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 double-M concept evolved from an attempt to eliminate shoreline damage caused by surging water from the wakes 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 a lighter weight structure, more interior payload space and faster construction, and produces a vessel capable of greater speeds 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 ap-proximately 16.3m2/175 ft2 each and a 139.4m2/1,500 ft2 working deck, totaling more than 186m2/2,000 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, with 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 might see global torsion loads in the two “tunnels” that run the hull’s length and channel water beneath the hull (see diagram, top of this page) 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°) 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 was 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 provide a smooth 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.