Marine Propulsion

When it comes to marine propulsion, a lot has changed in the past 60 years. The sterndrive was a new idea. A 100-hp Kiekhaefer Mercury was the most powerful outboard money could buy. A diesel inboard often belched soot that coated a transom with an oily film. By comparison, today’s engines are cleaner, quieter and more efficient, and would be unimaginably powerful to a boater in the early 1960s.

Two factors have influenced the evolution of marine propulsion: intense competition among key market players, and exhaust emissions regulations. In fact, the technology created to control auto emissions has enabled the development of today’s gasoline and diesel marine engines. In the future, the investment auto manufacturers make in hybrid and electric vehicles will likely trickle down to marine power too.

The modern sterndrive was introduced as the Volvo Penta Aquamatic 100 at the 1959 New York Boat Show. It paired an inboard 100-hp engine with a transom-mounted outdrive that was steerable and tilted “to combine inboard efficiency and safety with outboard flexibility and speed.” The Aquamatic also had 30 more horses than Mercury offered in an outboard.

Kiekhaefer Mercury countered in 1961 with the MerCruiser sterndrive, packaged with marinized Chevrolet auto engines at 120 and 140 hp. In 1964, a more robust MerCruiser II drive was offered directly to boatbuilders with familiar American engines rated up to 310 hp — three times the power of any contemporary outboard.

In 1990, following Brunswick Corp.’s acquisition of Kiekhaefer Aeromarine, Fred Kiekhaefer led an effort to transition Mercury Hi-Performance from racing into the production of engines for high-performance pleasure boats, a segment booming in the wake of the Miami Vice television frenzy. One of the early products was the MerCruiser Hi-Performance HP525SC, the first mass-produced, supercharged and warrantied marine engine. The 7.4-liter V-8 was rated at 490 hp on 92-octane pump gasoline. By 1992, Hi-Performance would add 600- and 750-hp supercharged engines to power a generation of performance craft.

Through the 1980s, all popular outboards used a two-stroke powerhead burning gas-and-oil fuel, a combination that produced tremendous hydrocarbon emissions. In the early 1990s, the outboard manufacturers were given about 10 years’ notice that emissions regulations would be forthcoming. This raised debate inside engine companies. The premise of the outboard motor was that it was lightweight for its power; manufacturers could not imagine how a four-stroke engine would ever be light enough. At Mercury and OMC, there were factories with teams of engineers tooled up to manufacture two-stroke engines. But when Honda introduced its 45-hp four-stroke in 1991, it got attention from other manufacturers. The three-cylinder, 808cc BF45 weighed about 200 pounds and was much smoother, quieter and more fuel efficient than contemporary two-strokes. Honda followed with a BF90 model in 1998, pitching its four-stroke motors as “power in tune with mature.” They were a hit with freshwater anglers who could troll for hours without inhaling two-stroke exhaust smoke.

1995 Common Rail Diesel Fuel Injection -A desire to reduce exhaust emissions in auto and truck diesel engines revived interest in an old idea: the common rail injection system. The advent of modern electronic engine controls made it possible to replace traditional mechanical diesel injectors with a hollow rail of highly pressurized fuel running the length of the cylinder head. The high-pressure common rail system atomizes fuel into fine particles that ignite easily and burn completely, significantly reducing emissions. Nippon-Denso introduced the first common rail system for truck engines in 1995, and the technology soon appeared on marine diesels from Volvo Penta, Caterpillar, Cummins and others. The system was further refined with the introduction of pulse injectors, which deliver fuel in multiple bursts in each cycle.

Doubting the viability of a large four-stroke outboard, and stuck in the prevailing two-stroke culture, Mercury Marine and OMC pursued a low-emissions two-stroke solution: direct fuel injection (DFI). The system would inject atomized fuel directly into the combustion chamber, timed electronically to prevent unburned fuel from escaping through the exhaust port, which is the main source of hydrocarbon emissions. The Mercury OptiMax 200 with DFI debuted in early 1996, and Evinrude and Johnson models with Ficht RAM DFI came to market in 1997. Yamaha would introduce its HPDI (High Pressure Direct Injection) DFI two-stroke models in 2000. Each company’s DFI technology cut emissions by about 30%, enough to meet the government’s 2006 emissions regulations, and improved fuel efficiency by 30%. Citing near-complete customer preference for four-strokes, Yamaha dropped its HPDI two-strokes in 2015. Mercury replaced its last OptiMax Pro XS models with V-6 and V-8 four-strokes in 2018. BRP persisted on the two-stroke track, introducing Evinrude G2 motors in 2014, but after 20 years of trying to win over boaters with DFI technology, BRP discontinued Evinrude production in 2020. Yamaha pushed the four-stroke outboard further into the mainstream with its F115 model. It applied casting technology developed for 450cc four-stroke motocross motorcycles to reduce powerhead weight. At 401 pounds, the F115 was 40 to 50 pounds heavier than a DFI two-stroke, but about 70 pounds lighter than a four-stroke Honda BF115. The Yamaha was smooth and quiet. As four-stroke outboards became more powerful and were rigged to larger boats, the weight difference became less significant. In 2001, Yamaha introduced a V-6 F225 four-stroke that was as compact and lightweight as a two-stroke.

The joystick has been called one of the 20th century’s most distinctive contributions to the interface between people and machines. It was initially related to flight, but a lot of manufacturers, including Hinckley Yachts, saw that it could be used for other things. In 1998, the builder introduced its JetStick, a joystick that simultaneously integrated control of the boat’s main jet drive and side thrusters. With it, the driver could rotate the boat almost within its own length, or slide it directly sideways to snug up to a dock. In 2005, the era of digital joystick controls arrived with the introduction of the Volvo Penta IPS. In 2007, the Cummins MerCruiser Zeus pod drive was introduced with all-digital controls that included the SmartCraft Joystick Docking System. IPS and Zeus achieved the level of close-quarters maneuverability offered by the Hinckley JetStick, but without the use of thrusters.

Digital steering and throttle/shift control made the joystick possible. With constant refinement by Volvo Penta, Mercury Marine and Dometic’s Seastar Solutions, joystick control is now available for all steerable propulsion systems. Linking digital steering with GPS has enabled station-holding features and digital autopilot systems. Today, we are on the cusp of workable autonomous docking and avoidance systems made possible by being able to independently control the steering, shifting and throttle of multiple engines.