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May / June 2003

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Hybrid Hulls


Hybrid Hulls
UMaine engineers helping the U.S. Navy to build lighter, faster boats using composite materials

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The U.S. Navy is exploring the technology for a new generation of high-speed support ships with the help of the University of Maine and companies in Maine, Hawaii and England.

The vessels would combine traditional steel infrastructure with underwater bodies or hulls made of composite materials. The resulting hybrid could lead to ships that meet the Navy's needs for faster and lighter mid-size vessels, says Associate Professor of Mechanical Engineering Vince Caccese, UMaine's project coordinator. The technology also could have civilian applications.

This Modular Advanced Composite Hull Form project, known as MACH, has attracted more than $4.5 million in federal funding, as well as support from U.S. Navy research labs. The Navy takes a conservative approach to ship design, and every aspect of the new technology must meet stringent performance criteria.

"The idea here is to use a metallic skeleton and a composite skin," says Caccese, whose research has included testing joint designs and composite structures for government agencies such as NASA and companies from Maine-based Bath Iron Works to Aegis Bicycles. "Whenever you do anything below the waterline, it's risky. You need to make sure that whatever you do doesn't leak. We're being cautious, looking at different kinds of joints (between composite and steel)."

Composite materials, such as glass fiber-reinforced polymers and high-strength carbon fibers, have long been used in racing yachts and other marine crafts. In the last decade, Scandinavian boat builders have extended that technology to military purposes.

Keith Berube
University of Maine mechanical engineer Keith Berube makes composite panels in Crosby Laboratory. The manufacturing process uses multiple layers of fiberglass fabric and synthetic resins.

The inspiration for MACH comes from the Navatek division of Pacific Marine and Supply Company, a shipbuilder in Hawaii. Like the technology behind hydrofoils boats that ride above the waves on wing-like panels below the water MACH vessels would incorporate sleek underwater structures that, while moving, could lift most of the ship out of the water. Power would come from an electric motor that also rides under the ship.

"Pacific Marine found that using an underwater body could result in a dramatic increase in speed using the same horsepower. This approach gets a lot of the ship out of the water and reduces drag," says Caccese.

Partners in the project include Pacific Marine in Honolulu; Applied Thermal Sciences, an engineering consulting firm in Sanford, Maine; and Nigel Gee and Associates, a naval architecture firm in Southampton, United Kingdom; as well as U.S. Navy labs in Maryland and Rhode Island.

At the University of Maine, students are collaborating with faculty and professional engineers to design, build and test hybrid hulls that can withstand underwater speeds of 50 knots or more. The challenge is to increase structure strength through the manner in which the composite panels are fastened to the internal steel frame. Moreover, the hull must be easily manufactured and maintained all at a cost the Navy can afford.

In addition to UMaine mechanical engineers, electrical and computer engineering professor Bruce Segee is leading a team that is developing a computing and data management system to monitor structural stresses in the hull and relay information to the ship's crew. Their approach uses sensors that are built into the panels.

The idea is to give the crew an immediate view of the hull structure. "Our focus is computing power and networking so that multiple sensors can be placed inside the panel, but the number of wires going to the panel remains small and independent of the number of sensors. All you need for the electronics is power, ground and a network connection," says Segee.

Many of the experimental composite parts are being designed and made from scratch. Working with engineers Keith Berube and Randy Bragg, students use a process known as resin transfer molding to make panels of different shapes. As many as 16 sheets of woven fiberglass fabric may go into a single panel. Liquid resin the color and consistency of maple syrup is pumped through the fabric layers. One panel can be made in about a day.

The dry composite panels are studied in UMaine's Mechanical Engineering Structural Testing Lab in Boardman Hall. There they are bolted to steel I-beams; then the joints are stressed to determine just how much pressure they can withstand without bending or breaking.

The tests have produced some interesting results. In one case, applying a high force bent the three-quarter-inch-thick steel structure, demonstrating the strength of the undamaged composite. In others, cracks in the panels helped the engineers determine how stress is distributed in the bolted hybrid connections. Before they are done, the UMaine engineers will study dozens of panels and I-beam joints.

While steel hulls are still essential for large naval vessels, Caccese sees a bright future for hybrid structures on smaller ships. Composite hulls are lighter, allowing for greater fuel efficiency and heavier payloads. Composites also are the materials of choice for stealth technology intended to avoid detection by radar.

With MACH, UMaine engineers and their partners are pushing shipbuilding into uncharted waters. It will be a long-term effort, says Caccese, with the potential to transform the nation's maritime industries.

by Nick Houtman
May-June, 2003

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