Inside Manufacturing: All-carbon composite stealth technology joins the U.S. Navy
Designed to quickly and quietly close in on trouble spots in littoral (near shore) operations, the U.S. Navy’s new 24.4m/80-ft M-80 Stiletto is the largest Navy vessel yet built entirely of carbon/epoxy and is faster, more agile and provides a much smoother ride than the 25-m/82-ft MARK V currently in use.
The M-80 features a very low profile, particularly appealing to the military for its stealth, or invisibility to radar. Unlike metal ship structures, the M-80’s carbon composite exterior makes its hull and deck structures radar transparent, while the ship’s multi-angled design presents no surfaces perpendicular to the water’s surface, which prevents reflection of a radar signal back to the source. But the Stiletto is distinguished not only by radar transparency but also by a patented shape that is the creation of M Ship Co., a privately held design firm in San Diego, Calif. M Ship’s unique hull configuration produces minimal wake that cannot be easily discerned by aircraft overhead. The unusual design, which the company refers to as a double-M (see photo) channels water and air beneath the ship, converting the energy of the bow wave to vertical lift and, thus, reducing drag. Fully loaded, the craft has a draft (hull under the water line) of only 0.9m/3 ft.
The double-M concept evolved from an attempt to eliminate shoreline damage caused by surging water in the wake of powerboats in the canals of Venice, Italy, says M Ship founder and naval architect Bill Burns. Burns was approached by the Venice Water Authority in 1999 to see if his team could come up with a creative design to reduce boat wake. "We combined the best features of catamaran, air-supported hovercraft and conventional mono-hull designs and came up with a package that performs better than each of those individually,"Burns says.
The result was a single-M design that, for the Stiletto — M Ship's first defense contract — was successfully scaled up to double-M configuration with an unusually wide 12-m/40-ft beam. "Nobody's ever built a planing ship that has a length-to-beam ratio of 2:1 and still goes 50 knots [57.5 mph/93 kmh]."The strength-to-weight ratio of the carbon composite materials translates into lighter weight, more interior payload space, faster construction and produces a boat capable of greater speed than comparably sized vessels.
Carbon composites also enabled engineers to construct a hull with fewer bulkheads, resulting in a lightweight craft with three climate-controlled compartments of approximately 16.3 m² (175 ft²) each and a 139.4m² (1,500 ft²) working deck, totaling more than 186m² (92,000 ft²) of open interior space — unprecedented in a boat of this size. The large space gives the Navy a staging area that it can freely adapt for different missions: one day equipping a ship for a mine counter mission; the next, outfitting it for special operations.
Yet, despite the openness, the hull’s strength-to-weight ratio facilitates a strong, durable structure, says Burns. "The Stiletto has the capability of not only carrying a larger payload for its size but also of tolerating shifts in that payload."Although the vessel's profile is unusually low, the underside of the M-80’s main deck clears the inside of the hull bottom by about 1.5m to 1.8m (5 ft to 6 ft), providing enough vertical distance to accommodate engines (located in the aft section, just below the waterline), electrical systems and other equipment.
Wide beam, high torsion loads
Because the double-M concept mandates a craft with an unusually wide beam and the imposed loads on the hull’s bottom were likely to vary significantly across its width, M Ship defined hull and superstructure loads based on a finite element analysis (FEA) of the entire craft performed by Gurit (Magog, Quebec, Canada, and Newport, Isle of Wight, U.K.). Loads on the hull bottom include hoop (90° or circumferential) loads imposed up and over the tunnels and axial (0° or longitudinal) loads on the bottom of the double-M hull between superstructure bulkheads — as well as global and local torque loads.
"We have a loading scenario in which one corner of the ship — say, the forward port side — could be in one wave, and the aft starboard corner in another wave, and the craft suspended in between,"explains Bruce Sutphen, M Ship’s construction manager. "So we had to address the global torsion loads in the deck, the hull bottom and the sides and bulkheads."The ship also can see global torsion loads in the two "tunnels"that run the hull’s length and channel water beneath the hull (see diagram) due to flexure and hydrostatic loading.
Because fibrous composites are nonisotropic and the strength of the fiber is in its axial direction, the fiber architecture was oriented in the load paths identified for different sections of the ship. Woven (0°, 90°) carbon fiber layers were selected to handle the circumferential and longitudinal loads, and stitched double-bias (±45°) or and sometimes triaxial (±45°, 90°) fabrics were chosen to laminate the joints between adjacent sections of the hull layup and between separately fabricated deck panels.
The thickness of the structural core and the laminate were increased in highly loaded areas. The FEA identified buckling modes around the opening in the transom and lengthwise in the tunnels. Longitudinal girders and other reinforcements were added in these areas to prevent buckling.
The laminate architecture for the hull’s curved tunnels is designed to allow a small amount of flex under load conditions on the high seas. "They have some elasticity, similar to the wing of an airplane,"Burns notes, to protect the structure from sudden shock loads and also to help damp the effects of rough seas and smooth the ride.
One hull of a mold
Hull and deck structures were built to M Ship’s specifications by Knight & Carver Yacht Center (San Diego, Calif.), with some parts made by Seemann Composites (Gulfport, Miss.). The curved hull pan tool was wet laminated in a single 40-ft/12m wide, 80-ft/24m long, wooden female (negative) mold. For this one-off structure, M Ship and Knight & Carver collaborated on the design, and the latter built inexpensive plywood jigs and molds. The layup tool for the hull pan, with the double-M's alternating concave/convex curves, was created with fiberglass over a fiberboard frame. If the Stiletto goes into production, says Johnny Smullen, special projects manager for M Ship, "we would build a more detailed mold that would allow molding multiple parts."The super size tooling was tented to control the temperature and humidity during the building process. Before lamination, the mold was prepared with Locktite Frekote mold release made by Henkel Corp. (Madison Heights, Mich.) and epoxy gel coat from Gurit.
Because the hull was so big, it was constructed in stages in a process that spanned three to four months. Starting at the bow, a section of the hull mold was prepared with mold release and gel coat, and then the outer skin was layed up using Gurit’s RC416T 2×2 twill, a woven fabric made from T-700, in which the warp yarn runs alternately over two fill yarns and under one fill yarn, and Gurit’s Ampreg 22 epoxy resin, which is formulated to exhibit low exothermic reactions even when used in a thick section. Wet out of the carbon fabrics was accomplished by a dedicated impregnating machine that also wound each impregnated fabric into a roll for easier handling (see Step 1, on p. 3). During layup, the wetout fabrics then were manually unrolled and arranged in a variety of paths and orientations to meet the FEA design load requirements. After layup and the addition of bleeder, peel ply and wicking consumables, that section was individually tented and the layup was vacuum bagged. The part was compacted under a 0.95-atmosphere (14 psi) vacuum for eight hours while the epoxy resin reached its green state but not full cure.
Then the vacuum bag and consumables were removed and thickened. Ampreg 22 was spread over the compacted, green outer skin, and Gurit’s P-800, 155 kg/m3 (10 lb/ft3) Corecell linear core was laid over the epoxy-coated outer skin. The core was vacuum bagged, and the section was again heated to 52°C/125°F for 12 hours to bring the resin between the core and outer skin to green state. Sutphen explains that "thickened AM22 was applied to both the laminate and foam surfaces, and the excess was removed during the vacuum consolidation through perforations in the foam."The bag and other consumables were then removed and the wetout fabrics for the inner skin were rolled out in mirror image to the outer skin, vacuum bagged, and similarly consolidated and brought to green state. The rest of the hull was layed up in successive sections using the same process with each new layup overlapping adjacent green-stage sections.
Resin film infused for autoclave quality
At this point, oven-cured bulkheads and longitudinal girders were set into the still-green hull pan. These carbon-fiber reinforced, flat sandwich structures had been prefabricated on flat vacuum tables, using Gurit’s trademarked and patented SPRINT resin film infusion materials and molding process and its P1200 Corecell structural foam core with specified local reinforcements. Skins were built up from SPRINT, which consists of a layer of carbon fiber reinforcement on either side of a precast, precatalyzed epoxy resin film with an optional light tack film on one face. Unlike conventional prepregs, the fibers remain dry and unimpregnated by the resin until the curing process. Gurit contends that this gives SPRINT materials good breathability and, therefore, the potential to produce autoclave quality laminates from vacuum bag processing.
Gurit provided the dry SPRINT materials prestacked, building up the fibrous layers interleaved with resin film according to the ply schedule specified to meet design loads, and then consolidated the stacks under vacuum with epoxy resin film between the layers, effectively providing Knight & Carver with a preform for each skin. Sutphen says the resulting reduction in labor and the resin film infusion's fast processing time significantly reduced the M-80’s total build time.
The skins and core for each sandwich panel were positioned on the layup table then vacuum bagged and oven cured. The cure "recipe"involved gradual ramp-ups to intermediate temperatures, permitting about one hour of dwell time at each level, until the final cure temperature (90.5°C/195°F) was reached and then held for eight hours. The staged cure ensured sufficient time to evacuate and consolidate the laminate and permitted the resin film to melt, flow and saturate all fibers before the matrix gelled and hardened to full cure.
After the hull, bulkheads, girders and other elements were assembled and bonded, the hull and joint bonds were cocured to full cure under the hull tent at 52°C/125°F for approximately 10 hours. Heat was applied by spatial heaters equipped with convection fans and monitored with infrared-sensing thermal measurement devices.
Concurrently with hull fabrication, the superstructure and deck were assembled from flat panels fabricated by Knight & Carver and Seemann Composites via essentially the same method used for the structural bulkheads and girders. The flat panels for the superstructure were laid up inside the curing oven (see Step 9) and were cored with Gurit’s P500, P600, P800 or P1200 Corecell structural foam with densities ranging from 6.2 lb/ft3 (110 kg/m3) to 13.7 lb/ft3 (220 kg/m3). The core density for each panel was selected according to the FEA-determined design requirements.
When the flat panels were cured, Knight & Carver cut panels to shape for the superstructure using a laser projection system to mark positions of the carbon fiber in specified load paths and assembled the cut pieces over a wooden construction frame called a location jig. There, all the panels for the pilot house, top deck and side decks were glued together, edge-to-edge, using the Spabond adhesive, and then the joints were reinforced identically, inside and outside, with overlays of Ampreg 22-impregnated carbon tape. "By fabricating the panels with an adequate rebate,"Sutphen notes, "the tapes are aesthetically undetectable.
The superstructure, now sufficiently stabilized to bear the stress, was transferred, along with three bow panels and the transom, to a lifting jig and were hoisted atop the hull. Then the superstructure and hull were joined, and these secondary bonds were postcured in sections, with each section tented and heated for 12 hours to about 52°C/125°F. As each section was cured, the tent was repositioned to the next section but with an overlap onto the adjacent cured section.
Rather than demold these huge parts from the tool, Knight & Carver carefully cut the wooden molds, a section at a time, from the cured ship sections.
After demolding, holes were drilled into the superstructure and deck for attachment of intakes, outlets and other fittings. The fittings were either bonded directly to the skins or through-bolted. In all cases, the sandwich structure was locally reinforced at the cut by triaxial carbon fiber to ensure it would take the load imposed by the fitting. "We left no core exposed,"Smullen says. "If we cut a hole — for a door, for example — we capped the hole, effectively linking the skins together, so there was no exposed core.
The hull was painted with bottom paint beneath the waterline. Top sides were finished with an epoxy-based primer with a camouflage top coat of high-solids linear polyurethane in colors of grey specified by the U.S. Navy.
Navigating the course
Built as a test vessel, the M-80 Stiletto is currently undergoing 18 months of testing by the U.S. Navy off the East Coast. While the military future of the vessel was unknown at press time, M Ship’s Burns sees great potential for the double-M hull concept elsewhere, citing other military boats, commercial ferries and even recreational boats as potential applications. M Ship has other double-M hulls — including fiberglass versions — in the design stage, scaled up and down in size from the Stiletto but, at this writing, none are yet under construction.
"Below the Stiletto size there is interest in unmanned surface vehicles [USVs],"Burns adds, noting that the challenges of operating on the water are actually more extreme in smaller sizes because the size of the waves relative to the ship is greater, making loading conditions more critical. "But the single-M hull and the double-M design offer benefits to smaller craft to operate on the water more efficiently and with more stability than traditional boat designs,"Burns adds. He also believes that craft larger than the Stiletto will be considered for support of future littoral combat operations.
Sutphen observes, however, that the huge amounts of T-700 carbon needed to build a ship of this size were not easy to come by in today’s tight market given the already high demand for carbon in military and commercial aviation applications. "The carbon fiber used for building Stiletto is equivalent to the amount of carbon used in five fully-rigged 90-ft America’s Cup boats,"Sutphen says. Burns agrees, noting that the shortage of carbon, in fact, delayed the Stiletto project for several weeks. Although Gurit eventually was able to procure all the carbon fabric needed for the M-80 from various worldwide sources, the carbon fiber supply will be a factor to be reckoned with, should M Ship begin to tap the large potential market for its double-M carbon composite ships.