• The new Cox CXO300 is the largest diesel outboard engine built today.

  • This motor was designed to be a dedicated outboard engine, only, from the very beginning.

  • The Cox Diesel outboard is heavier and initially more expensive than gasoline outboards, but is also more efficient and should last much longer.

  • Tender owners and long-range sportfishermen should be they types of boaters interested in the CXO300.

Cox Marine has begun production on the CXO300 diesel outboard engine, a twin-turbo beast that unlike most marine diesel engines was designed to be an outboard from the moment it was conceived. It’s also destined to be the most powerful diesel outboard available today. We got our first glimpse of the Cox at the Fort Lauderdale International Boat Show, where we shot this quick First Look Video.



Unlike some manufacturer’s past attempts at creating diesel outboards, the Cox (which is built in the U.K.) actually looks like it was meant to be an outboard in the first place and doesn’t appear to be abnormally large. In fact, we saw a pair perched on the transom of an Intrepid 345, where they looked perfectly at home and proportional to the boat. Wait a sec—why put a pair of diesels on a performance boat like the Intrepid? Actually, this is an ideal test-bed for the CXE300s. Intrepids are popular tenders for mega yachts, which run on diesel fuel. And carrying an assortment of different fuel supplies is always problematic—not to mention dangerous. By allowing yachters to utilize a single-source fuel supply and eliminate gasoline from the mix, Cox expects tenders to be one of the types of boats their diesel outboards are most popular on.

A pair of 300 HP Cox outboards look perfectly normal, mounted on the back of an Intrepid 345.

A big part of the reason why other diesel outboards have looked so odd in the past is that they were created by taking an engine developed for an entirely different use, standing it on end, and hitching it to a lower unit originally designed and built for a gasoline engine. Not so, in this case. Cox started fresh, and designed the CXO300 from the ground up to sit vertically, mate with a lower unit that’s not at all unlike those used by other outboard manufacturers, and fit inside a relatively compact package. This also enabled them to locate the engine’s service points in convenient spots, and outfit it with a NMEA 2000-complient ECU and a traditional mounting bracket.

Having these pieces-parts designed for outboard use from the start allows the Cox to play nice with the rest of your boat. That NMEA 2000 brain, for example, means you can use just about any modern data display for your engine monitoring. In fact, you can pipe the data right to your MFD. And the engine can function with off-the-shelf steering systems, even advanced versions like the Optimus joystick controls.

Since it began life as an outboard, the Cox is able to interface with other marine systems.

Running with a diesel outboard will net you a boost in reliability, longevity, and economy. So, why doesn’t everyone do it? Aside from initial cost – the CXE300 is expected to go for around $50,000, or about twice the cost of a gasoline outboard with the same amount of power – weight is a big issue. And yes, these engines do weigh significantly more than their gasoline counterparts. The new 4.6-liter Mercury V8 FourStroke, for example, has a listed weight of 538 pounds; Yamaha’s F300 Offshore V6 weighs 562 pounds; and the Suzuki DF300A tips the scales at 639 pounds. The Cox tops them all significantly, with a dry weight of 826 pounds.

The good news? In most cases, boats designed to handle engines of this size have a significant amount of displacement, and won’t be affected too terribly much by a couple hundred extra pounds. In fact, as a percentage of weight in a twin-engine application like that Intrepid, a few hundred pounds is almost negligible.

Plus, there are some additional advantages to running with diesel. Although they have a higher initial cost, operating expenses drop as there’s less need for regular maintenance and an efficiency increase of around 25-percent. Added bonus: that means a range increase of 25-percent too, which Cox believes will attract some offshore anglers into the diesel outboard fold. Finally, one of diesel’s greatest advantages is the ability to create torque – lots and lots of torque. According to Cox, about 60-percent more than a gasoline outboard with 50 additional horses. While this doesn’t make your boat go any faster, it does mean that these engines can swing larger props and a pair of Cox engines will be able to get large, heavy boats onto plane. In some cases, boats that would otherwise require triple outboards to get over the hump can perform just fine with the twin diesels.

Torque, and the corresponding ability to swing big props, is one of the major advantages a diesel holds over gasoline powerplants.

Will we see diesel outboards replace gasoline models in marinas all across the nation? Not likely. As we’ve noted they have plenty of down-sides as well as advantages. However, the Cox is likely to fill a very real niche in the boating world. Commercial outfits and governmental agencies that put thousands of hours a year on their engines are likely to find a diesel outboard quite attractive. And there’s a very real draw for those tender and fishboat owners, too.

Crazy about outboards? Be sure to read our feature on the Best Outboard Engines.

Written by: Lenny Rudow
With over two decades of experience in marine journalism, Lenny Rudow has contributed to publications including YachtWorld, boats.com, Boating Magazine, Marlin Magazine, Boating World, Saltwater Sportsman, Texas Fish & Game, and many others. Lenny is a graduate of the Westlawn School of Yacht Design, and he has won numerous BWI and OWAA writing awards.