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Gasoline-powered outboards have long ruled the roost, but is gas really the fuel of the future?

There was absolutely no smell at all. Which is what got my attention, really. I mean, prior to coming aboard, I’d expected to catch at least a whiff of diesel hovering over the twin 300-hp Cox CXO300 diesel outboards installed at the rear of our test boat—an Intrepid 345 Nomad center console—especially since our prototype engines hadn’t been cranked in a while. But as I got close, like nose-within-inches close—nothing! And they were quiet, too. Maybe not as quiet as a pair of gasoline-fired Yamahas, Mercs, Evinrudes or whatever, but all the same—67 decibels is fairly discreet for the slow-mo realm. After all, 65 decibels is the level of normal, breakfast-table conversation.

Maybe they’re slightly larger and heavier, but otherwise the diesel-powered Cox CXO300s were indistinguishable from gasoline-fired outboards in terms of driving experience.

Maybe they’re slightly larger and heavier, but otherwise the diesel-powered Cox CXO300s were indistinguishable from gasoline-fired outboards in terms of driving experience.

I was surprised by this state of affairs, I guess, because I figured I had a fairly solid understanding of the marine diesel genre thanks to years of knuckle-busting experience. Diesels, after all, smell like diesels, right? And many of their pumps, hoses and other components tend to feel a little oily to the touch, at least once you put a few turns on the ol’ hour meter. And also, diesels are relatively expensive, comparatively heavy (sometimes wickedly so), often prone to the emission of blue, black or white smoke (depending on the health of a variety of internal components) and, in general, give the environment a slightly harder time than most gas engines.

The ensuing sea trial of the Nomad blew just about all of these preconceptions out of the water. And, at the same time, it also got me to thinking: What is outboard propulsion going to look like in a couple decades? And, more to the point, which source of energy will hold sway? Certainly, given the sophistication of the gas-powered outboards on the market today, with technologies that are cleaner, more reliable and more powerful than ever before, gasoline is likely to continue to lead the parade, at least for the forseeable future.

But things inevitably change, even dominant sources of energy. And at some point I believe gasoline is going to give way to some other type of fuel or power source for outboards. But what will it be?

Smaller Torqeedo outboards feature an electric motor housed in the outboard’s torpeedo. 

Smaller Torqeedo outboards feature an electric motor housed in the outboard’s torpeedo. 

Considering the virtues that the twin Cox CXO300s manifested as I throttled the dickens out of ‘em on the St. Johns River in Northern Florida, I’m compelled to opine that diesel is, without a doubt, an excellent candidate, although there are a couple of other possibilities as well. Electric, for one, is a serious up-and-comer these days. I drove a flashy aluminum RIB built expressly for electric power in Germany last summer and the boat had performance to spare. And then there’s propane or LPG, a marine fuel that’s risen to commercial prominence over the past decade or so, in large part due to its environmental friendliness. But is the stuff even practical beyond a modicum of outboard horsepower?

Argument for Diesel

The average full-throttle speed I recorded for the Nomad was 38.2 knots at 3600 rpm, a top hop that’s fairly competitive with most modern gas outboards with equivalent horse­power, especially in view of the Cox corporate policy that governed our CXO300 prototypes some 400 rpm below the production version’s max of 4000 rpm. Moreover, the total fuel burn at 3600 rpm was 30.3 gph, a figure that generated a 1.26-nmpg WOT operating efficiency. Mid-range speeds of 19.9 knots and 27.4 knots, at 2500 rpm and 3000 rpm respectively, upped the ante ­considerably to 1.64-nmpg in both cases.

What’s the big deal here? When you stack these numbers up against web-based sea trial data from competitive gas outboards (pushing the same weight), you see that diesel is a rousing, range-extending money-saver, particularly for boaters who need lots of low-end torque to push heavy loads. And you also see that Cox’s claim that the CXO300 burns 25 percent less fuel than its gasoline-fired competitors is perhaps just a tad conservative.

A technician building an electric outboard.

A technician building an electric outboard.

Handling and performance? Besides discovering that driving a CXO300-powered boat was virtually indistinguishable from driving a gas-powered center console, I also noticed an almost total absence of particulate exhaust, the stuff that characteristically blackens the transoms of diesel-powered boats and often entails a little end-of-the-day washdown elbow grease.

Unquestionably, the electronically controlled, common-rail diesel technology that first appeared on marine inboards over a decade ago had lots to do with all this. Marine diesels used to get a bad rap for emitting black, greasy soot. But thanks to common rail, initially developed by automotive manufacturers to satisfy tougher and tougher emissions standards, the modern marine diesel has undergone an environmentally friendly facelift. More to the point, marine diesels have way more to recommend them than the mere fact that they burn unleaded, as opposed to leaded, fuel. They also produce less carbon monoxide, fewer hydrocarbons, less carbon-based ­particulates, less C02 and fewer nitrogen oxides than non-catalyzed gasoline engines.

“Our approach to common-rail is quite sophisticated,” says Cox Chief Engineer Ben Sherwood. “We are using a relatively high, 1,800-bar pressure in the system and this pressure has ramifications. First, the system’s integrity—from the CXO300’s pre-filter to its combustion chambers—is secure. It’s tightly and safely sealed, which explains the absence of traditional diesel smell that you experienced during your sea trial. And second, the combustion process itself is much improved because of common-rail technology.”

Sherwood further explains that, at 1,800 bar, the atomization of injector spray inside the cylinders of the CXO300 is exquisitely fine and accurately directed. In addition, its delivery is electronically timed with extreme precision. These two factors promote exceptionally thorough, clean-as-a-whistle combustion. By comparison, lower delivery pressures associated with older diesel engines (150 bar or thereabouts) produce sloppier, poorly timed spray patterns with much larger droplets that burn unevenly or not at all. Carbon builds up on cylinder walls as a result. “Which,” Sherwood concludes, “is where the soot comes from.”

Binnacle-type controls for electric outboards exhibit virtually no clutch-related delay.

Binnacle-type controls for electric outboards exhibit virtually no clutch-related delay.

Case Against Diesel

If convenience, efficiency and environmental friendliness constituted the whole diesel-outboard story, then I’d be personally compelled at this point to conclude, based on my sea trial of the Nomad, that diesel is likely to supplant gasoline as the fuel of choice for the outboard of the future. But there’s more to the story. Both the Cox CXO300 and the only other major-league diesel outboard on the market, the OXE 300 from Swedish manufacturer OXE Diesel, have a few comparative disadvantages.

First, both are heavier than their gas counterparts—Cox reports a weight of 840 pounds for the CXO300 and OXE says its OXE 300 diesel outboard tips the scales at 871 pounds. A 300-hp Yamaha F300 weighs in at just 562 pounds. And gas-fired powerheads, in general, are somewhat smaller too.

Then there’s cost. Cox quotes a retail price of approximately $55,000 for its CXO300 and the OXE outboard retails for a few thousand less. What this basically means is that opting for a diesel outboard or outboards will cost you about twice what you’ll pay for gasoline. Sure, you’ll amortize some of your investment via operating efficiencies later on. And diesel engines tend to live longer than gasoline engines. But do diesel outboards, at present, really make sense for boaters who aren’t putting oodles of hours on their boats? Maybe not.

Then finally, there’s after-market customer care. Presently, Cox is touting a build-out of service centers and distributorships for its “clean-sheet-of-paper,” British-engineered, turbocharged, V-8-configured CXO300. And Laborde Products of Louisiana, a purveyor of numerous established diesel brands, is the distributor for the Scandinavian-based OXE 300 diesel engine, with its marinized, horizontally-mounted General Motors powerhead and belt-driven lower unit. But how will maintenance and other issues that arise with these new diesel products be handled? Will owners get the same competence and speed they’ve come to expect from the long-established outboard players? Only time, as they say, will tell.

A marinized BMW i3 battery of the type that powers the sporty ZenPro 580.

A marinized BMW i3 battery of the type that powers the sporty ZenPro 580.

Argument for Electric

The flashy, electric-powered aluminum RIB I referred to earlier—the ZenPro 580—was built by Naviwatt, a relatively new French company specializing in waterborne e-mobility. The boat, as implied by her designation, is 5.8-meters (19-feet) long, 8-feet wide and weighs just over 2,000 pounds. She had an 80-hp-equivalent Torqeedo Deep Blue 50 electric outboard mounted on her transom and a marinized 40-kWh BMW i3 lithium-ion battery installed beneath her deck. Naviwatt’s engineers had designed and built the 580 as an exclusively electric vessel, especially useful as a harbor boat for marina usage.

The 580’s performance was genuinely impressive. I sea-trialed the feisty little RIB on a large, choppy, “all-electric” lake in Germany, one of many lakes in Europe where governments are either resisting combustion-type marine propulsion by requiring radically expensive permits or banning combustion engines altogether. Not only did she evince a reasonable top-end average of approximately 25 knots, she did so with speedboat style. Indeed, her average time to plane was about five seconds.

Dockside maneuverability was extraordinary. Because the Torqeedo, in line with most e-mobility power sources these days, generated loads of torque instantaneously, the 580 tended to back down with unerring directionality, especially by comparison with combustion outboards that, when going astern, usually need to gather way before they take effect. And there was virtually no lag time between shifting from forward to reverse—thus, no wasted seconds in neutral détente. And finally, I found boat handling itself way more precise, thanks to prop speeds I could slow to a crawl if necessary.

Of course, e-mobility on the marine scene presently has one major drawback—lack of range. The 580, according to the Naviwatt rep on board, could sustain a speed of 13 knots for only about two hours on a full battery charge and a mere 3 knots (a rather unreasonable velocity) for approximately 13 hours. Top hop, the rep added, would drain the battery in about 40 minutes.

Mercury Marine’s 5-hp propane-powered outboard is a lean, clean, green little machine, with decent range.

Mercury Marine’s 5-hp propane-powered outboard is a lean, clean, green little machine, with decent range.

The Winner Is?

But just think. When it was first introduced in 2014, the capacity of BMW’s i3 battery was only 22.6kWh. Last summer, the capacity of the same battery, marinized by Torqeedo and installed under the deck of the 580, was 40kWh. In just six short years, advancements in lithium-ion technology had almost doubled the battery’s capacity and potential range. And the beat goes on, apparently. According to Sean Streven, president of the forward-leaning European powercat builder Cheetah Marine, Cheetah is gearing up to market a vessel with 200-hp-equivalent EV Marine electric outboards energized by a lithium-ion battery pack with a 90kWh capacity, again doubling the capacity and range of extant marine technology today. “Most likely, we’ll offer a twin application eventually,” he says, “but we’ll start with a single outboard on a smaller vessel. End of this year or thereabouts. That’s the time frame we’re looking at.”

There’s a theme afoot here, obviously. Battery technology (and therefore range) is advancing rapidly, pushed primarily by the automotive sector, which historically leads and also predicts progress on the marine front. And what’s more, the automotive sector is going electric quite speedily these days, with plug-in and battery-electric automobiles flourishing all over the planet. In addition to fully-electric sedans and SUVs from Tesla, Jaguar, Toyota, Volvo, Kia, Nissan, BMW, Volkswagen, Porsche, Fiat, Honda and others, American icons like Ford and General Motors are currently adding pickup trucks to their existing e-mobility lineups.

Recently, a friend asked if I thought propane might eventually be the clean, green substitute for gasoline. I replied that I didn’t think so but promised to check with the folks at Mercury Marine. Just last year, Merc added a cool little 5-hp propane outboard to its internal combustion array.


“Yes, propane’s certainly a clean fuel,” Mercury’s propane expert Jim Hergert said when I called him up, “but realistically, you’re looking at a 15-hp cutoff. Anything more powerful than that and, in order to get any kind of reasonable range out of the outboard, fuel stowage becomes an issue. With a 9.9-hp application or even a 15-hp application, you’ll get enough range out of a grill tank—something in that size range—to make propane worthwhile. But beyond that, it doesn’t make much sense for outboards.”

Hergert’s take matches mine. Some years ago, I owned a small boat with a propane outboard manufactured by Lehr Marine. Sure, the fuel for the engine was environmentally friendly, reasonably effective and more or less easy to use, but at length I was forced to concede—the grill-type tank for fuel stowage sniffs the edge of practicality, at least for small-boat applications.

So? Are big, propane-powered outboards looming on the horizon? No, I think not. And although diesel is certainly an attractive possibility, I’m presently constrained to take a wait-and-see attitude towards diesel outboards being the way of the future. But battery-supplied electricity for big, high-powered, long-range electric outboards? Yup, I’d say that’s the coming thing. Ready or not. In 10 to 15 years—tops.

This article originally appeared in Outboard magazine.