Floating o'er the waves: Seattle boatbuilder constructs large hovercraft to serve remote Alaskan village.
When King Cove in southwest Alaska finally obtained federal funding to build a reliable transportation link to the nearest all-weather airport, a vehicle was needed that could cross eight-mile-wide Cold Bay. Cold Bay is a stretch of water that has no natural harbors, is open to storm-force winds and can freeze over in winter. A hovercraft fit the bill.
Since 1962, hovercraft have routinely been used by commercial and military operators in conditions and surfaces--including ice floes, mudflats, beaches tundra, etc.--that bring other vehicles to a halt. On water crossings, the hovercraft also has the unique ability to function without a traditional dock or harbor.
King Cove's primary requirement was for a craft that could carry an ambulance or a fuel truck with a maximum axle weight of 18,000 lb., and a deckhouse seating 49 passengers. To find the best design, the King Cove planning committee was assisted by the Canadian Coast Guard, which operates two British-built hovercraft from a base in Richmond, British Columbia. They visited the center of hovercraft development on the south coast of England where they found a design that suited their needs, the Hoverwork BHT130.
The BHT 130 is based on a platform 100 ft. x 45 ft. overall (including the inflated skirt) that offers a very flexible layout. Hoverwork's engineers gave the craft, named Suna-X, a 38 ft. x 21 ft. cargo deck reinforced with box-section beams and fitted with eye plates to lash the vehicle down. The design can also be configured with a full-length deckhouse, seating 130 passengers or large cargo deck with bow ramp. The first of this class was launched on the Solent by the Duke of Edinburgh in June 2006. That model was the passenger-only version for use by Hovertravel, the parent company of Hoverwork, which has a 40-year history of hovercraft ferry operation.
All the early hovercraft built in the U.K., including the 185 ft. SR.N4, used aircraft construction techniques and gas turbine engines. In the 1980s the industry followed the lead of conventional boatbuilders and began using welded aluminum and lightweight diesel engines to improve durability and lower cost.
King Cove selected Kvichak Marine Industries, Seattle, Wash., to build its hovercraft. Kvichak has built a national reputation for its ability to deliver complex projects on time, including such challenging designs as pilot boats, fast catamarans and small craft for the U.S. Navy. In 2003, it delivered a 41 ft. Griffon hovercraft for use on the North Slope oil fields.
The Suna-X is powered by four MTU 2000 diesel engines, built in Detroit, Mich., and supplied by Pacific Power Products, Kent, Wash. Two of the engines are V16s used for thrust, with the remaining pair of V12 engines used for lift. All four engines are cooled by custom-designed radiators from AKG, have a displacement of 2.0 L/cyl and incorporate the DDEC IV system to provide individual control to the high-pressure (25,000 psi) electronic unit injection pumps. The engines have dual turbochargers and are EPA Tier 1 certified.
Two 16V 2000s, rated 1205 hp at 1800 rpm, are located in the aft end of the deck house. Power is transmitted to the air screw via a 13 in. wide-toothed Gates Power Grip belt that passes through the roof of the engine room. The upper pulley provides a 1:1.5 reduction of the engine's maximum speed to 1200 rpm and turns a 16 ft. long, 13.6 in. diameter carbon-fiber drive shaft. The 11 ft. 5 in. diameter, five-bladed, adjustable-pitch airscrews are made in Germany also from carbon-fiber. They spin in fiberglass ducts 14 ft. 7 in. across, that reach 25 ft. into the air. The towers that hold them are tripods 10 ft. high with heavily reinforced bases.
Each engine is cooled by two radiators, one for the engine jacket water and one for charge-air cooling. The engines also drive three Auragen 8 kW inductive generators that provide the craft's electrical supply, including heat for the deckhouse during winter. The steering vanes on the back of the ducts and the propeller blade variable-pitch controls are hydraulic, driven by Parker PGP series pumps located in the engine room. The craft carries 951 gal. of fuel, consuming approximately 130 to 160 gal/hr during typical service, and 185 gal/hr at full power.
The second pair of 12V 2000 engines are rated 905 hp at 2100 rpm and are positioned under the port and starboard side decks. They turn a shaft with three centrifugal fans at 1800 rpm. The first fan forces air back over the engine and through the radiator. This airflow is then ducted through the large nozzles port and starboard. These nozzles can be rotated by the helmsman and their thrust used to control any sideways drift of the craft. The second and third fans pump air downward to create the air cushion inside the skirt and keep the craft hovering 5 in. above the surface. The safety rules for this unusual cooling system specify that if engine temperature should rise, the air flow can be immediately vented through the sidewalls of the cargo deck via approved fire dampers.
The 5 ft.-deep, flexible skirt forms a virtual seal between the hull and the surface below, and is built to enable the craft to pass over irregularities onshore and conform to choppy waves without losing its seal, providing it with good seakeeping characteristics. The skirt was built in England from high-strength natural rubber and nylon fabric 0.05 to 0.08 in. thick. All the specialized hovercraft equipment--the skirt, propellers, shafts, ducts, fans, nozzles and controls--was supplied by Hoverwork.
The bare hull of the BHT-130 is an oval platform only 32 in. deep. Since the flat bottom does not come into contact with the water when the craft is underway, the numerous small stringers that are needed for stiffening can be fitted on the outside, leaving a clean interior with numerous watertight compartments. The flat bottom panels were built from long, 0.11 in. aluminum sheets to save weight and ensure high quality. The panels were sub-contracted to a specialist metal shop--Advanced Joining Technology, Waukesha, Wis.--that is licensed to use a friction stir welding technique.
In the friction stir process, welds are made without the application of heat. Instead, a heavy tool with a profiled probe is rotated and slowly plunged into the joint line between two pieces of metal, which are butted together. Frictional heat is generated, which causes the parts to soften without reaching the melting point and allows the tool to traverse along the weld line. The pool of plasticized metal moves from the leading edge of the tool to the trailing edge and forges the two parts together, leaving them solidly bonded. This method is particularly suited to relatively soft aluminum alloys and the technique offers very low distortion with high strength.
The finished 40 ft. x 10 in. panels were trucked to Seattle and welded into place on the bottom. The deckhouse was pre-fabricated and craned into place inside Kvichak's assembly hall. Dry weight is 64.5 tons, with a maximum load of 23.7 tons.
This is the largest hovercraft ever built in the U.S., Kvichak said, and the first to be Coast Guard-certified as a Sub-Chapter T class, to carry 49 passengers. In reasonable weather, fully loaded, cruise speed will be around 35 knots, and it will take less than 20 minutes to cross Cold Bay to the airport. However, the BHT 130 can operate in strong winds and waves up to Sea State 4 at a reduced speed. On flat water with a light load, speeds in excess of 50 knots can be maintained.
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|Title Annotation:||INNOVATIVE USES of HORSEPOWER|
|Publication:||Diesel Progress North American Edition|
|Date:||Dec 1, 2006|
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