The turtle's back: rebuilding the first U.S. submarine required some guesswork, but the casting process that created its critical hatch component was a no-brainer.

The Turtle, the first submarine to be used in warfare back in the 18th century, was in some ways a failure, but the metal castings that helped keep it sealed worked just fine.

"The Turtle was designed to perform covert operations to blow up British ships," said Roy Manstan, a retired engineer for the Naval Undersea Warfare Center (NUWC). "The downside was they were never successful. There were successful tests, but no successes against the British."

The idea was simple and brilliant. The Turtle, piloted by a single crewmember, would sink below the hull of a moored British ship. It would bore a hole into the boat, implant an explosive fitted with a timer and sail safely away.

The sub indeed dropped below the hull of a British ship in Boston Harbor in 1776 during the Revolutionary War, the pilot allowing a controlled amount of water to enter the base of the cabin, thus descending the entire vessel. Once an appropriate depth was achieved, the captain found buoyancy equilibrium by pumping out a small amount of water. The ship then sidled up to the British target through the use of two hand propellers. But complications arose when the pilot attempted to implant explosives, and the maiden mission was aborted.

After failing its initial battle test, the Turtle sank into the abyss of history, but it has resurfaced several times in the shallows of the past. Most recently, Manstan and Fred Frese, a self-described "boat builder" with previous experience recreating the world's first battle-tried undersea vessel, undertook a replication project that employs several critical cast components.

"Our job is to try to put together, as best as we can, the mechanical components needed to sail around the Connecticut River," Manstan said. "I'll be piloting so I have a vested interest in making sure it works correctly. The cast component that is of interest is the hatch."

Hatching a Plan

No drawings of the original Turtle exist to give present day explorers a glimpse of the design developed by David Bushnell more than two centuries ago. Manstan and Frese instead had to rely on written descriptions to recreate the entire submarine. Following is an excerpt from a letter Bushnell wrote to Thomas Jefferson:

"The entrance into the vessel was elliptical and ... surrounded by a broad, elliptical iron band, the lower edge of which was let into the wood of which the body of the vessel was made ... Above the upper edge of this iron band, there was a brass crown, or cover, resembling a hat with its crown and brim, which shut water tight upon the iron band."

Using this and other descriptions, Manstan and Frese, working through a partnership between the NUWC and Old Saybrook High School, Old Saybrook, Conn., developed sketches of the Turtle and its various components. The available writings indicated with a good deal of certainty that the ship's hatch was originally a cast component. Other pieces seemed to be candidates for casting, though Manstan and Frese could not pinpoint the method of their manufacture. Frese said that this time, he intends to cast those pieces, including the hub of the propellers, the rudder steering arms and the keel.

Turtle Patternmaking, Fast as a Hare

Developing the pattern for the Turtle's hatch was the most complicated part of the casting project and proved to require some outside help. The team's plan was to create a hatch that matched the original design closer than any previous effort to recreate the Turtle.

So Manstan got in touch with Scott Boyd, another engineer at the NUWC. He and Boyd then sat down with Sharon Hertzler, Mystic River Foundry, Mystic, Conn., and discussed the best way to produce the pattern quickly and efficiently. Hertzler suggested highly machinable polyurethane and put the curious historians in touch with the man who could deliver it. Frank Banner entered the picture as the last member of the restoration team.

"We showed Frank the dimensions of the part and asked him for suggestions," Boyd said. "There are different grades and densities of the material. and he suggested the 302 version of the product. He was balancing the machinability vs. the durability in the casting process based on the dimensions of the thing and how we were going about casting it."

Back at the NUWC, computer technicians busily mined Manstan's recreation of history into a modern marvel a solid 3-D model, or CAD file. That file was used to manually program a vertical machining lathe and carve out a plastic model of the Turtle's hatch.

"You give [the CNC machine] a file that is readable, but a human has to be there to tell it what tools to use," Boyd said. "The human's involved to tell the machine which 12 tools we are going to use to machine the part."

The finished model looked like a top hat with a brim and seven portholes, per the written descriptions.

Cast from the Past

For all the newfangled rapid prototyping that went into the creation of the pattern for the Turtle s hatch, the actual casting processes were relatively simple and traditional. After all, this was a component that had to be functional more than 200 years ago.

"Ours is relatively old school technology," Hertzler said of her metalcasting facility in Mystic. "We use standard, old practices here."

Polyurethane pattern in hand, Hertzler and her associates created a mold in nobake sand. The Mystic River Foundry also uses a standard green sand, but with a pattern the size of the hatch--24 in. in diameter and 10 in. tall--they wanted a molding media that could be hardened to maintain its integrity. This represents the one significant advantage the modern metalcaster had over the historic one. To cap off the project, the hatch was poured in silicon bronze, copper alloy number 867.10.

Frese expects the Turtle to launch early this summer. The finished product will like the original, measure 4 ft. wide, 6 ft. long and 7 ft. tall. And with a working hatch keeping the vessel sealed, pilot Manstan is optimistic that the Turtle's back will serve as sufficient protection against the murky waters of the Connecticut River.

For More Information

"RP Checks Out Fast," S. Gibbs, MODERN CASTING, August 2006, p. 43-45.

Shea Gibbs, Assistant Editor

RELATED ARTICLE: Learning from the turtle.

Fred Frese had made Turtle replicas before. They were successful. But what made his most recent effort special was that he was able to use the design to teach students about various manufacturing processes.

"We're in the third year of building the boat, and we're going to finish this year," Frese said.

Because the ship is made primarily of wood with cast components fitted in various locations, Frese has been able to use it to show the students at Old Saybrook High School how to work with a variety of building materials.

The program is augmented through an educational partnership agreement between the Naval Undersea Warfare Center (NUWC) and the Old Saybrook school system.

While the students weren't involved extensively in the patternmaking process for the hatch, they did craft patterns for the boat's keel and the hub of the propellers. They also were able to visit Mystic River Foundry, Mystic, Conn., and see the processes that went into casting the hatch for the Turtle.

"[Owner] Sharon Hertzler was a great host to the students and gave a good description of how the techniques she uses are similar to what was available in the 18th century," said Roy Manstan, a retired engineer for the NUWC who was instrumental in starting the re-creation process.

According to Frese, Old Saybrook High School dissolved its metalcasting program years ago, and the Turtle project is as close as they can get to reviving it.

"The kids are involved in every piece of this project, whether their hands were on it or there eyes, they were brought through the process," said Scott Boyd, the engineer who oversees the manufacturing processes at the NUWC.