Charleston, S.C.— Archeologists and other researchers are much closer to unlocking the mysteries surrounding the disappearance of the H.L. Hunley, the first fully submersible submarine to sink an enemy ship—the USS Housatonic—in combat. This submarine went down the night of February 17, 1864 and was lost until 1995 when author Clive Cussler and the non-profit National Underwater and Marine Agency found it in Charleston Harbor. Once found, the question became how to raise a 136-year-old submarine, buried in silt, without damaging its fragile hull.
Lifting the Hunley from the harbor required years of planning and preparation. "One of the first things we did was perform finite element analysis," says mechanical engineer Mark Van Emmerik of Oceaneering International Inc., the company that planned the recovery and designed the rigging used to raise the Hunley. He used Ansys (Cannonsburg, PA) FEA software, Version 5.6. "The FEA indicated the hull could deform or split," he says. Although the water provided buoyancy to the hull when submersed, no one knew how the submarine would react when lifted into the air.
Van Emmerik considered fabricating neoprene pads to wrap around the submarine's hull. "I felt comfortable with what I new about neoprene," he says. "But there was a problem. The divers would have to wrestle with the neoprene underwater because of its buoyancy. It wasn't practical."
Van Emmerik determined that the tensile load for lifting the Hunley was 900 lbs on each of a series of lifting slings positioned every 15 inches along the submarine's hull. The question then became how to cushion the submarine from the intense lifting pressure when raised.
Raising the Hunley involved lowering a steel recovery truss over the top of the 40-ft vessel. Van Emmerik placed thirty-two sling belts under the hull and attached them to the truss. At rest, the boat listed 40°to starboard, the side that would bear most of the stress during initial phases of lifting.
Fearing the hull might collapse from the stress imposed by the slings, Van Emmerick designed five-foot vinyl-impregnated bags attached to the inside of the slings. The bags were filled with Froth Pak, a two-part polyurethane foam from Flexible Products Inc. and donated by NCS Supply (Marietta, GA). During the salvage operation on August 8, 2000, divers placed the foam's mixing nozzles in the tops of the bags. After filling, the bags expanded and set up, taking the shape of the hull as they expanded. Froth-Pak cradled the Hunley, successfully distributing the lifting stresses evenly across the entire hull with no fulcrum points. Currently, archeologists are discussing the best ways to examine the submarine's contents.
Additional applications for Froth-Pak include insulation and support structures for NASA spacecraft and insulation for fiberglass pools, spas, and tubs.