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The combat experience of the New Ironsides’s crew and those of the monitors would be a heavy weight in the scales of battle. Despite scores of hits from heavy Confederate ordnance, the New Ironsides emerged unscathed. The monitors’ combat experience against the harbor forts of Charleston had led to the introduction of a sheet metal inner sheathing to the turrets to prevent injuries to the crew from exploding rivets propelled by shot striking the outer surface.

Their twenty-foot-diameter turrets, with ten inches of armor in one-inch molded plates, were invulnerable to any ship-mounted guns in the world. Their hull armor was five inches, a half inch thicker than the New Ironsides and Warrior class, but the low freeboard meant most of the hull would be under water. Deck armor was only an inch, thus requiring the expedient remedy of a layer of sandbags. Designs of the Passaic class had called for twin XV-inch Dahlgrens, but production lags forced them to substitute one XI-inch. The turret was powered by its own small engine and could rotate 360 degrees in thirty seconds. Four iron beams in the deck served as slides for the gun carriages. “The slides would allow the guns to recoil a maximum of 6 feet, while the friction mechanisms could reduce the recoil to as little as two.” The carriages rested on brass rollers, so that large crews were not needed to run the guns out. Oval-shaped, armored gun ports swung open to allow the guns to fire and then were shut as soon as the guns discharged. Because the guns were muzzle-loaders, the long rammers had to be manipulated through the open gun ports. At such times, the turret would be turned away from the enemy, loaded, then turned again toward the enemy, and re-aimed.

This necessity was the greatest drag on the guns’ speed of fire, a problem the broadside ironclad did not suffer. The XV-inch gun in the monitor’s turret took an average of six minutes to fire, which could sometimes be reduced to a minimum of three. The XI-inch gun’s speed was about half that average. The broadside-fired Dahlgrens were another story entirely. The IX-inch could be fired every forty seconds, and the XI-inch every 1.74 minutes for about an hour. Comparing monitor and broadside ships, it was a trade-off between invulnerability and rate of fire.

There was no trade-off in punch. The XV-, XI-, and IX-inch Dahlgren shells with explosive charges weighed 350, 136, and 72.5 pounds, respectively. Their shot was even heavier, weighing in at 400, 169, and 80 pounds. The largest British shot, on the other hand, were the 110-pounder Armstrongs and the standard 68-pounder. Most British guns remained the venerable 32-pounders. Unlike the Armstrongs, the entire Dahlgren line was a byword in safety and reliability. The captain of the sloop the USS Brooklyn believed his IX-inchers to be well-nigh perfect and stated that “the men stand around them and fight them with as much confidence as they drink their grog.” Unlike most British guns, the Dahlgrens had been designed to fire both shell and shot, giving them a powerful versatility. Because of the Monitor-Virginia standoff the year before, Dahlgren had rigorously tested his guns against armor plate. He found that they could easily stand to increase their powder charges by 50 to 100 percent in order to easily drive holes through iron plate and wood backing of thickness and composition similar to the British Warrior class-“4- to 4.5-inch thick iron faceplate, bolted to about twenty inches of wood, sometimes with an inch-thick iron back plate, set up against a solid bank of clay. In most cases, the Dahlgren shot penetrated clean through the target and embedded itself deep into the clay bank, even with the target angled steeply as fifteen degrees.”

The exploding shell was all the rage in naval ordnance, and no gun delivered the destructive charge against wooden ships better than the series of guns Dahlgren had devised. Against ironclads, the shells were less effective. Dahlgren’s guns were equally able to deliver solid shot. Dahlgren had also taken to heart the error that had prevented the original Monitor’s XI-inch Dahlgrens from punching holes right through the CSS Virginia’s iron casemate. Navy policy had mandated that a powder charge only 50 percent of the maximum tested charge be used. Subsequent tests by Dahlgren showed that not only could his robust guns fully stand the use of a 100 percent charge for long use but that they would tear 5-inch armor apart. At the time, Dahlgren had been particularly incensed by the comment of the British First Lord of the Admiralty that his guns were “idle against armor plate.”During the Civil War two opposing schools of thought arose as to the best means of destroying armored warships. The “racking” school held that projectiles should smash against the ship, distributing the force of the blow along the whole side and dislocating the armor. Once the armor had been shaken off, the vessel would be vulnerable to shell fire. The “punching” school held that the projectile should penetrate the armor, showering the ship’s interior with deadly splinters in the process.… Dahlgren had concluded that racking was more effective against armored vessels than punching. Racking favored his low-velocity smoothbores over high-velocity rifled cannon.

Dahlgren had had the satisfaction in June of seeing the monitor USS Weehawken force the ironclad CSS Atlanta to strike its colors after hitting it at three hundred yards with only a few XV-inch shot. One shot alone dislodged the armor plate, sending a mass of iron and wooden splinters into the casement battery, spilling all the shot in its racks, and killing or wounding forty men. The prize had been repaired at Port Royal and was on the way to Philadelphia when Dahlgren intercepted her. He asked for volunteers for a scratch crew and captain and found himself with more than enough seamen and with a score of ambitious lieutenants who could smell promotion in the air. Of them he chose Lt. B. J. Cromwell.

As the Philadelphia was essentially unarmed, the admiral transferred his flag to the New Ironsides, from which he had commanded the previous attacks on Fort Sumter and the other harbor forts. His son, Ulric, had insisted on staying with the Navy rather than be sent ashore to join Gilmore’s troops. “New Ironsides has been in action more than any ship of the squadron. Where else would I be, Papa?” he asked. He had already hit it off with the ship’s captain, Stephen Clegg Rowan. An Irish immigrant, Rowan had joined the Navy in 1826 and fought “in the Seminole and Mexican Wars and had distinguished himself in the North Carolina sounds early in the Civil War.” Under his command, the New Ironsides became the most feared of all Union ships to the defenders of Charleston.