With its new 350-hp DF350A, Suzuki has introduced more than just its largest outboard to date. It’s also the first high-output outboard with counter-rotating propellers.
Despite the wow factor of six gleaming stainless-steel blades, the purpose of the twin-propeller design is a practical one.
“We are an engineering company,” says Gus Blakely, vice president, marine sales and marketing, for Suzuki Motor of America Inc. “That’s what we’re known for, and that being said, we felt like we developed this great motor, but we couldn’t get the power to the water.”
A big engine that makes big power needs a big propeller, and on a typical single-prop outboard that means big gears. Suzuki engineers knew the shape of the lower unit and the design of the propeller would be critical to performance. A bulbous large-diameter gear case would create hydrodynamic drag, and that translates to subpar performance.
By spreading the load over two counter-rotating (Suzuki calls it contra-rotating) stainless-steel props, the company achieved two goals. First, the second propeller improved the motor’s overall grip of the water. Second, the torque per propeller was decreased, which allowed for smaller-diameter gears. Suzuki did strengthen the material of the gears and the heat-treating process to improve longevity and reliability. A reduction in gear diameter led to the design of a smaller, more hydrodynamic gear case.
Another factor contributing to the sleek gear case design is that the motor’s shifting takes place higher in the engine’s midsection, instead of down in the gear case. “We have a full system that’s up in the midsection where the shifting takes place,” says Blakely. “It allowed us to make the gear case very hydrodynamic so we could improve speed over the standard 300 gear case.”
In addition to using the new DF350A’s power more efficiently, the counter-rotating props improve overall handling and slow-speed maneuverability. “We feel like the counter-rotating system grips the water and propels the boat much more efficiently in forward and reverse than a single propeller system,” says Blakely. “We think that after seeing the way this [engine] performs, it’s a great application for a single-engine pontoon.”
As a kind of mascot for the outboard, Suzuki uses the Japanese character for Geki, which means “parting seas.” The product literature reads, “A force to match the power of nature and the sea, representing Suzuki’s identity and heritage, a symbol of our passion and commitment to the ultimate in marine innovation.”
The DF350A, which was three years in development, was introduced to more than 35 marine journalists from the United States and around the world at the Boca Raton (Fla.) Resort and Club on June 9. The company had a number of boats, including a couple of 35-foot-plus center consoles with quad DF350As and a pair of pontoon boats powered by the motors.
Toshihiro Suzuki, representative director and president, CEO and COO of Suzuki Motor Corp., led a group of executives from Suzuki’s corporate headquarters in Japan who made the trip for the unveiling.
Previously the biggest outboard Suzuki made was a 300-hp 4.0-liter (244.1-cubic inch) V-6. The new model is a V-6 with a maximum operating range of 5,700 rpm to 6,300 rpm, a displacement of 4.4 liters (268.4 cid) and a bore and stroke of 3.86 inches (98mm) by 3.82 inches (97mm).
This is the biggest displacement V-6 on the market, and Suzuki didn’t want to lose too much ground in the weight department. Weighing in at 727 pounds for the 25-inch shaft length and 747 pounds for the 30-inch model, the DF350A is about 87 pounds heavier than the DF300, and Blakely says the bulk of that is in the longer gear case and extra propeller.
Speaking of propellers, all of the available wheels for the motor are 15½ inches in diameter, and pitch ranges from 19½ inches to 31½ inches.
Because we’ve already started looking at details of the DF350A by checking out the bottom of the engine, let’s continue with the gear case design. Suzuki used computational fluid dynamics and lots of good old-fashioned test drives to eliminate what the manufacturer called “cavitation voids.” These are areas where the water was not passing the gear case surfaces smoothly. By eliminating the voids, Suzuki wound up with a hydrodynamically efficient lower unit with two sets of water pickups to ensure that enough cooling water gets up through the midsection to the powerhead. The main water intake is positioned just below the prop shaft at the front of the gear case, with secondary inlets just above the skeg, also on the leading edge.
Working our way up, some Suzuki technology that has been proven on previous models is still in use on the DF350A. Suzuki Selective Rotation is available, and it means that any model can be programmed to rotate in either direction. Also, the two-stage gear reduction results in a large reduction-gear ratio (2.29:1) that gives Suzuki the ability to simultaneously prop the boat for quick acceleration and a higher top end.
“We had to work really hard to get this engine to have the same top-end speed as a single propeller,” says Blakely. Not surprisingly, though, it is the DF350A’s acceleration that has Blakely even more excited. “The biggest differences are that at zero to 30 and zero to 50, we’re seeing two- to five-second increases in speed over single-propeller engines,” he says.
Also unchanged is Suzuki’s offset driveshaft design, which places the crankshaft in front of the driveshaft through the use of the upper reduction gear at the top of the engine’s vertical shaft. It moves the engine’s center of gravity forward, resulting in better weight distribution and balance, more directional stability and less vibration.
Now that we’ve worked our way up to the power part of the engine, there are two key design innovations on the DF350A. Suzuki’s goal was to develop 80 hp per liter, but a block with too much displacement would have meant more weight, and the designers didn’t want to use a supercharger or turbocharger to boost the power.
The solution was to increase the motor’s compression ratio to 12.0:1, the highest ratio for a production outboard engine. Yamaha’s 350-hp outboard is built on a V-8 block that displaces 5.3 liters (323.4 cubic inches) and has a compression ratio of 9.6:1. Mercury’s 350-hp Verado is a V-6 that displaces only 2.6 liters (158.7 cubic inches), but it’s supercharged and the compression ratio is not published. A high-compression ratio can often make an engine prone to knocking, so Suzuki developed new fuel-injection and air-intake systems to work with the high-compression ratio.
One of the best ways to prevent knocking is to keep the air-fuel mixture as cool as possible and finely atomized when it’s injected into the cylinder. To keep the air cool as it flows to the injectors, Suzuki engineers developed a dual louver system on the DF350A’s cowling, designing blades with a dogleg design to capture and deflect water particles away from the air in the intake flow. The outer row of blades removes the spray from the boat and the inner louvers capture and prevent remaining mist, keeping water out of the combustion process. The result is that the intake air is free of moisture and never gets hotter than 10 degrees above ambient temperature.
Because Suzuki decided to use two propellers to more effectively put the DF350A’s power to the water, it makes sense that the new outboard has a dual-injector system with two smaller injectors per cylinder. This provides more precise atomization of the fuel at the most efficient angle before it’s injected into the combustion chamber.
Speaking of the pistons, they were also redesigned to better withstand the greater forces of the high-compression engine. The piston surface is shot-peened, which creates fine dimples on the surface to evenly distribute the pressure created during combustion. The connecting rods and bearings have also been beefed up.
Even with the high compression ratio, the DF350A is rated to run on 90-octane fuel, the same as its competition. The DF350A has a 55-degree cylinder bank, which is important because it means the engine can still fit on 27-inch centers when installed in a multi-motor configuration. That, and the fact that it takes conventional hydraulic steering, make the DF350A a good candidate for re-power projects. Suzuki offers it with a joystick system from SeaStar Solutions and can add SeaStation position keeping. Suzuki pairs the DF350A with its C10 helm display, and it can cross-reference with virtually any instrument that is NMEA 2000-compliant. Retail price for the DF350AX is $31,565 for the 25-inch version in black. It is also available in white.
Suzuki expects to have a limited number of the new outboards available to manufacturers this summer so they can have them for late-season dealer meetings. The DF350A should be at dealerships by the end of the 2017 season.
Although early interest in the DF350A will more than likely be for offshore fishing boats, such as center consoles and pontoons, Suzuki is well positioned for it to have far-reaching appeal as manufacturers begin using outboards on family craft, such as runabouts and deckboats.
“Not only is it about the boats you’re going to build today, it’s about the boats you’re going to build tomorrow,” Blakely says.
This article originally appeared in the July 2017 issue.