There are two descriptions of the H. L. Hunley that come from people who had their hands on her. One is William Alexander’s, written at Mobile in June 1902, thirty-nine years after he helped build her, with the warning that “all dimensions are from memory.” The other is the site assessment made by divers of the National Park Service, the Naval Historical Center and the South Carolina Institute of Archaeology and Anthropology in May 1996, with the boat still under a metre of sand four miles off Sullivan’s Island. They agree better than a memory and a tape measure usually do.

The hull

Alexander’s firm, Park and Lyons of Mobile, had built one boat already, an iron hull of oblong section about 25 feet long, 5 wide and 6 deep, which was lost under tow off Fort Morgan early in 1863 with no one aboard. “We decided to build another boat, and for this purpose took a cylinder boiler which we had on hand, 48 inches in diameter and 25 feet long … We cut this boiler in two, longitudinally, and inserted two 12-inch boiler-iron strips in her sides; lengthened her by one tapering course fore and aft, to which were attached bow and stern castings, making the boat about 30 feet long, 4 feet wide, and 5 feet deep. A longitudinal strip 12 inches wide was riveted the full length on top.”

The divers in 1996 found a hull 39 feet 5 inches long from the tip of the bow to the aftermost point of the upper hull, not counting the propeller, its guard ring and the rudder, which they did not uncover; 3 feet 10 inches wide; and 4 feet 3 inches from the top of the shell to the bottom of the keel ballast. The inserted strake they could see measured 9 inches, not 12. The bow and stern “narrow almost to a point at either end, but the narrowing is gradual, and the entire hull appears fair.” In 2000 the Friends of the Hunley described her as about 40 feet and about 20 tons, and when the concretion came off in 2014 and 2015 she turned out to be a far more carefully shaped thing than a boiler with ends on it, which is what the first draft of her history had called her.

Tanks, cocks, pumps and gauge

“At each end a bulkhead was riveted across to form water-ballast tanks (unfortunately these were left open on top); they were used in raising and sinking the boat.” That bracket is the design flaw that killed the first crew at the wharf and the second in the harbour. A tank open on top inside the boat is a tank that, once the boat heels or the sea cock is left open, pours into the crew compartment.

“In connection with each of the water tanks there was a sea-cock open to the sea to supply the tank for sinking; also a force pump to eject the water from the tanks in the sea for raising the boat to the surface. There was also a bilge connection to the pump. A mercury gauge, open to the sea, was attached to the shell near the forward tank, to indicate the depth of the boat below the surface.”

The conservators removed the after pump in January 2008 and the forward pump that May, and the pipework that linked the two tanks in 2005. Their reading of the pumps’ settings, as the boat was found, is that they were not set to pump the crew compartment out.

Ballast that could be dropped

Besides the water, “the boat was ballasted by flat castings, made to fit the outside bottom of the shell and fastened thereto by ‘Tee’ headed bolts passing through stuffing boxes inside the boat, the inside end of bolt squared to fit a wrench, that the bolts might be turned and the ballast dropped, should the necessity arise.”

When the keel blocks were finally removed and conserved, there were eight of them, some over 500 pounds, and three could be released from inside by a turnkey, dropping more than 1,000 pounds together. In October 1863 the bolts were found “partially turned, but not sufficient to release it.” In 2000 they were found tight.

The fins

“A one and a quarter shaft passed through stuffing boxes on each side of the boat, just forward of the end of the propeller shaft. On each end of this shaft, outside of the boat, castings, or later fins, five feet long and eight inches wide, were secured. This shaft was operated by a lever amidships, and by raising or lowering the ends of these fins, operated as the fins of a fish, changing the depth of the boat below the surface at will, without disturbing the water level in the ballast tanks.” The port dive plane the divers exposed in 1996 was 6 feet 10 inches long.

The men and the shaft

“The boat was operated by manual power, with an ordinary propeller. On the propeller shaft there were formed eight cranks at different angles; the shaft was supported by brackets on the starboard side, the men sitting on the port side turning on the cranks. The propeller shaft and cranks took up so much room that it was very difficult to pass fore and aft, and when the men were in their places this was next to impossible. In operation, one half of the crew had to pass through the fore hatch, the other through the after hatchway. The propeller revolved in a wrought iron ring or band, to guard against a line being thrown in to foul it.”

The excavators reached the crank in March 2001 and by April had exposed seven handles, which told them where seven men had sat; the eighth, Dixon, stood forward with his head in the hatch. The crew’s wooden bench came out for conservation in January 2005.

Hatches and air

“There were two hatchways, one fore and one aft, 16 inches by 12, with a combing 8 inches high. These hatches had hinged covers with rubber gasket, and were bolted from the inside. In the sides and ends of these combings glasses were inserted to sight from. There was an opening made in the top of the boat for an air box, a casting with a close top 12 by 18 by 4 inches, made to carry a hollow shaft. This shaft passed through stuffing boxes. On each end was an elbow with a 4-foot length of 1½ inch pipe, and keyed to the hollow shaft; on the inside was a lever with a stop-cock to admit air.”

The divers measured the hatches 16 feet 3 inches apart on the centreline, their covers hinged to open towards each other, and the air box 4 inches behind the forward hatch, 14 inches by 15, with the stumps of two snorkel pipes 3½ inches across. Forward of the forward hatch ran a cutwater 3 feet 4 inches long. They also found a row of small glass deadlights along the top that no written account had mentioned, ten of them, each with an iron cover. All ten were found shut.

The torpedo

“The torpedo was a copper cylinder holding a charge of ninety pounds of explosive, with percussion and friction primer mechanism, set off by flaring triggers. It was originally intended to float the torpedo on the surface of the water, the boat to dive under the vessel to be attacked, towing the torpedo with a line 200 feet after her, one of the triggers to touch the vessel and explode the torpedo.” After the towed torpedo proved unmanageable at sea, “we then rigged a yellow-pine boom, 22 feet long and tapering; this was attached to the bow, banded and guyed on each side. A socket on the torpedo secured it to the boom.”

The spar she carried on 17 February 1864 was hinged at the bow and about sixteen feet long, and the torpedo on it was a copper Singer torpedo holding 135 pounds of black powder with a pressure trigger: the drawing of “the torpedo used for blowing up the Housatonic” survives, and so did pieces of the copper case, peeled back over the end of the spar and conserved with it. The charge had grown by half since Mobile, and the distance between it and the crew had shrunk from 200 feet to the length of the spar.

Working her

“All hands aboard and ready, they would fasten the hatch covers down tight, light a candle, then let the water in from the sea into the ballast tanks until the top of the shell was about three inches under water. This could be seen by the water lever showing through the glasses in the hatch combings. The seacocks were then closed and the boat put under way. The captain would then lower the lever and depress the forward end of the fins very slightly, noting on the mercury gauge the depth of the boat beneath the surface; then bring the fins to a level; the boat would remain and travel at that depth. To rise to a higher level in the water he would raise the lever and elevate the forward end of the fins, and the boat would rise to its original position in the water. If the boat was not under way, in order to rise to the surface, it was necessary to start the pumps, and lighten the boat by ejecting the water from the tanks into the sea. In making a landing, the second officer would open his hatch cover, climb out and pass a line to shore.”

Her speed, Alexander said, was four miles an hour in smooth water. Engineer Tomb, who towed her, said “about 3 knots” with seven or eight men “working at their best,” and that “she was very slow in turning.” The crew for the attack was nine in Alexander’s day: a first officer forward to steer and dive her, a second aft “to the after-tank and pumps and the air supply,” and seven at the cranks. When she was raised there were eight.

Sources

• W. A. Alexander, “Thrilling Chapter in the History of the Confederate States Navy. Work of Submarine Boats,” Southern Historical Society Papers, volume 30 (1902), pp. 164–174. Every quotation above not otherwise attributed. archive.org
• National Park Service, Submerged Cultural Resources Unit, with the Naval Historical Center and the South Carolina Institute of Archaeology and Anthropology, H. L. Hunley Site Assessment (1998), chapter 8, “Hull Description.” The 1996 measurements. archive.org
• Notes from the papers of First Assistant Engineer J. H. Tomb, C. S. Navy, in Official Records of the Union and Confederate Navies, series I, volume 15, pp. 334–335. Her speed and turning. archive.org
• Friends of the Hunley, “Scientific Timeline,” “The Evidence,” “Conservation” and “Keel Blocks Offer Key Evidence in Hunley’s Disappearance” (18 July 2018). The pumps, the crank, the bench, the deadlights and the keel blocks. hunley.org, keel blocks
• Rachel M. Lance and others, “Air blast injuries killed the crew of the submarine H.L. Hunley,” PLOS ONE, 23 August 2017, and its supporting figures S1 and S4: the torpedo drawing and the conserved spar with the remains of the copper case. journals.plos.org