A diesel outboard engine light enough to be used on an inflatable.

The lightweight diesel outboard, seen by many in the commercial and military sectors as the holy grail in terms of small-craft propulsion, has been a long time coming ‒ that is, until now. British company Amaroq have been developing a 72kg 697cc 50hp diesel outboard engine, which following a UK patent was launched this year. Due to be revealed at the cancelled 2020 Seawork show, this motor will be demonstrated on the water to interested customers during the early part of 2021.

There has been a big demand from the military for this type of outboard, as this motor complies with NATO’s single-fuel directive. It will be hugely appealing to the commercial sector, especially the superyacht world where there is likewise a need for a single fuel supply. The crucial question is how this diesel outboard weighs some 30kg less than Suzuki’s DF50, 40kg less than Yamaha’s F50 and 100kg less than its only diesel outboard competitor, the turbocharged 50hp Yanmar Neander Dtorque. There will be many factors keeping weight down, but the fact that it is a 2-stroke diesel engine has a lot to do with it.

This motor revs to 5500rpm, just 500rpm short of what a 4-stroke petrol outboard engine spins to. It produces maximum torque at 4000rpm, which is about 500rpm short of the point where its petrol counterparts produce their peak torque. In terms of output, the Amaroq puts out 70Nm of torque measured at the prop shaft, which though short of the Dtorque’s 111Nm is unbeatable in terms of torque to weight. Making an exact torque comparison against a petrol outboard is hard, as none of the manufacturers publish any figures in this area, but a 50hp or even a 60hp petrol outboard will fall short of developing 70Nm.

As is to be expected, it uses common-rail fuel injection running at 2500 bar, direct to the cylinder head. Induction is via reed valve/crankcase and transfer ports, in the same manner as a 2-stroke petrol engine. Lubrication is also like a 2-stroke petrol engine – direct injection to the bearing components and cylinder bores, using a specific TC-W3 lubricant. Ignition is via controlled heater plugs when cold. When running under load, the engine, like any other diesel motor, relies on compression to provide the required engine temperature to ignite. It runs on a relatively low 15:1 compression ratio, unlike the 18:1 ratio normally used in conventional turbocharged 4-stroke diesel engines. However, unlike a conventional diesel engine, and like a conventional outboard, it uses a raw-water cooling system. This can cause a drop in engine temperature at low engine speed, especially at idle when the engine is not loaded. The heater plugs can ‘kick back in’ later if needed to maintain clean smokeless combustion, as this engine is EPA Tier 3, IMO Tier 2 and RCD compliant. I am told this feature is crucial when operating in the northern latitudes.   

Exhaust is conventional through the prop, and the gear ratio is 1:85. Shaft lengths are 15in, 20in and 25in, with trim being either manual or powered. The clutch is a mechanical dog type, and the three-bladed prop is available in pitches of between 7 and 15 in. The steering is either manual or remote. Price TBA.



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