Yanmar D-Torque diesel outboard
The outboard engine market is dominated by petrol engines which are mainly designed for the leisure market so it was a new experience to start up a diesel outboard, the first to bear the Yanmar badge for 10 years.
Yanmar ceased production of its D38 diesel outboard mainly because it could not meet modern emission standards. Since then the market has had not access to diesel outboards apart from the recently introduced Oxe 200hp unit. Now Yanmar is back in the diesel outboard business with their new 50hp unit that has been developed by Neander Shark in Germany.
Neander has spent more than 5 years developing this unique diesel outboard which is aimed squarely at the commercial and military markets where the demand for diesel power is high. In their tie up with Yanmar, Neander is responsible for the engineering development and production whilst Yanmar is undertaking the marketing, support and servicing on a worldwide basis. This has advantages for both participants with Shiori Nagata, the president of Yanmar International saying that they welcome being back in the diesel outboard market where demand for diesels is high and Lutz Lester the managing director of Neander saying the link with Yanmar gives them ready access and support for the engine in international markets.
The D-Torque diesel was originally developed for use as a motor-cycle engine but that market never developed and so development focussed on the outboard engine sector. The engine is a two cylinder unit with a unique twin crankshaft system. “We chose the twin crankshaft arrangement because it proved challenging to make a two cylinder diesel engine run smoothly,” commented Lester. “With the two crankshafts and two connecting rods for each piston the forces are much better balanced and we have a smooth running diesel”.
Weight was also an important consideration for an outboard motor and unusually for a diesel the main body of the engine is cast from aluminium. The three sections, the cylinder head, the cylinder block and the crankcase are joined together with long through bolts to get the necessary strength and rigidity. The engine is turbo-charged and intercooled and a common rail fuel injection system is used similar to those on most modern diesels. It is unusual to find a common rail system fitted to such a small diesel but this ensures that the engine not only meets modern emission standards but is also fuel efficient.
In other respects the D-Torque features current outboard motor technology. It has power trim and tilt and it is available in standard and long shaft versions. It can be fitted with remote control or tiller steering. The exhaust discharges through the propeller hub and a range of propellers sizes is available. Most of these features were found in the lower unit of the outboard which is a modified unit from the Selva range of outboards. In order to handle the torque of the diesel the internal parts for the leg have been strengthened from standard.
We tested the D-Torque on several small workboats/RIBs including a Tideman 550 constructed from high density polyethylene and a Zodiac Milpro 5.30 metre RIB. Start up was quick and smooth with no smoke in evidence but there was a significant noise difference when compared with a petrol unit. This continued when the throttle was opened and is probably part of the penalty of having diesel power but at no time was the noise oppressive and you soon got used to the difference. The torque for getting onto the plane was excellent and even the heavily loaded Milpro came onto the plane smoothly and quickly and would run at 20 knots with 5 people on board. The performance of a larger twin engine boat was equally impressive.
Yanmar see the market for the D-Torque as almost entirely in the military and commercial sectors. This partly because of the price, which is close to double that of an equivalent petrol outboard, should prove to be a significant deterrent for leisure users. This high initial price has to be balanced against a significant reduction in the fuel cost with consumption claimed to be around half that of a petrol outboard. A commercial operator using the engine on a daily basis would be able to overcome the high initial investment with fuel savings.
Then there is the availability of fuel with diesel being widely available compared with petrol and the fuel is the same as might be used by a mother ship of by other equipment such as trucks and generators. For a commercial operator this could be a significant advantage and for many military operators it has now been mandated that no petrol should be carried on ships that might operate RIBs or daughter craft.
Yanmar see a wide range of possible commercial and military users ranging from harbour craft through to the military. For contractors and dredging companies the advantages of diesel are obvious whilst, with its SOLAS approval the engine could work for rescue boats and MOB craft. Probably one of the largest potential markets will be for inshore fishing boats and for fish farm service craft and many governments are actively supporting a switch to diesel for these craft.
Yanmar consider that this new diesel outboard with its worldwide availability and support and servicing will be a game-changer. Whilst the current model on offer is rated at 50hp Yanmar considers this to be a very conservative rating and they anticipate that more powerful versions will be available once more operating experience has been gained. They are talking about possible 80hp versions which will widen the market considerably. Costs may also reduce as production is ramped up.
By Dag Pike
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