How effective were siege guns and gun boats compared to field artillery in the 19th century?

Is there something approaching a 1- to 1- correspondence between the value of siege artillery and gunboat guns compared to field artillery?

The problem with this sort of question is it's trying to add up a lot of different variables and come out with a "score". This would be like trying to determine who's food is better by assigning numbers to and adding up all the stuff in our refrigerators; doesn't make much sense.

Let's drag this back into reality by asking a more direct question: which guns would have been most useful to defend Pittsburg Landing against Confederate attack: siege, field or gunboat?

Let's have a look at this battle.

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The Union army is surprised on April 6th and over the course of two days falls back. Their lines were in a crescent shape with their flanks and rear protected by the Tennessee River and gunboats. The Confederate lines are in contact.

By eyeballing landmarks on Google Maps we can get a scale for the map. On the north/south scale, Snake Creek to Lick Creek is about 2.5 miles. For east/west, the Tennessee River to the road junction is about 2 miles. Thus the map depicts an area about 4 miles high by 4 miles wide. This means the lines are less than 1000 yards apart and the Union position has, at most, only two or three miles between them and the river for their artillery.

This means...

• There's not a lot of time for artillery to set up, dig in and sight.
• There's not a lot of space either.
• Artillery will be doing short range butcher's work.

Now, some notes on Civil War era artillery. Today we think of artillery as big guns firing at high arcs using complicated math from far behind the lines. And some Civil War artillery did that, especially siege guns, but Pittsburg Landing was a hasty battle fought at short range. Field artillery would often be direct fired like a normal gun: see that mass of enemy over there? Shoot them. Estimate range and wind, adjust sights, fire, observe fall of shot, correct, repeat.

While much Civil War artillery was smoothbore, many were rifles (especially Union). At the ranges involved (a mile) they could be fired quite accurately, even the smoothbores. Here's some contemporary accounts.

At 1600 yards [1500 m] the Whitworth gun fired 10 shots with a lateral deviation of only 5 inches.

Our guns were 12 pound brass Napoleons, smooth bore, but accounted the best gun for all round field service then made. They fired solid shot, shell, grape and canister, and were accurate at a mile.

The Yankee three-inch rifle was a dead shot at any distance under a mile. They could hit the end of a flour barrel more often than miss, unless the gunner got rattled.

Field artillery and gunboats had more than cannon balls. They were supplied with different kinds of ammo for different situations. Solid shot (ie. a cannonball) for battering fortifications and plowing through columns of infantry. Canister/grape is like a big shotgun: great for firing at a massed charge at a few hundred yards. For longer range work, explosive shells had a fuse which would explode and send pieces of the case flying. Case/shrapnel improved on the idea by adding more pieces and fused to burst in the air and rain down on the enemy (thus "bombs bursting in air"). I don't have the details of what they had at Pittsburg Landing, but note that the standard 12-pounder Napoleon Federal artillery battery supply was 37% solid shot, 37% shrapnel, 12% canister, and 12% shell. They had choices.

Gunboats

Specially the USS Lexington and USS Tyler carrying smoothbore 10 8 inch and 3 32 pounder guns between them putting them well in the realm of siege artillery.

Pros

• Always mobile.
  • Difficult to hit.
  • Quick to relocate for a new target.
• Fast (5-10 mph).
• Carries their ammunition with them.
• Typically carries solid shot, explosive shell and anti-personnel rounds.
• Gun crews are protected.
• Cannot be overrun.
• Each carries numerically as many guns as a typical field battery (six).
• Heavy guns (8 inch and 32 pounders) typical of siege artillery.
• Can provide flanking fire from a relatively safe position.
• Excellent mounting.
• Fairly accurate (they are designed to fire at moving targets).
• Fast firing.

Cons

• Can only move in water.
• Heavy seas can hamper accuracy.
• Expensive (what with the "boat" part and all).

None of these cons were a concern on the day of battle.

Of the gunboats at Pittsburgh Landing, Major General Leonidas Polk of the CSA reported...

"...[the Confederate forces] were within from 150 to 400 yards of the enemy's position, and nothing seemed wanting to complete the most brilliant victory of the war but to press forward and make a vigorous assault on the demoralized remnant of his forces. At this juncture his gunboats dropped down the river, near the landing where his troops were collected, and opened a tremendous cannonade of shot and shell over the bank, in the direction from where our forces were approaching."

General Grant stated of the victory "in this repulse much is due to the presence of the gunboats."

Field Artillery

Federal field artillery was generally 6 to 24 pound guns. Smoothbore or rifled. Long guns or howitzers. It's not clear what Grant had available at Pittsburg or even how many. Wikipedia says...

As with the Hornets Nest, the estimate of the number of guns varies widely. Grant, in his memoirs, recalls "20 or more." Daniel, p. 246, and Grimsley, p. 109, account for 41 guns; Sword, p. 356, states there were "at least 10 batteries"; and Cunningham, p. 307, cites historical accounts that vary from 42 to more than 100.

The Union Order Of Battle includes these units, but it's not clear how many were engaged in the fighting, nor how many or type of guns they possessed.

• 1st Illinois Light Artillery
• 2nd Illinois Light Artillery
• 3rd Illinois Light Artillery
• 6th Indiana Battery
• 9th Indiana Battery
• 1st Ohio Light Artillery
• 5th Ohio Battery
• 8th Ohio Battery
• 13th Ohio Battery
• 2nd Michigan Battery
• 1st Minnesota Battery
• 1st Missouri Light Artillery
• Missouri Light Artillery, Mann's battery
• 5th U.S. Artillery

Pros

• Relatively cheap (not a concern once battle is joined).
• Typically carries solid shot, explosive shell and anti-personnel rounds.
• Very good against exposed soldiers.
• Can go anywhere the army can.
• Rifles can be very accurate.
• Fast firing.

Cons

• Immobile while firing, slow to "limber up".
  • Counter-battery fire can zero in on their position.
  • Slow to relocate for a new target.
• Slow when moving (16 miles a day, max).
• Vulnerable to being overrun.
• Vulnerable ammunition train.
• Unless given time to dig in, gun crews are exposed.
• Lighter guns (6 to 24 pounders, typically 12 pounders).
• Needs to feed and maintain horses (6 to 8 for a 12 pounder).
• Smoothbores can be very inaccurate.

Siege artillery

Large rifles and mortars from 24 to 300 pounders.

I don't believe any siege artillery was available to the Army of the Tennessee at Pittsburg Landing. The previous Battle of Fort Donelson featured only light artillery and the army was lacking siege artillery during the Siege of Vicksburg a year later. They were so desperate, they made mortars out of wood.

What if Grant did have siege artillery? What would he have had in April 1862? We can take a guess. At the same time as Shiloh, the Feds were plastering Fort Pulaski using mostly 10" and 13" heavy seacoast mortars: very inaccurate, very heavy, very immobile, and probably of zero use in a mobile field battle like Shiloh. A mobile army like the Army of the Tennessee would have used smaller and lighter, but even less accurate, 8" and 10" siege mortars. They still weighed 2000 to 4000 pounds and required a prepared bed before firing.

They also had a few rifled James and Parrot rifles. While more accurate than a mortar, and at the "small" end of siege artillery, they still weighed 5000 lbs and up and could not be moved around the battlefield quickly.

Finally they had 8" and 10" smoothbore Columbaid cannons clocking in at 10,000 to 15,000 pounds.

For comparison, your typical field artillery piece is about 1000 pounds and it still required a team of six to eight horses to pull it.

The Battle of Shiloh was a surprise for the Feds and the Pittsburg Landing defense was hastily put together. With Federal units marching to the sound of battle, it's unlikely they had time to take their siege train with them nor to unload (it traveled primarily by water) it and its ammunition nor emplace the huge guns.

Pros

• Heavy guns able to breech fortifications.
• Able to fire from a relatively safe distance.
• Mortars can fire over defensive structures.
• Fires shot and shell.
• Rifles can be very accurate.

Cons

• Very slow to move and setup.
  • Of little tactical value.
  • Counter-battery fire can zero in on their position.
  • Very slow to relocate for a new target.
• Can only be moved over limited terrain.
• Unlikely to carry anti-personnel rounds.
• Vulnerable to being overrun.
• Vulnerable ammunition train.
• Unless given time to dig in, gun crews are exposed.
• Mortars can be very inaccurate.
• Very slow to fire.
• Expensive.
• Often required specialty ammunition.

Few of the pros apply to Pittsburg Landing defending against a hasty Confederate attack.

Siege guns are only useful in static situations such as a fixed defense (like defending a fort or city), or a siege. In a battle such as Pittsburg Landing they would have been worse than useless. There would have been no time to move them into position nor set them up. They would have limited close range anti-personnel value. And they would have fallen into Confederate hands who desperately lacked heavy artillery.

Field artillery were fairly mobile, at least to be hastily put into position for the defense of Pittsburg Landing. They had good performance against exposed soldiers, especially firing right into the teeth of a massed charge with canister shot. However, they fired a relatively small shell and were quite vulnerable to being overrun.

Gunboats combine the best of both. Carrying heavy artillery like a siege gun, but on a very mobile platform and with plenty of anti-personnel rounds. From the Tennessee River they could engage the enemy with flanking fire from a position of relative safety. While they would often anchor to fire more accurately, they could quickly weigh anchor and steam rapidly to provide firepower at a new hot spot ("quickly" and "rapidly" being relative to having to limber up artillery to a team of horses and haul it across rough terrain).

In conclusion, if you're fighting along a river get all the gunboats you can. Otherwise, field artillery is what you want in a hasty defensive position. The value of siege artillery is less than zero unless you're conducting or under siege.