Composite rigid inflatable boats adapt for hard work, safe play
Whether it’s a military or emergency response team looking for a dependable workboat or a leisure boater seeking a comfortable cruise, the rigid inflatable boat (RIB) is fast becoming the watercraft of choice. A RIB is a lightweight, high-performance, high-capacity boat constructed with a rigid hull that is fitted, at the gunwale (the boat sides above the waterline), with an inflatable tube. This tube absorbs vibration and the shock of water swells, increasing the craft’s stability and yielding a smooth ride.
RIBs are giving conventional boat constructions a run for their money for other reasons as well: “RIBs are more efficient because they can achieve the same speed as a hard-sided boat with less horsepower and less drag,” says Matthew Velluto, director of marketing for RIB manufacturer RIBCRAFT (Marblehead, Mass.). RIBCRAFT’s director of operations Matt Provenzano adds that RIBs also can carry heavy payloads.
The roots of RIBs can be traced back to demand for offshore vessels that could handle extreme sea states. The first designs that combined a rigid hull with inflatable buoyancy tubes appeared in 1967; a design developed at Atlantic College in Wales, U.K. was patented in 1969. The first commercial RIBs were used as lifeboats in Solent, U.K. in 1970. After the original patent expired (circa. late 1980s), RIB construction expanded significantly. One estimate puts the number of RIB manufacturers as high as 1,000 worldwide.
Although the greatest growth in the RIB market has been in Europe, Velluto says business in the U.S. is surging. “It is the only segment of the marine industry that is showing significant growth in the U.S.,” he claims, crediting much of the gain to the fact that the boats “play in the business and recreational segments.” RIBCRAFT’s business, he notes, is approximately 30 percent recreational and 70 percent professional.
They are popular because of their ease of use, says Christian Stimson, founder and director of Stimson Yachts (Cowes, U.K.). “For the leisure market, it is great to hop in a boat that [is] virtually unsinkable because of its buoyancy,” he says. RIBs vary in length from as little as 2.7m/9 ft to more than 24.4m/80 ft, with larger models often earmarked for military applications. According to Stimson, the average size of a RIB is about 9.1m/30 ft.
The use of composites has become commonplace in the marine industry — a statement that holds true for RIBs, which make use of a host of composite materials, including glass, aramid and carbon reinforcement as well as polyester, vinyl ester and epoxy resin systems, depending on the desired characteristics of the boat and the size of the customer’s wallet.
Low Weight, High Performance
Boat manufacturers and designers turn to composites to achieve lower weight and greater corrosion resistance, yet achieve performance on par with aluminum constructions. Stimson notes that RIB designs save considerable weight. “The hull and deck surface areas weigh approximately 20 percent less for a RIB than for a hardboat of the same construction,” he explains. For a 9m/29.7-ft RIB, “that would translate directly to a 100 kg/220-lb weight savings on the composite shell.”
In terms of stiffness, Stimson calculates that a RIB made from E-glass — solid laminate, no core — offers about three-fourths the stiffness of a comparable aluminum construction. Carbon, however offers twice the stiffness (see chart, upper right). To optimize stiffness, Stimson Yachts uses sandwich constructions cored with foam. Compared with a solid laminate, the sandwich laminate, with a core equal in thickness to the skins, does not increase boat weight significantly or reduce laminate strength, but increases its stiffness sevenfold. During design, Stimson optimizes designs with two CAD packages: Rhino from McNeel North America (Seattle, Wash.) and SolidWorks from SolidWorks Corp. (Concord, Mass.). Stimson uses one or more of three types of closed-cell (water-resistant) foam core to avoid taking on water: Corecell high-density styrene acrylonitrile (SAN) foam from Gurit (Magog, Quebec, Canada); Airex PVC (polyvinyl chloride) and PET (polyethylene terephthalate) foams from Alcan Airex AG (Sins, Switzerland); or Divinycell rigid PVC foams from DIAB Inc. (DeSoto, Texas).
Velluto notes that 90 to 95 percent of RIBCRAFT’s business is done primarily in glass-reinforced composites. “We stick to fiberglass mainly out of customer demand,” says Velluto. “It is what is known. It is more mainstream with boats.”
Production methods vary depending on the model. For its 7m/24-ft RIB, for instance, RIBCRAFT uses an infusion method and builds the boat to military specifications. For the infusion process, the company uses separate molds for the hull, deck and the stringer/stiffener grid. Layups are vacuum bagged, ambiently cured and then parts are joined using methyl methacrylate adhesive and secondary tabbing.
While fiberglass is typically the material of choice, RIBCRAFT turns to carbon for the transom on its 9m/29.7-ft RIB. For this model and several others, RIBCRAFT uses DuPont Advanced Fiber Systems’ (Wilmington, Del.) Kevlar aramid fiber strategically in the keel and other damage-prone areas to increase abrasion and impact resistance. RIBCRAFT sources composite materials through distributor Composites One (Arlington Heights, Ill.), including core products manufactured by DIAB Inc. and vinyl ester resins from both Cook Composites & Polymers (CCP, Kansas City, Mo.) and Valspar Composites (Wheeling, Ill.).
When RIBs are used as ship-to-shore tender boats — that is, service boats that are deployed from and then hauled back aboard larger vessels — unidirectional fabrics are used to strategically reinforce the boat’s lifting points, the location of which can vary based on each boat’s center of gravity. “There is an incredible amount of load at this point,” says Provenzano.
Custom Made to Order
Composite construction has enabled RIB builders to offer customers a basic boat design that is reconfigurable before the build, rather than a standard boat that must be altered after the sale to meet specific requirements. RIBCRAFT’s deck molds are modular, enabling the company to mix and match parts to meet customer needs.
“Each of our molds can easily allow for customer personalization, including different console, seating, storage and power configurations,” says Velluto. The company’s 5.85 professional model (5.85m/19.2-ft), for example, can be had in a number of different formats depending on the choice of subcomponents. “We have multiple console and seating layouts,” he explains, permitting customers to choose, for example, from a “rescue” control console or a “voyage” console, seat pods or leaning bolsters. “The mounting flanges and bolting patterns are the same for all our consoles and seats,” he adds. “All we have to do, internally, is mount the part specified by the customer, which can be done with relative ease. Use of modular parts cuts manufacturing times and increases standardization and overall quality of the product.” The 5.85’s internal grid, for example, can support a single outboard, twin outboard or single inboard engine arrangement, he adds, “all on the same platform without structural changes.” The only changes required to adapt the boat for the various engines are in the deck panels and the control console.
Similarly, Stimson Yachts offers a semicustom RIB design service, using deck and hull tooling designed to permit easy changes to boat configurations. “The tooling of the deck is basically flat, with a vertical bulwark to the tube carrier. On that flat deck, a variety of submoldings can be fitted in various locations, depending on the configuration — inboard or outboard engines, console forward or aft, bench seats or jockey seats,” he explains. “We also have the option to insert rails into the deck to allow, say, a set of bench seats to be changed out for a set of dive tank racks and jockey seats, to reconfigure the boat.” Further, the company says its high-cure-temperature composite tooling was developed to enable several build techniques and material combinations to be used to meet specific client requirements, “High-temp in this context means our molds can accept 80°C-plus [176°F] postcure temperatures, when working with SP SE84 carbon/epoxy prepreg,” Stimson explains. “The molds are of 120°C [250°F] carbon prepreg to ensure consistent rates of thermal expansion.” SP SE84 carbon/epoxy prepreg is supplied by Gurit (Isle of Wight, U.K.). Because the tooling handles carbon/epoxy prepreg with a high-temperature oven postcure, the same tools can be used with the company’s relatively inexpensive and less technically challenging wet layed glass/polyester solid laminate, and also enable fabrication of sandwich laminates wet layed with E-glass, Kevlar and/or carbon infused with either epoxy or vinyl ester, with or without oven postcure.
Stimson acknowledges that the upfront costs of composites vs. aluminum fabrication are greater because an investment must be made in tooling, but he notes that in the case of RIBs, parts are produced in sufficient numbers to amortize that upfront investment. Further, once the molds are built, actual production time vs. aluminum is reduced. The net result is a overall reduction in per-unit cost.
Stimson Yachts, from day one, embraced computerized 3-D modeling for its designs. The 3-D visualization offers the surface area and centroid (the center of the surface around which the boat surfaces act, similar to the center of gravity, around which all the component weights of a boat act.) “With that centroid and other surface area data,” Stimson says, “we can apply a laminate schedule and derive a weight for that component, which can be added into the boat’s center-of-gravity calculations.” Using 3-D modeling software, he says, “there is no need for estimates. It is much more accurate.” Master models or plugs are cut on 2-, 3- or 5-axis CNC machine tools, with the 5-axis mill used for more complex parts. “Computer capability is continuously improving so, in turn, we are more confident that what we see on the computer screen, we will see on the production floor,” he maintains. “Individual, complex components can be made from one computer software program and manufactured in different locations, and yet still fit together accurately.”
The Value of Vinyl Ester
While RIB manufacturers are open to using several resin types, vinyl ester is often selected for its cost efficiency and its durability — specifically, its resistance to osmotic blistering. Vinyl ester is less porous than polyester and therefore better resists water ingress through the outer resin layer where the moisture then reacts with chemical residues (e.g., curing agent, fiber sizing) in voids, forming an acidic mix that ultimately creates pressure beneath the surface, raising a blister on the part surface.
Vinyl ester also offers another advantage. The core and reinforcements carry most of the load, “once you get them in the right order and orientation,” says Richard Downs-Honey, CEO of marine design firm and distributor High Modulus (Auckland, New Zealand and Hamble, U.K.). “You shoot for gains with the resin system after you get the dry laminate structure decided.” For most RIB designs, a primary goal is to optimize the laminate in the bottom of the hull shell because this is where dynamic loads are greatest. Resin selection is a big part of this optimization. Although some manufacturers use polyester, “the load level at which initial damage occurs will be higher in vinyl ester and epoxy,” Downs-Honey says. Vinyl ester, however, has higher elongation properties than epoxy, so it can reduce microcracking — the point at which the laminate reaches a stress level where the resin begins to crack away from fiber reinforcements that are not aligned with the applied load.
Likewise, Stimson considers vinyl ester paired with E-glass fabric the most cost-effective alternative for its RIBs because vinyl ester offers the production ease of polyester, but yields structural properties near those of much more expensive epoxy prepreg.
That said, the company recently fabricated one of its most sophisticated designs from carbon/epoxy prepreg, a Stimson Yachts’ 2006 design for a RIB tender for Peter Harrison’s Farr 115 ketch Sojana. When the designers began, the goal was to create a lightweight tender to the dimensions specified by the client. However, the Sojana’s transom garage, where the RIB would be stored, was smaller than the tender’s specified size. The solution was to design a 2m/6.6-ft removable bow unit on the tender that allowed the 7.5m/24.6-ft-long tender to fit diagonally into the 6m/19.7-ft-long compartment. Because the boat would be hoisted by cables attached to the ketch’s mizzen boom and inserted into the transom though an opening in the aft end of the ketch (see photo), weight was a primary design constraint. Therefore, the boat’s primary hull structure and removable bow section are sandwich constructions, featuring DuPont Nomex (aramid honeycomb) and Corecell foam core between skins made from SP SE84 carbon/epoxy prepreg and cured at 90°C/194ºF. This achieved weight control superior to that possible with wet layup, according to Stimson. The bow/hull joint is accomplished with a dihedral (split) bulkhead, with one of its matching panels laminated to the main hull and the other affixed to the bow section at the parting line, with three locking pins and titanium fasteners that ensure alignment.
“Suitability determines material choice,” Stimson sums up, but notes that in all designs, there is a tradeoff between the materials technology, cost and processing methods. While material selection sometimes can be dictated by what the customer can afford, careful design can help mitigate the per-pound premium of carbon fiber. “Six hundred grams of E-glass can be replaced with 450g/15.9 oz of carbon,” he explains, “which means builders can use less resin, material and labor.”
Kits Streamline Rib Manufacture
Growth in the RIB market is stimulating interest in cutting and kitting services among builders of smaller boats. Cut/kit programs are part of a trend among materials distributors toward offering value-added services that can include assistance with product design, protoyping, toolmaking, testing and even limited short-run production. High Modulus uses customer-provided design files to generate instructional code for automated cutting systems supplied by Eastman Machine Co. (Buffalo, N.Y.) that precut and prelabel core and reinforcement materials for the company’s boatbuilding customers. When High Modulus developed what it calls the B3 (Build Boats Better) SmartPac program, it was envisioned as an attractive option for manufacturers of small boats (less than 7.6m/25 ft) with limited engineering expertise. Early customers were primarily builders of larger boats (9.1m/30 ft to 15.2m/50 ft) that, according to Downs-Honey, understood the value of an engineered structure. Among small boat manufacturers, he says, “Uptake has been quicker with RIBs because they are structurally more complicated than the typical runabouts.”
High Modulus says its SmartPac program and the company’s design expertise are helping RIB shops streamline production and improve boat quality. In recent work with a boatyard in China, High Modulus reviewed the boatyard’s first 9m/29.5-ft performance RIB, identified several build issues and then provide a kitted solution that resulted in a boat that now matches the expectations of the designer and has a more consistent and repeatable build quality, according to Downs-Honey. “The first boats off the production line in the 9m/29.5 ft were heavier than expected, despite a well-developed, engineered and drawn specification,” he says. “Quite often, it is a culmination of a multitude of minor amendments to the drawings and specification on the shop floor that create a difference of this order of magnitude.” (In one case, High Modulus reports, hulls and liners were 50 percent heavier than expected.) Downs-Honey points out that the unexpected weight wasn’t due to the presence of excessive resin in the laminate — often the assumed culprit. Instead, the weight accrued as substitutions of more readily available “equivalent” materials were made on the shop floor. “This happens when the specification is developed without regard for the actual styles of material used regularly in the particular yard. More often than not, the builder will ‘up size’ to be sure of no structural issues,” he explains, noting that calculated fabric weights are often increased by a 10 to 20 percent “fudge factor” to cope with process variability.
A factor that contributes to overbuild is the common practice of simplifying the specification — and the build — by selecting, for instance, a single thickness of a particular core or one weight of multiaxial for use throughout the laminate structure. For the Chinese firm’s RIB, High Modulus laminate engineers took the opposite tack with a more complex specification. They performed a detailed analysis of the structure and, as a result, specified two core materials in 13 thicknesses using 11 reinforcements, tailoring laminates in various zones to optimize weight and reduce both material cost and labor. The strategy trimmed the weight of the original design by 18 percent. The company’s cutting/kitting software enabled the High Modulus team to digitally “unfold” the boatbuilder’s 3-D model and create flat patterns for every piece of fiberglass and core. Because materials were precut and prelabeled, says Downs-Honey, “the increased complexity wasn’t an issue for the builder.”
Lightweight, super-buoyant RIBs are a value segment in the marine industry, given their cross-market appeal in both the commercial and recreational sectors. They have become a profitable option for the boatbuilder as well, because the relatively simple concept lends itself to modular composite construction and can be easily reconfigured to the specific needs of the buyer in both markets. “It is,” RIBCRAFT’s Provenzano sums up, “a healthy business.”