Review: Nanni Diesel N4.50 engine

By: Andrew Norton

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  • Trade-A-Boat

The Nanni Diesel N4.50 marine engine is a logical repower for older displacement cruising boats.

Review: Nanni Diesel N4.50 engine
The Nanni Diesel N4.50 engine is an ideal choice for displacement hulls on accounts of its simplicity and low-end torque.

I have fond memories of being aboard Halvorsen hire boats on the Hawkesbury River ever since I was a child. The still mornings on the upper reaches around Sackville will be forever in my mind.

The last time I was aboard one of these hire boats was in 1976 when a friend and I took a 26 up to Windsor. When younger, my parents had always hired the 25 for the four of us but the 26 had one huge difference: a shower! It also had a hydraulic gearbox meaning fingertip control of throttle and gear shift, instead of the tram car gear lever of the 25.

Both these boats were powered by the old straight-six Chrysler Crown petrol engine which developed between 45 and 102 brake horsepower, with straight-drive to the prop.

A simple cooling loop along the keel kept the engines cool and prevented river silt from reaching the engines’ water passages. Running the engine at fast idle at anchor provided enough hot water to power the shower.

However the big drawback of the Crown was its points ignition, which was prone to failure in the damp bilge. Also, no gas blower was fitted, so starting always involved lifting the saloon floor to vent the engine compartment and crossing fingers, there was no stray spark.

Now as the Crown has long gone, what to fit? As this engine was de-rated right down to provide around 45bhp, a four-cylinder diesel of around 50bhp would do the job. This should provide at least 7kts maximum and an all-day cruise of around 6kts, adequate to reach a destination without getting old doing so. And one blue diesel stands out for its blend of torque, fuel efficiency and inherent simplicity, the Nanni Diesel N4.50.



The Nanni Diesel N4.50 has a Japanese Kubota tractor base engine and is marinised in France. The four-cylinder 2197cc base engine is Nanni Diesel’s most powerful naturally-aspirated marine engine and develops 49.3bhp at a relatively low 2800rpm. Combined with the standard TMC60 mechanical box with 2.15:1 reduction ratio, this translates to 1300rpm at the prop, right where it should be for a full displacement hull such as the 26. The overhead valve rockers are operated by pushrods and a gear-driven camshaft, so none of that newfangled belt-driven OHC stuff in this engine!

Indirect injection on the Nanni Diesel N4.50 marine engine with a 21:1 compression ratio and glow plug pre-heat is used. This way you never have to suffer embarrassment by trying to hand start the engine. The simple inline injection pump pressurises the fuel to 140bar or 2000psi, with timing set at 18-degrees BTDC, so like all mechanically injected diesels this one should be run continuously in its upper rpm range.

The voltage regulated alternator pumps out up to 100amp and like the starter motor is mounted high up and away from bilge water, though in a timber boat a high-sided drip tray should be fitted under the sump as diesel loves to rot planking. Heat exchanger cooling with a large capacity of 9lt is fitted but the Nanni Diesel N4.50 can be set-up with keel cooling. Diesels have lower exhaust gas temperatures than petrol engines, so running the dry exhaust out through the topsides shouldn’t be a problem providing it’s well lagged.

The standard engine instrumentation includes the C4 panel, which has an analogue tachometer with digital hour meter and analogue oil pressure and water temperature gauges, along with the usual audible alarms.

With the TMC60 box the Nanni Diesel N4.50 is 932mm long, 544mm wide and 632mm high, so it’s not exactly bulky. The dry weight is 235kg and, for you yachties out there, the saildrive version is 258kg and has a 2.15:1 gear ratio.

Like all small diesels primarily designed for yachts the gear-driven sea and belt-driven freshwater pumps and engine oil filter are at the forward end of the engine, along with the evacuation pump for the large six-litre oil sump. Unfortunately the drain point for this is at the front of the sump, so when the engine is inclined, getting all the old oil out should be fun! The recommended sump oil is SAE 15W40 with minimum CD rating and oil/filter changes every 200 hours or annually after the first 20 hours.



Tractor and industrial diesels make great choices for repowering displacement hulls because of their bottom end torque and inherent simplicity. The Nanni Diesel N4.50 marine engine complies with all current exhaust emission legislation and Nanni Diesel has servicing agents in all states of Australia, including three in Tassie and two in Queensland, so you’re never far from help.

To avoid blurred vision and giving the planking fasteners a workout at lower rpm, just remember to use flexible engine mounts, as there’s no way the Nanni Diesel N4.50 will ever match the almost vibration-free running of the Crown. 

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TYPE Inline four-cylinder diesel marine engine

RATED HP 49.3 at 2800rpm


BORE X STROKE 87 x 92.4mm

DRY WEIGHT Shaftdrive 235kg (inc. gearbox); saildrive 258kg (inc. gearbox)


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Originally published in Trade-A-Boat #454, July 2014. Why not subscribe today?


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