus forts - it was that three guns afloat were required to match one gun ashore, because of the way in which the sailing ship attackers would be unable to manoeuvre as they wanted.
In the age of wooden steam ships, the formula was altered to allow for the major paradigm shifts brought about by shell-firing guns and steam power. Now one gun afloat was worth one ashore.

The ironclad upset these calculations again, but if it appears as though the "steam formula" should show a draw or very narrow British defeat - remember, the British also have four ships of the line present, each of them with about a 45-gun broadside.

Instead of pointing this out, however, we shall go back to looking at things as though the ironclads alone are engaging.




The walls



We happen to have good information on the penetration into brickwork and masonry of the Armstrong guns (the 40 pounder and 110 pounder).
https://books.google.be/books?id=EP...e&q=armstrong gun penetration masonry&f=false
This shows that, at a range of 1,000 yards, the Armstrong guns could penetrate about four feet into a brickwork and masonry Martello tower with shells - in granite masonry, this would mean penetration of over two feet and would do serious damage to the walls of American masonry forts (which tended to have a thickness of six feet). A few hits to the solid front of the casemate would collapse and destroy the casemate.



The other examples


There are a few examples from the real Civil War of what happened when Third System forts ran into modern rifles. At Fort Pulaski, for example, it took the concentrated effort of 36 guns of all calibers (including ten rifles: two 84 pounder, two 64 pounder, one 48 pounder and five 30 pounders) less than two days to chew the fort to pieces and induce a surrender, despite all pre-bombardment assumptions that the fort would hold out indefinitely. The 30 pounders had a bursting charge of 1lb 8oz; even the smaller British rifle (the 40 pounder) had a bursting charge with common shell of 2lb 4oz, and the larger 110 pounder had a bursting charge of 7lb 10oz.
In total weight of rifle shell fired, each broadside by the British ships in Maine would produce one ton (2,250 lbs) of rifle shells, while each volley from the Union guns at Fort Pulaski would produce less than a quarter of this (494 lb); the British guns are also breech loaders, so would be able to be fired faster.

The Union siege artillery at Pulaski, counting the smoothbores, consisted of 12 mortars, 10 rifles and 14 smoothbores with a total weight of 4,138 lbs of shell; the British bombarding squadron here consists of four mortars, 23 rifles and 28 smoothbores with a total weight of 4,941 lbs of shell - of which a greater quantity is heavy rifle shells.




The likely outcome

It is hard to see how Forts Preble and Scammell could withstand more than a single bombardment from the British ironclads. Neither fort has a weapon able to damage Defence or Warrior, and the guns carried by the British ironclads could easily shatter the walls at 1,000 yards range (let alone 600 yards).
In short, the four ironclads and two mortar frigates TFSmith has repeatedly bombard Portland's forts for two months should have finished the job on the first or second day, rendering their targets unable to further resist by doing several times the damage to them that it took to disable Fort Pulaski. (Incisive readers may have noticed that Cochrane's ironclads could have fought their way through the ports during the sixteen-hour-long day they first arrived at Portland, and wonder why they didn't bother; the answer, of course, is that this sort of thing is what the Union does.)



The explanation

Of course, to TFSmith none of this can be allowed. It is too much like what "grognards" do, which is to say to focus on details - the Union will win in the abstract, so any details which conflict with this are unimportant, and so we get the result of a Royal Navy turning up at a port to fight their way inside and then not bothering to fire more than a few desultory cannonballs for nearly three months.

Incidentally, the British never do take Portland. That would apparently be effort.