Plastics take to the water

Engineering plastics and boat engines already go together like ducks and water. With their corrosion resistance and ever-improving structural capabilities, plastics have taken the place of metal in components both inside and outside the engine compartment. Just look at the newest outboards from Mercury Marine for a lesson in all that plastics can do. The company's 225 to 275 horsepower, six-cylinder Verado engines, the first supercharged four-stroke models in their class, make use of polymers for their air-intake manifolds, cam covers, engine control unit housings, resonators, plenums that help reduce engine noise during idle, and most of their fuel modules.

Three is Better than One

Marine engine covers long ago went into plastics, mostly single-piece designs made from sheet molding compound (SMC) or thermoformed sheet. Verado's new engine enclosure, by contrast, consists of three injection-molded plastic cowls. The topmost cowl stands out for its sheer size. Made from a 33-percent glass-filled nylon 66, this part measures 33.5 x 22.9 x 16.4 inches and weighs 11.3 lbs. Mercury plastics engineer Mitesh Sheth calls it the "largest injection-molded cosmetic nylon part in the world," a claim that DuPont Engineering Polymers, the material supplier, confirms. A structural rear cowl made from the same nylon mates with the top cowl along a diagonal split line and attaches to an aluminum structural member that joins the engine to the boat. A smaller front cowl molded from two pieces of glass- and mineral-filled DuPont Minlon nylon completes the upper engine housing.

A few other molded plastic parts go into the cowl assembly. A PC/PBT air dam cap, which channels air into the engine, attaches to the top cowl. And the rear cowl has a separately molded nylon 66 structural rib that Mercury attaches with a urethane adhesive. Finally, the Verado sports a lower cowl to cover the drive shaft housing. Though it's mostly conventional in its design, Mercury engineers chose to mold this lower cowl out of a DuPont Sorlyn ionomer, a type of thermoplastic known for its ability to support high-gloss, molded-in color.

The whole assembly might seem a bit complex at first glance. After all, marine engine top cowlings have until now had simple, single-piece shapes. John Zebley, Mercury's director of design, goes so far as to describe these earlier designs as "inverted trash cans with a seal." Yet using injection molded plastics and splitting the cowl into pieces worked to Mercury's advantage in several ways.

Reducing cost is a big one. Sheth explains that the nylon components can reach a Class A finish with less paint than the SMC parts. "They also have fewer defects, like orange peel, which increases our yields," he says. And the lower cowls, with their molded-in color, don't need any paint at all. Sheth estimates that these paint line savings contribute to a roughly 46-percent reduction in piece-part costs compared to SMC.

Weight reduction is another advantage. Tom Walczak, cowling project engineer, reports that traditional SMC cowling represents as much as 20 percent of the entire engine weight. At 35 lbs, Verado's cowling weighs 30 percent less than SMC. And that savings makes a big difference in an industry that equates performance with power density.

Some of that weight loss comes from the thermoplastic's specific gravity advantage versus SMC. But the design freedom from molded plastics contributed on this score as well. Sheth notes that plastic parts have relatively thin nominal wall thicknesses while the SMC parts would have required "bulked up" regions to mold properly. And the injection-molded parts allowed molded-in features impossible in a compression-molded part. As an example, Walczak cites the Verado cowling's patented integrated latching system—in which formerly separate hinge mounting points, cable guides, and brackets have been molded right into the top cowl. This design integration saves some weight by eliminating bolted-on latches. It also contributes to a reduction in assembly costs on top of the 46 percent savings on the paint line.

The multi-piece nature of the cowl likewise furthered some important design goals—ones related to packaging the engine and sprucing up its visual appearance. The Verado's straight-six configuration and supercharger together resulted in a tall engine. Zebley says the diagonal split line and overall styling of the cowling combine to "help mask that height." In balancing this industrial design objective against engineering needs, Mercury's engineers and designers worked together from the beginning of the project and often had to make some compromises. For example, the engineers wanted to leave some clearance in the engine compartment for air flow. They also wanted to leave some wiggle room to allow relative movement between the engine block, on its mounts, and a cowling that rigidly attaches to the Verado's aluminum mounting structure. Walczak says that industrial design ended up "tweaking their styling lines a little bit" to give engineering that much-needed clearance.

Breaking up the cowling also solved a classic draft angle problem. Getting a tall, thin, bucket-shaped cowl out of a mold would have been nearly impossible without adequate draft. Even a small amount of draft would have flared out the bottom of the cowl. And that width would have interfered with the 26-inch spacing needed to put more than one motor on a boat. "Splitting the cowl made the draft angle of the individual parts less of an issue," Walczak says.

Finally, the split cowling added an important ease-of-use feature. Rather than struggling to lift off a heavy, single-piece cowl from outside the boat, boaters can now reach across the top cowl, unlatch it, and remove it from inside the boat.

Tough Material

Aside from its role in dressing up the Verado visually, the cowling also has to play a structural role in order to protect the engine. Mercury Marine designs the cowlings to resist deflection under static loads corresponding to a 200-lb person with size 10 shoes standing on top of the engine cover. "People shouldn't get in the boat that way, but they do," Walczak says. And the cowling must hold up to impact loads on all sides of the outboard—both above and below the waterline. One of Mercury's tests involves smashing a fast-moving fake log into the leading edge of the submerged gearbox, which forces the engine to pivot violently out of the water. According to Walczak, this test can result in 300G impact loads, which ultimately transfer through the cowling components. So rigorous is this test that it makes some of the company's other impact tests almost a formality. "If the cowling survives the log test, it survives everything else we throw at it," says Walczak.

Making sure the cowling did survive called for some design intervention as the cowling matured. For instance, Walczak and Sheth added a structural rib to the inside of the rear cowl to help tie it to the engine structure. Mercury glues in the rib using a structural urethane adhesive that gives them a joint with a lap shear strength of more than 500 psi. Molding in the rib would not have worked in this case because the rib's stiffness would have interfered with the unique ejection system used to get this horseshoe shaped part out of its mold—a proprietary "floating island" design from CDM Tool & Manufacturing Company. "The cowl needs to flex in the tool to allow ejection from the mold," Sheth says.

The Right Material

A good understanding of material properties also helped the parts meet the structural requirements in the face of environmental and thermal factors that often reduce plastics' working mechanical properties. Sheth had to account for a number of these factors in picking the right material for the job: The cowling parts experience temperatures up to 230F as well as exposure to UV light, salt water, rain, snow, and chemicals ranging from fuel to suntan lotion. The need for a Class A surface further complicated the choice. "Glass-filled nylon 66 was the least costly material that was both structural enough and aesthetic enough," Sheth says.

But even picking the right nylon wasn't straightforward given the conditions in which the cowls operate. Before picking a material, Mercury engineers performed extensive finite-element-analysis work on the cowl using ANSYS software. And this work told Sheth that he needed a material with a flexural modulus of at least 700 kpsi. That figure does fall well within the capabilities of reinforced nylon 66. In fact, the grade that Mercury uses has flexural modulus of roughly 1,300 kpsi. Yet Sheth's experience with plastics told him that the material wouldn't perform anywhere near that well in the field. "We needed something with 700 kpsi after accounting for the effects of moisture, heat, and chemicals," he says. "And that's not something that FEA will tell you." It took extensive material testing to tell him what he did need to know—that the 1,300-kpsi grade under consideration would have a flex mod closer to 800 kpsi in the engine's actual operating conditions. "That was still adequate for our application," he says.

Sheth also put a great deal of effort into predicting and controlling glass fiber orientation in the cowl parts. Orientation obviously matters for structural reasons, but it additionally influences shrinkage and warpage that could have hurt the fit of the cowl components. "The shrink on the top and bottom had to be exactly the same," Sheth says. What's more, the location of the fibers relative to the surface of the part can hamper the ability to get Class A surface—if the fibers read through the resin surface.

During the design stage, DuPont performed a mold-filling analysis to see how the fibers lined up in the part under different gating scenarios and molding conditions. Later on, he worked with experts from Mercury's molder, Bemis Manufacturing Inc., and from DuPont to fine-tune the molding methods and conditions influencing fiber orientation. They decided, for instance, to mold the top cowl in a sequentially valve-gated tool, which helps manage fiber orientation by controlling filling patterns.

Keeping Quiet

Another important role for the Verado's cowling involved its contribution to reduced noise, vibration, and harshness (NVH). Not all of the engine's hushed sound comes down to the cowling. "Many things have been done right to make this engine quiet," says Tim Reid, director of engine design for Mercury R&D. He cites a short list for the Verado that includes improved gear quality in the drive train, a well-tuned air intake system, the plenum that tunes the idle exhaust, and a resonator that cancels out some of the rotor noise from the supercharger.

But the cowling undeniably helped quiet things down and offset some of the noise added by the supercharger. "Prior to designing the cowling, noise measurements taken from early engine prototypes gave us some concern," Walczak recalls. "Initial noise measurements of the uncowled Verado came in higher than the noise measurements of some of our uncowled competitors."

Yet the design team didn't just settle for a noisy engine. Walczak reports that they put extra effort into optimizing the air intake system, including the geometry of the air dam on the top cowl as well as the intake-manifold passages. Noise considerations also informed the material selection process. Sheth says that he sifted through dynamic mechanical analysis data and asked for sound transmission tests on a panel samples before selecting the material for the cowlings. Their efforts have paid off. "We believe we now have the quietest engine in its class," says Walczak.