For nearly 150 years, the story of the Hunley’s attack on the USS Housatonic has been Civil War legend.

And it has been wrong.

Scientists have discovered a piece of the Confederate submarine’s torpedo still attached to its spar, debunking eyewitness accounts that the Hunley was nearly 100 feet away from the explosion that sent a Union blockade ship to the bottom of the sea off Charleston in 1864.

Instead, the Hunley and its eight-man crew were less than 20 feet from the blast. And that changes everything about the story — and possibly even provides a clue as to why it sank.

“I would say this is the single-most important piece of evidence we have found from the attack,” said Maria Jacobsen, senior archaeologist on the Hunley project.

Basically, Hunley conservators found a piece of the torpedo’s copper shell, peeled back from the blast, when they removed a century of hardened sand and shell from the submarine’s 20-foot spar. The torpedo was bolted to the spar, contradicting the conventional wisdom that the torpedo was planted in the side of the Housatonic with a barb like a fishing hook, slipped off the spar and then detonated by rope trigger when the sub was a safe distance away.

Instead, the Feb. 17, 1864, attack off Charleston was a dangerous, close-quarters assault that risked the sub and crew.

“This changes some things,” said Lt. Gov. Glenn McConnell, longtime chairman of the state Hunley Commission. “They were much closer to the explosion than we believed, but I don’t believe this was a suicide mission.”

When the Hunley was built in 1863, it was originally intended to attack ships using a contact mine towed from the end of a long rope.

The idea was that the submarine would dive under a ship and drag the mine into its flank, by which time the Hunley would be safely on the other side of the ship.

After the towline got fouled in the Hunley’s rudder and propeller during a test run in Charleston Harbor, engineers decided to refit the sub with a spar similar to the ones used by ironclads, picket boats and Davids, which were low-profile stealth boats.

The engineering quickly evolved through trial and error. In October 1863, a David attacked the USS New Ironsides outside Charleston Harbor, ramming a torpedo into its flank. The blast didn’t sink the ship, but did serious damage.

The explosion also threw a plume of water into the air, some of which extinguished the fire powering the David’s steam engine. Jacobsen said that attack prompted Confederate engineers to refine their method of attack. If the main thrust of the blast was up, the mines would have limited success hitting the side of a ship. They would do more damage if they were planted under the ships.

The Hunley was equipped with an adjustable spar that could be raised or lowered. The torpedo was fixed on the spar at an angle, so that when the spar was lowered for an attack, the torpedo was sitting dead horizontal.

Jacobsen knows this because of a detailed drawing of the “torpedo used to sink the Housatonic” that survives in the papers of Confederate officials in Charleston during the war. But until Hunley scientists found the remains of that exact torpedo, they couldn’t be sure those drawings were accurate.

The torpedo, like the Hunley, had been upgraded through trial and error. Because triggers and detonators on these torpedoes were woefully unreliable, the Hunley’s torpedo had three triggers, any one of which would blow the charge.

And, because the David’s 65-pound torpedo did not sink the Ironsides, the Hunley’s torpedo was packed with more than double the gunpowder — 135 pounds.

The Hunley left Sullivan’s Island shortly after 6 p.m. on Feb. 17, 1864. Two hours later, it was spotted off the port bow of the Housatonic when it was several hundred yards out.

Instead of directly ramming the sloop-of-war, Jacobsen said, Hunley commander Lt. George E. Dixon maneuvered the Hunley around the Housatonic’s bow and aimed for the starboard rear flank.

On the Housatonic, the ship’s hull curved upward and inward toward the stern. The Hunley planted its charge on the side of the ship beneath the bilge line, ensuring that an upward blast would go through the ship.

And it did.

The blast left a hole in the Housatonic so large that accounts say a couch floated out of the breach sideways.

But what did that blast do to the Hunley and its crew, which were also above the blast and less than two dozen feet away?

“This is a riveted iron structure. How well would it hold up against shock waves?” Jacobsen said.

That is a question that may not be answered soon. The Hunley’s hull is still covered with a shell of hardened sand — concretion — that scientists are leaving in place to protect the metal until the conservation process begins. When that concrete-like casing is removed (current plans are to do so next year), scientists at the Warren Lasch Conservation Center should get a better idea of what, if any, damage the blast caused the sub.

It could have buckled hull plates, allowing enough water into the sub to sink it. But the crew, under enormous pressure to break the blockade, had no way to test the effects of shock waves from the blast on the sub.

“They were pressed for time, they were pressed for resources, but nothing indicates this was a suicide mission,” Jacobsen said. “They just had to get the job done.”

In other words, the crew had to take a risk.

McConnell said that there is one tantalizing clue that suggests a shock wave hit the Hunley hard. Dixon’s pocket watch is stopped at almost the exact moment the Housatonic crew said the Hunley attacked. Did the blast actually stop a clock?

“I think we are now narrowing our focus some to look at what effect the concussion of that blast might have had,” McConnell said.

Until the sub itself is examined more closely, scientists will use this new information and data to simulate the blast. Jacobsen said that will offer a better idea of what impact the blast had on the Hunley. She said that will be a time-consuming and costly project, one that will require the lab to partner with an outside source.

They will begin with computer simulations and may eventually move to scale models of the attack. And that ultimately may shed further light on one of the most mysterious legends of the Civil War.