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The Latest Technology In Sterndrives


For decades, gas inboards were big, simple, marinized automotive blocks. But now the technology is at a crossroads of sorts, and it’s anything but simple.

A new generation of sterndrives are quieter, stronger, and more fuel efficient.

The inboards on my last boat were based on a V-8 design from 40 years ago, popular, I was told, in taxicab fleets. Robust to be sure, but they were also noisy, smelly, and reliant on the vagaries of the four-barrel carburetors that sat atop each block like carbon-smudged crowns. It may be a cliché, but one of the first things you notice while being around modern inboards is, well, they start. As in the first time you turn the key, just like your car. And that’s no accident, considering there are very few differences between the engine powering today’s boats and the one in your light truck.

Our marine inboards have always shared a basic design with automotive engines. But until recently, the biggest inboard manufacturers, Mercury Marine and Volvo Penta, relied on blocks from General Motors that were a generation or two behind the times. That’s all changed now. In a major departure from the status quo, Mercury announced in June 2014 that it would produce a block of its own design for the MerCruiser engines. Volvo went in the other direction, and it began basing its designs on newer-generation GM blocks. These two different approaches to the same problem — how to improve the boating experience for those of us with gas inboards — can be boiled down to opposing beliefs.

“Six years ago, GM notified us it was discontinuing manufacturing the blocks that we’d used as the basis of our engines for the past 20 years,” says Marcia Kull, vice president of sales for Volvo Penta North America. “At that time, we needed to make a decision. We asked ourselves, are engines more like a basic commodity, or are they an important component that influences the boating experience?” Volvo Penta believes that all the engineering work and technology built into those GM power plants pay dividends to boaters in measurable ways, so the company moved forward using the very latest GM Gen 5 blocks.

Mercury, on the other hand, surveyed its options, then chose a different direction.

“Most of what’s happening in automotive is introducing a level of tech that’s very good for an automotive application but delivers little benefit to marine purposes,” says Facundo Onni, director for product management at Mercury Marine. “Given the marine-duty cycles that require higher rpm and high torque at all times, the automotive fuel-efficiency focus adds complexity and cost without much upside.” Mercury believes starting with a blank slate — designing a new marine power plant from scratch — provides advantages that can’t be achieved with the GM block. As consumers, we now have a real choice between inboards, and here’s why that’s a good thing.

Common-Rail Direct Injection Comes To Gas Inboards

While “Common-rail direct injection” has become the norm in diesel applications, its use in gas-powered inboards only recently made its way into the marine market. With a common-rail system, a high-pressure pump feeds fuel into a single pipe or “rail” to which all the injectors (one per cylinder) are connected. The pressure in the rail can reach more than 22,000 psi for a gas system, and as much as 44,000 psi in a diesel one. The enormous pressures are necessary to overcome the force created by the piston compressing the air inside the cylinder and also to fully pulverize the fuel into tiny, tiny droplets upon injection. The injectors are effectively gateways into the cylinder — miniature valves that open and close, controlled by the engine-management computer. There’s no need for the injector to push the fuel in; rather, the high pressure built up in the rail ensures fuel delivery at the precise time it’s needed.

Between the computer-controlled timing and the atomization of the fuel, direct-injected engines can burn fuel more completely, which means they can use a bit less fuel per combustion cycle and still produce the same power. Plus, the combustion process results in less waste and by-products, so there’s less pollution as well. Bottom line, these systems add some complexity but make your gas inboard cleaner and leaner.

Same Power, Fewer Cylinders

Both builders have rolled out new V-6 engines with horsepower ranging from 200 to 250, the sweet spot for sterndrive power formerly occupied by small-block V-8s. These power-dense engines are truly remarkable in that they’re putting out nearly identical power to their bigger, older cousins while using less fuel and creating fewer emissions — and they weigh less. In the case of Mercury’s 4.5-liter V-6, with its newly designed block, the 130-pound weight savings over the old 5.0-liter V-8 comes at a cost of only 10 horsepower, noticeable only when the throttle is wide open; the V-6’s impressive fuel savings run around 15 percent.

The new Volvo V6 produces 240 horsepower.

Volvo’s 4.3-liter V-6, based on a Gen 5 GM block released to the automotive market just 18 months ago, produces 240 horsepower and weighs less than its predecessor but features common-rail direct fuel injection for clean, efficient combustion. Plus it has variable valve timing, for more oomph across a wider range of rpm, and a host of other advanced technologies. It runs an 11:1 compression ratio, normally the stuff of race-tuned engines, without requiring high-octane fuel, thanks to the combustion-chamber design, ignition timing, and piston-cooling oil jets.

Both Volvo and Mercury have made servicing these engines easier. On the Volvo, the company not only located all the service pointson the front of the block, where you typically have the best access, but also designed these parts to be common to all the inboards in its lineup, V-6s and V-8s alike. This means your mechanic will be very familiar with servicing your motor, no matter which model it is. For the do-it-yourselfers, easy access means you’re less likely to put off a maintenance task simply because it’s inconvenient.

On the performance side, the variable valve timing and wideband oxygen sensors in the Volvo engines allow it to start and run well in all conditions while using a variety of fuel qualities. A higher rpm range (6,000 is the max for the V-6 models) is more like that of an outboard, and this gives boatbuilders greater flexibility in propping their boats for best performance.

“We like the new-generation automotive blocks,” says Kull. “They’re just stronger, more powerful engines. Boats are getting the same performance out of smaller, lower horsepower blocks. The boater isn’t paying more for larger engines up front, or paying to feed those big engines all the time,” she continues. “There is a replacement for displacement.”

The new Mercury block has zero GM parts.

Starting from a blank slate, Mercury focused on ease of maintenance and making the engine smooth and quiet. For example, a lightweight flywheel makes for smoother shifting. Mercury also moved the throttle body, and all the noise associated with it, to the back of the block. The throttle body usually faces forward on an automotive engine, away from the driver of a car. But on a boat, that points the throttle body right into the cockpit. Mercury says the change reduces engine noise by 3 decibels. In addition, the plastic cowling material on top of the engine cuts down on high-frequency noise, making the engine less harsh to the ear. Even the oil pan has noise-deadening properties. Mercury even decided that the fuel-pump noise was irritating, so it changed that as well. All of these refinements add up to a quieter, more pleasant engine to be around.

Mercury also makes owning one of its engines easier with one-touch drainage of the (raw) water portion of the cooling system. Turn a knob, and all the water drains from the block, making winterizing a snap. “We call that the season extender,” says Onni.

A New Way To Drive

Volvo has been busy, creating not only a whole new line of inboards but also an entirely new drive to attach them to. While some might call it backward, it’s really forward, as in a tractor drive, with the props located in front of the skeg. This revolutionary new Forward Drive, as Volvo calls it, promises to change watersports for the better, particularly wakesports.

One of the biggest things to hit the water in recent years has been the advent of wakesurfing. A rider on a short surfboard is pulled up behind the boat, then releases the towrope, freed to surf the wake created by the towboat. But that wake must necessarily be substantial, and the boat needs to be inboard powered for safety. Until recently, only dedicated wakeboats fit this bill, ones that carried water ballast or some sort of tab or wedge to create a reliably big wake and located the prop safely up underneath the hull, well away from the rider.

Now, Forward Drive gives boaters a chance to have a boat that’s capable of serving as a wakeboat but can also cruise the lake all day as well. One of the first to arrive is the Four Winns TS222. The combination of water ballast and the Forward Drive, which can be trimmed to lift the bow just as with a normal sterndrive, throws a clean, symmetrical wake. The six blades on two hubs provide excellent bite (and low-speed maneuvering) for keeping the boat at the 10- to 12-mph sweet spot for wake surfing. Also, the exhaust exits at the back of the drive, well underwater, so most of it bubbles up long after the rider has gone past; this means there’s no “station-wagon effect” of carbon monoxide building up inside the cabin.

While this drive neatly solves the wakesports puzzle, don’t expect it to remain in that niche. Having the props tucked up just under the transom — about 27 inches forward of a traditional sterndrive — also means a cleaner transom for fishing applications. It also improves steering response. Volvo expects a pair of Forward Drives mated to a joystick to perform much like a boat equipped with pod drives, so we may see midsize cruising boats with these in the near future as well.

New Perks

There’s a lot to like about both of these new blocks. Digital engine control means a boat powered by either brand won’t bleed speed while turning or require input from the helm to keep a steady speed when towing a skier, making anyone look like an expert driver. “In lots of cases, this is the entry platform for many boaters,” says Onni. “If we can lower the learning curve for those new boaters, I think we’re all going to be better off.”

It’s easier to breathe near both of these new designs, too. While all new inboards are required to have catalytic converters to transform hydrocarbons (unburned fuel) and carbon monoxide in the exhaust to less harmful water vapor and carbon dioxide, it’s hard to overstate what a big change this is from just five years ago. For example, Volvo says that its new engine at idle emits 95 percent less carbon monoxide than previous designs.

Bigger, Better

This year and next, both Mercury and Volvo are expected to roll out new models featuring more horsepower. Later this summer, a 280- hp version of the Volvo V-6 should arrive, followed by 300- and 350-hp V-8s based on a 5.3-liter block. In 2016, the 380- and 430-hp Volvo models will be updated onto the Gen 5 blocks, completing the transfer to the latest tech. Mercury was tighter lipped but hinted at a fall release for its next motor.

For those of us brought up on engines with carbs and distributor caps, lifting the engine hatch and seeing the smooth plastic cover of one of these modern blocks may be a bit of a surprise. But as soon as you turn the key and push the throttles forward, the improved performance over their forebears should begin to wow you.

Michael Vatalaro

Contributor, BoatUS Magazine