Review: John Deere 6068 diesel engine

By: Andrew Norton

Steel trawler Steel trawler
John Deere 6068 parts John Deere 6068 parts
John Deere marine diesel shaft John Deere marine diesel shaft

The John Deere 6068 diesel engine is built to last on boats.

Review: John Deere 6068 diesel engine
The John Deere 6068 diesel engine (or specifically, the 6068TFM50-M4 model) was rated to 800 running hours per year at this level of “tune”.

My mate Ken is a smart guy. He knows how to set-up diesel engines to not only get the best performance but also be easy to maintain and service.

So when he was shopping around for two diesel engines to replace the existing units in his 44-foot (13.4m) steel trawler yacht, he went for industrial marine engines and keel cooling with dry exhausts to ensure easy spares availability around Australia and much lower maintenance than heat exchanger cooling. As he pointed out to me, he didn’t want saltwater anywhere near his engines.

Ken’s boat Noble Tasman was built in Tassie from 6mm steel plating with closely-spaced transverse framing and longitudinal stringers. Built to survey she displaces 26 tonnes and has a masthead sloop-rigged steadying sail arrangement to provide some reaching or running sailing ability. Unlike most trawler yachts her size there’s a separate engineroom with work benches and standing headroom, not just an engine compartment. She’s more like a little ship than a motoryacht.


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Like many Tasmanian (and West Country UK) trawlers, main and wing engines are fitted with the auxiliary to port and keel cooling loops to starboard. The idea is that the wing engine has sufficient power and torque to get the boat home should the main fail, yet the starboard side is left free for net or pot hauling. This setup is also way cheaper than having two main engines.

Ken told me he was amazed about the attitude of engine importers. One wanted $30K upfront for just the main engine with a six-month delivery time, whereas the John Deere agent, Marine Diesel, could supply both main and wing engines for not much more than that. Ken reckons the entire repower, with his mates and him doing the installations, was less than $50K.



The John Deere 6068 series is available in naturally aspirated, straight turbo and turbo inter-cooled variants, with various outputs in each of these categories. Ken opted for the highest-output straight turbo model known as the 6068TFM50-M4 developing a maximum of 225 brake horsepower at 2600rpm. At this level of "tune" the John Deere 6068 is rated to 800 running hours per year.

Maximum torque output is 784Nm at 1800rpm, so the M4 is still under-stressed. A naturally aspirated diesel of this displacement would pump out around 450Nm, with an inter-cooled engine producing around 1200Nm. The M4 produces 450Nm at 1000rpm, 762Nm at 1600rpm, 734Nm at 2200rpm and 628Nm at 2600rpm.

The 6.8lt straight six has a gear-driven camshaft operating 12 valves via push rods and conventional inline mechanical direct fuel injection and a compression ratio of 17:1. Replaceable wet cylinder liners allow for easy liner renewal without having to hone the cylinder bores and fit oversize pistons and rings.

Standard is a 24V 50amp voltage regulated alternator and Ken has also fitted a 12V 140amp alternator for onboard electronics plus a hydraulic pump for powering the bowthruster. This arrangement is way better than an electric thruster because it has a 100 per cent duty cycle, not just bursts of a few seconds at a time.

In bobtail form (minus gearbox) the M4 is 1141mm long, 712mm wide and 881mm high and has a dry weight of 730kg. Marine Diesel fitted a Dong-I hydraulic gearbox with 3.1:1 gear reduction and the ability to handle full engine torque in forward or reverse. This added about 60kg.

Originally Noble Tasman had a V8 Perkins diesel main engine mounted relatively low in the bilge, made worse in that the gearbox output shaft was in line with the crankshaft. The 6068 has a step-down gearbox output flange and being an inline engine, allows for access not only right around the engine but also under it. Even my bulk would fit between the sump and bilge and I wouldn’t get dirty because Ken keeps the bilge clean enough to eat from. I love the company of people as nerdy as me!



Swinging a 33 by 23in three-bladed prop the M4 was able to rev out to 2600. Left over from the Perkins installation the prop was a lefthander, so Ken set the controls to provide ahead with the box in reverse where it runs through three gear sets instead of two for forward. There was some gear whine as the engine came under load but surprisingly, no turbo whine. The engine started instantly cold with no grey exhaust smoke appearing, nor was there any black smoke under maximum load.

As expected of a steel hull and a large diameter, slow turning prop there was the usual stern-gear rumble but no vibration from the flexibly-mounted engine. Noise levels were low too, considering no soundproofing was fitted to the engineroom deckhead, directly beneath the helm station.

Using the combination of responsive engine and bowthruster Ken was able to put Nobel Tasman exactly where wanted alongside a jetty.



How refreshing to test a diesel in a large-displacement cruiser. The M4 is beautifully engineered with traditional design features I love in diesels, and with regular scheduled maintenance will provide countless years of long-distance cruising enjoyment. Visit for more information.



Single 225hp John Deere 6068TFM50-M4 turbo-diesel engine with two-way runs on Lake Macquarie, NSW, using onboard GPS and John Deere fuel flow curves, calm water.


BHP (absorbed by prop)


































* Minimum and maximum recommended continuous cruising rpm.



TYPE Six-cylinder turbo-diesel engine

RATED HP 225 at 2600rpm

TORQUE 784Nm at 1800rpm


BORE X STROKE 106 x 127mm

WEIGHT 790kg (dry w/gearbox)


Originally published in Trade-A-Boat #246, October / November 2014. Why not subscribe today?

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