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Navy Craft Absorbs the Hits

The 7M Advanced Combatant Craft (7ACC), displayed at CAMX 2018, represents a new approach to small craft construction for the U.S. Navy. The boat is significantly lighter and more resilient than its predecessors, and its innovative design reduces the risk of injury to sailors who navigate it through rough seas. It took years of work, the development of several new technologies and the close cooperation of many companies to create the 7ACC.

The project began in 2009, when Structural Composites won a grant from the Congressionally-funded Small Business Innovation Research Program (SBIR) to work with the Navy on improving its advanced combatant craft. At that time, the boats’ decks were bonded to the hull, creating a stiff structure that transmitted the shock loads from ocean waves to passengers’ bodies. This caused a high number of injuries. To solve the problem, Structural Composites built upon some work it had done for rescue boats in Norway and for an amusement park’s stunt boat.

“We changed the whole framing of the [advanced combatant] boat; it looks like airplane construction,” says Scott Lewit, president of Structural Composites Inc. and Compsys Inc. Instead of using large stringers – the typical supports for a hull – the new advanced combatant craft utilizes a skeleton grid of PRISMA®, a composite preform made of fiberglass knit fabrics and flotation-grade polyurethane foam.

Rather than attaching the deck to the hull, Structural Composites used large PRISMA cross sections to form the deck framing. The deck is then connected to the boat’s gunnel – the top edge of the sides – so it floats suspended over the hull. When the small 7ACC craft takes a hit from a wave, the shock waves are first absorbed by the boat’s more flexible hull, then passed through the sides to its suspended deck. That greatly reduces the impact on passengers.
There was another benefit to this configuration. Using the grid approach, Structural Composites was able to use a thinner laminate for the hull. This reduced the boat’s weight, saved money and made manufacturing simpler.

However, with a less rigid laminate hull, Structural Composites needed a different surface coating. “One of the challenges that you have if you’re going to make things move around, flex and absorb energy is the gel coat,” Lewit says. Flexing in the hull or deck can crack the brittle gel coat.

With funding from Florida’s SBIR enhancement grant program, Lewit experimented with other coatings. By blending polyesters with polyurethanes, he created a coating called CoCure™ with greatly increased elongation properties. CoCure contains four components: a gel coat, an initiator, a B-side polyurethane component (polyol) and an A-side polyurethane component (isocyanate). A gel coat with no urethane has about 1.4 percent elongation. CoCure 15, a coating with 15 percent urethane, has an elongation of 3 to 3.5 percent, and CoCure 20 has an elongation of about 10 percent. With this greater elasticity, the gel coat is more resilient and less likely to crack, says Lewit.

» Author: Mary Lou Jay

» Publication Date: 06/11/2018

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This project has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° [609149].

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