The result is a new boat design that uses standard two-pound density marine foam urethane and glass fibers instead of high-density expensive core materials. These are very inexpensive materials whose density restricts their use as a core and are instead used to form shapers that hold the framing in place and support compression boards that keep the frame from buckling. The design allows the frames to carry the load by having enough glass on them.
Instead of using stringers to connect the deck and hull, the boat’s structure is formed from a grid of preformed composites. Its design looks like an air frame construction, with the deck de-coupled from and suspended on the hull bottom.
The composite hull of the 850B prototype has a sandwich-free construction, low section framing and a single-skin transom.
The boat is designed so when it hits a wave, the hull and deck respond independently. Since they’re not coupled, the boat absorbs the shock energy and uses the deflection of the hull-to-deck cavity to reduce the lock loads. The design mitigates shock and solves what Lewit says is a big problem: “Sixty percent of boat operators report lost time due to wave impact injuries. There are broken bones, back problems and occasional fatalities. The idea of protecting the guys they’re carrying and the driver and mitigating shock is important.”
Lewit’s team implemented the new technologies in two boats for the Naval Combatant Craft Division: an 8.5-meter working demonstration craft named the 850B and an 11-meter prototype that is currently undergoing tests. Both versions rely on composite materials to significantly reduce hull and deck weight. The 850B is 40 percent lighter and the 11-meter is 30 percent lighter than typical rigid inflatables used by the Navy.
Removing significant weight from the structure allows for much higher-powered propulsion. To demonstrate this, the 850B was outfitted with twin 185Hp Optimax outboard engines modified to use jet fuel. Jet fuel was used not for speed, but because the Navy would prefer to use fuel that is readily available. All Navy ships carry diesel and aircraft fuel, and gas engines pose dangerous fire hazards.
Configured this way, the 850B weighed 3,600 pounds – much lighter than the Navy’s typical 7-meter rigid hull inflatable, which weighs 4,600 pounds. The 850B also comes in significantly under the weight limit (5,500 pounds) for cranes on Navy ships that lift small combatant craft into and out of the water. “It’s hard to change the crane,” says Lewit. “Better to change the boat.”
The lightweight 850B also offers much higher fuel efficiency. “With the weight and fuel savings, it can support an additional 6,500-pound payload,” says Lewit.
In addition, the new laminate design and framed construction is repairable and easier to maintain than a rigid inflatable, says Lewit. The design focuses failures onto the frame. The craft’s first failure mode – if, for example, it runs aground on rocks – is designed so the bottom will deflect and the frame’s sidewall will buckle outward, bending the glass. “The glass can take quite a bit of bending before it cracks. When it does, it is easy to see where the crack happened on the frame and repair it by grinding and patching,” says Lewit.
From elite fighter jets in the sky to small, but indispensable craft in the sea, the military turns to composites for cutting-edge technologies.
Navy ships are relying more heavily on composites, from small components to large structures. Military fleets now use smaller components such as deck drain inserts, electrical boxes, pumps and stanchions. These complex structural parts offer important money- and time-saving benefits when compared to steel or other materials. They can be used in places where fire dangers are high, such as ventilation ducts in engine rooms, since they can be fire-hardened. They are easier to repair or replace, have longer lifetimes and can have lower acquisition and/or life-cycle costs, especially when produced in large quantities.
Composites also are cropping up for the first time in larger marine structures, where they demonstrate significant benefits to military customers. Composite deckhouse structures provide lower infrared signatures. Submarine cover plates offer acquisition cost savings when compared to steel. Composite fairwaters allow divers to perform maintenance in place instead of requiring docking.
