Engineering Insights: All-Composite Hovercraft Rises to Performance Challenge
Envisioned as early as the mid-1800s, the hovercraft is a unique amphibious vessel designed to float on a cushion of air, enabling it to travel over water and land. Closer to airplanes than boats in concept, 20th Century hovercraft designs were made practical with aircraft-like aluminum monocoque hulls light enough to be lifted off the ground by powerful fans. Today, the U.S. Navy operates one of the largest hovercraft fleets, deploying approximately 90 LCAC (landing craft air cushion) mostly aluminum hovercraft, which are used to move troops and equipment from ship to shore. A smaller number of commercial hovercraft ferry passengers and cars on water routes throughout the world.
Now, a Florida startup company has developed an all-composite hovercraft design — one it hopes will replace aluminum designs for commercial applications — and has given CT an exclusive look at the design and materials. Despite higher initial material costs, All Terrain Land and Sea Hovercraft (ATLAS) Inc. (Green Cove Springs, Fla.) claims its ultralightweight design can be manufactured with low-cost methods and dramatically reduces fuel consumption, permitting ferry operators a rapid return on investment.
"We're pioneering a whole new way to manufacture these very unique vessels,"notes Kurt Peterson, ATLAS' CEO. The two-year-old company is producing and testing its patented design for a commercial ferry company and already has a vessel under contract.
A CLEAN-SHEET DESIGN
A hovercraft generally starts as a flat-bottomed, rectangular barge. Powerful lift fans placed underneath the flat deck blow air into a large flexible rubber tube or "skirt"extending downward from the deck. Vents in the inner surface of the skirt allow the air to fill the space under the hull while it is contained by the skirt. The air creates a giant low-pressure bubble that lifts the deck above the water or the ground. Additional large "thrust vector"fans mounted above the deck drive high-velocity air over moveable rudders to propel and maneuver the craft as desired.
The ATLAS team started with a "clean sheet"in developing its all-composite hovercraft design because nothing comparable to it existed. Finite element analysis (FEA) was used to size the structural framework for the 100-ft long by 50-ft wide by 30-ft tall (30m by 15m by 9m) AH-100-P hovercraft's anticipated passenger and vehicle loads. RHINO 3D modeling software from McNeel North America (Seattle, Wash.) and Prolines hull design software from Vacanti Yacht Design LLC (Renton, Wash.) helped the team create the vessel's exterior form. The 3-D forms were input to AutoCAD (Autodesk, San Raphael, Calif.) for producing shop drawings for actual parts, says Peterson.
"A hovercraft doesn't experience the same loads as a conventional boat,"he explains, "because the hull is supported above the water's surface, with no wetted hull. While still considered in the design, slamming loads and drag forces aren't the same. Instead, we tried to achieve the lightest possible structure to maximize the lift energy from the fans.â€
The air bubble that supports the vessel acts as a large frictionless bearing, notes Peterson. For a given vessel weight, less energy is needed to drive it forward, as compared to conventional boats with hulls and propellers, because there's very little drag. Low drag means high speed: the AH100P is designed to carry 150 passengers at 52 knots/60 mph, which is more than twice the speed possible with a commercial catamaran (i.e., two parallel hulls) ferry, while consuming less than half the fuel.
HOVERCRAFT AND COMPOSITES A GOOD FIT
Composites were deemed the best material solution for the design concept. They provide greater strength-to-weight than aluminum, lower overall vessel weight and greater durability and corrosion resistance, which translates to reduced maintenance costs. In addition, ATLAS' patented technology and manufacturing process uses no metallic fasteners, employing high-strength adhesive instead, which further reduces weight and corrosion, reports Peterson.
The frame for the AH-100-P is made with pultruded structural I-beam profiles manufactured by Creative Pultrusions Inc. (Alum Bank, Pa.). The profiles, fabricated with E-glass and flame-retardant vinyl ester resin, are about 6 inches/150 mm from flange to flange and are joined to form a grid pattern with Weld-On methacrylate adhesive from IPS Corp. (Durham, N.C.). According to ATLAS, testing has shown that the adhesively bonded joints are comparable in structural strength to metal welds: "We have had several independent labs verify the strength of the construction and bonding methods as part of the U.S. Coast Guard approval process for new commercial vessel construction,"says Peterson. "The results prove the composites meet and often exceed the USCG and industry requirements.â€
The assembled frame is covered with either pultruded or resin-infused composite sandwich panels, both flat and complexly curved. Curved panels that form the cabin areas and pilothouse are made via a vacuum-assisted resin transfer molding (VARTM) process, developed by ATLAS, using AME6000 vinyl ester resin from Ashland Performance Materials, Composite Polymers (Columbus, Ohio). Three-dimensional, 77-oz woven reinforcements from 3TEX Inc. (Cary, N.C.) form the inner and outer skins, and 1-inch/25-mm thick styrene acrylonitrile (SAN) closed cell CoreCell foam manufactured by Gurit (formerly SP, Oakville, Ontario, Canada) comprises the central core.
Selected parts, such as the rectangular ductwork that's mounted along the perimeter of the deck's lower surface to deliver the air from the lift fans to the skirt, are made with ZPlex, a fairly new material from 3TEX. This three-dimensional reinforcement has fabric skins and a core made up of slender individual foam "noodles"or cylinders held in place with through-thickness stitching. The core noodles can be 0.25 inch, 0.375 inch or 0.5 inch (6.35 mm, 9.52 mm or 12.7 mm) in diameter; woven 0°/90° skins above and below the core vary in areal weight from 32 oz/yd2 to 76 oz/yd2 and could be constructed with any type of fiber, although ATLAS is using E-glass. The noodles provide integral resin flow channels that help speed the infusion process (see photo, above). Because ZPlex is highly conformable, it's ideal for the deep rectangular shape of the ducts, says Peterson, and when cured, the z-direction stitches add strength and toughness without weight penalty.
"We need very high strength-to-weight parts for this project, and ZPlex is an innovative material that gives us that while being easy to work with,"he states. "3TEX has been a partner in the project and helped with material selection and design for manufacture."ZPlex also is being used to fabricate the complexly curved housings for the lift fans and thrust vector fans described above. All infused parts are made in-house on low-temperature composite and/or metal tooling designed and fabricated by ATLAS.
Pultruded composite panels from Creative Pultrusions comprise the vessel's flat deck. While the company has infused simple flat panels for the hovercraft's bulkheads and interior walls, Peterson says that the team is likely to select the commercial pultruded panels for all bulkheads in future production: "The pultrusion technology produces a structural component that's superior in strength and consistent in dimensional tolerance that's not easy to achieve with other methods,"he explains.
ATLAS aims to make full-scale production as efficient and cost-effective as possible through the use of automation. Assembly robots will help streamline bonding of the vessel's structural frame and installation of the panels to limit touch labor as much as possible. "We are driving costs down and demonstrating that we can build a composite vessel faster than a metal vessel, without a shipyard,"declares Peterson. Previous hovercraft designs used aircraft engines, propellers and rudders for propulsion and control, which gave them a long-standing reputation for being loud, expensive and unreliable in commercial operation, says Peterson. ATLAS has solved these problems with a custom diesel/electric propulsion system, designed in-house in conjunction with several outside suppliers. Diesel engines drive direct-current (DC) electrical generators, which generate electrical power to drive the large fans that fill the skirt with air, while the thrust vector fans propel the craft. According to Peterson, the diesel/electric system provides superior fuel economy and the "lowest noise levels possible.â€
The first prototype hovercraft will undergo sea trials in early 2007, with strain gages installed in strategic locations to measure operational loads. The ATLAS hovercraft falls under the U.S. Coast Guard's "Tboat"classification for small passenger vessels (fewer than 150 passengers), and the agency considers the composite design an "emerging technology,"requiring rigorous testing, explains Peterson, from basic structural analysis to flame and fire tests. Most of those tests have already been completed.
The first AH-100-P Hovercraft has been contracted by Anthony Difiglo Sr. of D & D Yacht Charters, a Chicago-based passenger and dinner cruise company, and Peterson doesn't rule out the potential for search and rescue or military orders in the future. Goodrich Corp. Engineered PolyÂmer Products Div. (Jacksonville, Fla.) has joined the ATLAS team as a development partner in the area of composite thrust and lift fan technology, which could potentially improve the drive system and fuel economy, he says.
"The composite hovercraft has anywhere from a 15 to 20 percent higher cost up front,"Peterson notes, "but it uses one-half the fuel over a five-year period, which means the fuel savings alone will pay for the craft — it makes a pretty compelling economic case for composite design.â€
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