America's Definitive Marine Engine Magazine
Some aficionados adamantly insist that with powerboats what's most important is how many horses are on tap to spin the propeller. Agreed, acceleration and top speed are a factor. But in my book the reputation of the company that builds the engine is just as important as its horsepower rating. A good example is the John Deere PowerTech diesels.
In business since 1918, JD recently celebrated the building of its five millionth engine. Its power plants are found in trucks, tractors and boats the world over. The John Deere lineup include diesel motors rated from 49- to 600-horsepower. PowerTech marine diesels include 4.5 Liter , 6.8 Liter, 8.0 Liter and 12.5 Liter displacements with both naturally aspirated and turbocharged models available.
For the purposes of this discussion, we're most interested in JD's Turbo-charged 6.8L marine motor, denominated as the 6068TRFM. Simply put, this 1,000 pounds of gray iron castings and machined steel is an engineer's dream come true, and not only for its sophisticated design and performance. But also for its reliability.
The John Deere Marine Diesel M1 version of the 6068, rated 158 hp @ 1800 rpm, is intended for boats that run their engines 24-hours a day at wide-open throttle. That's more than 3,000 hours of severe duty logged every year. Think work boat tough. Think thousands of dollars worth of fuel bills.
The slightly more powerful John Deere Marine M2 version, rated 178-hp @ 1900 rpm, is intended for boats that run wide-open throttle (WOT) for up to 16 hours a day. Or a little less than 3,000 engine hours per year. The most powerful of the bunch is the John Deere Marine M3, rated 201 hp 2000 rpm. It's an appropriate choice for boats that throttle up to full power for only about four out of every 12 engine hours.Typically these boats would log no more than about 2,000 hours per year.
For the most part, the difference between John Deere M1, M2 and M3 horsepower is measured by fuel flow and turbo boost. The more pounds of diesel fuel and the greater the volume of air jammed into the combustion chamber, the higher the rpm and the stronger the output. As illustrated in the accompanying chart, all three variations on the theme burn nearly the same amount of fuel until mid-range rpm and higher. Notice how the higher revving M3, with 200 more rpm in its torque curve, burns more fuel at wide-open throttle.
It's also important to know the John Deere 6068 series marine diesels are high torque, low rpm motors. In other words the PowerTech engines thrive swinging a large diameter propeller at a low propeller shaft speed. And low propeller speeds are more efficient at converting every last drop of horsepower into forward motion. The added thrust of a big, slow turning propeller also pays big dividends in maneuverability. Docking is more precise because the propeller throws a bigger volume of water, forward or aft, at your command. For the same reasons close quarters maneuverings also improved. Aesthetics benefit from low rpm in that propeller shaft vibration is minimized, so you don't feel the propeller shaft resonated the deck beneath your feet.
In the beginning of this story we mentioned how the John Deere 6068s are turbo motors. Both the turbine and its exhaust manifold are water cooled, a configuration that quiets exhaust resonance, significantly reduces sound levels. The cooler engine room environment also extends the life of the turbine bearings. One really nice touch is the way John Deere engineers minimized the number of connections and hoses in the cooling system, greatly adding to the engine reliably. Fewer hoses and connections mean fewer chances to spring a leak.
No big surprise, given the start of the art, JD diesels are electronically controlled and self diagnostic. The electronic controls mean that a precise volume of fuel is injected into the combustion chamber at the exact moment it's required (relative to crankshaft position.). The ECM calculates fuel flow based on a number of variables such as rpm, throttle positions, engine temperature and the like. The net result is maximum horsepower and minimal fuel consumption. Beyond microchip electronics, another variable in the equation purely mechanical, a set of low-friction piston rings that pay big dividends by way of enviable fuel economy. Less horsepower is fled away to friction and freed up to do work. Personally, I like the way the engine can be pre-flighted (dipstick) from either side. The oil filter is remote mounted so its draining hot crankcase oil doesn't result in scorched fingers or bloodied knuckles.
Even though the Mean Time Between Overhaul (MTBE) runs for many thousands of hours, eventually the day will come when the engine requires repair. With that in mind the wet liners are replaceable. Rebuilds are easy with the replaceable wet cylinder liners and replaceable valve seats. Don't misunderstand, these component are designed to fail. It's just that when they do wear out it's easier to replace them. For those few souls who are unfair with the terminology wet cylinder liner, it simply means the piston bore is surrounded by coolant that carries away heat. Finally, PowerTech engine are clean machines, exhaust emissions US EPA Tier II compliant.
6068 Marine Diesel
|Bore X Stroke||4.56" x 5.06"|
|Horsepower||375 hp @ 1900 rpm|
|JD 6068||Fuel Flow||Gallons||Per Hour|