Engineering Insights: All-Composite Hovercraft Rises to Performance Challenge
An exclusive look at a lightweight pultruded and resin-infused design that sets a new standard for amphibious craft.
By Sara Black, Technical Editor | February 2007
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.
Source: Karl Reque
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.
Source: ATLAS
An artist’s rendering of the yet-to-be-completed hovercraft.
“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.”
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.”
Source: 3TEX
A close-up view of the ZPlex 3D material used for some of the composite components on the hovercraft, including lift fan ductwork below the vessel’s deck. The foam “noodles” are clearly visible, held in place with through-thickness stitching.
