Advanced composites put to sea in Navy's new ship.
Stiletto, designed by M Ship Co. of San Diego (photos at www.mshipco.com), will be used to test littoral or coastal operating tactics. Littoral capabilities are part of the Navy's emerging strategy for 21st-century warfare, which calls for a swift and aggressive response to conflicts that are more likely to involve coastal fighting than ocean confrontations. The Navy will still commission large ships for deepwater operations, but plans to build 56 littoral combat ships (LCS) between 2006 and 2022, some 27% of total ship construction during that period.
To maximize speed, maneuverability, range, and payload capacity, LCS must be lightweight and strong; hence the interest in epoxy and carbon fiber. (Not all LCS, however, will be fabricated of composites.) Stiletto is manufactured with a relatively new laminate technology called Sprint, which was developed by SP Systems, a British company based on the Isle of Wight (North American headquarters are in Magog, Quebec, Canada). Sprint laminates yield a controlled loading of resin that improves the strength and bonding properties of the composite, says Alex Shimell, client and engineering manager. Sprint can reduce the weight of a structure by up to 40% compared with conventional fabrication methods involving glass fiber and polyester or vinylester resin.
Reducing weight without sacrificing strength is critical to Stiletto's design. The hull incorporates planning tunnels that slope aft to the waterline. These capture the ship's bow waves, forcing them through the tunnels. As the tunnels increase water and air pressure in the bow waves, lift is created that raises the boat in the water by 1.5 ft (0.45 m), reducing drag and making speeds of 50 to 60 knots achievable, says William F. Burns, executive director of M Ship Co. Without the use of composites, this wouldn't be possible.
Cargo needs could create opportunities for composites
The light weight and strength of epoxy and carbon fiber may also revive construction of small and medium-size cargo vessels by the U.S. shipbuilding industry. With international shipping to the U.S. projected to double to 2 billion tons/yr by 2020, the Maritime Administration, part of the U.S. Dept. of Transportation, is promoting greater use of inland and coastal waterways for cargo distribution as a means of reducing port and highway congestion. This concept, called short-sea shipping, would require fleets of small and mid-size ships capable of rapidly transporting goods. Navy Cdr. Gregory E. Glaros, a transformation strategist at the Pentagon's Office of Force Transformation, which oversees Stiletto, believes composites can be used to quickly and economically build such ships. Information developed from tests on Stiletto could form the basis of a computational database that civilian and military boatbuilders access to expand the use of composites in naval architecture, he says.
Of course, composites have long been a staple of recreational shipbuilding. These are mostly polyester or vinylester and glass-fiber structures. The use of advanced composites in commercial and military ships, however, has been pioneered by non-U.S, boatyards, notably in Scandinavia and Europe. The Swedish navy has the largest composite ship in the world, the 236-ft (72-m) Visby-class corvette. Glaros says that engineers at the U.S. Office of Naval Research think it is possible to build an all-composite ship 250 to 300 ft long (76 to 91 m).
One expert who believes the Navy's use of advanced composites could spur interest among U.S. shipyards is Bob Lacovera, director of technical services at the American Composites Manufacturers Assn., Arlington, Va. "I think it's accurate to say that assuming this program is successful, it's going to provide a lot of trickle-down to the commercial boatbuilding industry." As to how long this might take, Lacovera says that if innovative companies champion advanced composites, it could be 5 to 10 years.
Fabrication process controls resin loading and laminate strength
Stiletto was built by Knight & Carver YachtCenter, National City, Calif. The Sprint fabrication technique is simple and effective. Each laminate used in construction of the ship is made up of two layers of dry carbon fiber with a precast layer of epoxy resin between them, one side of which has a light tack film for adhesion to vertical surfaces. Two laminates are combined with a high-density (7 to 8 lb/cubic ft) structural-foam core of styrene acrylonitrile (SAN) to form a panel. The core is fabricated of Core-Cell foam from ATC Chemicals Inc., Burlington, Ontario, Canada. SAN was specified because of its durability and impact resistance. Each panel is placed on a vacuum table, covered with a vacuum bag, and exposed to 0.85 bar of vacuum. This eliminates voids and ensures that the epoxy, when heated to 80[degrees]C (176[degrees]F) in a subsequent step, flows freely through the two layers of carbon fiber, completely wets them out, and bonds to the core.
Shimell says that compared with conventional prepregs, this technique permits greater control over resin diffusion. The amount of resin used in each laminate is precisely dispensed at the SP Systems factory, so there is little chance of loading mistakes during fabrication. The result is a laminate that is strong and tough with a high-quality surface.
Advanced Composites Put to Sea in Navy's New Ship
The nominal carbon-fiber loading of a Sprint panel is 45% (Stiletto's average carbon-fiber loading is 43%, according to Bums). Shimell says the loading can be tailored to meet structural demands in specific parts of an application. Some parts of Stiletto, for example, will be exposed to high torsional stress during operation. For these areas, panels were built up with as many as 15 layers of laminate without the need for labor-intensive debulking.
Because some of the Stiletto panels are 40 ft long and 10 ft wide (12 by 3 m), a special vacuum table and autoclave had to be built. The autoclave featured six computer-controlled heating elements, and the process generally lasted 11 hr at 93[degrees]C (200[degrees]F).
The panels were assembled on an aluminum-frame jig. Knight & Carver used a toughened adhesive developed by SP Systems, which created strong joint lines. On top of the joint lines, the boatbuilder applied a precut carbon-fiber tabbing impregnated with epoxy. Once this cured, the bond was virtually indestructible.
While carbon fiber was specified for Stiletto, Shimell says that Sprint laminates can use any reinforcement. The epoxy is supplied by SP Systems.
Sprint has been used on yachts and, according to Shimell, by major auto OEMs for fabrication of body panels. He declines to identify the OEMs, citing confidentiality agreements. Recent work includes developing ribs for powered boats used by Britain's Royal National Lifeboat Institution, a rescue group.
One claim for the Sprint process is that it is less labor-intensive than other composite techniques, which improves its cost-performance ratio. Shimell says that while it would be possible to duplicate the strength and light weight of Sprint with conventional composite-fabrication methods, it would be labor-intensive and repetitive, thus increasing cost.
Stiletto was on schedule to be delivered to the U.S. Navy on Dec. 22, 2005. Despite its length and 40-ft (12-m) keel, Stiletto's draft is only 3 ft (0.9 m), which suits it for extreme near-shore operations. The ship displaces 67 tonnes (148,000 lb) and carries a fuel load of 10 tonnes and payload (personnel, equipment, and weapons) of 15 tonnes. It has a range of 500 nautical miles at full load and maximum speed. Stiletto will generate 6200 hp from four Caterpillar 1650 diesel engines. It can operate independently or as part of a battle group, and its arsenal includes unmanned aerial vehicles (i.e., drones), which can be launched from a rear deck.
One interesting feature of the ship is modular electronics that permit "plug-and-fight" capabilities. The ship can change missions by installing different software programs on an "electronic keel," i.e., a data bus, thus expanding its flexibility in dealing with different threat scenarios.
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|Title Annotation:||United States. Navy|
|Comment:||Advanced composites put to sea in Navy's new ship.(United States.|
|Author:||Toensmeier, Patrick A.|
|Date:||Feb 1, 2006|
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